Gluing blue sytyrofoam to plywood

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Old 01-31-09, 07:30 AM
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Gluing blue sytyrofoam to plywood

I have a simple 32" square opening in the ceiling to get into the attic, it is trimmed with molding and a piece of 1/2" plywood lays neatly on top of smaller molding that acts as stops (which I put whether stripping on to stop airflow).

I have been insulating my attic, I am almost done, one of the few things I need to do is insulate the plywood door. My idea was to use 2 2" thick pieces of that blue insulating styrofoam on top of each other to make a thick barrier. I can cut them so there is a tight tolerance with the joists giving a decent seal.

My question is what type of glue will give a very strong bond between the plywood and the bottom piece of styrofoam and also between the bottom piece of styrofoam and the top piece that will sit on top of it? The glue needs to hold in hot and cold weather, since I live in New Jersey.

Thanks!
 
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Old 01-31-09, 08:00 AM
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PL300 is one popular brand of foamboard adhesive. Usually the best foam adhesives will specifically SAY they are for foam, since other types of construction adhesives (PL200 and PL400) may have solvents that will actually dissolve the foam before they cure.

I think Liquid Nails also makes a foamboard adhesive. Just be sure the label says it's safe to use on foam. You'll probably want to tack a roofing nail or something through the foam to hold the foam into the glue so that it doesn't move.
 
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Old 01-31-09, 08:07 AM
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And since theres really no stress on it, you could probably just use latex caulk, unless you happen to have some foam adhesive that you need to use. I used latex caulk (DAP 230) with my attic access doors (plywood and white 2" styrofoam) and its holding just fine after 18 months. I bet my attic gets much hotter than yours, though not as cold.
 
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Old 01-31-09, 01:45 PM
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So a normal caulk will work? I got plenty of Phenoseal and similar.
 
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Old 01-31-09, 01:53 PM
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Well, not familiar with that brand....just saying that 6-8 beads across a 2' x 4' panel is holding just fine. I'm sure the styrofoam would break before the bond did. DAP Dynaflex 230 is an elastomeric latex. You definitely don't want any solvent based stuff.
 
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Old 01-31-09, 06:54 PM
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Yes, the main thing is no solvents. A latex based sealant will be fine, especially if you have a fastener to help hold the foam in place until the sealant cures.
 
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Old 01-31-09, 07:03 PM
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Sounds good guys, thanks!

BTW, I'm waiting on a couple pieces of that blue styrofoam from my Father who has a lot extra on one of his jobs. What R value would 2 2" pieces give me? I assume this stuff is the best for insulating the plywood ceiling door to my attic?
 
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Old 01-31-09, 07:28 PM
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about R=5 per inch. three pieces wouldn't hurt. include a fuzzy air seal around the edge. A friction fit will not have friction all the way around.

good job
Bud
 
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