Unvented raised heel scissor truss
DBars
| Posted in Expert Exchange Q&A on
I have an unvented assembly with a raised heel scissor truss (Zone 4C, under Seattle modified 2018 irc).
1. Is it worth considering a permeable roof underlayment, like slopeshield plus SA (30 perm) to allow outward drying?
2. Would the class II VR at the ceiling drywall impede inward drying?
3. Do you see any drawbacks with using Smart Baffle by DCI Products to hold up the batt insulation?
The folks at DCI gave it their approval. I see this approach as more foolproof than tying-up the batts with string, especially considering the importance of having the batts stay tight to the sheathing, and the reality that access into the truss void to push a sagging batt back into place would be nearly impossible after the drywall ceiling is up.
4. Do you see other issues with the assembly?
Thanks, Daniel
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I would tack up some 2x2 bellow where the fluffy would go and use faced batts. These you can staple to the facer to the 2x2 to keep the insulation in place.
There is no issues with a gap to drywall or the permeability of drywall. If you do use baffles to hold the fluffy in place, they need to be permeable as this assembly needs to be able to dry towards the interior.
The permeable roofing underlayment doesn't do anything in your assembly since the limited drying through the shingles.
I would move the peel and stick under the rigid (this also gets you dried in before the rigid and 2nd deck goes on). This would be your main air barrier, so it should connect to your walls.
Even there, you can use ZIP roofing or tape the seams of the sheathing and use standard underlayment. Finally you can use regular synthetic roofing underlayment under the shingles.
P.S. See if your truss supplier can make the trusses with 2x10 top chord. This would simplify your insulation a fair bit.
Thanks for the comments.
I can see how tacking up 2 x 2's with kraft faced would work. Good idea.
The smart baffle material is corrugated plastic - so pretty much impermeable, but they would spaced apart and there would also be 1-1/2" gaps from eave to ridge in between each truss bay. Wouldn't those gaps be enough to allow drying to the interior?
If I use a taped bottom layer of sheathing to become the main air barrier, I'd then be filling it with holes from all the screws holding down the top layer of sheathing and rigid. Would that end up compromising it as an air barrier?
If the smart baffles are spaced apart, than it is fine. Still not sure if it is worth the cost. You can also use insulation wires, heck of a lot cheaper.
A hole with a fastener in it is not a hole, no air will leak through there.
Your top layer on the other hand will have thousands of holes for the shingles. When you re-roof, all those nails come out and new nails put in so you add thousands of small holes. If the air barrier is bellow the foam, this is not an issue. Also a roof deck with peel and stick on it is very difficult to remove which makes any roof deck replacement hard. This is why regular roofing underlayment is a better idea as the top layer. It can simply come out and be replaced during a re-roof.
I'm concerned that with a typical 2x4 top chord truss you would be limited to a few truss web members for placing the wire supports. However, as you suggested, if the top chord could become a 2x10, then there would be ample opportunity to put in wire supports. Are there different types of wire supports or are they all similar to the Simpson IS24 R supports?
Good point about the many holes from the roofing nails and moving the air barrier down in the assembly.
Regarding what you suggested previously, about using zip sheathing or taping the seams of osb sheathing, is taping the joints on standard osb all that is necessary to make the osb layer into an air barrier? What is a good type of tape for standard osb joints - is zip tape a solid choice or are there different tapes that are more effective?
Thanks again!
You can tape OSB or CDX with a good quality tape:
https://www.finehomebuilding.com/2013/11/07/backyard-tape-test-10-months-later
Zip now has a roofing system similar to their wall system, you can use that instead of plain OSB.
How effectively the insulation stays in place depends on the spacing of the wires and how stiff the batts are. High density batts (HD fiberglass or mineral wool) would work better. You can also use webbing to hold the insulation in place.
Daniel,
1. It's not a straightforward decision. The trade off is between some small amount of outward drying (a long as the polyiso is not faced), and an increased risk of water penetration from above. RDH looked at this issue and concluded that in the PNW an impermeable underlayment may make more sense. See page 212 in this link: https://www.rdh.com/wp-content/uploads/2017/08/RCI-2015-Problems-with-and-Solutions-or-Ventilated-Attics-GFINCH.pdf
2. The only interdiction Martin makes when discussing the interior vapour-retarders on these assemblies is not to use poly. But I don't know enough about your code to say what is required. Practically a class 3 seems preferable. See assembly #3 in this link: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
3. I saw your thread on the Smart Baffles and think they sound like a good solution. I would make sure that the ceiling was detailed as an effective interior air-barrier though to stop moist air getting up into the batt insulation. See this discussion: https://www.greenbuildingadvisor.com/question/michael-maines
4. No. it looks like a good solution for a difficult roof type.
Thanks much for the responses and links.
Regarding #2 , I agree a class 3 would seem to make more sense to promote inward drying. But with my wall assembly, which doesn't have exterior insulation or vented cladding, the Seattle IRC calls for class II vapor retarder. So that means I'm stuck with a class II on the ceiling as well, right?
3. With the ceiling as the interior air barrier, have you heard of success using ultra thin LED fixtures for recessed lights, like the Lotus LY6RCS with acoustical sealant on the flange to really seal it well?
I'm up in Canada, so while I poke around in the IRC to answer questions here, I'm not that familiar with your code.
I don't think you would have any problems with either thin recessed LEDs, or those that fit in standard light boxes.
Thanks. Your advice is much appreciated!
Deleted
Perhaps you have already constructed this assembly in which case you can ignore the following comment, on the off chance this is still in design though….
One method we have used with scissor trusses many times, albeit in CZ-5 is to install 2x2 strips on each side of the truss top cord about 2-3” below the roof deck. This allows us to install 1/4” plywood or if it’s available 1/2” MSL wood fibre panels (picture Ten Test without the asphalt coding) to create a continuous vent path from soffit to ridge. The roof deck can then be installed and waterproofed in a traditional manner before the finished roofing material is installed.
In terms of insulation below the baffle, dense pack, fibreglass (BIBS) or cellulose can work well as it fits itself nicely around truss webbing, batts can also be an option, but are more difficult to install in our experience. On the ceiling side of the truss, a continuous, smart vapour control membrane, Majrex or Intello can create an excellent vapor/air management system. Installing a 2.5” service cavity (2x3 on edge) allows for electrical boxes, thin, recessed, lighting, and wiring to be run through the ceiling without penetrating the membrane.
We have found this system, quite simple to install and also cost-effective.
Thanks for the reply and the photos which illustrate what you've done very clearly! In my situation I need to go with an unvented assembly.
sommerbros,
The OP asked about un-vented assemblies. If we want to expand the discussion out from the OPs question there are all sorts of variations we can consider. Whether the baffles need to extend the whole length of the trusses depends on how deep the insulation is, and the height of the trusses. Other option is covering the top of the trusses with a permeable membrane and venting above.
I guess you put the 2x2 strips on while the truss is on the ground?
Deleted