aspn-defa14a_20170426.htm

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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on June 14, 2017 ASPEN AEROGELS, INC. Meeting Information Meeting Type: Annual Meeting For holders as of: April 20, 2017 B Date: June 14, 2017 Time: 9:00 AM EDT A ASPEN AEROGELS, INC. 30 FORBES ROAD, BUILDING B NORTHBOROUGH, MA 01532 Investor Address Line 1 Investor Address Line 2 Investor Address Line 3 Investor Address Line 4 Investor Address Line 5 John Sample 1234 ANYWHERE STREET ANY CITY, ON A1A 1A1 234567 1234567 1234567 1234567 Location: Offices of Mintz, Levin, Cohn, Ferris, R Glovsky and Popeo, P.C. C 1 Financial Center Boston, MA 02111 O D E You are receiving this communication because you hold shares in the above named company. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. 1234567 See the reverse side of this notice to obtain proxy materials and voting instructions. Broadridge Internal Use Only

Before You Vote How to Access the Proxy Materials Proxy Materials Available to VIEW or RECEIVE: 1. Annual Report 2. Proxy Statement How to View Online: Have the information that is printed in the box marked by the arrow following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER or E-MAIL Copy: (located on the If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET: www.proxyvote.com 2) BY TELEPHONE: 1-800-579-1639 3) BY E-MAIL*: sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked by the arrow g (located on the following page) in the subject line. Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before May 31, 2017 to facilitate timely delivery. How To Vote Please Choose One of the Following Voting Methods Vote In Person: Many stockholder meetings have attendance requirements including, but not limited to, the possession of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any special requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the information that is printed in the box marked by the arrow g available and follow the instructions. Internal Use Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. Only

Voting items The Board of Directors recommends you vote FOR the following: 1. Election of Directors Nominees 01 Steven R. Mitchell 02 Richard F. Reilly 03 Donald R. Young B The Board of Directors recommends you vote FOR the following proposal: A R 2. The ratification of the appointment of KPMG LLP as Aspen Aerogels, Inc.'s independent registered public accounting firm for the fiscal year ending December 31, 2017. C NOTE: Such other business as may properly come before the meeting or any adjournment or postponement thereof. O D E 23456789012 12345678901 123456789012 12345678901 123456789012 12345678901 123456789012 12345678901 123456789012 12345678901 12345678901 Broadridge Internal Use Only xxxxxxxxxx xxxxxxxxxx Cusip

Reserved for Broadridge Internal Control Information NAME THE COMPANY NAME INC. - COMMON 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS A 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS B 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS C 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS D 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS E 123,456,789,012.12345 THE COMPANY NAME INC. - CLASS F 123,456,789,012.12345 THE COMPANY NAME INC. - 401 K 123,456,789,012.12345 Broadridge Internal Use Only THIS SPACE RESERVED FOR SIGNATURES IF APPLICABLE Job # Envelope #