Exhibit 1.1

EXECUTION COPY

INTERSTATE POWER AND LIGHT COMPANY

(an Iowa corporation)

5.100% SERIES D CUMULATIVE PERPETUAL PREFERRED STOCK

(Liquidation Amount $25 Per Share)

UNDERWRITING AGREEMENT

Dated: March 14, 2013

TABLE OF CONTENTS

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INTERSTATE POWER AND LIGHT COMPANY

(an Iowa corporation)

5.100% SERIES D CUMULATIVE PERPETUAL PREFERRED STOCK

(Liquidation Amount $25 Per Share)

UNDERWRITING AGREEMENT

March 14, 2013

Barclays Capital Inc.

745 Seventh Avenue

New York, NY 10019

J.P. Morgan Securities LLC

383 Madison Avenue

New York, NY 10179

Robert W. Baird & Co. Incorporated

777 East Wisconsin Avenue

Milwaukee, WI 53202

Wells Fargo Securities, LLC

550 South Tryon Street

Charlotte, NC 28202

Ladies and Gentlemen:

Interstate Power and Light Company, an Iowa corporation (the “ Company ”), confirms its agreement (the “ Agreement ”) with Barclays Capital Inc. (“ Barclays ”), J.P. Morgan Securities LLC (“ J.P. Morgan ”), Robert W. Baird & Co. Incorporated (“ Baird ”) and Wells Fargo Securities, LLC (“ Wells Fargo Securities ”), and each of the other underwriters named on Schedule A hereto (collectively, the “ Underwriters ”, which term shall also include any underwriter substituted as hereinafter provided in Section 10 hereof), for whom Barclays, J.P. Morgan, Baird and Wells Fargo Securities are acting as representatives (in such capacity, the “ Representatives ”), with respect to the issue and sale by the Company and the purchase by the Underwriters, acting severally and not jointly, of the respective principal amounts set forth on Schedule A of 8,000,000 shares of the Company’s 5.100% Series D Cumulative Perpetual Preferred Stock, par value $0.01 per share, with a liquidation preference of $25 per share (the “ Securities ”). The Securities are to be issued in book-entry form and will be registered in the name of Cede & Co. as nominee of The Depository Trust Company (“ DTC ”) pursuant to the blanket letter agreement, dated October 1, 2003, between the Company and DTC.

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The Company understands that the Underwriters propose to make a public offering of the Securities as soon as the Representatives deem advisable after this Agreement has been executed and delivered.

The Company has filed with the Securities and Exchange Commission (the “ Commission ”) an automatic shelf registration statement on Form S-3 (No. 333-178577-02), including a base prospectus (the “ Base Prospectus ”), for the registration of certain securities, including the Securities, under the Securities Act of 1933, as amended (the “ 1933 Act ”), and the offering thereof from time to time in accordance with Rule 415 of the rules and regulations of the Commission under the 1933 Act (the “ 1933 Act Regulations ”), which automatic shelf registration statement became effective under the 1933 Act upon filing with the Commission. Promptly after execution and delivery of this Agreement, the Company will prepare and file a prospectus supplement relating to the Securities in accordance with the provisions of Rule 430B (“ Rule 430B ”) of the 1933 Act Regulations and paragraph (b) of Rule 424 (“ Rule 424(b) ”) of the 1933 Act Regulations. Any information included in such prospectus supplement that was omitted from the Registration Statement at the time it first became effective but that is deemed to be a part of and included in such registration statement pursuant to Rule 430B is referred to as “ Rule 430B Information .” Such registration statement, at any given time, including the amendments thereto to such time, the exhibits and schedules thereto, at such time, and the Rule 430B Information, is referred to herein as the “ Registration Statement .” The final prospectus supplement, together with the Base Prospectus, in the form first furnished to the Representatives by the Company for use in connection with the offering of the Securities, is referred to herein as the “ Prospectus .” A “ preliminary prospectus ” shall be deemed to refer to any preliminary prospectus supplement, together with the Base Prospectus, used in connection with the offering of the Securities that omitted the Rule 430B Information. For the purposes of this Agreement, all references to the “Registration Statement,” the “Prospectus” or any preliminary prospectus shall also be deemed to include all documents incorporated therein by reference pursuant to Item 12 of Form S-3 under the 1933 Act at such time and the documents otherwise deemed to be a part thereof or included therein by the 1933 Act Regulations. For purposes of this Agreement, all references to the Registration Statement, Prospectus or preliminary prospectus or to any amendment or supplement to any of the foregoing shall be deemed to include any copy filed with the Commission pursuant to its Electronic Data Gathering, Analysis and Retrieval system (“ EDGAR ”).

All references in this Agreement to financial statements and schedules and other information which are “contained,” “included” or “stated” (or other references of like import) in the Registration Statement, Prospectus or preliminary prospectus shall be deemed to mean and include all such financial statements and schedules and other information which are incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in, as of such applicable date, the Registration Statement, Prospectus or preliminary prospectus, as the case may be; and all references in this Agreement to the Registration Statement, amendments or supplements to the Registration Statement, Prospectus or preliminary prospectus shall be deemed to include the filing of any document under the Securities Exchange Act of 1934, as amended (the “ 1934 Act ”), which is incorporated by reference in or otherwise deemed by 1933 Act Regulations to be a part of or included in, as of such applicable date, the Registration Statement, Prospectus or preliminary prospectus, as the case may be.

