Scissor trusses and wood fiber batts
user-1185720205
| Posted in GBA Pro Help on
I am already sheepish about posting a question about insulating a cathedral-ish ceiling, using scissor trusses, and not having a definitive insulation plan dialed in..
But here goes:
We are building our own house in western Massachusetts, climate zone 5. We’re planning to use scissor trusses that have a 9 pitch roof and a 7 or 8 pitch ceiling. I wanted folks opinion on a plan to use the wood fiber batts from TimberHP and put 3 layers of 5.5” batts in the ceiling. They seem rigid enough that they could friction fit in the truss bays though filling between the webbing is a question- any opinions there? If they have a snug fit, is there a reason we can’t just install them such that they are flush to the bottom cord of the truss and all the space above creates a natural vent channel and would vent soffit to ridge? The truss design has a 2’3” heal cut but the triple stack of wood fiber batts would only equal 16.5” of insulation for an R-value of roughly 49. We are using intello as an interior air barrier so that would be attached to bottom cord, then 2x strapping for service cavity, then drywall. If this could work, we could avoid making or buying vent channel baffles or doing the double sheathed roof with battens and mento which would be cool to avoid if possible. If that could work, we could sheath on top of trusses and then use a non vapor smart wrb and then metal roof?
For the wall assembly, we are planning for drywall, 2x service cavity, intello, 2×6 frame with timberhp batts, 5/8” plywood, mento WRB, 2” rigid insulation (most likely recycled xps), 1x or 2x rain screen battens, wood siding (likely vertical)
Does this all make sense?
Thanks for any thoughts,
Kate
Thanks for any thoughts,
Kate
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Replies
Cutting insulation to fill the webs will be vastly more labor than detailing vent baffles. Depending on the depth of the truss you will only need baffles extending for the first few feet off the eave. I think using anything besides cellulose for this situation would be crazy.
strausjw,
On an 8/12 scissor trussed roof you need to enclose and dense-pack the cellulose or it will all shift. The truss space gets higher as it rises, so you would also need some way of limiting it to reasonable depth, or you would end up too much near the peak.
Kate, cutting TimberHP batts are quite stiff, nothing like fiberglass batts, more like mineral wool batts but even stiffer. They make a nice friction fit between framing members. Working with them is essentially carpentry, so you would need to notch carefully around every truss web. You would need to fill large gaps with pieces of insulation. I think it's very do-able, just a lot of work. Have you considered having wood fiber or cellulose blown in instead? That's usually much simpler when working with truss roofs.
Otherwise your plan sounds fine. There is no need to have a variable permeance membrane under your roofing; regular, synthetic underlayment will be fine.
Thanks for the feedback. All that notching does sound like it might not be worth it…
In a combined follow up question to these responses, if we did blown in, it makes sense that it would need to be dense packed because of the pitch. Could we install baffles to hold back the blown in at a reasonable depth towards the peak? Would they be parallel to bottom cord, creating say a 2’ cavity for insulation all the way to the ridge but with 3” of vent at the eaves and obviously more as it goes up?
Lastly, is it reasonable to rely on the Intello with the service cavity battens already installed to sufficiently hold back a dense pack?
Thanks so much for the advice…
Kate