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My home is 1 1/2 story, built 1946, has 2 x 4 rafters on 24″ spacing resting on knee walls. The space behind the knee wall is used for storage. Currently uninsulated. Space above ceiling has 12 x 16 inch access and I am afraid my circumference is greater than that perimeter so will use blown insulation there. Roof is cedar over skip sheathing with comp over that and will need replacing in about 3-5 years. No soffit vents or soffit for that matter or ridge vent. How can I get good R-value insulation in that space? Are air channels between the rafters necessary in western Oregon? Have thought about rigid board applied horizontally across the rafters. Am open to any suggestions. Ron Peterson
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Listeners write in about haunted pipes and building-science tomes, and they ask questions about roof venting and roof leaks.
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My questions are ditto (in western Oregon too!). Is there anyone out there that can help us? I've been wondering how best to maximize R-value and perhaps elimainate venting by using foam, Icynene, cellulose or other.
*We see this in a lot of rehabs and usually in spaces that are pretty small. The right way to do this is to lower the ceiling enough to add vent space and insulation. The major problem is that adding that much depth to the roof often renders the living space much less usable. What we usually do is stuff the 2x4 space (in our case it is half round logs) with fiberglass, cover the framing with 1 1/2 to 2" foil foam board, strap with 1x3 and sheetrock. It is a compromise but it will work. Do not put any electrical penetrations in this if possible.
*You could install insulation on the roof when your new roof is installed. It will require some trim work. My guess is you will be replacing the cedar with asphalt shingles , so you will remove everything, install new plywood,insulation board, trim to hide the boards at the edges, then shingle with nails of proper length through board into plywood. Good Luck Ron Rosa ,All Trade Roofing.
*Hey guys,I had the same problem in my house here in Fernie, BC and I put on a new roof. Incidentally the layout sounds exactly the same with the knee walls, which I eliminated, but more on that some other time. Here's what I suggest you do, if I read your message correctly. You have a vaulted ceiling, right? You're gonna rip off the cedar and such in a coupla years, right? So don't waste your time now. It'll be super quick to reframe the roof anyway then, and you can raise the roofline instead of lowering the ceiling. With the added structural strength you cud probly get rid of your knee walls and have added accessible space too! You can reframe the roof from above if you want, just insert 2x8's (or whatever meets code for your span, but the load shudn't be great if you're not in heavy snow country) next to the 2x4's. You may have to cut a coupla buried wires and replace them (in whole, not in part) but it shud be an easy retrofit. Then you shud at least be able to stuff R-28 insulation in there with an inch and a half gap for ventilation. And probly get rid of your knee wall. And on a steep angle, R-28 will give an effective R in excess of 35 as it is on an angle, therefore thicker in cross section, to the heat which travels up. At least that's what our building inspector says.To vent the roof, you shud run two or three strips, about 1-2 inches wide, of 1-1/2" Styrofoam (buy a sheet or two and cut strips out of it) to hold the insulation down. (but, before you naysayers get in there, not compressing it!) For roofing I can't tell you how much I recommend tin, so long as you don't have too many doodads and valleys in your roof. Tin rocks! It's cheap, comes in many colors, and is super quick and easy to put up. Use Cobra Stripping for your ridge vent (looks like really thick scotchbrite) You do need ventilation in your roof to prevent heat buildup, moisture, and so on. The slats (is that skip sheathing?) will promote cross ventilation too, which is good! Of course you'll need to run 30lb roofing felt under your tin to catch the condensation, before it drips off the underside of your tin roof onto the insulation. But any roofer will tell you that. If you choose a roof pattern with good-sized ribs, you'll get venting that way too. Here's a picture of my house as it then was after reroofing. I know this sounds a bit disjointed but ask me more and I'll tell you more.Marko :^]
*I knew it sounded too easy.To insulate properly you'll have to use a vapor barrier (at least you'd have to do so up here) and that wud mean taking off the ceiling. So much for just dropping it all in from above. BTW in an old issue of FHB there was a good article on condensation problems in houses where vapor hit the vapor barrier and condensed, causing trouble in the house. I would read this article and consider your insulation options carefully, in light of the fact you seem to live in a damp climate, and that the rest of your house probably is not vapor sealed. Nothing is simple, is it?
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My home is 1 1/2 story, built 1946, has 2 x 4 rafters on 24" spacing resting on knee walls. The space behind the knee wall is used for storage. Currently uninsulated. Space above ceiling has 12 x 16 inch access and I am afraid my circumference is greater than that perimeter so will use blown insulation there. Roof is cedar over skip sheathing with comp over that and will need replacing in about 3-5 years. No soffit vents or soffit for that matter or ridge vent. How can I get good R-value insulation in that space? Are air channels between the rafters necessary in western Oregon? Have thought about rigid board applied horizontally across the rafters. Am open to any suggestions. Ron Peterson