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SECTION 1. Representations and Warranties .

(i) Registration Statement, Prospectus and Disclosure at Time of Sale . The Company meets the requirements for use of Form S-3 under the 1933 Act. The Registration Statement is an “automatic shelf registration statement,” as defined in Rule 405 (“ Rule 405 ”) of the 1933 Act, that automatically became effective not more than three years prior to the date hereof and no stop order suspending the effectiveness of the Registration Statement has been issued under the 1933 Act and no proceedings for that purpose have been instituted or are pending or, to the knowledge of the Company, are contemplated by the Commission, and any request on the part of the Commission for additional information has been complied with. The Company has not received from the Commission any notice pursuant to Rule 401(g)(2) (“ Rule 401(g)(2) ”) of the 1933 Act objecting to use of the automatic shelf registration statement form and the Company has not otherwise ceased to be eligible to use the automatic shelf registration form.

At the time the Registration Statement became effective (including without limitation any effective dates of any amendments thereto and each deemed effective date with respect to the Underwriters pursuant to Rule 430B(f)(2) of the 1933 Act Regulations) and at the Closing Time, the Registration Statement and any amendments and supplements thereto complied or will comply, as the case may be, in all material respects with the requirements of the 1933 Act and the 1933 Act Regulations and did not or will not, as the case may be, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading. Neither the Prospectus nor any amendments or supplements thereto, at the time the Prospectus or any such amendment or supplement was issued and at the Closing Time, included or will include an untrue statement of a material fact or omitted or will omit to state a material fact necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading. The representations and warranties in this subsection shall not apply to statements in or omissions from the Registration Statement or Prospectus made in reliance upon and in conformity with information furnished to the Company in writing by any Underwriter through the Representatives expressly for use in the Registration Statement (or any amendment thereto) or Prospectus (or any amendment thereto), it being understood and agreed that the only such information consists of the information contained in Schedule E hereto.

Each preliminary prospectus and the prospectus filed as part of the Registration Statement as originally filed or as part of any amendment thereto, or filed pursuant to Rule 424 under the 1933 Act, complied when so filed in all material respects with the 1933 Act Regulations and each preliminary prospectus and the Prospectus delivered to the Underwriters for use in connection with this offering was identical to the electronically transmitted copies thereof filed with the Commission pursuant to EDGAR, except to the extent permitted by Regulation S-T.

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As of the Applicable Time (as defined below), neither (x) the Issuer Free Writing Prospectus(es) (as defined below) listed on Schedule B and the Statutory Prospectus (as defined below), all considered together (collectively, the “ General Disclosure Package ”), nor (y) any individual Issuer Free Writing Prospectus, when considered together with the General Disclosure Package, will include any untrue statement of a material fact or omit to state any material fact necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading.

At the time of filing the Registration Statement, at the earliest time thereafter that the Company or another offering participant made a bona fide offer (within the meaning of Rule 164(h)(2) of the 1933 Act Regulations) of the Securities and at the date hereof, the Company was not and is not an “ineligible issuer,” as defined in Rule 405.

As used in this subsection and elsewhere in this Agreement:

“ Applicable Time ” means 3:05 P.M. (Eastern time) on the date of this Agreement.

“ Statutory Prospectus ” as of any time means the prospectus relating to the Securities that is included in the Registration Statement immediately prior to that time, including any document incorporated by reference therein and any preliminary or other prospectus supplement deemed to be a part thereof.

“ Issuer Free Writing Prospectus ” means any “issuer free writing prospectus,” as defined in Rule 433 (“ Rule 433 ”) of the 1933 Act Regulations, relating to the Securities that (i) is required to be filed with the Commission by the Company or (ii) is exempt from filing pursuant to Rule 433(d)(5)(i) because it contains a description of the Securities or of the offering that does not reflect the final terms, in each case in the form filed or required to be filed with the Commission or, if not required to be filed, in the form retained in the Company’s records pursuant to Rule 433(g) or is exempt from filing pursuant to Rule 433(d)(8)(i) relating to road shows.

Each Issuer Free Writing Prospectus, as of its issue date and at all subsequent times through the completion of the public offer and sale of the Securities or until any earlier date that the issuer notified or notifies the Representatives as described in the next sentence, did not, does not and will not include any information that conflicted, conflicts or will conflict with the information contained in the Registration Statement or the Prospectus, including any document incorporated by reference therein and any preliminary or other prospectus deemed to be a part thereof that has not been superseded or modified. If at any time following issuance of an Issuer Free Writing Prospectus there occurred or occurs an event or development as a result of which such Issuer Free Writing Prospectus included or would include an untrue statement of a material fact or omitted or would omit to state a material fact necessary in order to make the statements therein, in light of the circumstances, not misleading, the Company has notified or will notify

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promptly the Representatives so that any use of such Issuer Free Writing Prospectus may cease until it is amended or supplemented. The foregoing two sentences do not apply to statements in or omissions from any Issuer Free Writing Prospectus based upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives expressly for use therein, it being understood and agreed that the only such information consists of the information contained in Schedule E hereto.

