When selecting insulation thickness and
installing insulation for the rafter portion, the
building code requires 21/2" of ventilation air
space between the insulation and the roof
sheathing with no obstructions
Keep in mind, this of course is the Canadian Building Code. The regulations may be different in your area. I personally don’t agree with the turning the roof into a wall theory, the extreme temperatures that shingles incur is much higher than the temperature of your walls. Touch either one on a 90 degree day and i’ll bet the farm on which one you’ll pull your hand away from first.
So I am getting a quote on spray insulation on Monday (gulp!)
I figure it is worth it to see what that will run me vs. doing all the batt work myself. Also if I do from the soffit all the way and seal it completely, I don’t have to chase air leaks around the ducts, pipes, outlets, etc.
Ok - so planning this project got away from me and the holidays hit and, well, here we are…
I never heard back from the spray foam guy with an estimate, so I have to start that again. I did talk with my county inspectors and they said I need r30. Furring my 2x8s would work they said. So I figure on this math: existing 2x8 = 7.25" minus 1" baffle = 6.25" left. R30C for cathedral ceiling is 8.25". If I rip a 2x4 in half that gets me 1.75" plus the 6.25" = 8". The inspector said close enough. Granted the effort to fur the rafters - might as well do the whole 2x4…
So I will still get spray foam quotes and do the math on my materials for furring and batts and compare. Thoughts on the furring plan?
I’ll agree with the foam if you can afford it. I’m not convinced of the steel though. A lifetime architectural shingle will hold the snow up there for a bonus layer of insulation. Snow slides off of steel, and if you happen to be fetching the paper at the time it decides to cut loose, they’ll find you next spring:D
Steel is not an option. would be completely out of place where I live in Richmond VA - and snow load is minimal at best here (except the last 2 weeks!). My rough estimate for insulation and furring - with my labor is about $22-2500. Let’s call it 3K to be safe in materials. That is a far cry from the 6K for foam…
That may be true but one can not seal the area up tight (less heating/cooling cost), with batt insulation, like one can with the spray foam ( http://www.icynene.com/en-us )…pay now and have the comfort of living, or pay later with drafts while living. Just my opinion… I heat and cool ~2,400 ft. sq. for ~ $500/year incl/hot water.
Notwithstanding what the others are saying about ventilation, I would never use sprayfoam because I remember the 1970’s when insurance co.s made everybody rip it out even if it was properly installed because of the off gassing problem. I would use styrofoam boards, R 5 per inch or thereabouts.
I am also thinking that that roof sheathing and shingles with styrofoam on the back is going to be cooler than roof sheathing and shingles in contact with very hot attic air, even if it is moving up and out, but that would have to be field tested.
I am not surprised.
I live in a 120 yr old home no insulation in the walls and an R-60 in the attic no vapor barrier under it, 2 gable vents, no frost ever in the attic I look often well not as much any more (I have stairs and an insulated hatch). No problems with ice damning and I pay 650-700 per year forced air and to heat the water, natural gas. I spray foamed all openings in the attic and basement before we insulated (there was none when I moved in).
The temperature of the roof sheathing of an unvented roof will rise by no more than 10 degrees F over a ventilated attic, but that could result in a 10% drop in shingle life. That being said, the color of your shingle and the orientation of your roof plane to the sun can affect it’s lifespan more than foam insulation.
Warranties are riddled with weasel clauses. It might void your warranty if you installed your roof when Mercury was in retrograde
If spray foam is installed under the roof deck, the condensing surface temperature is controlled on the roof deck.
I take it your Air Handler is in the attic? Not the compressor. Your ductwork should be well insulated so that it doesn’t become a place of condensation.
Richmond is not particularly humid, so my opinion is that the closed cell foam is your better choice, but if you lived in Georgia or Dallas, I’d be concerned with how much humidity would be held in the living space that would not be able to dissipate through normal air exchange.
Make sure your roofing and flashing are in good shape, put a window in that sucker and then foam it.
Richmond can get quite humid. My upstairs air handler is in the attic and the compressor it outside next to the house.(ground level). The system is a new Trane unit as of last August. No window will be added because I don’t want to mess with the Dryvit exterior and add another water infiltration point. It is a kids playroom/media room so windowless is fine.
I heat and cool ~2,400 ft. sq. for ~ $500/year incl/hot water.
