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Old 12-05-2008, 01:05 PM   #1
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Insulating Garage / Workshop with Free Foam?

Hi all, I'm new here.

I have a question about insulating my garage but don't know the first thing about home improvement so go easy if I say something stupid please.

I have access to a bunch of free styrofoam sheeting. This is a 3/4 to 1.5 inch thick white material that is used for the protection of goods while shipping. (packing foam). The sheets I'm getting are between 24 and 30 inch in size and I would like to use them to insulate the space between the 2x4 rafters in my garage.

My garage has no drywall and the exterior is brick. The 2x4 rafters are wide open all the way to the peak of the roof. I would like to keep them that way because I use the space above for storage of light weight items like fishing poles and such.

Anyhow, the shingles on the roof are nailed to a plywood substrate and that is nailed to the 2x4 rafters. (normal construction I believe)

My question: Is it ok if I use liquid nails to glue sheets of foam between the 2x4's to the plywood substrate the shingles are nailed to? I've already done a few rows and it works great. My concern is that a buddy of mine mentioned something about moisture so I stopped working until I get a good answer.

Again, I am custom cutting the foam sheets to fit snuggly between the 2 x 4 rafters and I am using 4 to 6 dabs of liquid nails on each sheet to glue it to the plywood.

Advice? Is this ok to do or am I asking for problems?

Thank you!!!!
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Old 12-05-2008, 03:49 PM   #2
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My only concern would be the extreme flammability and ease of ignition. You definately need to install sheetrock on the walls that have foam in them to act as an ignition barrier.

Personally, I'm not a fan of doing rigid foam in walls. Give me batts or cellulose any day.
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Old 12-05-2008, 04:12 PM   #3
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Flamibility is a big issue. You will need to cover the foam, as mentined, with drywall. Don't think so? take one of those panels and take a match to it and see what happens once it starts to burn. They are an oil based product.

How is your roof ventred right now? Ridge vent? or those little square vents? or something else? How about the venting on the underside of the soffits?

You will still need to allow the air in at the soffits and it needs to go up to where the roof vents are. While doing so it needs to travel along the under side of the roof to help control the moisture. Ie. move it out if it is there and also help keep the roof deck cooler in hot weather.

By gluing the foam directly to the underside of the plywood deck you are creating little pockets where moisture will form and collect. this is beacuse you will not achieve 100% attachments to the plywood deck. Not good.

If you could it would be better to leave a gap against the plywood deck for air travel. Say at least 2 inches. This would be similar to the foam things used in attics to keep the insulation out of the soffit while allowing the air to pass through up to the roof vents.

Of course if you were using a spray foam that actually adheared to the plywood and sealed every gap this would not be a problem but would cost $$$$$.

Either way you will need to cover with drywall when finished.

The dryawwll dosent' have to be pretty but you need that sheetrock btween the foam and a possible ignition source. Hopefully you could find a source of scraps of that too.
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