(ii) Company is a Well-Known Seasoned Issuer . (A) At the time of filing the Registration Statement, (B) at the time of the most recent amendment thereto for the purposes of complying with Section 10(a)(3) of the 1933 Act (whether such amendment was by post-effective amendment, by incorporated report filed pursuant to Section 13 or 15(d) of the 1934 Act or by form of prospectus) and (C) as of the date hereof, the Company was and is a “well-known seasoned issuer” as defined in Rule 405.

(iii) Incorporated Documents .

(A) The documents incorporated or deemed to be incorporated by reference in the Registration Statement, the Statutory Prospectus and the Prospectus, at the time they were or hereafter are filed with the Commission, complied or will comply, as the case may be, in all material respects with the requirements of the 1934 Act and the rules and regulations of the Commission thereunder (the “ 1934 Act Regulations ”), as applicable, and, when read together with the other information in the Registration Statement, the Statutory Prospectus and the Prospectus, did not or will not, as the case may be, contain an untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading.

(B) The description of regulatory matters to which the Company is subject, as disclosed in the Company’s filings with the Commission under the 1934 Act and the 1934 Act Regulations and as incorporated by reference into the Registration Statement, is true and correct in all material respects, except to the extent such description in any specific filing has been superseded, updated or supplemented by such description in a subsequent filing under the 1934 Act or the 1934 Act Regulations made prior to the date hereof or by such description in the Statutory Prospectus and the Prospectus.

(iv) Independent Accountants . Deloitte & Touche LLP, which has certified certain financial statements of the Company and its consolidated subsidiaries and delivered its report with respect to the audited financial statements of the Company and related schedules included in the Registration Statement, the Statutory Prospectus and the Prospectus, is an independent registered public accounting firm with respect to the Company and its subsidiaries as required by the 1933 Act and the rules of the Public Company Accounting Oversight Board.

(v) Financial Statements . The financial statements included in the Registration Statement, the Statutory Prospectus and the Prospectus, together with the

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related schedules and notes, present fairly in all material respects the financial position of the Company and its consolidated subsidiaries at the dates indicated and the statements of income, changes in common equity and cash flows of the Company and its consolidated subsidiaries for the periods specified; said financial statements have been prepared in conformity with generally accepted accounting principles (“ GAAP ”) applied on a consistent basis throughout the periods involved. The supporting schedules included in the Registration Statement, if any, present fairly in all material respects in accordance with GAAP the information required to be stated therein. The selected financial data and the summary financial information included in the Statutory Prospectus and the Prospectus present fairly in all material respects the information shown therein and have been compiled on a basis consistent with that of the audited financial statements included in the Registration Statement. The interactive data in eXtensible Business Reporting Language (“ XBRL ”) incorporated by reference into the Registration Statement, the Statutory Prospectus and the Prospectus present fairly in all material respects the information called for and was prepared in accordance with the Commission’s rules and guidelines applicable thereto.

(vi) No Material Adverse Change in Business . Since the respective dates as of which information is given in the Registration Statement, the General Disclosure Package and the Prospectus, except as otherwise stated therein, (A) there has been no material adverse change in the condition, financial or otherwise, earnings or business affairs of the Company and its subsidiaries considered as one enterprise, whether or not arising in the ordinary course of business nor have there been any developments involving a prospective material adverse change of the Company and its subsidiaries considered as one enterprise, whether or not arising in the ordinary course of business (a “ Material Adverse Effect ”), (B) there have been no transactions entered into by the Company or any of its subsidiaries, other than those in the ordinary course of business, which are material with respect to the Company and its subsidiaries, and (C) except for regular dividends on the common stock, $2.50 par value per share, of the Company and the 8.375% Series B Cumulative Preferred Stock, $0.01 par value per share, of the Company in amounts per share that are consistent with past practice, or the terms of such preferred stock, there has been no dividend or distribution of any kind declared, paid or made by the Company on any class of its capital stock.

(vii) Good Standing of the Company . The Company has been duly organized and is validly existing as a corporation under the laws of the State of Iowa and has corporate power and authority to own, lease and operate its properties and to conduct its business as described in the General Disclosure Package and the Prospectus and to enter into and perform its obligations under this Agreement; and the Company is duly qualified as a foreign corporation to transact business and is in good standing in each other jurisdiction in which such qualification is required, whether by reason of the ownership or leasing of property or the conduct of business, except where the failure so to qualify or to be in good standing would not result in a Material Adverse Effect.

(viii) No Significant Subsidiaries . The Company has no “significant subsidiary” as defined in Rule 1-02 of Regulation S-X.

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(ix) Capitalization . The authorized, issued and outstanding capital stock of the Company as of the date hereof is set forth in the General Disclosure Package and the Prospectus in the column entitled “Actual” under the caption “Capitalization” (except for subsequent issuances, if any, pursuant to this Agreement, pursuant to existing reservations, agreements or employee benefit plans, or pursuant to the exercise of convertible securities or options outstanding on the date hereof or pursuant to any dividend reinvestment plan). All of the issued and outstanding shares of capital stock of the Company have been duly authorized and validly issued and are fully paid and non-assessable and none of the outstanding shares of capital stock of the Company were issued in violation of the preemptive or other similar rights of any securityholder of the Company. As of the date hereof, except for 6,000,000 shares of the Company’s 8.375% Series B Cumulative Preferred Stock, all of the issued and outstanding shares of the capital stock of the Company are owned by Alliant Energy Corporation, a Wisconsin corporation (the “ Parent ”), free and clear of all liens, encumbrances, equities or claims. Immediately prior to the Closing Time, except for 6,000,000 shares of the Company’s 8.375% Series B Cumulative Preferred Stock, all of the issued and outstanding shares of capital stock of the Company will be owned directly by the Parent, free and clear of all liens, encumbrances, equities or claims. The Parent is a “holding company” as defined under the Public Utility Holding Company Act of 2005 (the “ 2005 Act ”). The Company is a “subsidiary company” within the Alliant Energy Corporation “holding company system,” each as defined under the 2005 Act.

