Insulating cathedral ceiling w/ inadequate truss space
Hi All,
New build in CT is finally rough framed, and I discovered the truss package didn’t include enough room everywhere to accommodate the R-60 blown-in insulation + 2″ for ventilation baffles we specified, and I’m trying to determine the best path forward.
From what I’ve read here and elsewhere, the options are as follows:
1) Deal with having less insulation than anticipated
2) Put foam on top of the roof deck
3) Use CCSF on the inside of the roof deck
We worked to make sure there wasn’t going to be any spray foam inside the house, but I think that’s probably what we’ll have to go with, now. Back in the planning stages we discussed putting foam on the roof deck, and the GC couldn’t find any subs who were comfortable with doing it – I can’t imagine that’s changed.
Beyond blocking off all the soffits intentionally designed into the trusses, is there anything else that has to be done to the roof deck or attic prior to putting in CCSF? Silver lining is that I caught this before any interior work has been done, so we can still muck around up in the attic as needed. Thank you!
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Replies
How much loss are we talking? Is the issue where the trusses connect to the wall top plates there isn't enough space height wise to allow for the complete installation of the insulation? If this is the case, could you use polyiso, eps, or xps for the baffles to get you there or use a different insulation with a higher r-value for part or all of these areas?
I think the issue is precisely what you're saying - there's not enough space where the trusses connect to the top plates, height-wise. I specified raised-heel trusses with enough room at all sides for the requisite insulation.
I have to get up there with a tape measure - I'm not 100% what the total truss height is at the top plate, but it looks to be less than the 20" inches I expected (18" for fill, 2" for baffles).
I would just use polyiso for the baffle material to get a little extra r-value for that area since it looks like you are close to getting the requisite 18". Can you use a different brand of blown cellulose that offers a slightly higher r-value? It looks like the Greenfiber brand at home depot has an r-value of 3.7/inch which would get you to r-60 in about 16.25".
These baffles will just be at the low point where the truss meets the wall plate right, and not the entire width of the roof? I think there may be some more requirements if they are over the whole roof and not just the lower section.
I was under the impression that the baffles would run the width of the roof on the non-gable sides. Is that incorrect?
Heel height is measured at the outside face of framing. It looks like yours are about 10".
I think we're saying the same thing, but yes, the baffles should go approximately where your red arrow is in the photo above.
Oh, I see where the confusion is, what I mean is you won't have baffles over the entire area of your roof, right? If there are baffles over the entire area of the roof they have to be able to allow water vapor to pass through to allow any warm moist air to escape through venting. Polyiso or XPS won't allow this so you wouldn't be able to use them in that situation but if you just need them for the first few feet near where the truss meets the wall then you should be ok because your blown in insulation would be able to release any moisture through the rest of the vent since it would be exposed to the cellulose insulation.
David,
I agree with Krackadile. No need to throw out the baby wiith the bathwater if the problem is only at the eaves. Stick with a vented assembly, and use foam for the baffles at the exterior walls.
Thanks to both of you - I'll get up there tomorrow, measure to make sure I can meet code at the eaves (which looks possible) and then talk w/ the GC about using foam for the baffles. I found a FHB detail on this to follow.
Can you get at least R-30 at the eaves without foam? If not, you could cut-and-cobble rigid foam into the truss bays. Otherwise, closed-cell foam would be the easiest approach. I avoid foam when reasonably possible but reducing the risk of ice dams is important. Installing rigid foam over the entire roof rather than using a little spray foam inside the building envelope doesn't seem like a good use of resources to me.
I'll take a look tomorrow, and assuming I can get R-30 at the eaves, I'll likely follow the advice above from Malcolm and Krackadile and just use foam for the baffles. Thank you!