Half vented, half unvented house
Hi There,
I live in Raleigh, NC with a climate zone of 4A in a 1200 sqft ranch. I currently have a completely vented attic space. I am planning on vaulting approx 800 sqft to the ridge by using metal tie rods as roof ties 1/3″ up the rafter height. Because of headroom, I am thinking of doing this vaulted section unvented with 5″ spray closed-cell in between my 2×6 rafters, 1″ rigid polypro, 2×3 furring on face 24″ spacing then 1/2″ drywall. This will leave me in the R39 range which is plenty for the R30 cathedral ceiling code. My question is that can I leave the remaining 400 sqft vented, unconditioned attic space with the plethora of cellulose that was blown up there? From the ridge to the ceiling joists maxes out at 5′ so that 2×4 wall will extend to the underside of the rafters. Is this acceptable? If so what should be the R value of an interior 2×4 attic wall?
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Replies
Your assembly should work well. You can simplify a bit by mounting the drywall straight over the rigid with longer drywall screws. I've done 2" of foam like this without issues.
A cheaper assembly might be to fur out your rafters with strips like here:
https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge
And only spray about 2" of closed cell SPF. If you fur out your rafters with 1.5" of foam and 1/2 plywood strips, you can fill the cavity with either R21 or R24 batts. This is a higher overall R value assembly and much cheaper to do.
For your vertical mini wall in your attic, I would insulate the same R value as your ceiling. Fill the stud bays with fluffy insulation than lay 2x6 batts on the outside against the wall held in place with a couple of long screws and insulation washers.
There is nothing wrong with a mixed vented/unvented roof, just make sure your air barrier stays continuous when you do this transition.
I appreciate the idea and input. The furring cavity is for electrical and low profile IC rated LED recessed fixtures. In my zone I need at least R15 of air-impermeable insulation under the roof sheathing for Unvented space. Maybe I could use 2.5" Polyiso, then I could fill with R24. The smallest R24 still has 5.5" depth which would require some depth to the furred boards.
Your R15 is correct for an R49 roof. The important part is the ratio which is 30% of overall roof R value, so if you have a lower R value roof, you need less impermeable insulation.
Rigid insulation won't work on the inside as an alternative for SPF. For an unvented assembly your option is either SPF under the deck or rigid above the deck, can't put the rigid under the deck.
My suggestion was to fur out the rafters after the 2" of SPF so that there is still space for the 5.5" thick batts. The foam furring strips from the link do a decent job of reducing the thermal bridging from the studs and although the center of cavity R value is less than your initial assembly, the overall assembly R value is more.
That isn't how I understand our code. I am in zone 4A. I will certainly fur out the rafters that way, I appreciate that advice. Where does the 30% ratio come from?
Thanks,
The 30% comes from here, Table 1:
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
I'm not familiar with your local code, but if it specifically calls for R15, then you should install it. 2.2.5 to 2.5" of closed cell SPF is not all that much extra.