(x) Authorization of Agreement . The Company has all requisite corporate power and authority to execute and deliver this Agreement and to perform its obligations hereunder. This Agreement has been duly authorized, executed and delivered by the Company.

(xi) Authorization of the Transfer Agency Agreement . The Company has all requisite corporate power and authority to perform its obligations under the Transfer Agent Services Agreement (the “ Transfer Agency Agreement ”), dated January 8, 2006, between the Parent and Wells Fargo Bank, National Association. The Transfer Agency Agreement has been duly authorized, executed and delivered by the Parent and constitutes a valid and binding agreement of the Company enforceable against the Company in accordance with its terms.

(xii) Authorization of the Securities . The Securities have been duly authorized and, at the Closing Time, will have been duly executed by the Company and, when issued and delivered against payment of the purchase price therefor as provided for in this Agreement, will be validly issued, fully paid and non-assessable, and will have the terms set forth in the Articles of Amendment Regarding Designation and Authorization of the Securities.

(xiii) Description of the Securities . The statements relating to the Securities contained in the Prospectus and the General Disclosure Package conform in all material respects to the terms of the Securities.

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(xiv) Absence of Defaults and Conflicts . Neither the Company nor any of its subsidiaries is (A) in violation of its articles of incorporation or bylaws, each as amended, or (B) in default in the performance or observance of any obligation, agreement, covenant or condition contained in any contract, indenture, mortgage, deed of trust, loan or credit agreement, note, lease or other agreement or instrument to which the Company or any of its subsidiaries is a party or by which it or any of them may be bound, or to which any of the property or assets of the Company or any of its subsidiaries is subject (collectively, “ Agreements and Instruments ”), except for any such defaults with respect to this clause (B) that would not result in a Material Adverse Effect; and the execution, delivery and performance of this Agreement and the Securities, the consummation of the transactions contemplated herein and in the Registration Statement (including the issuance and sale of the Securities and the use of the proceeds from the sale of the Securities as described in the General Disclosure Package and the Prospectus under the caption “Use of Proceeds”), and compliance by the Company with its obligations hereunder and under the Transfer Agency Agreement and the Securities have been duly authorized by all necessary corporate action and do not and will not, whether with or without the giving of notice or passage of time or both, conflict with or constitute a breach of, or default or a Repayment Event (as defined below) under, or result in the creation or imposition of any lien, charge or encumbrance upon any property or assets of the Company or any of its subsidiaries pursuant to, the Agreements and Instruments except for such conflicts, breaches or defaults or liens, charges or encumbrances that, singly or in the aggregate, would not result in a Material Adverse Effect, nor will such action result in any violation of the provisions of (I) the articles of incorporation or bylaws, each as amended, of the Company or any of its subsidiaries or (II) any applicable law, statute, rule, regulation, judgment, order, writ or decree of any government, government instrumentality or court, domestic or foreign, having jurisdiction over the Company or any of its subsidiaries or any of their assets, properties or operations, except for any such violations with respect to this clause (II) as would not, individually or in the aggregate, result in a Material Adverse Effect. As used herein, a “ Repayment Event ” means any event or condition which gives the holder of any note, debenture or other evidence of indebtedness (or any person acting on such holder’s behalf) the right to require the repurchase, redemption or repayment of all or a portion of such indebtedness by the Company or any of its subsidiaries.

(xv) Absence of Labor Disputes . No labor dispute with the employees of the Company or any of its subsidiaries exists or, to the knowledge of the Company, is imminent, which would reasonably be expected to result in a Material Adverse Effect.

(xvi) Absence of Proceedings . Except as disclosed in the General Disclosure Package and the Prospectus, there is no action, suit, proceeding, inquiry or investigation before or brought by any court or governmental agency or body, domestic or foreign, now pending, or, to the knowledge of the Company, threatened, against or affecting the Company or any of its subsidiaries which would reasonably be expected to result in a Material Adverse Effect, or which would reasonably be expected to materially and adversely affect (A) the properties or assets of the Company and its subsidiaries or (B) the consummation of the transactions contemplated by this Agreement or the performance by the Company of its obligations hereunder. The aggregate of all pending

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legal or governmental proceedings to which the Company or any of its subsidiaries is a party or of which any of their respective property or assets is the subject which are not described in the General Disclosure Package and the Prospectus, including ordinary routine litigation incidental to the business of the Company and its subsidiaries, would not reasonably be expected to result in a Material Adverse Effect. The Registration Statement is not the subject of a pending proceeding or examination under Section 8(d) or 8(e) of the 1933 Act, and the Company is not the subject of a pending proceeding under Section 8A of the 1933 Act in connection with the offering of the Securities.

