Insulation And Airtight Drywall In Room With Sloped Walls And Flat Ceiling? - Insulation - DIY Chatroom Home Improvement Forum
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Old 01-09-2017, 01:17 PM   #1
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Insulation and airtight drywall in room with sloped walls and flat ceiling?


I'm still working on reinsulating one of the upstairs bedrooms in my 1953 cape. So far I have installed all the air baffles using 2" XPS, furred out the rafters with 3/4" XPS to help with thermal bridging and now I'm filling the remaining space with R-15 Roxul. After that I'll install Certainteed Membrain smart vapor retarder then I'll put up 1/2" drywall using the airtight method.

My question is since the sloped walls run into the flat ceiling won't air from the attic be able to penetrate the Roxul? Airtight drywall can stop air from the room traveling upward to the attic but what about the air in the attic traveling down? Can't this degrade the Roxul's R-value? Should I use rigid foam to create blocking where the ceiling joists meet the wall rafters? In the picture below the red arrows are pointing to the area I'm referencing.



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Mark

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Old 01-10-2017, 10:09 AM   #2
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Re: Insulation and airtight drywall in room with sloped walls and flat ceiling?


Something like this - I made a block for the top of the bay from scrap foam to stop the cold attic air from penetrating the air permeable Roxul.

Is this necessary? I'm thinking it is otherwise the bays won't be air tight but I wanted to double check since it will be a pain to cut and install all these blocks. I wish I had a normal room where all the walls had top plates...ugh... lol!



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Mark

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Old 01-10-2017, 03:07 PM   #3
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Re: Insulation and airtight drywall in room with sloped walls and flat ceiling?


Fiber insulation does best when enclosed on all six sides, but ceilings have always had the problem that they can't be covered on top so they just use more to compensate. In your case air sealing to keep inside air away from the cold exterior surfaces is the number one priority. Where you are using Roxul there will be very little infiltration downward and if any it will not be carrying excess moisture with it, it will be fine.

Your example looks fine. The only concern I would have and I'm not sure it is even a concern, would be closing off the upper direction for drying. But any moisture can still go through the framing and into the attic, wood is reasonably permeable.

My guess would be, it you did one area one way and another area the other I wouldn't be able to see a difference with my infrared camera. I did some A-B testing on my house and was disappointed that I saw nothing. But at least both were good.

Bud
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Old 01-11-2017, 04:07 PM   #4
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Re: Insulation and airtight drywall in room with sloped walls and flat ceiling?


Thanks Bud. I'll probably skip the blocking since cutting the foam is a lot of work plus I'd have to make sure the spacing is perfect otherwise it could cause problems when installing the drywall (if they extend a little too far the drywall won't fit flush). I'm also planning to top off the attic with as much cellulose as I can fit so that should help prevent convection loops.
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Old 01-11-2017, 05:07 PM   #5
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Re: Insulation and airtight drywall in room with sloped walls and flat ceiling?


I still have to finish upgrading the slopes and flat area on my cape and I'm jealous, good job.

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Old 01-11-2017, 08:28 PM   #6
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Re: Insulation and airtight drywall in room with sloped walls and flat ceiling?


Looks like a good job to me. Check on vapor barrier though. Where I am the recommendation is NO vapor barrier when using blown in cellulose. Ron
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Old 01-13-2017, 04:00 PM   #7
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Re: Insulation and airtight drywall in room with sloped walls and flat ceiling?


Thanks guy! I've been working on this project for months. Air sealing a cape is no easy task!

I'm in climate zone 5 and code says I should use one (although I'm not convinced it's really necessary). Either way I bough a smart vapor retarder (Certainteed Membrain) so I should haven't issues. Since I had access to the attic from this project (normally it's sealed), I also removed the old insulation that had kraft paper from the ceiling in my other bedroom and bathroom and replaced with Roxul. I'll be topping that off with cellulose as well. I didn't worry about a vapor retarder there because A.) it would have been really hard to apply because the room has drywall and B.) the house is over 50 years old and the drywall has many layers of oil based paint which acts like a vapor barrier. I'll probably repaint the ceilings again with vapor barrier paint just to be on the safe side. Only reason I'm adding the smart vapor barrier to this room is because I took it down to the studs so it's much easier.
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