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Old 12-21-2015, 12:12 PM   #16
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By foil-faced, I assume you're talking about a product like this:

http://www.homedepot.com/p/Super-TUF...8426/100322374

Double-faced poly-iso. But, isn't poly iso very bad at dealing with water? The foil-facing prevents a problem, but the narrow edges on each side will be unfaced.

So, would you recommend sealing the four unfaced edges with aluminum tape, for a continuous foil-barrier, before installing?
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Old 12-21-2015, 10:39 PM   #17
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Could someone offer a link to the foil faced product for the rim joist area, so as to clear up my confusion, thank you to all who participate in this thread
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Old 12-22-2015, 08:56 AM   #18
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Quote:
Originally Posted by Mml665 View Post
Could someone offer a link to the foil faced product for the rim joist area, so as to clear up my confusion, thank you to all who participate in this thread
I linked a foil-faced polyiso, in the post just before yours. Home Depot carries it. Lowes, to my knowledge, does not.

However, polyiso (even foil faced) has a much higher water absorption percentage over XPS foam -- and especially if the foil deteriorates or is penetrated.



For foil-faced polyiso, I would, at least, think that sealing the four exposed edges with additional aluminum tape would be wise.

And, I've also heard people recommend XPS over polyiso for these humid applications.

Last edited by Alketi; 12-22-2015 at 09:06 AM.
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Old 12-22-2015, 12:19 PM   #19
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Is this for an interior or exterior application? If it is interior, the absorption concern is moot.

Isn't this interior foam?
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Old 12-22-2015, 07:02 PM   #20
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Sorry if I caused any confusion. The rims should be faced foam- plastic, foil, anything with a low permanence to stop winter/summer moisture drives/condensation.


" However, especially in retrofit situations, the amount of exterior insulation that can be installed (if any) may not be sufficient to provide adequate thermal insulation (at least R-10). Thus in these cases, the alternative is to install rigid insulation (such as foil-faced polyisocyanurate) flush against the interior side of the sheathing. This still allows air drying of the rim joist cavity but reduces the potential for interior summer condensation by decreasing the condensation surface to the bare minimum of the interstices of the insulation/ sheathing interface. In the winter, the insulation is protected from interior-sourced water vapor by the foil-facing."


Granted, you wouldn't see a lot of condensation unless you added fibrous insulation in the floor cavity (directly inboard of the foam) to reduce the foam board temps there As that interface wouldn't be warmed by the room as much.

More importantly is the facing on the rim side of the foam to stop any moisture from entering the foam to keep the wood wet longer, with more time to mold. The interior faced, if foil, would also support radiant heat resistance, a good thing- if 3/4" gap. Don't use any facing and you will still be waaay better than none at all.

IMO, use a faced product on the walls as you have an interior drainage system and want to stop any moisture through the CMU to weep down and pump out. Poly will also work but only with a drainage system interior.

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