(xvii) Accuracy of Exhibits . There are no contracts or documents which are required to be described in the Registration Statement, the General Disclosure Package, the Prospectus or the documents incorporated by reference therein or to be filed as exhibits thereto which have not been so described and filed as required except that this Agreement, an opinion of Godfrey & Kahn, S.C. with respect to the validity of the Securities and the Certificate of Designation setting forth the terms of the Securities will be filed as exhibits to a Current Report on Form 8-K which shall be filed with the Commission in accordance with the 1934 Act and the 1934 Act Regulations prior to the filing of the Prospectus.

(xviii) Absence of Further Requirements . No filing with, or authorization, approval, consent, license, order, registration, qualification or decree of, any court or governmental authority or agency is necessary or required for the performance by the Company of its obligations hereunder, in connection with the offering, issuance or sale of the Securities hereunder or the consummation of the transactions contemplated by this Agreement or for the performance of the Transfer Agency Agreement by the Company, except (A) such as have been already obtained, (B) such as may be required under the 1933 Act or the 1933 Act Regulations or state securities laws, and (C) such as may be required by the 2005 Act, solely with respect to the filing of a Report of Securities Issued required to be made with the Federal Energy Regulatory Commission subsequent to the Closing Time (such 2005 Act filings to be made by the Company).

(xix) Possession of Licenses and Permits . Except as described in the General Disclosure Package and the Prospectus, (A) the Company and its subsidiaries possess such permits, licenses, approvals, consents and other authorizations (collectively, “ Governmental Licenses ”) issued by the appropriate federal, state, local or foreign regulatory agencies or bodies necessary to conduct the business now operated by them in the manner described in the Prospectus except where the failure to possess any such Governmental Licenses would not have a Material Adverse Effect, (B) the Company and its subsidiaries are in compliance with the terms and conditions of all such Governmental Licenses, except where the failure so to possess or comply would not, singly or in the aggregate, have a Material Adverse Effect, (C) all of the Governmental Licenses are valid and in full force and effect, except where the invalidity of such Governmental Licenses or the failure of such Governmental Licenses to be in full force and effect would not have a Material Adverse Effect and (D) neither the Company nor any of its subsidiaries has received any notice of proceedings relating to the revocation or modification of any such Governmental Licenses which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would result in a Material Adverse Effect. Without limiting

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the foregoing, the Company has received an authorization letter from the Federal Energy Regulatory Commission (the “ FERC Authorization ”), dated October 14, 2011, authorizing the issuance of the Securities and such issuance is in compliance with the terms and conditions of such authorization. Such authorization is in full force and effect and has not been amended, supplemented or otherwise modified. No proceeding to review, suspend, limit, modify, restrict or revoke such authorization has been instituted.

(xx) Title to Property . The Company and its subsidiaries have good and marketable title to all real property owned by the Company and its subsidiaries and good title to all other properties owned by them, in each case, free and clear of all mortgages, pledges, liens, security interests, claims, restrictions or encumbrances of any kind except such as (A) are described in the General Disclosure Package and the Prospectus or (B) do not, singly or in the aggregate, materially affect the value of such property and do not interfere with the use made and proposed to be made of such property by the Company or any of its subsidiaries; and all of the leases and subleases material to the business of the Company and its subsidiaries, considered as one enterprise, and under which the Company or any of its subsidiaries holds properties described in the General Disclosure Package and the Prospectus, are in full force and effect, and neither the Company nor any of its subsidiaries has any notice of any material claim of any sort that has been asserted by anyone adverse to the rights of the Company or any of its subsidiaries under any of the leases or subleases mentioned above, or affecting or questioning the rights of the Company or any of its subsidiaries to the continued possession of the leased or subleased premises under any such lease or sublease, except where such would not have a Material Adverse Effect.

(xxi) Investment Company Act . The Company is not, and upon the issuance and sale of the Securities as herein contemplated and the application of the net proceeds therefrom as described in the General Disclosure Package and the Prospectus will not be, an “investment company” or an entity “controlled” by an “investment company” as such terms are defined in the Investment Company Act of 1940, as amended.

(xxii) Environmental Laws . Except as described in the General Disclosure Package and except as would not, singly or in the aggregate, result in a Material Adverse Effect, (A) neither the Company nor any of its subsidiaries is in violation of any federal, state, local or foreign statute, law, rule, regulation, ordinance, code, policy or rule of common law or any judicial or administrative interpretation thereof, including any judicial or administrative order, consent, decree or judgment, relating to pollution or protection of human health, the environment (including, without limitation, ambient air, surface water, groundwater, land surface or subsurface strata) or wildlife, including, without limitation, laws and regulations relating to the release or threatened release of chemicals, pollutants, contaminants, wastes, toxic substances, hazardous substances, petroleum or petroleum products (collectively, “ Hazardous Materials ”) or to the manufacture, processing, distribution, use, treatment, storage, disposal, transport or handling of Hazardous Materials (collectively, “ Environmental Laws ”), (B) the Company and its subsidiaries have all permits, authorizations and approvals required under any applicable Environmental Laws and are each in compliance with their requirements, (C) there are no pending or, to the knowledge of the Company, threatened

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administrative, regulatory or judicial actions, suits, demands, demand letters, claims, liens, notices of noncompliance or violation, investigations or proceedings relating to any Environmental Laws against the Company or any of its subsidiaries and (D) there are no events or circumstances that might reasonably be expected to form the basis of an order for clean-up or remediation, or an action, suit or proceeding by any private party or governmental body or agency, against or affecting the Company or any of its subsidiaries relating to Hazardous Materials or any Environmental Laws.

