Removing foil facing from GPS foam board
I just purchased some GPS rigid foam board (15 4×8 panels) and it was not advertised as being foil facing or having any kind of vapor barrier, but it has foil facing on one side and a thin plastic sheet on the other. So I’m stuck with these 15 boards and I’m hoping I can salvage and use them for my project. I’m finishing part of my basement and the plan was to use 2″ EPS/GPS/polyiso rigid foam to put against the cinder blocks that form the walls of the basement. However, I wanted the foam to be vapor permeable. Given that I have foil on one side and thin plastic on the other, can I attempt to peel these off the boards and still use them? Is there a better alternative for salvaging these boards?
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Per Joe Lstiburek (one of the originators of the 'foam needs to let the basement wall dry' concept), a Class I vapor barrier is actually not that much of a problem:
Joe Lstiburek Discusses Basement Insulation and Vapor Retarders
An all-day seminar with Dr. Joe yields two news stories, a six-digit idea, and plenty of quotable opinions and conclusions
https://www.greenbuildingadvisor.com/article/joe-lstiburek-discusses-basement-insulation-and-vapor-retarders
Here’s the news story: Lstiburek now says that the advice he gave in the Builder’s Guide was wrong. Lstiburek said, “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”
Lstiburek’s former recommendation to use vapor-permeable foam was due to a concern that imperfections in the foam installation might allow some warm interior air to contact the cold concrete, leading to condensation. However, experience has shown that most foundation walls, even those with small amounts of condensation, experience some drying activity at the top of the foundation wall.
If you still wanted greater vapor permeance to the wall, I'd consider perforating the facers, rather than peeling them off, unless they come off easily. Looks like you could repurpose a wallpaper perforating tool for this purpose.
I think you'll find the foil facing VERY difficult to remove without severely damaging the board. When I've worked with polyiso, if the facer peels, it tends to take chunks of foam with it too. I've found the poly facers to peel off easily with XPS though, but I've never tried with anything else.
I agree that perforating the facer is probably a better option. Take a small 4d finishing nail and punch a bunch of holes in the facers if you want more vapor permeability, but don't go all the way through the foam -- only pierce the facing.
Personally, I wouldn't worry about it. It's common to use foil-faced polyiso to insulate basement walls, or to put a poly liner over the wall and insulate over that. You don't really need vapor permeance here. The only place you might want vapor open insulation is your rim joist area, the concrete wall doesn't really need it.
Bill