Zone 7 Wall Design Review – Deep Winter Homestead
Hi All!
We are making a modest 25×28′ addition onto the 1bd/1ba cabin we live in, to convert it into a family home. The existing structure is 2×6 conventionally framed with R-21 fiberglass bats.
For the addition I am thinking of something like this:
1. Drywall
2. MemBrain, Siga Majrex, or Intello vapor barrier with seams sealed with Siga tape and glued to electrical boxes, top plate, etc. with Tremco acoustical sealant.
3. T-Stud 24″ OC + BIBS -or- dense-packed cellulose
4. Steel strap bracing -or- OSB/plywood
5. 2” GPS Foam (R-10), foam-glued (PL 300 or similar) around sheet perimeter to the studs, and glued to each other on joints
6. Furring (1/2″ or 3/4″ plywood strips with Cor-A-Vent along bottom and top -or- a roll-out rain screen)
7. Vertical steel siding
We are re-siding and re-roofing the existing building too, so layers 5-7 will continue onto there to help make the whole building more consistently insulated.
Questions:
1. it was suggested to me to use J-channel along bottom of wall to support the 2″ GPS and protect the exposed edge from pests. This seems to me like an invitation to hold water and rust the J-channel. Water will not be absorbed by the GPS, but it doesn’t seem like it would be good sitting there, either, if it found a way in.
2. A contractor mentioned to me a product which can be “rolled out” to serve as a rain screen/gap and integrated rain screen ventilation to keep pests out of the rain screen layer. I am picturing it being made out of a composite material, so I am not sure if siding could be fastened to this like we could do with plywood strips. What are your thoughts?
3. We aim not just to build an efficient home, but a healthy one. We always choose low/no-VOC products when available. We plan to use a CERV2 which detects and manages VOC. But reading about Tremco Acoustical Sealant, which I know is great stuff for air tightness, it contains distillates, petroleum, and mineral spirits. I am sure this is why it’s so useful, never cures, etc. But honestly if this would lead to months of off-gassing making my kids sick (regardless of how well the CERV2 might work) I’d rather have a drafty house. Thoughts?
4. Any concerns with steel strap bracing instead of sheathing? I like the openness of this wall, but also because we live in a forest, I am worried this would be asking for trouble from mice.
Thanks everybody! We are so fortunate to have this forum.
Cheers.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Patrick,
I'll leave it to others to comment of the merits of your overall wall assembly, and just reply to your specific questions - but will say there is big difference in the ease of building a structure when it has exterior sheathing, not just foam.
1) The suggested J channel should be made of perforated-stock which will allow both air-movement and water to drain. It should be sized to cover the bottom of both any exterior insulation you include, and the rain-screen gap. Here is an example: https://www.menzies-metal.com/metal-flashings/perforated-j-channel-rain-screen-low-back/
2. The mesh rain-screens are best used under cladding like cedar shingles that require horizontal wood battens - which limit their usefulness. These mesh products do not provide support for the fasteners used for siding.
3. Tremco acoustical sealant does smell. An alternative that contractors here on GBA have suggested with similar attributes is Pro Climate HF. If you do use the Tremco, only use it where you are sandwiching it between the membrane and the framing - and secure the materials with staples. It isn't a glue, and if you use it around electrical boxes or other exposed spots, you will find it migrates onto your tools, clothes, truck, family, pets, friends and acquaintances. I'm speaking from experience.
thank you Malcolm!
1. With the perforated J-channel, I am assuming carpenter ants could still be a problem, right? Would something like that go at the top of the wall too, with a gap to the eave, or somehow seal it to the eave and let the soffits vent?
2. That makes total sense. So as long as we follow your J-channel suggestion, we'd have venting along the bottom and the rain screen layer could be furring strips, right?
3. I will check out the Pro Climate - thanks!
Any thoughts on the steel strap bracing (essentially putting the GPS right onto the studs) vs. using some sort of sheathing? The only concern I'd have with not using traditional sheathing is it seems like sheathing would add a layer to be a little less welcoming to mice (we live in the forest), but I don't like the idea of sheathing blocking moisture.
Patrick,
1. My (unfortunately) frequent experience with carpenter-ant infestations in buildings here has been that they will find multiple ways to enter the envelope. The best strategy is keeping a small buffer zone around the foundation clear of vegetation and being vigilant. That said, the holes in the flashing are too small for them to enter. The only insects I've had make it into wall cavities protected by them have been tiny sugar-ants.
2. To me furring strips are the easiest and best way to go with most cladding.
3 (b) I build in a high-seismic region where omitting the sheathing isn't an option, but some builders have done so successfully. In a wall with exterior foam, the worry of the sheathing "blocking moisture" is very rarely an issue, as the foam typically has a much lower permeability.
> Tremco Acoustical Sealant
I recommend reading the manufacturer's information, where I don't think you will find a recommendation for use as you propose. Consider Dymonic 100.
I'd put DrainWrap under the foam.
From the Tremco Site:
Basic Uses
Tremco Acoustical/Curtainwall Sealant was developed for acoustical sealing of drywall partitions, curtainwalls, corridors and party walls.
This sealant also is used as a lap joint and perimeter sealant for polyethylene vapor barriers over fiberglass batt or other insulations and may be used in contact with polystyrene.
Maybe I'm being too picky, but that sounds like "We developed this screwdriver to turn screws. Some people use it to pound nails". Membrain (and probably the others) isn't made of polyethylene (again, perhaps too picky).
So let's look elsewhere - do the membrane manufacturers recommend acoustical sealant with their products? A quick look - no. I see no sign of Tremco Acoustical meeting the "must conform" spec in one CertainTeed (MemBrain) document where it says:
• Approved Sealant/Caulk
- Silicone-based sealants
- Must conform to ASTM C920 or C834 specifications
• Approved Sheathing Tape
- Canadian Construction Materials Centre (CCMC)
evaluated or other code-approved sheathing tape.
Siga says "Seal only with SIGA system components".
Pro Clima says to use tape or Orcon F (not an acoustical sealant).
With no manufacturer support for it, does someone have good, long term (not just blower door) data showing that acoustical sealant works better than alternatives in a case like this?
I know of a local passive house builder who uses it on every build, near Duluth, MN. But I won't name drop to protect the innocent. He swears by the stuff. But as I said, I've got little kids, so I care about healthy air even more than I care about efficiency and sustainability, so maybe it's not a fit for me. Maybe just using tape and Orcon F is what I should be looking at. He also does things like take a 1' wide strip of barrier and put it at any joints between interior walls and exterior, so there is a continuous barrier to seal against, and uses the Tremco all along the top plate. I don't have the A-B data to say whether this makes a difference in a blower door test. Maybe we can do just as well without it.
Tremco is quite common for the application I had in mind. I am only concerned about the potential for off-gassing.