(xxiii) Accounting Controls and Disclosure Controls . The Company together with its consolidated subsidiaries maintains a system of internal accounting controls sufficient to provide reasonable assurances that (A) transactions are executed in accordance with management’s general or specific authorization, (B) transactions are recorded as necessary to permit preparation of financial statements in conformity with GAAP and to maintain accountability for assets, (C) access to assets is permitted only in accordance with management’s general or specific authorization, (D) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences and (E) interactive data in XBRL included or incorporated by reference in the Registration Statement, the Statutory Prospectus and the Prospectus is prepared in accordance with the Commission’s rules and guidelines applicable thereto. Except as described in the General Disclosure Package and the Prospectus, since the end of the Company’s most recent audited fiscal year, there has been (I) no material weakness in the Company’s internal control over financial reporting (whether or not remediated) and (II) no change in the Company’s internal control over financial reporting that has materially affected, or is reasonably likely to materially affect, the Company’s internal control over financial reporting.

The Company together with its consolidated subsidiaries employs disclosure controls and procedures that are designed to ensure that information required to be disclosed by the Company in the reports that it files or submits under the 1934 Act is recorded, processed, summarized and reported, within the time periods specified in the Commission’s rules and forms, and is accumulated and communicated to the Company’s management, including its principal executive officer or officers and principal financial officer or officers, as appropriate, to allow timely decisions regarding disclosure.

(xxiv) No Price Stabilization or Manipulation . The Company has not taken, directly or indirectly, any action designed to or that could reasonably be expected to cause or result in any stabilization or manipulation of the price of the Securities.

SECTION 2. Sale and Delivery to Underwriters; Closing .

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Payment of the purchase price for, and delivery of certificates for, the Securities shall be made at the offices of the Company at 4902 North Biltmore Lane, Madison, Wisconsin 53718, or at such other place as shall be agreed upon by the Representatives and the Company, at 10:00 A.M. (Eastern time) on the fifth business day after the date hereof (unless postponed in accordance with the provisions of Section 10), or such other time not later than ten business days after such date as shall be agreed upon by the Representatives and the Company (such time and date of payment and delivery being herein called the “ Closing Time ”).

The Company will deliver, or cause to be delivered the Securities to the Representatives for the account of each Underwriter against payment by or on behalf of such Underwriter of the purchase price for the Securities by wire transfer of immediately available funds to the account specified by the Company to the Representatives at least twenty-four hours in advance, by causing a registrar to register the Securities in the name of Cede & Co., or such other nominee as DTC may designate, and shall cause DTC to credit the Securities to the account of Wells Fargo Securities at DTC. It is understood that each Underwriter has authorized the Representatives, for its account, to accept delivery of, receipt for, and make payment of the purchase price for, the Securities that it has agreed to purchase. Wells Fargo Securities, individually and not as a representative of the Underwriters, may (but shall not be obligated to) make payment of the purchase price for all or a portion of the Securities to be purchased by any Underwriter whose funds have not been received by the Closing Time, but such payment shall not relieve such Underwriter from its obligations hereunder.

SECTION 3. Covenants of the Company . The Company covenants with each Underwriter as follows:

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SECTION 4. Payment of Expenses .

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SECTION 5. Conditions of Underwriters’ Obligations . The obligations of the several Underwriters hereunder are subject to the accuracy in all material respects of the representations and warranties of the Company contained in Section 1 hereof or in certificates of any officer of the Company or any subsidiary of the Company delivered pursuant to the provisions hereof, to the performance in all material respects by the Company of its covenants and other obligations hereunder, and to the following further conditions:

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SECTION 6. Indemnification .

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(i) against any and all loss, liability, claim, damage and expense whatsoever, as incurred, arising out of any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement (or any amendment thereto), or the omission or alleged omission therefrom of a material fact required to be stated therein or necessary to make the statements therein not misleading or arising out of any untrue statement or alleged untrue statement of a material fact included in any preliminary prospectus, any Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto), or the omission or alleged omission therefrom of a material fact necessary in order to make the statements therein, in light of the circumstances under which they were made, not misleading;

(ii) against any and all loss, liability, claim, damage and expense whatsoever, as incurred, to the extent of the aggregate amount paid in settlement of any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or of any claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission; provided that (subject to Section 6(d) below) any such settlement is effected with the written consent of the Company; and

(iii) against any and all expense whatsoever, as incurred (including the fees and disbursements of counsel chosen by the Representatives), reasonably incurred in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever based upon any such untrue statement or omission, or any such alleged untrue statement or omission, to the extent that any such expense is not paid under clause (i) or (ii) above;

provided , however , that this indemnity shall not apply to any loss, liability, claim, damage or expense to the extent arising out of any untrue statement or omission or alleged untrue statement or omission made in reliance upon and in conformity with written information furnished to the Company by any Underwriter through the Representatives expressly for use in the Registration Statement (or any amendment thereto), or any preliminary prospectus, Issuer Free Writing Prospectus or the Prospectus (or any amendment or supplement thereto), it being understood and agreed that the only such information consists of the information contained in Schedule E hereto.

