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Penetrating interior basement wall foam insulation?

wayno_from_vt | Posted in Green Building Techniques on


We have 2.5” Thermax insulation on the interior of our basement walls below grade. The original tape is pulling away and has torn at most of the corners. It looks like regular foil tape with a color backing to match the Thermax color. There are also quite a few tears on the interior visible part of the Thermax sheets.

So, can I pull off the old tape where possible and retape? If so, what tape is better?

Can I just tape the tears in the inside part of the Thermax or does that even matter since the portion against the basement wall is OK?

I’ll need to try and reseal everywhere as moisture was clearly getting between the Thermax and foundation wall creating condensation when I pulled a few sheets off to inspect. 

Also, I planned to use the 1×3 strapping technique demonstrated by TOH to save some floor space by NOT building full 2×4 walls. See video below at ~7:20 mark. They use spring nails through the insulation and  1×3 strapping to create “studs”. Seems as if you’d want to attempt to seal around spring nails.

Thank you!

Wayne “DIYer overwhelmed by all the choices” Maceyka

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hey Wayne.

    You can remove and reapply the tape. In my experience, peeling tape can peel the foil facing from the insulation with it. Have a utility knife ready to cut the foil as you go so you don't do more harm than good. Read this for info on what tape to use when you re-tape the insulation: Backyard Tape Test

    And yes, seal everything really well, including the insulation perimeter. You need to keep indoor air from reaching the foundation to prevent condensation.

    1. wayno_from_vt | | #2

      TY Brian. After more research, I'm getting quotes for spray foam on these walls. I can use the Thermax panels in the garage. Seems that sprayfoam has the insulation and sealing in the same application, which looks like a much better long term solution. Also, with a 2x4 wall framed ~1" from the foundation wall and the the spray foam applied, it's about the same in terms of square footage consumed.

      I can just see the sealing around the panel seams being a problem down the road.

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