cathedral ceiling insulation with rigid foam
I’m going to finish my bonus room above the garage of my cape cod. I’m trying to determine the most cost effective way of insulating the 2×6 roofline in the bedrooms. Any insight would be appreciated. I plan on having the assembly vented. My initial plan to create the 1″ air gap from the soffit to ridge vent was to use thin plywood nailed to 1×1 pieces of wood. However, it seems a better use of money would be to use 1 inch of rigid polyiso foam for the vent channel. The foam is more expensive, but at least it gives me an r value of 6. So to get my R value of 38 my assembly would be:
plywood roof underlayment
1″ air gap
1′ polyiso r6
fur roof joists out 2′ to accept r 19 fiberglass batts.
2″ rigid polyiso foam board r12
total r value of darn near 38
any thoughts? my concern is sandwiching the fiberglass between the 2 layers of foam
Replies
Nice post
I have been contemplating something very similar.
I too wonder about traping the FG and hence possibly moisture inbetween the two foam boards. Obviously air sealing the room side of teh insulation to keep moisture out is very important. You still have to consider the "what if" senerio. Secondly, people have said to air seal the upper foam (the baffle). That sounds good as FG is air permeable and could loss heat through air moving around the foam baffle), but it also sounds bad as air sealing would trap moisture. ?????????
What type of foam gives you an R6? And what is its permeability rating?
Hopefully someone WITH knowledge will chime in for you. And hopefully flordia will not recommend white glue.
Insulation
A recent project required 4-1/2" "iso" board, which I believe will get you close to R-38. With the 2x6 rafter space, you would have your 1" of airspace for ventilation, quick and easy. The cost of the board is steep, but the labor savings will make you feel better.
foam in place?
The thick "iso" board, do you cut it wide to force a tight fit (seal airing) or loose and use spray foam to seal it in place?
insulating 2x6 cathedr. ceiling
If you can increase the depth of the rafters I would use another 2x6 fastened to the existing with metal straps to give you extra depth. String wires crosswise at the 1" space under the roofply,(it will act as stop for the insulation) install 2 layers of R19 Roxul, strap 2x2 perpendicular under the 2x6 and install 1 1/2" Type II EPS, install 6 mil poly vaporbarrier. The 2x2 will give you a nailingbase for the drywall or other finishmaterial.
With the installation of 2" rigid polyiso - how will you fasten your ceiling finish?
Not sure what climate zone
but 1" polyiso as the top layer might not have enough R-value to keep the underside of that layer above the dewpoint... in which case as mentioned above you would need to be concerned about condensation. I would probably start with a layer of 2" just to avoid thinking about it, although not necessary in mild climates.
You could install 2 layers of 2" polyiso between the rafters, with a 1-1/2" vent space at the top, then another layer of continuous 2" under the rafters, followed by 1x furring, then your ceiling drywall.
You could also do some furring and install 6" total of polyiso between the rafters, but you don't pick up the thermal bridging that way. You do get to nail drywall right to the rafters, though.
Whatever you do, you want the board airsealed at all joints and edges. Buy a $30 foam gun, avoid the Great Stuff cans with straws.
foam and more foam
I'd tuck unfaced 3-1/2" FG batts into the rafter bays. R-13.
Then rip 2" polyiso to fit between the joists. Push it up so the bottom face of the 2" thick polyiso is flush with the bottom edge of the rafters. You can do friction fit of the framing is pretty steady. If the framing is a bit wobly and the rafter spacing varies, then rip them a 1/4" - 3/8" narrower than the rafter bays are wide. Set them in the bays, use a couple of small scraps of wood screwed into the rafter edges to hold the polyiso in place. Then use canned foam to fill that 1/4" gap. As it expands, it'll push the polyiso tight to the other rafter. Rips that butt end-to-end? Gap them, then seal that gap with foam. Another R-12.
Now hang full sheets of 2" polyiso on the rafters, just like hanging drywall. Leave a 1/4" to 3/8" gap between the edges of the sheets as you hang them. The ends of the sheets do not have to fall on the rafters, they can land over a rafter bay. I hang them with nails with round cap plates. You only need 4 per full sheet. Another R-12.
Once hung, peek into the gaps to find the rafters. Use a sharpie marker and mark the face of the polyiso so you know where the rafters are. Now use canned foam to seal the gaps.
Use scrap wood, real furring strips, or rip a sheet of 3/4" ply into 1-1/2" to 2" wide strips. These will be your furring strips. Run these horizonally across the ceiling. Using the sharpie marks as your guide, use 4-1/2" screws to screw through the furring strip, through the foam, and into the edge of the rafter. The ends of the furring strips do not have to fall on the ends of the rafter. They can also land mid-span. But offset the ends of the furring strips from the ends of the sheets of polyiso. If the furring strips are bowed, set them so the ends of the strips touch the polyiso. When you screw down the center, it'll pull the ends even tighter.
Hang drywall off the furring strips. The drywall plus air gap is worth a little over R-1.
Total R-38.
The beauty of it all is the foam will thoroughly outperform FG. There will be no air infiltration through the ceiling plane. There will be no radiant gain in the summer, so your space will be cool in the summer.
