How to insulate parallel chord roof trusses?
Hello,
I Am currently building a small house with an attached garage. Over the house are parallel chord trusses with a 5/12 pitch. I have an energy heel on the trusses, and have about 28 inches from the bottom of the truss plate to the roof sheathing. I would like to use blown in cellulose. A concern I have, which may be unfounded, is the celluse “rolling” down from the top. Should I install some sort of breaks every so often, such as a piece of two inch styrofoam? Also should I put my venting all the way to the top of the roof, or just enough to clear the cellulose? I am trying for an R value of 54 or more as I live in northern Minnesota. Thank you for any help!
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Replies
Robert,
You are aiming for an insulation layer that is about 16 inches thick. You told us the vertical height from the bottom of the truss to the roof sheathing, but you didn't tell us the total depth of the parallel chord trusses (measured perpendicular to the slope).
If I were insulating these trusses, I would install baffles to create a ventilation gap from the soffit to the ridge, and I would then install the blown-in insulation under the baffles. For more information on ventilation baffles, see these two articles:
Site-Built Ventilation Baffles for Roofs
How to Install Cellulose Insulation
To read more about insulation slumping in sloped applications, see Insulating scissor truss.
That's what I meant when I said 28 inches. I have that much space perpendicular to the slope. Would you recommend building baffles in place then?
Robert,
With a 5/12 pitch, I'm guessing that the cellulose will mostly stay in place. I would probably end up installing ventilation baffles on the bottom third of the roof, in case there was some cellulose slippage. The upper 2/3 of the roof should be OK without baffles.
i am looking at a plan with parallel-chord trusses in a vaulted ceiling,
but they are at a 12/12 pitch, could something like that be insulated with cellulose?
How deep are your trusses, and how much R-value do you want? 12:12 roofs can be dense-packed with no issues but the deeper the space the more challenging it gets. (My record so far is a 36" cavity.)
hi michael,
the trusses aren't spec'd yet, but this would be in central VT, so target is r-60.
makes sense now that the architect i was chatting with suggested 16".
couple of related questions:
1) what do you normally use for dense-pack r-value calcs?
2) is this type of assembly vented, and if so how?
Buildzilla, I usually do 18" deep trusses when parallel chord; that leaves space for 1 1/2" Accuvent, a readily available insulation baffle tough enough to hold up to dense-packed cellulose, leaving 16.5" for cellulose. I use R-3.6/in for dense-packed cellulose, which results in R-59.4, which I'm comfortable rounding up to R-60.
While some people have had success using unvented cellulose in roofs, and I have a few of my own on past projects, it's not compliant with the IRC and is not a safe or resilient assembly. In other words, this type of roof should always be vented.
If you want an unvented roof, you need to control the dewpoint, which means using enough closed-cell spray foam or exterior insulation to keep the condensing surface warm. In climate zone 6, at least 50% of the total R-value should be in the foam layer. But on new construction it should be easy to avoid such a climate-damaging as foam.
thanks michael, that's great info.
i like your 18" + accuvent strategy.
would something like intello be sufficient for double-duty as
dense-pack webbing and air-barrier?
Intello+ can certainly work, but the challenge is getting enough air out of the insulation cavity for a proper dense-pack, which for a space that large should be close to 4.0 pcf, rather than the 3.5 pcf most installers aim for and the 3.0 pcf they often end up with. I recommend blowing behind Insulweb and going over it with Intello or another variable permeance membrane after the insulation is in place. But talk with your installer about it; there are a few workarounds, such as using a double-wall pipe for installation. But if they just say it's no problem, ask how much experience they have dense-packing large cavities.
hi michael, circling back to this thread as i noodle more on it, if using trusses, do people ever apply insulweb across each truss from ceiling-side to roof-side to create a bay effect to facilitate dense-packing between the trusses?
also, not familiar with double-wall pipe strategy, can u elaborate?
Hi Buildzilla, yes that's standard practice for filling open cavities. You can separate each bay or possibly go up to 2-3 bays at a time. I have also had installers put a layer of insulweb parallel to the roof to make a 24" deep cavity into two 12" deep cavities, which also makes it easier to get good density.
My friend Jon Riley is an insulator and building science nerd; he wrote about how to do dense-pack right: https://www.finehomebuilding.com/project-guides/insulation/dense-pack-cellulose-insulation-done-right