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Insulation and vapor

 
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losthighway
carpal tunnel


Joined: 14 Apr 2008
Posts: 1584

PostPosted: Mon Aug 03, 2015 5:29 pm    Post subject: Insulation and vapor Reply with quote

Hiya, folks. Finally getting ready to put up some insulation/do some interior framing now that my structure, roof and siding have all passed inspection.

Here's the question:
I'm building a room within a room where the outside walls and roof are to be insulated, as well as the separated interior walls. My builder has no problem with putting insulation in between the studs on the outside wall, with plastic over it as a moisture barrier. He's concerned about insulation up in the roofbeams, and isn't too familiar with problem solving it, since in a typical house you have insulation in your ceiling *not* in the actual roof beams.

The options we've thought of are these:

A) Put batts of insulation between the roof joists flat up against the roof boards. Match the thickness of the insulation to the beams for maximum R-value and sound absorption.

B) Put batts of insulation between the roof joists *however* use a smaller thickness and leave several inches between the insulation and the OSB that the roof shingles etc are on. The framers put typical roof vents in, as my builder thought we could always cover them if needed for sound containment.

C) Put insulation in not in the roof joists, but following the collar ties (picture the bottom of an obtuse triangle) this would put the insulation against the roof on the outside edges, but then as the eaves slope up to the peak, the insulation would be a couple feet away. It's quasi 3 leaves this way because sound would pass through the ceiling of the live room (double drywall with insulation), hit an air gap, hit insulation, hit a small air gap, hit the roof. That's the main thing that I dislike. My builder likes it best for venting moisture.

D) Not insulate the roof. Sound would go through the ceiling of the live room, air gap, and then the OSB/shingles of the roof. There'd be some leakeage through the two roof vents unless I plugged them. I don't think I like this option.
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floid
re-cappin' neve


Joined: 03 Jan 2006
Posts: 741
Location: in exile

PostPosted: Mon Aug 03, 2015 10:01 pm    Post subject: Re: Insulation and vapor Reply with quote

I forget the proper name for it, but there is Styrofoam channel that is designed to be stapled to the underside of your roof decking between each pair of rafters. It provides roughly a one inch airspace for ventilation when putting batts between rafters. It sized to work with both 16 & 24 inch centers.
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losthighway
carpal tunnel


Joined: 14 Apr 2008
Posts: 1584

PostPosted: Wed Aug 05, 2015 1:13 pm    Post subject: Re: Insulation and vapor Reply with quote

I bet the insulation contractors know about that stuff. The more I think about it the more that seems to be the best option. The mass of the insulation is still connected to the rafters so I think it's a functioning single leaf, the only loss is a couple inches.... I think.
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