1.5 story cathedral ceiling fool proof simple exterior roof insulation amount and type. Do I have to fill insulation cavity completely?
Hello! I’ve in a 1.5 story cape built in 1944 near Albany, NY zone 5b with cathedral ceilings on second floor. The house is in the shape of an L with two shed roof dormers and two valleys because of the L shape. There is a ridge vent and a gable vent. One part of the second floor is newer than the other and was built differently. The newer part has insufficient insulation but more than the old section. The old section has about r-13 foil faced Batts with an air gap under the roof decking. The newer part has r-19 paper faced Batts with an air gap under the roof decking. I will need a new roof in 5-10 years. I experience ice dams and big temperature fluctuations on the second floor. I have very little room to build down from the ceiling to increase insulation thickness. My question is: if I were to strip the shingles off, cut off the rafter tales, tape roof decking seams creating air tightness, and add exterior insulation (fiberglass z girts, 2×4 strapping with long screws, lots of questions) is there an amount of insulation that makes the roof bulletproof and unable to have a condensing surface underneath the roof decking? I would ideally have an air gap between the mew decking above the insulation to the ridge, maybe with cross strapping to allow air flow at valleys. I don’t want to have to tear down the whole interior ceiling to get new insulation against the bottom of the roof decking or tear up the decking from above to add insulation to fill the cavity completely. It’s an expense I might not be able to take on and also a time commitment if I do it myself. What I would like is the ability to leave the decking in place, foam or rockwool over the decking g, add new decking with venting underneath and call it safe. If I can avoid an entire interior renovation in order to insulate the roof that’d be ideal. A foam over-roof job is probably already approaching $30k+ including the new roof so having to tear down most of the interior surface area in order to do exterior roof insulation makes the job more out of reach. Has anyone done exterior roof insulation in an old house and left the insufficient insulation, leaky ceiling and air gap in place? Can I seal the gable vents and leave them empty or do I have to fill them with insulation or do I have to raise the ceiling higher than the current collar ties.
Also, is there a successful sip product to more quickly and cheaply add the maximum amount of insulation yo achieve this? Or is multiple layers of reclaimed or new foam, rockwool the answer?
Thanks!
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In zone 5 you want a minimum of 40% of your total R-value to be above the roof sheathing for an unvented roof. This percentage is extrapolated from IRC table R806.5 which requires R-20 continuous insulation for an R-49 roof in zone 5.
3.5" of polyiso insulation is R-20, and conveniently is the same thickness as a 2x4 if you want to put new eaves on with the method shown in photograph 3 of this article: https://www.buildingscience.com/documents/insights/bsi-081-zeroing-in
As far as installation details and products and all that go, have you read this article: https://www.greenbuildingadvisor.com/article/installing-rigid-foam-above-roof-sheathing and all of the related cathedral ceiling articles it links to? I think those articles will answer a lot of your questions.
Ice dams aren't a big issue where I work, so I'm not an expert, but Joe Lstiburek talks about the vented overroof similar to what you are describing briefly in this article in figure 8: https://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam
The only purpose of the venting is to control ice dams. Aside from ice dams, this type of assembly doesn't need to be vented.
Thanks for the info! Do you think that old insulation with an air gap that used to act as a ventilation channel under the old decking is a problem if left alone? I've seen articles that it undermines the above decking insulation. I'm concerned about moisture build up in the air gap as well.
You'll definitely want to seal up the vents that served the old ventilation channel. With the vents sealed up, that dead air space might not be optimal, but it shouldn't undermine the above-deck insulation as long as the vents are sealed. The old ventilation channel may somewhat undermine the existing r-13 and r-19 batt insulation, although I'm not exactly sure by what mechanism. The gap might not be avoidable without working from the interior. Perhaps another commenter will have a clever way to fill in that gap!
It would be worth seeing if you can access the existing rafter cavity at the top of wall (perhaps from the outside?) to make sure that you have insulation between the underside of the existing sheathing to the top of your wall's top plate. You don't want a gap between your wall insulation and your roof insulation.
Thank you for your thoughtful input!
>"I don’t want to have to tear down the whole interior ceiling to get new insulation against the bottom of the roof decking or tear up the decking from above to add insulation to fill the cavity completely."
I've not personally done this, but I believe you can blow cellulose in over the batts. Ask a cellulose installer. They can drill holes through the exterior sheathing. Whether or not the gap is truly a problem, I don't know.
"Can I seal the gable vents and leave them empty or do I have to fill them with insulation or do I have to raise the ceiling higher than the current collar ties."
So you have a small attic triangle? You will, at the least, need to insulate the gable walls (where the gable vents are) to make that part of your thermal envelope, making a contiguous connection with your roof insulation plane and wall insulation plane. In theory, the amount of insulation on the floor of the attic triangle (ceiling of 2nd story) needs to at least match the ratio rule outlined by Brendan above. But there is also some concern about 'splitting' insulation like that. I don't feel I've ever seen an unequivocal answer as to why exactly that is (the mechanics of it) including in this blog of Allison Bailes: https://www.energyvanguard.com/blog/3-reasons-to-remove-attic-floor-insulation-in-a-spray-foam-attic/
But assuming you do want to remove that insulation and you don't otherwise have good access, perhaps you could enter the attic from above by cutting back some sheathing during construction (you'll need to somehow insulate the gable wall either way).
That is very clever thinking. I did realize I would have to seal and insulate the gable ends. I suppose there will have to be some removal of decking removal from above at some point.