How best to insulate a 30'x45' corrugated aluminum roof -- closed cell spray foam vs vinyl wrapped fiberglass vs ?

Asked by John Patterson
Versailles, KY

Roof is over 2x4 purlins on edge over 2x12 rafters on 7' centers. Though 66 yrs old, the bldg is structurally sound with no corrosion and is used as a woodshop in central kentucky


Lucas Johnson

Answered by Lucas Johnson

Seattle, WA

Cascadia Consulting Group

November 19, 2012

As usual, the answer is “it depends”.

The first question to ask is whether you want to do-it-yourself.

  • If yes, then you are probably stuck with vinyl backed fiberglass or custom-cut rigid foamboard.
  • The foamboard will be more difficult to install, but it will most likely look a lot better and provide better performance per inch (because the foamboard insulation is better and also integrated with an air barrier).

If you want to hire an insulation contractor, you need to make sure they are set up to install closed cell spray foam. This means that they own the proper equipment and have the specific certification (if applicable) for installing closed cell spray foam.

  • This is the highest performance, but most likely, highest cost option. 
  • In addition, especially if using closed cell foam, you need to make sure there aren’t any moisture issues of concern (like holes in the aluminum roof) because insulation can hide/trap any moisture.
  • Because you mentioned vinyl-backed fiberglass, I am guessing there have been moisture issues? 

To make sure you do this job correctly, it is most likely worth hiring a building scientist to stop by your project to review your plans. The Building Performance Institute is a national accreditation organization that trains and manages building scientists (i.e. contractors and energy raters).

  • You can find a list on their website (
  • However, I’d recommend looking for a BPI Building Analyst (or BPI Accredited Contractor) with experience installing insulation in your area.
  • Check the Find a Pro directory here on GreenHomeGuide to find an energy auditor near you. 

Kind regards,

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