We have a vaulted ceiling with true 2 x 4 rafters (3 3/4″ deep) and are trying to figure out the best way to insulate before we sheetrock. Can we use wall insulation? Rigid? Foil-faced? Unfaced? Any recommendations? What about venting, etc?
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Please give us an idea of where you live. Then you should get specific answers.
My house is in Oakland California. The rafters are doubled and even tripled and supported mid-span as well as with a beefy ridge-beam. The rafter bays are 24 inches on center. I think it's structurally fine, and I want to keep the vaulted space because it adds volume to the living space. I'm looking for an affordable option and I'm not sure about hiring someone to spray the rafter bays. What kind of off-the-shelf solutions are there? Any ideas?
In that shallow of a rafter bay foam is going to give you the best r-value. Spraying it would be the best performance and theoretically eliminate the need to vent, though with the summer heat in Oakland, venting would probably help prolong shingle life some.If it was me, I would either spray foam it and be done, or screw 2" ridged foam to the bottom side of the rafters and blow cellulose behind the foam, either with or without a vent channel. Without venting if I could get away with it, with if the inspector was going to require it.Doing rigid foam across the bottom of the rafters would help mitigate all the thermal bridging going on with those doubled and tripled rafters.Steve
Edited 6/21/2009 8:54 am by mmoogie
Good post Steve!!
Ten years ago that post would have started a flame war about venting here....
What?? Didn't someone post "Flame war #27" a few posts up???
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
I personnally like a vented roof, but she ddn't have any option.
I second steve; you're going to have an awful lot of thermal bridging with so much lumber up there, although in your climate it may not be as big of an issue.
Abandon all hope ye who enter here.
2 inch rigid foam attached to the bottom of the rafter to allow for a 1 and a half air channel. How would I attach the rigid foam? Good question. Be creative. Since they're fairly thick, I would just "toe screw" them in place. That should hold long enough untill the sheetrock goes on.
2X4 rafters seem very undersized to me. What kind of span do they have? Do you have enough headroom to sister a larger 2x alongside?
As far as insulation goes, closed cell sprayed foam is the best way to go. Highest R-value, eliminates any drafts, plus you don't need to vent the roof with it.
You have a very marginal situation. a better description and location would help immensely in giving you good advice - perhaps a photo or two of the details.
and what a nice screen name you have!
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hey lovely.... since it's vaulted you can drop the ceiling by say 16" and dens - pak the space with cellulose
get you about an R-60Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore