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Old 01-14-2011, 01:29 PM   #16
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Drywall over 1-1.5" rigid foam?


stephen 1234 can you provide a drawing the explanation is confusing

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Old 01-15-2011, 12:13 PM   #17
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Drywall over 1-1.5" rigid foam?


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Originally Posted by oh'mike View Post
I've done exactly what you are thinking about----never had a call back,or a crack---
I did the exact same thing - on a vaulted ceiling of a family room I added on to our house - many years ago.

It was tougher to install the drywall, required extra long screws, etc. But we were living in Minnesota at the time, and I wanted the extra insulation.

We lived in the house for 5 years after I built the family room. No cracking or problems during that time. We then sold the house and moved out of state, so I cannot say what happened in the ensuing years.
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Old 01-15-2011, 01:12 PM   #18
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Drywall over 1-1.5" rigid foam?


I built an 1850 sf lake home using all masonry construction in northern MN (cold winters, warm summers) over few year period. The scissor roof trusses spanned 30', starting at 9' and going up to to the peak (to about 12'). When I originally built it, I put 10" of the fiberglass junk between the trusses spaced at 24" o.c. and used poly to hold it in place for a few years. When I went to finish the ceiling a couple of years later, I opted for 1 1/2" of extruded polystyrene since the home was so energy efficient (turned off breaker so no heat from sometime in October until the middle of December with a few periods on below zero. The thermostat was still showing a temperature higher the low limit of 65F, with quite a few days of with below 0 F.). I went to 1 -1/2" instead of 2" and very liberally slashed the old poly "vb". I had a contractor put in the 1-1/2" rigid XPS and drywall with 5/8" sheet rock (traditional, but long screws and had the foam and drywall joints staggered in both directions). After a year, I had the ceiling taped. After 2 more years there is no sign of problems with screws or taping.

It worked with no problems despite the long span, but I have a feeling the uniform temperature may have had an effect.

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Old 01-16-2011, 11:38 PM   #19
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Drywall over 1-1.5" rigid foam?


I believe you may have trouble with the interior (room) moisture leaving through the outside wall, depending on your climate. The idea of foam on the outside is to have it thick enough to keep the moisture from condensing on the sheathing by raising the temperature of the inside face of the foam. If the heat is stopped at the drywall, the moisture will condense on the insulation and the sheathing after going through the foam because they are colder, the reverse of this; http://www.greenbuildingadvisor.com/...foam-sheathing

The insulation needs to be outside to work with the vapor flow; http://www.buildingscience.com/docum...e-perfect-wall

Another BSC article explaining the different thickness of foam required for different locations as to changing dew point temperature; http://www.buildingscience.com/docum...apor-retarders

Of course the ceiling would be different than a wall, especially if vented, to get rid of the moisture from inside. I'm not an expert on this subject, just surmising, open to discussion.....


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Old 05-18-2011, 05:14 PM   #20
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Drywall over 1-1.5" rigid foam?


If you can't find the adjustable electrical boxes, or don't want to mess with the existing wiring, there are box extension rings that will fill the gap between the front of the box, and the face of the sheetrock. These are required by code. You can't just use longer screws on the switches and receptacles.
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Old 05-18-2011, 05:53 PM   #21
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Drywall over 1-1.5" rigid foam?


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Last edited by concretemasonry; 05-18-2011 at 05:56 PM. Reason: duplicate
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Old 05-21-2011, 03:48 PM   #22
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Drywall over 1-1.5" rigid foam?


I don't know... I'm not sure I see any problems with doing this at all. He should be using 1/2" drywall, not 3/8".. but the foam shouldn't act any different than the vapour barriers we use here in BC. As far as screws popping... why? As long as everything gets sucked down tight when it's properly screwed off there shouldn't be problems. And he could always GLUE the drywall to the foam.

I don't understand some of the responses. But then maybe some here have more experience than I with this specific idea.
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Old 05-21-2011, 11:36 PM   #23
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Drywall over 1-1.5" rigid foam?


Foam board and plastic vapor barriers are not the same. The foam will insulate (stop the heat) from reaching the sheathing better than cavity insulation alone, letting any moisture condense on the inside sheathing because you changed the dew point location and temperature. Read the links on post #19.

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Old 05-22-2011, 03:07 AM   #24
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Drywall over 1-1.5" rigid foam?


The only thing that strikes me is the amount of space you are building out your surface, especially if it's a ceiling. Unless it's a high ceiling, or something like a cathedral ceiling, you are going to decrease your ceiling height by an inch and a half or so, more if you use plywood strapping to attach the drywall.

Something to keep in mind with this idea.

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