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Old 02-17-2012, 12:12 PM   #1
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1/2 inch drywall over ridged foam


Trying to add another inch of insulation to my detached shop loft ceiling and cover the rafters. Plan is to nail directly over rafters. Can I then nail drywall directly over foam or must I use some sort of furring strips?

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Old 02-17-2012, 04:53 PM   #2
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1/2 inch drywall over ridged foam


Is the cathedral ceiling vented?

http://www.buildingscience.com/docum...n-roof-venting

Gary

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Old 02-17-2012, 05:06 PM   #3
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1/2 inch drywall over ridged foam


Why not just add insulation on top of the ceiling joist?
You need 5/8 sheetrock for a ceiling not 1/2".
Use screws not nails.
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Old 02-18-2012, 02:34 AM   #4
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1/2 inch drywall over ridged foam


Anyone ever done this? Fastened sheetrock through ridged foam right to the rafters?
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Old 02-18-2012, 02:42 AM   #5
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1/2 inch drywall over ridged foam


Sorry. I must agree, just put more insulation in
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Old 02-18-2012, 03:31 AM   #6
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1/2 inch drywall over ridged foam


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Sorry. I must agree, just put more insulation in
Put more insulation in where?
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Old 02-18-2012, 03:42 AM   #7
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1/2 inch drywall over ridged foam


Quote:
Originally Posted by bullhome

Put more insulation in where?
In the rafters, regular batted fiberglass, or in the joists like Joecaption says. The rigid insulation hasn't nearly the needed r-value.
And the sheetrock will never be completely flat nailed over "semi" rigid insulation, even with fir strips.
It'll be wavy.

Last edited by titanoman; 02-18-2012 at 03:46 AM.
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Old 02-18-2012, 03:48 AM   #8
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1/2 inch drywall over ridged foam


There are no joists. Cathedral. Have bats between 2x6 already. Want therml break. 2 more inches ridged gives me another r-10.

Wavey. Interesting.
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Old 02-18-2012, 03:53 AM   #9
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1/2 inch drywall over ridged foam


I'm just thinking it could be wavy, unless you're real good and can dish sheetrock nails or screws without the rigid insulation crushing a little, especially near the edges, and be consistent.
But maybe not.
I'm just hypotherizing and thinking out loud.
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Old 02-18-2012, 04:06 AM   #10
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1/2 inch drywall over ridged foam


I read somewhere that if I pre-drill, the likelihood of the foam crushing is less, but I haven't tested this. Want to use the Owens Corning extruded polystyrene (XPS) rigid foam. Maybe an inch would be better it's still an extra r-5.

Thanks for the replies.
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Old 02-18-2012, 04:17 AM   #11
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1/2 inch drywall over ridged foam


I can see that working.
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Old 02-18-2012, 08:01 AM   #12
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1/2 inch drywall over ridged foam


Is there insulation up there now?
Do you have foam baffles in the bays, soffit vents and and a ridge vent on the roof?
Is this a shingled roof?
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Old 02-18-2012, 12:54 PM   #13
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1/2 inch drywall over ridged foam


Quote:
Originally Posted by joecaption View Post
Is there insulation up there now?
yes
Quote:
Originally Posted by joecaption View Post
you have foam baffles in the bays, soffit vents and and a ridge vent on the roof?
yes
Quote:
Originally Posted by joecaption View Post
this a shingled roof?
yes
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Old 02-18-2012, 12:59 PM   #14
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1/2 inch drywall over ridged foam


Quote:
Originally Posted by joecaption View Post
Why not just add insulation on top of the ceiling joist?
You need 5/8 sheetrock for a ceiling not 1/2".
Use screws not nails.
1/2 inch drywall is fine for a ceiling.

It can go over the foam with no problems except maybe a little extra finishing.
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Old 02-18-2012, 11:29 PM   #15
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1/2 inch drywall over ridged foam


1/2" ceiling board is stiffer, we use it all the time.

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