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Old 11-21-2009, 06:33 PM   #1
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Insulation and furring strips question


I am insulating the my attic to create more usable space up there. I will be drywalling the underside of the slope rafters. Because they are uneven in spacing and slope (the guy who built this house was not....um...skilled, shall we say?), I was thinking of adding 1x3 furring strips to the underside of the rafters, to even them out a bit and give myself a better structure to apply the drywall to. This will leave a 3/4" airspace between the back of the drywall and the rafter edge. From an insulation standpoint, is this a bad thing? Will air currnets form behind the drywall and wick away heat from the room, or is the dead space an insulator in its own right, and help with the thermal barrier?

Thanks in advance for any replies.

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Old 11-21-2009, 08:00 PM   #2
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Insulation and furring strips question


You'll certainly create a chimney effect that will move air around. You are better off insulating the 3/4" gap. You'll never get it sealed up tight enough to create a truly dead air space.

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Old 11-21-2009, 09:04 PM   #3
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Insulation and furring strips question


I'm not sure if I agree. Our code wants something of low air permeance against the insulation and I'm assuming you have poly which would meet that particular code. The extra space, in my opinion, doesn't need to be insulated. I see M6's point about not having the vapour barrier sealed up 100% but if your furring is horizontal, I don't see much air moving around. That being said, insulate it with the cheap styrofoam if you really want to go all out. I say the cheap stuff because it isn't a vapour barrier and you don't want a double vapour barrier EVER. Just remember, at least 2/3 of the R value must be on the cold side of the vapour barrier and 1/3 max on the warm side or you have issues. Hope this helps.
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Old 11-21-2009, 09:53 PM   #4
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Insulation and furring strips question


Thanks, guys. I appreciate the input. One other question...I was thinking of taping all the seams between batts, just to make sure the vapor barrier is more robust. This is/was my first insulating project and there are some holes and gaps, especially where the ends of the batts are butted together. Is there a tape you recommend that would do the trick, or am I just wasting my time? I tried to patch a couple holes with duct tape, but that is clearly the wrong choice, as it peeled off basically overnight.
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Old 11-21-2009, 10:08 PM   #5
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Insulation and furring strips question


Is your roof deck vented?
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Old 11-21-2009, 10:15 PM   #6
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Insulation and furring strips question


Yes. There are foam channels in each rafter space leading from exterior soffits (which are continuous perforated vinyl panels) to a continuous ridge vent which was put in when the new roof was installed April 2008
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Old 11-21-2009, 11:46 PM   #7
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Insulation and furring strips question


"Will air currnets form behind the drywall and wick away heat from the room, or is the dead space an insulator in its own right, and help with the thermal barrier?" ------ Yes, an air space will negate the insulation: http://oikos.com/library/insulating_...lls/index.html
And the 1" dead air space will have movement, even if it was totally dead air ---1-4" = R- 4.

"I was thinking of taping all the seams between batts, just to make sure the vapor barrier is more robust. This is/was my first insulating project and there are some holes and gaps, especially where the ends of the batts are butted together." ---------- It is only a vapor retarder. It should be pressing next to the drywall surface which will lay any cuts flat again. If big holes, you could tape them (foil tape) if you want but I suggest painting with a vapor barrier primer after drywall.
The best job will use rigid foam board on the rafter bottoms to thermally separate them from the cold roof: under "cathedral ceilings": http://www.southface.org/web/resourc...lings_4pdf.pdf Notice the foam boards under the knee wall and at the vent supply holes to protect the insulation from the air supply stream.

Be safe, Gary

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