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SECTION 7. Contribution . If the indemnification provided for in Section 6 hereof is for any reason unavailable to or insufficient to hold harmless an indemnified party in respect of any losses, liabilities, claims, damages or expenses referred to therein, then each indemnifying party shall contribute to the aggregate amount of such losses, liabilities, claims, damages and expenses incurred by such indemnified party, as incurred, (a) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriters on the other hand from the offering of the Securities pursuant to this Agreement or (b) if the allocation provided by clause (a) is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause (a) above but also the relative fault of the Company on the one hand and of the Underwriters on the other hand in connection with the statements or omissions, which resulted in such losses, liabilities, claims, damages or expenses, as well as any other relevant equitable considerations.

The relative benefits received by the Company on the one hand and the Underwriters on the other hand in connection with the offering of the Securities pursuant to this Agreement shall be deemed to be in the same respective proportions as the total net proceeds from the offering of the Securities pursuant to this Agreement (before deducting expenses) received by the Company and the total underwriting discount received by the Underwriters, in each case as set forth on the cover of the Prospectus, bear to the aggregate initial public offering price of the Securities as set forth on such cover.

The relative fault of the Company on the one hand and the Underwriters on the other hand shall be determined by reference to, among other things, whether any such untrue or alleged untrue statement of a material fact or omission or alleged omission to state a material fact relates to information supplied by the Company or by the Underwriters and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission.

The Company and the Underwriters agree that it would not be just and equitable if contribution pursuant to this Section 7 were determined by pro rata allocation (even if the Underwriters were treated as one entity for such purpose) or by any other method of allocation which does not take account of the equitable considerations referred to above in this Section 7. The aggregate amount of losses, liabilities, claims, damages and expenses incurred by an indemnified party and referred to above in this Section 7 shall be deemed to include any legal or

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other expenses reasonably incurred by such indemnified party in investigating, preparing or defending against any litigation, or any investigation or proceeding by any governmental agency or body, commenced or threatened, or any claim whatsoever based upon any such untrue or alleged untrue statement or omission or alleged omission.

Notwithstanding the provisions of this Section 7, no Underwriter shall be required to contribute any amount in excess of the amount by which the total price at which the Securities underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages which such Underwriter has otherwise been required to pay by reason of any such untrue or alleged untrue statement or omission or alleged omission.

No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the 1933 Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation.

For purposes of this Section 7, each person, if any, who controls an Underwriter within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act shall have the same rights to contribution as such Underwriter, and each director of the Company, each officer of the Company who signed the Registration Statement, and each person, if any, who controls the Company within the meaning of Section 15 of the 1933 Act or Section 20 of the 1934 Act shall have the same rights to contribution as the Company. The Underwriters’ respective obligations to contribute pursuant to this Section 7 are several in proportion to the principal amount of Securities set forth opposite their respective names in Schedule A hereto and not joint.

SECTION 8. Representations, Warranties and Agreements to Survive Delivery . All representations, warranties and agreements contained in this Agreement or in certificates of officers of the Company or any of its subsidiaries submitted pursuant hereto shall remain operative and in full force and effect, regardless of any investigation made by or on behalf of any Underwriter or controlling person, or by or on behalf of the Company, and shall survive delivery of and payment for the Securities to the Underwriters.

SECTION 9. Termination of Agreement .

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SECTION 10. Default by One or More of the Underwriters . If one or more of the Underwriters shall fail at the Closing Time to purchase the Securities which it or they are obligated to purchase under this Agreement (the “ Defaulted Securities ”), the Representatives shall have the right, within 24 hours thereafter, to make arrangements for one or more of the non-defaulting Underwriters, or any other underwriters, to purchase all, but not less than all, of the Defaulted Securities in such amounts as may be agreed upon and upon the terms herein set forth; if, however, the Representatives shall not have completed such arrangements within such 24-hour period, then:

No action taken pursuant to this Section 10 shall relieve any defaulting Underwriter from liability in respect of its default.

In the event of any such default which does not result in a termination of this Agreement, either the Representatives or the Company shall have the right to postpone the Closing Time for a period not exceeding seven days in order to effect any required changes in the Registration Statement or Prospectus or in any other documents or arrangements. As used herein, the term “Underwriter” includes any person substituted for an Underwriter under this Section 10.

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SECTION 11. Certain Agreements of the Underwriters . Each Underwriter hereby represents and agrees that:

(a) it has not used, authorized the use of, referred to or participated in the planning for use of, and will not use, authorize the use of, refer to or participate in the planning for use of, any “free writing prospectus”, as defined in Rule 405 under the 1933 Act (which term includes use of any written information furnished to the Commission by the Company and not incorporated by reference into the Registration Statement and any press release issued by the Company) other than (i) a free writing prospectus that contains no “issuer information” (as defined in Rule 433(h)(2) under the 1933 Act) that was not included (including through incorporation by reference) in any preliminary prospectus or a previously filed Issuer Free Writing Prospectus, (ii) any Issuer Free Writing Prospectus listed on Schedule B or prepared pursuant to Section 3(o) above (including any electronic road show), (iii) any free writing prospectus prepared by such underwriter and approved by the Company in advance in writing, or (iv) any free writing prospectus prepared by or on behalf of such underwriter that would not be required to be filed with the Commission; and

(b) it has not and will not, without the prior written consent of the Company, use any free writing prospectus that contains the final terms of the Securities, unless such terms have previously been included in a free writing prospectus filed with the Commission; provided that the Underwriters may use a term sheet substantially in the form of Schedule D hereto without the consent of the Company; provided , further , that any Underwriter using such term sheet shall notify the Company, and provide a copy of such term sheet to the Company, prior to, or substantially concurrently with, the first use of such term sheet and, provided , further , that nothing in this Section 11 shall prevent the Underwriters from transmitting or otherwise making use of one or more customary “Bloomberg Screens” to offer the Securities or convey final pricing terms thereof that contain only information contained in the term sheet attached as Schedule D hereto.

