Air sealing and insulating trusses roof
I am ready to insulate my home which is a new construction. I originally planned to spray the roof deck with open cell and the walls with closed cell but decided to switch to sheep’s wool. My main concern comes with insulating the roof deck. I have engineered trusses with a bonus room.
My plan is to vent the roof deck with baffles and fur out the trusses to achieve at minimum an R-38.
My thought is to use 2” rigid foam to construct baffles from the top plate to the ridge, which would also act as an air seal (if that would provide an air seal which is my main question), outside of that would go wool insulation.
would the rigid board between trusses provide an air seal or would I need to rely on something like drywall?
Should I be concerned with the uncovered wood members of the trusses?
My aim is to keep the air seal and insulation against the walls and the roof deck.
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Replies
Hey Sammuel,
I have seen a lot of projects from designers and builders I trust where site made ventilation channels are installed as you describe. One thing you left out of your description is how you will seal the rigid foam to make it airtight. You'll need to seal it to the framing and seal any seams in the rigid foam. Canned spray foam, sealants, and tapes are all options, though I've heard lots of reports of canned spray foam being unreliable over the long term, and as wood moves. Air leaks are a common culprit for roof issues, so I'd still consider an air barrier at the drywall, either the drywall itself or a membrane. For others to give you helpful replies, they'll want to know what climate zone you are in (where the project is located). In the meantime, here are some helpful articles:
How to Build an Insulated Cathedral Ceiling
Site-Built Ventilation Baffles for Roofs
Thank you. I’m building in Riceville, TN which is in zone 4.
I have decided to go with a flash and batt. Using 2" of cc foam and 6" of blow in sheep's wool. I do not plan to use a vapor barrier (drywall or other types) outside of the sheep's wool. The spray foam contractor is concerned that with 2" of spray foam there will not be enough thermal mass to keep condensation from forming on the foam from the inside. Is this a genuine concern? The following article said this method of insulating performed well in my climate zone if I am following it correctly.
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
He proposed 3" of closed cell or 6" of open cell, spraying over the wood members to eliminate thermal bridging.
Sammuel,
The foam needs sufficient R-value so that condensation doesn’t occur in the underside. How thick that is depends on how much permeable insulation (batts) is below it. You can see how much that is for your climate zone in Assembly #4 in the link below:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Randy’s article provides a very good explanation:
https://www.greenbuildingadvisor.com/article/irc-2021-and-the-ratio-rule-for-hybrid-roof-insulation
That's good info.
Links:
https://codes.iccsafe.org/content/IRC2021P2/chapter-8-roof-ceiling-construction#IRC2021P2_Pt03_Ch08_SecR806
https://codes.iccsafe.org/content/IECC2021P1/chapter-4-re-residential-energy-efficiency#IECC2021P1_RE_Ch04_SecR402
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Given the right amount of cc spf is used on the underside of the roof deck and an appropriate amount of fluffy insulation, could the fluffy insulation remain uncovered?
Sammuel,
He is what Michael Maines wrote in another discussion:
"After my article on flash-and-batt came out in 2010 (https://www.greenbuildingadvisor.com/app/uploads/sites/default/files/Why%20Flash%20and%20Batt%20Makes%20Sense.pdf), I got many calls and emails from people who were convinced to use that system and found condensation on the interior face of the foam, sometimes running down the wall in quantity. That's when I realized how important it is to keep indoor air from easily circulating through the fluffy insulation.
An air barrier (such as drywall) is much more important than a vapor retarder, but because many things can go wrong, I prefer to make sure there is a vapor retarder as well. The IRC calls for a class 3 vapor retarder when you have a proper ratio, i.e. painted drywall, so that's a minimum. "
What if I ditched the flash and batt idea and went with a traditional vented roof using a 2” polysio board for a baffle, I could fur out to fit an additional 7” of sheep’s wool on top bringing the total R-Value to R-40. However if I wanted to maintain some head height by reducing the insulation in the top chord between the knee wall and the ceiling would I suffer any detrimental consequences. That portion is a 2x6 so without furring I would at most get an R-value of 20. I could transition from fluffy insulation to all rigid board for that portion of the truss. Something like a 2” vent channel 3.5” of rigid board between the trusses and an additional 2.5” on top of that bringing it to a total R-30 through that portion.
If rigid board in this application is a good option would foil faced or unfaced be ideal?
A standard vented roof works just fine so long as you make sure the whole roof is vented but you won't meet the code minimum values so you'll end up spending extra energy to heat and cool the house over its lifetime. R-38 is the current code there I am assuming. The current IECC specifies R-60 for that zone but it is yet to be adopted by your local AHJ.
I am at a decision point on roof insulation. My options are as follows
1. Vented roof asemby w/2″ of rigid board for a baffle, trusses furred out to accomodate R-30 of fluffy insulation for a total R-value of 40 However between the knee wall and top chord I would use all rigid board to limit the reduction in head height. Could I add rigid board as a last layer to add R-Value, & reduce thermal bridging to this whole assembly without causing moisture issues? If so what type would be best suited (faced, unfaced, etc).
2. Non Vented Flash & Batt w/3″ cc spf and 8″ of fluffy insulation (blown in sheeps wool @R 4.3 per inch), smart vapor barrier. I am worried that air sealing around my trusses will prove challenging.
3. Exterior continuous insulation – It is possible to remove my roofing (metal), install rigid insulation 3″ and reinstall the roof. The roof sits on horizontal 2×4’s 2′ apart. I could pull those up lay the rigid foam and reinstall on top (not ideal) or insulate around them, but those areas would have reduced insulation. Would that pose a problem? Also, I am unsure whether i could properly detail this type of assembly. The truss is designed with a bonus room
*zone 4a zip 37370 Thanks in advance.