If you wish to set up a combination of rigid foam in addition to your roof sheathing and air-permeable insulation between your rafters, you should be sure that your rigid foam is thick more than enough to keep the roof sheathing earlier mentioned the dew stage.
Based on section R806.five of your 2012 IRC, “Unvented attic assemblies (spaces amongst the highest-story ceiling joists as well as roof rafters) and unvented enclosed rafter assemblies (spaces involving ceilings which are utilized directly to the underside of roof framing members/rafters and also the structural roof sheathing at the best in the roof framing customers/rafters) shall be permitted†given that a number of problems are fulfilled.
I was skeptical of Bill Hulstrunk's declare that AccuVent baffles could face up to being dense-packed within an enclosed cavity.
Q. "If I install a polyethlene sheet among the bottom in the batting and also the sheetrock [It could be attached towards the purlins], wouldn't this be an inside vapor barrier?"
Any property remaining crafted much better than the absolute minimums need to with the incredibly least not use OSB for roof sheathing. Imo
Amazingly handy (five*) thread. So, if you need to use fiberglass insulation batts, so you dont have use of spray foam or closed cell boards, what do you do? I assume The true secret is to really make it unvented and add in other air-impermeable levels. I'm going to insulate a new all-steel cathedral pitched gable roof inside the Caribean. The plan should be to fill the cavity between the four" Z-purlins with R-eleven batting (vapor side going through the residing House). For extra warmth conduction resistance we want to roll A different thinner (filleted?) layer of this batting excessive in the purlins (ie involving the purlins as well as the sheeting).
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In a very phrase, no — the code explicitly forbids this technique. Cellulose can only be Employed in an unvented roof assembly if there is an sufficient layer of rigid foam previously mentioned the roof sheathing or an satisfactory layer of shut-cell spray foam underneath the this article roof sheathing. Cellulose alone received’t function.
Probably the technique to go is always to settle for 4" of foam around the cathedral ceiling, as an alternative to 8" of froth, instead of keep on the foam in excess of the vented attic. There will be described as a Bizarre jog within the roof line exactly where the foam begins, but it will eventually mostly only be visible within the back again of the home, not from the street.
When you are taking into consideration transferring to many places or simply just interested in distinctive areas of the country, you should evaluate various metropolitan areas.
On an unrelated matter: The Cold Weather Housing Study Middle has developed a online video (beneath) displaying a person technique of setting up an insulated cathedral ceiling. The tactic shown from the online video is particularly appropriate for builders in climates subject matter to ice dams.
You describe your proposed air barrier as "taped felt." I don't Imagine asphalt felt is often efficiently taped, and I do not are convinced asphalt felt makes a very good air barrier.
If you'd like to use rigid foam, the most beneficial location to put it is along with your roof sheathing, not beneath your roof sheathing.
Builders solve this problem by furring click to read down or scabbing on additional framing under the rafters to deepen the rafter bays.