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Old 02-06-2009, 10:11 PM   #1
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Attaching Drywall to 4" of Rigid Foam


I am trying to get my cathedral ceilings up to R45 using 2 x 10s. I am thinking of attaching 2 layers of 2" thick foam boards to the bottom of the rafters and attaching drywall to the bottom of the foam boards. I could use wood strapping with 6" long drywall screws but it gets kind of hairy to find the rafters through so much foam.

The construction is (starting from the bottom):
- 1/2" Drywall
- Strapping attached to the rafters by 6" screws 16" O. C.
- 2 layers of 2" thick Dow styrofoam (R20)
- R 30 Cathedral fiberglass insulation between the 2 x 10 rafters
- Tyvek AtticWrap
- 1 1/2" x 1 1/2" furring strips on top of the rafters for vents
- 5/8" plywood
- 3 tab shingles

Is there a better way than screws to attach this much foam?

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Old 02-07-2009, 12:47 AM   #2
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Attaching Drywall to 4" of Rigid Foam


yea i see your point.
How about a 5/4 x4 firring on the flat onthe bottom of the 2x10 then the foam then the rock? it would give you a bigger target

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Old 02-07-2009, 01:53 PM   #3
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Attaching Drywall to 4" of Rigid Foam


Wouldn't it be easier to have closed cell foam sprayed into the bays?
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Old 02-07-2009, 02:07 PM   #4
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Attaching Drywall to 4" of Rigid Foam


Nudura sells a product which is a insulated panel with furring strips built in to them.

http://www.nudura.com/en/Constructio...fproducts.aspx

You can also a glue, BUT make sure it is safe for use on foam because if it isnt it will eat the foam, I believe PL makes a product for foam, PL700(not sure if it the right # but it will say on the tube when you read it), but check it out at your local hardware store.

Last edited by choda_boy_04; 02-07-2009 at 02:43 PM.
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Old 02-07-2009, 03:48 PM   #5
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Attaching Drywall to 4" of Rigid Foam


Do you mean glue the drywall to the foam? Wouldn't the seams crack?

The Nudura product looks interesting. You get 3 1/2" with one layer of foam & only need a 5" screw.

Closed cell spray foam is prohibitively expensive for an R45 ceiling. I am thinking of it for the walls.
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Old 02-19-2009, 12:39 PM   #6
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Attaching Drywall to 4" of Rigid Foam


Quote:
Originally Posted by PerpetuallyRepairing View Post
I am trying to get my cathedral ceilings up to R45 using 2 x 10s. I am thinking of attaching 2 layers of 2" thick foam boards to the bottom of the rafters and attaching drywall to the bottom of the foam boards. I could use wood strapping with 6" long drywall screws but it gets kind of hairy to find the rafters through so much foam.

The construction is (starting from the bottom):
- 1/2" Drywall
- Strapping attached to the rafters by 6" screws 16" O. C.
- 2 layers of 2" thick Dow styrofoam (R20)
- R 30 Cathedral fiberglass insulation between the 2 x 10 rafters
- Tyvek AtticWrap
- 1 1/2" x 1 1/2" furring strips on top of the rafters for vents
- 5/8" plywood
- 3 tab shingles

Is there a better way than screws to attach this much foam?
Curious, how did you end up fastening the styrofoam? I am trying to figure out the same thing except using 1" foam. Also, did you use unfaced fiberglass or faced. Just curious if I can use foil faced foam board on top of the faced insulation?
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Old 02-19-2009, 02:48 PM   #7
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Attaching Drywall to 4" of Rigid Foam


With 1" foam you should not have any issues finding the studs. I have a real problem with ice dams because it is a larger roof & I get a lot of snow. (Northern Massachusetts) so I need the bigger R value to prevent ice dams. My roof went after only 16 years. It is leaking through the nail holes in the ice & water shield under the ice dams. My final solution is to nail 2 x 3 studs to the bottom of the 2 x10s to give me 2 1/2" for the 6" drywall screws to find the studs. I will use R38 Cathedral ceiling fiberglass insulation. I will look for unfaced but if faced is cheaper I will get that & slit the facing.
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Old 02-19-2009, 03:50 PM   #8
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Attaching Drywall to 4" of Rigid Foam


I think you're really going to end up regretting your decision to do your ceiling this way. The ceiling should be furred down with whatever amount of wood it takes to get a direct/solid connection between the framing and the sheetrock. I think you'll have nothing but problems with the sheetrock on this ceiling.
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Old 02-19-2009, 04:33 PM   #9
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Attaching Drywall to 4" of Rigid Foam


Use spray in closed cell foam but don't try to get the full R45 with the foam. Have them spray in 2" of foam then apply fiberglass over the foam to get R45. The foam will be the vapor barrier and normally you'd want the vapor barrier on the warm side but as long as you use at least 2" of closed cell foam the dew point will be within the foam so condensation will not be a problem. I think 2" of spray in foam is similar in price to 4" of foam boards but haven't priced it lately. And if your spray foaming the walls anyway you should be able to get a decent deal.

Oh, and you would eliminate the roof vents because the foam will be sprayed to the bottom of the roof sheathing.
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Old 02-19-2009, 05:24 PM   #10
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Attaching Drywall to 4" of Rigid Foam


The Tyvek AtticWrap provides the air barrier on the cold side so in this case the spray foam does not help as an air barrier. I don't think enough is known about spraying a thin layer of foam on the outside as a moisture barrier to feel safe about it. If there is R30 fiberglass over 2" of foam, I can guarantee that under some conditions you will get condensation on the foam. Look at a psychometric chart. For instance RH = 40%, T = 70 F --> W = .006 --> Dew pt. = 44 F. Outside air = 0F. 2/3 of the temperature drop is in the fiberglass. Thus the foam surface temp = 24F, which is 20 F below the dew pt.

The other advantage of closed cell spray foam is that if there is a roof leak, it will prevent it from going into the house. With AtticWrap the leak will simply drain to the eaves. I would prefer the draining so I can find the leak.

With or without foam, I realize that unvented assemblies have the advantage of preventing air motion in the insulation but the AtticWrap also does that. Spraying foam to a deck also makes it hard to replace that deck if it rots. I can see why contractors love spray foam because it is cheap to install & you don't have to be scrupulous with the air barrier. I am doing the work myself so the cost of putting in air channels & sealing everything is not high.

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