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The foam will inherently block some heat in the sense that as closed cell foam it develops R-value to reduce conductive heat transfer, even though that is not the typical use for the brand name I mentioned. Looks to be U value of 0.25 so R-4 per inch based on some data sheets I have seen. The stuff being sold for car audio is only 1/8” thick, so I would have to do multiple layers to get where I want or source some thicker material, which is not easy in self-adhesive form. I figure I would want at least R-2 on the inside, so ½” minimum. I have also been looking at cross linked polyethylene foam, I have seen up to 215 degrees F, which should be fine for an interior firewall application.
I am basically looking for the lightest reasonably priced material to add thermal insulation to the interior of the vehicle that can stand up to the service temperature, be flame retardant, and not be itchy. I have also considered a layer of foam and then a layer of some sort of synthetic batting. If I could use polyisocyanrate or extruded polystyrene I would be all set, as I have plenty of that lying around, but the temperature ratings/ flame retardency are a no go.
The exterior firewall will have a layer of Thermo-Tech Thermo Guard Flame Retardant with the edges taped, rated as 90% radiant barrier and at ¼” thick should be around R-1 or so. That should keep the temperature the interior firewall insulation is dealing with to a reasonable level.
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