Insulating 32" OC rafters


Old 03-14-06, 10:21 PM
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Insulating 32" OC rafters

First, any thanks to those of you experts who post replies regularly to questions posed by us "weekend warriors".

I have 32" OC 2x4 rafters in my attic. I will be converting the attic into living space which will involve building kneewalls and ceiling. I will scab on 1 x 8's to the rafters to get more depth (for better insulation) and use some venting material like raft-r-mate (owens-corning) to allow passage of air from the space behind the kneewall to the space above the to-be-build ceiling.

My first question is: How do I insulate 32" OC rafters? The stores in my area only sell insulation for 16 and 24" OC spaces. I haven't tried it yet but I am guessing that two 16" bats side by side would just fall down?

Second: Do I need any backing behind the kneewall to hold up the insulation? Won't it fall down over time?

Third: Can I use insulation meant for 2 x 6 spaces in a 2 x 4 framed kneewall?
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Old 03-15-06, 06:43 AM
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Ranman1973, Welcome to the DIY Forums.
I would really suggest having an engineer look at this situation. 2 x 4 rafters on 32" centers? I am amazed that your roof is not bowing. Adding sheetrock is going to increase the load on these rafters. Look at the link below for some ideas. Good luck.
Old 03-16-06, 09:36 PM
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Insulating 32" OC rafters

The structural question is big, but perhaps there are things that you did not describe that could minimize the problem (if there is one).

Who says you need to use fiberglass and scabbed 1x8s? - You could use rigid extruded polystyrene that really gives you far better real insualtion performance than fiberglass for the same "theoretical" Rvalue with no infiltration, thermal sort-circuiting from studs and problems with chutes and obstructing the necessary air flow for proper ventilation.

Old 03-17-06, 12:37 AM
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Thanks to both of you for your responses so far.

Regarding the structural problem/issue, the roof has stood the test of time so far... 85 years and counting. I guess the knee wall I erect will take some of the load as well.

Regarding the rigid extruded polystyrene, I appreciate the suggestion because I want to get the best R-value as possible without taking up a lot of space. The fact that it has a higher r-value per inch is great.

I want to make sure I understand this correctly: These are the rigid styrofoam-looking boards I see at home improvement stores, right? Should I apply this all the way along the rafters from bottom to top or just the portion of the roof that will become wall? In either case, don't I need to vent from the area behind the knee wall to the area above the new ceiling where the vents will be?
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