Making rafter bays deeper for vaulted ceiling insulation
Hey guys, I have a dumb question and I have looked it up but getting mixed reviews on how to do it. I am vaulted a ceiling in my house that currently has 2×8 rafters. (I have a roof thats only 6 years old so adding foamboard outside isnt an option). I have a vented roof/soffits/ridge etc and I want to keep it that way so I dont want to spray CC foam directly to roof deck. I am adding venting baffles lets day 2″ deep to each bay from the soffit to the ridge. My 2×8 are actually 7″ so that only leaves me 5″ left for insulation UNLESS I deepen the bays.
Im not a huge fan of covering the whole ceiling with 2x4x8 panels, then drywalling to that… I know its a common thing but it seems like I would get a lot of nail pops etc. I dont know, just doesnt feel right.
I thought I had more space in the bays and planned on doing 3 to 3.5 inch of foam on the baffles and 3.5″ r15 batt for a flash an batt set up to save some money and put me around r35-39.
I can still do this but I need 2 more inches of depth.., or 1.5 inches if I do a 1.5″ vent baffle instead of 2″ (I have to research these more).
Maybe I could find a 1.5″ vent with 2″ spray foam and 3.5″ batt but that would only take me to about r29.
If I added 2×2 (1.5×1.5) furring strips to the bottom of the rafters running parallel with the rafter (essentially just adding 1.5 to the bottom of possible?)… could I use some liquid nails and screws, or do I need to run them across? If I go across it kind of defeats the purpose and I dont have the straight depth that I need.
I know its a beat up subject and I do apologize.
Located in Southern New Jersey for reference. Zone 4
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Replies
B3llo38,
What you are suggesting will work fine. You effectively end up with a 2"x10" rather than the present 2"x8"s.
If you want more space, a 2"x3" is probably the deepest lumber you can easily install on edge with common screws. After that you can add 2"x4"s on edge and attach them with plywood gussets every 2 ft. Still want more depth - leave a space between the bottom of the rafters and the 2:"x4"
Okay, i just wasnt sure if I could add them parallel along the bottom of the joists. Not tons of info/pictures of that vs packing out a ceiling running them perpendicular
B3llo38,
Running them perpendicular to the rafters as strapping has the advantage of reducing thermal bridging, but as you say makes insulation more difficult.
Not sure what your local code requires as far as the ceiling insulation is concerned but it looks like in the 2021 IECC it requires an assembly U-factor of 0.024 (R-42) or use insulation with an R-value of R-60. I don't see how you'll get there without installing some type of drop ceiling to get some additional depth. Even with a high insulation R-value of R-6 it would take 10 inches to hit the R-60 value.
https://codes.iccsafe.org/content/IECC2021P2/chapter-4-re-residential-energy-efficiency
Local inspector said because its existing structure to aim for mid to upper r30s
Its a vaulted ceiling also. No drop ceiling
2x3 on edge parallel to the rafters would get you above R30 which is not too bad of an install.
If you want better, you can install 2x6 batts into the existing rafter bays. Cover them with a variable perm membrane taped at the seams and sealed to your wall plate as your main air barrier. Over this, cross strap with 2x4 on edge for 3.5" batts. From there it is standard install. This would also let you run the wiring through the 2x4 so you won't need to make any holes in the air barrier.
The 2x4s can be toe nailed where they cross the rafters and attached with long screws at the code required 2x4 rafter span to the existing rafters.
Whichever way you go, make sure your baffles are permeable (ie thin foam, cardboard, housewrap or thin OSB). You don't want plastic baffles.
Spray foam would melt the plastic ones? Or?
This is a good article about site-built baffles: https://www.greenbuildingadvisor.com/article/site-built-ventilation-baffles-for-roofs. There is potential for moisture to condense at the underside of moisture-impermeable baffles. As author Martin Holladay notes, it's not a problem often seen in practice, and I often specify Accuvent plastic baffles because nobody wants to make their own and flimsy foam baffles don't hold up to dense-packed cellulose or wood fiber insulation.
In case of spray foam, it doesn't matter. I was thinking you'll insulate with batts. If you can vent the whole thing, not sure spray foam is worth the cost or the risk.
Buy 4X8 sheets of 2" nailbase product (Atlas - ACfoam) & rip into 1.5" strips & screw them to the 2X8s to build them out (not perpendicular)
https://roof.atlasrwi.com/products/acfoam-nailable-polyiso-roof-insulation/acfoam-nail-base/
Take 1.5" foil faced polyiso board & score one side (2" from edges) to create the 1.5" roof vent baffles (R9) - see attachment
Spray 3" closed cell foam (R21)
3" Rockwool Cavityrock (R12.5)
R42 thermally broken - dries to inside ...
Hmmm i never seen that before wonder where i can get some local to Philadelphia