SECTION 12. Notices . All notices and other communications hereunder shall be in writing and shall be deemed to have been duly given if mailed or transmitted by any standard form of telecommunication. Notices to the Underwriters shall be directed to the Representatives at:

Barclays Capital Inc.

745 Seventh Avenue

New York, NY 10019

Attention: Syndicate Registration

Facsimile: (646) 834-8133

J.P. Morgan Securities LLC

383 Madison Avenue

New York, NY 10179

Attention: Investment Grade Syndicate Desk – 3rd Floor

Facsimile: (212) 834-6081

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Robert W. Baird & Co. Incorporated

777 East Wisconsin Avenue

Milwaukee, WI 53202

Attention: Matt Fiorita

Facsimile: (414) 298-7478

Wells Fargo Securities, LLC

550 South Tryon Street, 5th Floor

Charlotte, NC 28202

Attention: Transaction Management

Facsimile: (704) 410-0326

and notices to the Company shall be directed to it at 4902 North Biltmore Lane, Madison, Wisconsin 53718, attention of John E. Kratchmer, Vice President and Treasurer, or his successor (telephone: (608) 458-3413; facsimile: (608) 458-0124).

SECTION 13. Parties . This Agreement shall inure to the benefit of and be binding upon each of the Underwriters and the Company and their respective successors. Nothing expressed or mentioned in this Agreement is intended or shall be construed to give any person, firm or corporation, other than the Underwriters and the Company and their respective successors and the controlling persons and officers and directors referred to in Sections 6 and 7 and their heirs and legal representatives, any legal or equitable right, remedy or claim under or in respect of this Agreement or any provision herein contained. This Agreement and all conditions and provisions hereof are intended to be for the sole and exclusive benefit of the Underwriters and the Company and their respective successors, and said controlling persons and officers and directors and their heirs and legal representatives, and for the benefit of no other person, firm or corporation. No purchaser of Securities from any Underwriter shall be deemed to be a successor by reason merely of such purchase.

SECTION 14. No Fiduciary Duty . The Company hereby acknowledges that the Underwriters will be acting pursuant to a contractual relationship on an arm’s length basis and in no event do the parties intend that the Underwriters act or be responsible as a fiduciary to the Company, its management, stockholders, creditors or any other person. The Company and the Underwriters each hereby expressly disclaim any fiduciary relationship and agree they are each responsible for making their own independent judgments with respect to any transactions entered into between them.

SECTION 15. GOVERNING LAW AND TIME . THIS AGREEMENT AND ALL DISPUTES, CONTROVERSIES OR CLAIMS ARISING OUT OF OR RELATING TO THIS AGREEMENT OR A BREACH HEREOF SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK. EXCEPT AS OTHERWISE SET FORTH HEREIN, SPECIFIED TIMES OF DAY REFER TO NEW YORK CITY TIME.

SECTION 16. Effect of Headings . The Article and Section headings herein and the Table of Contents are for convenience only and shall not affect the construction hereof.

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SECTION 17. Counterparts . This Agreement may be executed in one or more counterparts and, if executed in more than one counterpart, the executed counterparts shall constitute a single instrument.

[Signature page follows]

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If the foregoing is in accordance with your understanding of our agreement, please sign and return to the Company a counterpart hereof, whereupon this instrument, along with all counterparts, will become a binding agreement between the Underwriters and the Company in accordance with its terms.

[Signature Page to Underwriting Agreement]

For themselves and as Representatives of the other Underwriters named in Schedule A hereto.

[Signature Page to Underwriting Agreement]

SCHEDULE B

ISSUER FREE WRITING PROSPECTUSES

Final Term Sheet, dated as of March 14, 2013

SCHEDULE C

PRICING INFORMATION

(b) The purchase price to be paid by the Underwriters for the Securities offered and sold pursuant to the institutional offering shall be $24.5000 per share.

The issuer has filed a registration statement (including a prospectus) with the SEC for the offering to which this communication relates. Before you invest, you should read the prospectus in that registration statement and other documents that the issuer has filed with the SEC for more complete information about the issuer and this offering. You may get these documents for free by visiting EDGAR on the SEC Web site at www.sec.gov. Alternatively, you may request a copy of these documents from Barclays Capital Inc. by calling 1-888-603-5847, from J.P. Morgan Securities LLC by calling 212-834-4533, from Robert W. Baird & Co. Incorporated by calling 1-800-792-2473 or from Wells Fargo Securities, LLC by calling 1-800-326-5897.

SCHEDULE E

UNDERWRITER INFORMATION