XPS attached on the ceiling and drywall after.


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Old 12-17-19, 09:31 AM
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XPS attached on the ceiling and drywall after.

Just wish to through an idea about adding a layer of 2" XPS foam on the ceiling attached directly on the trusses, then install the water barrier foil and drywall over in our new build. The ceiling is at 9'6" so no concern about reducing the height. Having this, I can gain R10 from this 2" of foam which will not change in time. Do you see any concern in doing this?

Thank you for any comment
 
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Old 12-17-19, 09:54 AM
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It will make it harder to hang the drywall if you can't see exactly where the center of each joist is. You would want to mark the walls and snap chalk lines after the foam is up. And using 3 1/2" long drywall screws doesn't sound like fun to me. But yes, it could be done that way.

Electricians will hate you since can lights will be hard to install.
 
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Old 12-17-19, 10:02 AM
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Hi roger, I assume you already have cavity insulation so the drywall is protected from outside temperatures.

IMO, adding a layer of rigid to the inside works just fine except for the details involved with the extra thickness, doors, windows, and electrical boxes. I suspect there are some videos on this to help.

Now, in terms of added r-value there is good news. That layer of rigid will be covering most of the framing so in terms of whole wall insulation value it adds a lot. Whole wall value usually is derated by something like 20%.

Not sure you need the "then install the water barrier foil" as the 2" of rigid is already a strong vapor retarder, just tape the seams to be sure there is no path for air.

Bud
 
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Old 12-17-19, 10:04 AM
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Yes, long nails will be used and will be a little tricky to hit the trusses. All my lights are recessed so no concern about frustration from electrical side...I'm doing that too.

Thanks for comments.
 
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Old 12-17-19, 10:28 AM
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Why the desire to do something "unusual"? If you have open trusses above why not just go thicker with the regular insulation? Most attics blow in the insulation so it's simple and inexpensive to just have more blown in. The blown in will be deep enough to cover the trusses so you don't have to worry about the truss affecting the R value much.
 
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Old 12-17-19, 06:44 PM
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Fiberglass insulation has around R2.2/in so I need to add 4.5in of insulation equivalent to 2in of XP foam. The cost is neutral but I agree is more work. The gain is over time as every year the blow insulation will compress itself (go thinner), almost 1"/year loosing in time the R value. Also, thermal bridge is present where the trusses ar in contact with the drywall for blow insulation and not exist if the XP foam is present.
 
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Old 12-17-19, 09:37 PM
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Not sure where you are getting your information on blown in FG but regardless I would use cellulose, higher r-value per inch and I wouldn't expect much change over time.

If you bury the ceiling joists in the attic that provides the same benefit of reduced thermal bridging as the rigid below. 6" of cellulose would give you in excess of an additional r-20, with far less hassle.

Bud
 
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Old 12-18-19, 05:56 AM
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I've had 2 homes with cellulose. you get a little bit of settling initially but it is not continuous and no were near your stated 1" per year!

As long as you get above your recommended initial R value at initial install you'll be above the recommended max with time.

Compared to the hassle of what your proposing adding that extra bit of insulation is far easier!
 
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Old 12-18-19, 07:07 AM
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My house has blown fiberglass. I measured it's thickness several years ago (because I was putting on an addition) and it was still above the required thickness and if it had settled in 12 years it was so minute I couldn't tell if it had settled at all.
 
 

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