More Q&A Spotlight
For many readers, ’tis the season to stay warm, and sometimes that means beefing up insulation. GBA-er “B3llo38” (B3) hails from Climate Zone 4A, specifically southern New Jersey. In this Q&A post, he writes that he’s in the process of insulating an existing vaulted ceiling. The roof is vented (soffits and ridge) and framed with 2×8 rafters. He doesn’t want to add rigid insulation to the roof’s exterior because the roof is only 6 years old, and he doesn’t want to spray the interior roof deck with closed-cell foam, as it would interfere with the venting.
He’s adding baffles to each bay for venting, and had planned a flash-and-batt strategy of 3 to 3-1/2 in. of spray foam and 3-1/2 in. of R-15 batts to achieve somewhere between R-35 and R-39. However, the baffles take up 2 in. of depth, leaving him with only 5 in. of space, so he needs to gain another 2 in. or so of space to get to the intended R-value.
Weighing his options, B3 says he doesn’t feel right about installing rigid insulation over the interior face of the rafters, as he thinks that will lead to nail pops in the drywall. He also wonders if he could use a shallower baffle—but that combined with 2 in. of spray foam and 3-1/2-in. batts, would only yield R-29.
His third idea is to add 2×2 furring strips to the bottom of the rafter, effectively increasing the depth by 1-1/2 in., but he’s not sure of the best way to install them, or if it would be better to run the strips perpendicular to the rafters.
So, what’s the best way for B3 to install the most insulation possible in his cathedral ceiling?
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11 Comments
I am framing a new vented cathedral 4:12 roof. Currently, I have 2x8 rafters already installed butted to the ridge beam. As hindsight would have it, we're now considering ways to increase insulation and realise we don't have wiggle room to bring the ceiling down as it is already lower than ideal. These are two options we have
Furring down system - 40.7 R value more if adding interior or exterior insulation
-Interior
-T&G wood ceiling
-Would 1" of rockwool 45 safe semi rigid prevent thermal bridging
-Siga Majrex air/vapour retarder
-2x4 gusseted with plywood strips to 2x8 rafter filled with dense pack blown cellulose
-Vapour open barrier - Tyvek commercial home wrap
-Is exterior roxul comfortboard 80 possible to put directly on top for thermal bridging or will it impact the ventilation of the cellulose
-1.5" air gap using 2x4s
-1/2" plywood sheathing
-Mento 3000 WRB
-1x3" strapping
-Standing seam metal roof
OR
Furring rafter on top system - 48 or 55.5 r value with no CEI
-Interior
-T&G wood ceiling
-Siga Majrex air/vapour retarder
-2x8 rafter with an additional 2x6 or 2x8 attached using 1/2" plywood gussets butting to a 2x ridge board sitting on top of the ridge beam with dense pack cellulose
-Vapour open barrier - Tyvek commercial home wrap
-Is exterior roxul comfortboard 80 possible to put directly on top for thermal bridging or is it better without
-1.5" air gap using 2x4s
-1/2" plywood sheathing
-Mento 3000 WRB
-1x3" strapping
-Standing seam metal roof
I have not seen many examples of people furring rafters on top and wondered if this was possible?
Also, in the Josh Salinger vented cathedral ceiling assembly which I used as a base model for this, how would you precent thermal bridging?
Thanks for your help. Kenny
“[Deleted]”
Concerning those brackets, great concept but it looks like they are made from plastic and over the course of the last 30 years most things I see made of plastic become brittle with time and break, especially plastic items exposed to heat which these being inside a roof assembly would definitely be exposed to heat. I would look for similar brackets made from steel or figure out how to do it with all the scrap wood that usually goes in a dumpster. Plastic is like spray foam, I try to use as little as possible.
Same here, Jud, but when balancing cost and performance, sometimes plastic wins. There are various standoffs available made of fiberglass or steel; they are just too expensive to easily justify for most residential projects, and metal standoffs defeat much of the purpose of having exterior insulation due to the high level of thermal bridging.
My favorite wood-based alternative is the Swinburne truss: https://www.greenbuildingadvisor.com/article/variations-on-the-larsen-truss
Jud,
I'm with you and Mike on plastic use, but maybe not as worried about the longevity in the same way. I use vinyl windows, pex waterlines, and ABS drain pipes. It's not those elements that are failing in the houses I renovate.
Personally, I put 2" of foam on the bottom of the rafters (Kingspan, R-16), then 2" strips of 3/4" OSB perpendicular to the rafters, screwed into them with 4" screws. I'm not worried about nail pops as 1-1/4" or 1-1/2" SCREWS are not going to pull out from the weight of drywall. (This was with deeper rafter bays to begin with, so more insulation overall--it's very close to a whole-roof R-60 really snug in there now!) Then again, ceilings are ridiculously high, so losing almost 3" was no big deal.
insofast,
It looks like an interesting product, but the terms of use for GBA preclude you using the discussions to advertise your company. You should delete your post.
I agree with Malcolm, but I think your product is a good one so I'll post a link instead.
Malcolm and Mike, I plan to address this. Thanks for stewarding.
Here is the product that the company had posted: https://www.insofast.com/x-bracket-system. I haven't used it yet but I have a few samples on my desk and I think they have a place in the building world. Re-reading the blog I'm not sure that it would be useful on the interior if you have little headroom to spare. You could screw them to the bottom of the rafters and attach furring to them to increase the depth of the insulation bay, but I have had no problems with using the gusseted rafter system that Malcom described in the post.
Where I could easily see the brackets being useful is if the roof were not so new; you could install them above the roof deck, install furring that would double as venting, and install sheathing above that, for a code-compliant, building-science-approved, foam-free cathedral ceiling without losing interior headroom. I've done that successfully using framing lumber but the brackets would be quicker, easier and probably less expensive.
Edit: the site isn't letting me respond to Malcom but here's my response to comment 6:
Malcolm, the site isn't letting me respond directly to you but I meant to include that when I've done the vented over-roof insulation, it was mineral wool batts. High density fiberglass, wood fiber or other types of batts would also work. Of course you would want to use ones designed for steel framing because they are full-width (16", 24").
Mike,
I think they are a very interesting product. I’ve seen variations on commercial jobs for decades, but they were metal V-shaped brackets which introduced a fair amount of thermal bridging into the roof assembly.
The video shows them making provision for the brackets on the roof with custom truss overhangs. If I were using them on a new build I'd similarly try and alter the framing on the walls so they were set in on the foundation to avoid that awkward flashing at the base.
No reason I can think of that the exterior insulation couldn’t just be high density batts.
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