Your exterior gable end walls? Fil them with FG, then add 1" polyiso over the studs. You can drywall right over the foam with no furring strips, just don't drive the screws so tight that you overcompress the foam. Over time you could get a bit of "bounce back" and the screw head could pull through the drywall face. Either hang your junction boxes proud to begin with, or get 1" box extenders to extend the boxes.
Mongo...
Mongo, in your senerio the 3 1/2 inch stuff is pushed tight to the underside of the roof sheathing, thus no venting? Yes?
And all that foam and multiple steps with associated labor is it worth it over sprayed in foam?
no...
"Mongo, in your senerio the 3 1/2 inch stuff is pushed tight to the underside of the roof sheathing, thus no venting? Yes?"
Correct.
"And all that foam and multiple steps with associated labor is it worth it over sprayed in foam?"
I got the impression that this was DIY. Spray foam will save you time, especially with all the diddling with the batts and fitting the polyiso between the rafters. The polyiso on the rafter faces goes quickly. But spray foam between the rafters will not give you the thermal break that the final 2" application of polyiso on the rafter faces will give you. If that thermal break matters to you. I never think the thermal break matters until winterime, when here in CT I see the snow melt lines from the rafters on everyones roofs. Except mine.
I can't believe I actually inserted a smiley face.
The horror of it all.
Cost-wise? He'd have to cost out foam locally. Closed-cell spray foam might run him $1-$1.50 a bdft. 2" polyiso runs about 55 cents a bdft.
So sure, there's labor versus price. Versus the no kidding upside that the 2" thermal break provides over the much simpler between-the-rafter insulation with no thermal break.
If he did want to DIY spray foam, there are the 2-part closed cell kits (like foamofoam or tigerfoam) that probably run about $1 a bdft.
Thanks for your reply , and ...
I think the posters rafters are 2 x 6 or 5 1/2 inches deep. I seems like your proposal is 3 1/2 inches of FG in the bay and then 2 inches of Polyiso in the bay for a total of 5 1/2 inches. Thus no vented spaces. Your mistake or mine ???
As for your second layer of polyiso, I like the added insulation on the underside of the rafters as well.
You did say
3 1/2 inch polyiso follwed by 2 inches of polyiso, right? And 3 1/2 inches gives you and R of 13 and 2 inches gives an R 12????
my bad...
"...3 1/2 inch polyiso..."
My mistake, I corrected it. Thanks.
On another note, If it was my house I'd have no issues using R-19 batts in the rafter bays with no venting. Then using 2" of polyiso on the rafter faces. Gapped, seal the gaps with canned foam. Tight as a drum. Furring strips and drywall. Roughly R-32 total.
My finished off attic is constructed in similar manner. I'm in CT, the roof gets full sun in summer. Summer attic temps with FG-only at the peak were over 120 degrees, after instlling the polyiso they were 77 degrees. Winter temps (I have no heat source in the attic) bottom out at around 66-67 degrees.
Before finishing off my attic, it had unfaced FG batts between the rafters with a poly VB. In winter the attic was way cold. Summer way hot. Typical attic. Sure adding the extra R-12 from the polyiso helped. But what helped more was that it was polyiso RFBI versus more FG batts. There is R-value, and there is effective R-value.
It's not just the insulation...but how effective the actual insulation is. Stopping air infiltration. Blocking radiant gain. Etc.
Stepping off the soap box...
Sorry, what is RFBI
I tried googling RFBI to no success. ????
Why do you discount venting? You said you would go with R19, filling the bay, and forget venting???
RFBI
Rigid Foam Board Insulation.
"You said you would go with R19, filling the bay, and forget venting???"
Yes.
"Why do you discount venting?"
Well, it comes down to answering "what would be the purpose of the venting?"
In cold climates, the primary purpose of roof ventilation is to keep the roof plane cold to prevent the formation of ice dams caused by melting snow. The second reason is to remove moisture that migrates from the living spaces into the attic.
The polyiso will act as a rock solid air infiltration and vapor barrier. Zero air infliltration means zero vapor tranmission between the living space and into the rafter bays. Zero air infiltration coupled with the excellent R-value per inch of the polyiso will limit conductive losses through the insulation. The roof will remain cold. Ice dams should not occur. Couple that with code-mandated ice and water membrane on the roof for ice dam protection?
It's certainly not the only way. But it's a way that I've found that does seem to work.
In hot climates, the primary purpose of attic ventilation is to cool off the attic space to reduce the cooling load on the building's HVAC system. Which is usually located in the house's unconditioned attic space. Unlike any thickness of FG batts, the polyiso will keep the attic much cooler than were it not installed. Not only does it keep the attic cooler in the first place, but with the HVAC system (the air handler and ductwork) now inside the thermal envelope of the house instead of outside the thermal envelope, it now performs much more efficiently.
thank you everyone for the responses
thank you everyone for the replies and great advice. my initial plan was to hang drywall right ontop of the rigid foam, bad idea? is hanging furring strips worth the extra step?
worth it...
I think the overall installation will probably go faster if you use furring strips.
You'll use shorter screws, and you'll easily find the furring strip.
You can snug the screws tight when using furring strips. When going right over foam, if you snug it toght sometimes the screw can slightly compress the foam. Over time the foam can rebound and push the drywall out, popping the paper.
And you'll probably end up with a fewer waves in the ceiling plane when going over furring strips.