25% of the roof has 2x10 rafters over a cathedral. The other 75% has only 2x6 rafters.
So it seems you are stating option#2 except with something to "keep the conditioned air inside and out of the insulation" ??? Which my thought was "Some type of underlayment for a air barrier? Maybe UDL 30". (Maybe I'm mixed up in my understanding here...)
I am hoping to make a flavor of Option #2 work to keep the cost as low as possible. If the roof is vented above the decking then I could fill the entire cavity (with 5 1/2" HD fiberglass or rigid with spray foam)
Thanks!!!
Guest User
2013-08-29 11:43:02.000000
Which of the following would be better (or neither is sufficient?):
Roof is 8/12. No Soffit to Ridge ventilation. The 26g metal, 30lb felt and 3/4" T&G are what must be used, I'm open to what else is needed for a solid design...
Option #1 (materials from bottom to top):
3/4 " Tongue and Groove Pine
unfaced fiberglass (batt)
1 1/2 " Closed Cell Foam
5/8 Decking
30lb roofing felt
UDL 30 synthetic underlayemnt
26ga metal (steel)
Option #2 (materials from bottom to top):
3/4 " Tongue and Groove Pine
Some type of underlayment for a air barrier? Maybe UDL 30
Faced fiberglass (batt only no 'flash'- save $ but lose R-value)
5/8 Decking
30lb roofing felt
1" battens (strapping) - creates airgap
26ga metal (steel)
Or combination of the 2?
Neal Young
2013-08-28 12:57:14.000000
What size are the rafters?
Right now, I am leaning towards neither as a system but parts and pieces of each.
You have to seal the T&G to keep the conditioned air inside the space and out of the insulation. You can do this with some sort of spray foam or applied sealant.
You could also put rigid foam board in the rafter bays, seal the edges and/or entire assembly with spray foam but that is a bit labor intensive.
You don't need an Class I vapor retarder give you location so OC SPF (Open Cell) will work fine if you are controlling the moisture in the home. Most moisture related issues are as a result of bulk air loss and not diffusion.
I do like the idea of an above deck venting. That is always a great idea.
Eric Novotny
2013-08-29 11:12:03.000000
Yes. The tongue and groove leaks like a sieve. You have to seal that much better and it has to be sealed at the tongue and groove (i.e. conditioned space) side of the equation.
This is why spray foam directly down to that is probably better.
On the 2x6s, I would just fill them full with OC SPF. Air seal and insulation in one shot. You don't need a vapor control layer in your climate.
The 2x10s, you could flash and batt as you were referencing.
