Insulating Deep Parallel Chord Trusses
Hello all, we are getting towards wrapping up our house plans and getting some structural details finalized. Our house has a vaulted room with a 12/12 pitch inside and out. The trusses are 22″ deep parallel chord supported by a girder truss as the center beam. The second complication is that we have no eaves. I let style win over practicality there. I know, Walta is going to scold me for this.
With that said, a 22″ deep cavity could easily have space for a ventilation gap and dense pack cellulose. However, just recently I read a post by Michael Maines that with no eaves, that vent channel presents a bad spot for water infiltration. So maybe a sarking membrane would be a smarter option? Our primary air barrier is going to be taped plywood on the walls. I can continue this over the wall and under the truss or over with a sarking membrane. My concern with the sarking membrane is the difficulty of installing that over a 12/12 pitch. And then having to dense pack 22″ deep cellulose with the space remaining. That seems excessive and has potential to not have enough density to prevent sagging. Filling the 22″ space with batts would be fine, but it would leave gaps between truss members I would think.
I would appreciate any thoughts from the group. Thanks!
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Replies
Tim_O,
The problem is almost certainly mine, but how does the sarking membrane negate the need for vents at the eaves?
It does not, however with a sarking membrane, that water is all outside your WRB. Assuming the sarking membrane attaches over your WRB on the walls. The vent channel would be over the sarking membrane.
Tim_O,
Sorry - I've got it now.
My vote would be to use batts. There will be a few voids, but at 22" you can afford a bit of inefficiency an still end up with a pretty well insulated roof. That avoids any worries about the sarking membrane bulging, and the difficulties of dense packing a deep cavity enough to avoid settling on such a steep slope. I've successfully used batts in similar roofs, but I've never tried to dense pack them, and I'll be interested to hear what others with direct experience doing that say.
Yeah, the simplicity of batts here seems good. Are you proposing going with the sarking membrane and using batts below?
My concern is still that the bottom cord is a 2x4 or 2x6 and exposed to exterior temp above that in that space. So even with R60 batts, you end up with R24 assembly.
This is just me thinking out loud. You need about a foot or so of insulation, with trusses that deep that will leave a pretty big gap above.
Because of the truss construction that gap will be pretty open along the whole width of the roof, so that means you can put some low gable vents into the bottom of this cavity and use those as intake. The exhaust vents could be standard ridge vents at the peak.
This means no need to figure out any soffit venting or deal with any membranes. If you insulate with batts as Malcom suggest, it would be a pretty simple job. Just remember to carry your wall air barrier under your trusses before you sent them to to tie it into the ceiling air barrier.
My target is R60. I don't have all the truss details yet, but I would figure 2x4 or 2x6 top and bottom chord. To hit R60, I think I'd need about 18" deep batts. With the top chord being a 2x4, I'd be afraid it doesn't leave enough cross flow between truss bays with 18" deep batts. Maybe strapping the top of the trusses with 2x3 purlins before sheathing would provide that path way. And that should be easier than a sarking membrane as it provides a walking surface.
Edit: I was looking at normal fiberglass batts. Going to a pair of HD R30 batts, I should be able to hit R60 in 14" or so.
But I still come back to my fear of the bottom chord being a 2x4 and no insulation is above that, and that 2x4 being directly exposed to the ventilation gap. I ran a quick calculation. A 2x4 24" on center with R60 between comes out to a true R value of R24 for the assembly.
Make one of the layers the same thickness as your bottom chord, this should be a regular wood stud sized batt.
For the layer above that, get batts for metal studs, these are true 16" or 24" wide so the batts above will touch, there won't be any void. Depending on how flexible the batt is, you might need to trim them a bit by the truss members. One challenge would be holding the top layer in place before the bottom layer goes in, insulation wires or some duct webbing might be needed.
As for venting, our code calls only for a 2x2 for providing cross venting to bays that are blocked, so only 1.5" gap. You'll have way more than that.
It is a lot more work but you can detail vent intakes bellow your gutters. If you use a wider piece of perf for this and put it a bit lower down but run the wall sheathing above this vent. Now this vent could be both your roof intake and wall exhaust vent and any water that does make it in can drain down through the rainscreen cavity. I would put a piece of trim over the perf so at least wind driven rain does not make it in.
The metal stud batts makes sense, that should expand in-between truss members fairly well.
A 2x2 for a bay next door, but what about when it's 10- 15 bays from the gable vent, that would be my concern. Likely I will continue the roof insulation the entire length of the house. It's 30x58 exterior dimensions.
The vent below the gutters might help. I think wind driven rain is one of the major concerns with the vent opening and the lack of eaves.
I think it's a perfect situation for a sarking membrane system. Installation is really not as bad as people who haven't done it think. It is more complicated than doing it conventionally but not by a lot, and you end up with a system that's superior in several ways.
I avoid batts whenever possible; I would dense-pack it with cellulose. A little sagging won't hurt a thing in this type of system and it would perform better than batts with gaps.
Thanks Michael, good to hear that! Maybe going with this as a primary plan and only switching to the alternatives if I can't find someone willing to do the install.
In the past, what membranes have you used for this? From Pro Clima, it seems like Mento Plus is the only one specified for Dense Packing. Which, given the lack of eaves, I would be more comfortable spending a bit more on a really good membrane. I just wondered if some of the Pro Clima "connect" options would be easier to work with, rather than fiddling with tape over a joint with no backing.
I have only done it with Mento Plus and Tescon Vana tape. I haven't used their Connect product but it looks like a good option. The membrane should be pulled very tight, and it's so tough that taping it isn't as hard as you might think.
Excellent! The plus might just be better to go with for the taping as well. The mesh probably keeps it stiffer and easier to tape.
I appreciate the feedback!
I got to thinking further on this. If I run into some resistance doing the sarking membrane, I wonder if using a hybrid method would be possible. Run a roll of WRB up the first 3 feet of the trusses and drape it down, lapped over the wall WRB. Leave it slightly loose between truss bays, not much, just a little. Then sheath as normal. Being left slightly loose, one should be able to stick a standard vent baffle in that first 3 feet, under the plywood, but above the membrane. 3 feet up the roof should be more than enough that any wind driven rain drips back out and down the wall WRB. Vent baffles would then continue to the ridge.
Does my explanation make sense? I'm not sure if there would be a downside to doing this. Possible damage to the WRB if it's too tight I guess.