A question, given that the IECC recommends R7.5 for exterior foam (in my area) is it asking for trouble to use the insulated vinyl products? It seems like they are mostly EPS, Mastic’s site claims it has a PERM rating of 5.0. But to my (limited) knowledge, no one claims more than R4. To be clear, I’m not planning on using it, so please no vinyl is evil diatribe. Just curious if this stuff is a disaster waiting to happen.
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Time will tell. The unknown variable is how much air leakage occurs with these products. (Air leakage would help avoid moisture problems but would greatly reduce thermal performance.)
In general, the logic behind these products is deficient. One of the best attributes of vinyl siding is that it hangs loosely on a building and is well ventilated. These are good attributes for a siding. Once the siding is aligned closely with thin thermal insulation, however, you've done three bad things:
1. You have greatly reduced the ventilation flow behind the siding (ventilation that acts like a rainscreen air gap).
2. You have basically glued the foam insulation to the siding (where it doesn't belong) instead of to the wall (where it does belong).
3. The foam is much too thin to keep the sheathing warm.
These factors are product design errors, in my opinion.