# 1/2 inch drywall over ridged foam



## Gary in WA (Mar 11, 2009)

Is the cathedral ceiling vented?

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

Gary


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## joecaption (Nov 30, 2011)

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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## bullhome (Jan 18, 2012)

Anyone ever done this? Fastened sheetrock through ridged foam right to the rafters?


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## titanoman (Nov 27, 2011)

Sorry. I must agree, just put more insulation in


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## bullhome (Jan 18, 2012)

titanoman said:


> Sorry. I must agree, just put more insulation in


Put more insulation in where?


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## titanoman (Nov 27, 2011)

bullhome said:


> 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.


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## bullhome (Jan 18, 2012)

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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## titanoman (Nov 27, 2011)

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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## bullhome (Jan 18, 2012)

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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## titanoman (Nov 27, 2011)

I can see that working.


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## joecaption (Nov 30, 2011)

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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## bullhome (Jan 18, 2012)

joecaption said:


> Is there insulation up there now?


yes


joecaption said:


> you have foam baffles in the bays, soffit vents and and a ridge vent on the roof?


yes


joecaption said:


> this a shingled roof?


yes


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## framer52 (Jul 17, 2009)

joecaption said:


> 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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## mae-ling (Dec 9, 2011)

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


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## Gary in WA (Mar 11, 2009)

Follow 13.1-----13.4 on page #13 here: http://www.lafargenorthamerica.com/GA-216-07 English.pdf

Gary


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## jbs11 (Sep 7, 2011)

I put 1/2" drywall directly over 2" rigid foam on the cathedral ceiling upstairs of my detached garage. It works fine. The foam is _way _to rigid to be compressed to any significant degree locally around the screws. I was a little apprehensive, but 3 1/2" and 4" deck screws worked fine (hard to find drywall screws that size, but they are out there). 

I did it for the same reason: a thermal break and better air sealing. The rafter bays have R-30 (2x10 rafters) fiberglass in them, and are vented.

Good luck. Post pics.


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## Jody1977 (Feb 21, 2012)

what about cutting the foam into 1.5" strips over the rafters, and then filling the space with fiberglass? then one 2X8 sheet of foam covers 16 8 foot rafters, saving $$$ there's no issue about any double vapour barriers....


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## Gary in WA (Mar 11, 2009)

In a garage, it's not a "double vapor barrier", unless there is vinyl flooring above. The full board acts as an air barrier against condensation and radiant barrier to cold garage/car. Remember to 5/8" Type X drywall for fire-barrier. http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/

On a sloped ceiling, be sure to ADA the drywall, *keeping all *indoor moisture out, though it would still dry to the inside (Asphalt shingles are a vapor barrier) through the foamboard 1"= 1.5perms, 2"= 0.75 perms; http://docs.google.com/viewer?a=v&q...iZ9vwo&sig=AHIEtbRlhNBCWID8xGZ54EyM707m0gPdXQ

ADA: http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Gary


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