New Hampshire wall assembly option help
Okay so I’m trying to make up my mind on which wall enclosure tech to use.
Background info:
This house build is in rural Southwestern New Hampshire. 1500sq ft. 2 story, 2 bedroom, no basement. Site is south-facing Hillside. House will be on the north edge of a 2 acre clearing. Site has really good exposure for PV panels. There is currently another house on the site that I built 4 years ago that is using the persist wall system and heated with firewood only until about a month ago when I finally got around to installing a Mitsubishi hyper Heat. I’m really loving the unit for its stable temps, quiet operation and its ability to modulate down because this time of year has been tricky for me historically trying to only put out a small amount of heat from a wood stove. The town I live in allows 2 dwellings on a site as long as you have enough acreage per dwelling. The existing house is in a totally different section of the site. Firewood collection is already part of my life for the existing house and shop.
Wall enclosure option 1:
Double stud wall, dense pack cellulose. I like this wall for its relative ease for traditional framers to build and for it’s lack of use of foam for environmental and ants reasons. Also because I like dense pack cellulose in theory as an insulation material. Total thickness of this wall I’m not certain on yet because of the law of diminishing returns on extreme insulation levels. As well as the cold sheathing/OSB issue. Although my reading on that now has been showing it not as big of an issue as some people once thought it to be, especially if you just choose a different sheathing other than OSB. I would probably go with plywood in this wall set up. This wall design is also relatively easy for window and door installation as well as other accessories and not terrible for all the sub trades.
Option 2 is the PERSIST wall:
I like this wall because of its easy air sealing and protection of the structure from rot and because I have already built a house with this system so I know what to expect. Downfalls of the system are the need to use rigid foam board for environmental reasons, Ants (I currently battle those in my existing house. Although it’s not a terrible issue at least that I know of yet) COST.
Wrapping a house in foam makes anything attached to the outside a tricky endeavor, ask me how I know LOL.
Foam options probably Iand in this order from my perspective:
Eps- although I have not found a local source this yet
Recycled polyiso foam- this is what my current house is wrapped in. Takes extra labor to work with and from what I am told has a reduced R-value due to age.
Virgin polyiso foam
Xps- too environmentally bad for me to use
Wall option 3:
2×6 wall dense pack cellulose with really good air sealing details. Possibly doing the BONFIG wall approach with this system. This wall is obviously not super insulated but with this building approach the money is instead spent on PV panels to offset the lack insulation with air source heat pumps. The more I research high R value wall systems and there down falls the most I think about this system.
I am anti any crazy wall systems including wacko I-joist stud walls and whatnot. LOL. Also I am not a fan of SIP panels
No matter what wall system is decided on this house will have a PV array. But any budget money not spent on insulation will go directly to the PV array size.
Another factor that I keep thinking about but maybe I am overly romantic about it is the desire for me to have a wood stove in this house. Partially because that’s what people do around here, partially because we lose power enough to like the security of a wood stove, partially because I have a woodlot, partially because there is nothing like sitting next to a wood stove mid-February while it’s dumping snow outside.
All that said I know first-hand how hard it is to have a wood stove in a small, well air sealed house. Draft issues, air quality issues, wood stove protrusions creating air leaks ect.
I think in my perfect world this house would be built with fairly conventional materials (easily sourced locally), use dense pack cellulose, avoid rigid foam board except for the foundation, run annually net zero from the use of a large PV array, air source heat pumps, HRV and supplemented with a small efficient wood stove. And be not so costly to build that I have to lie to the local climate change denier types LOL
So in summary I guess there are two questions here:
1- Given these site details what wall system would you choose?
2- Should I or should I not include a wood stove and how does that affect the wall system choice?
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Replies
Tech, for me it's a no brainer--I would go with the double-stud wall with cellulose. As long as the cellulose is densely packed and you have an interior vapor retarder (preferably variable permeance) and a vented rain screen detail, it's a safe assembly--as far as I know there are zero documented failures. It's even ok with OSB, though plywood is more vapor open and my current favorite, with seams taped as an airtight layer. The last one I designed, with ZIP sheathing, hit 0.25 ACH50. The returns start to diminish dramatically above about R-30 walls, with very little gain above R-40, but also not much difference in cost, so it can be worth aiming for R-40. (When using foam I tend to stop around R-30 to balance environmental impact, but recently consulted on a PERSIST-inspired system with R-20 cellulose-filled walls wrapped with R-40 Neopor nailbase panels.)
For below-grade rigid foam, contact Branch River Plastics in Rhode Island. They sell borate-treated EPS in various densities, so insects won't bother it, and they deliver for a reasonable fee.
If I had the choice between a wood stove and an air source heat pump with generator backup, I'd go for the latter, but I also like woodstoves so it would be a hard choice. I've been involved with many different types of high performance construction, including I-joist systems, and woodstoves work the same in all of them--high levels of insulation and airtightness make installation and operation more complicated than in a drafty house.
It's not directly one of the choices that you listed, but if you have experience with PERSIST/REMOTE, and you are concerned about ants and/or termites, you might consider something like the wall construction shown in BSI-096 by Building Science Corporation. Using ZIP sheathing and external mineral wool with internal dense pack cellulose, you get something that looks robust and minimizes insect and fire risks. You could also use plywood and some sort of peel and stick, or a fluid applied weather barrier, but ZIP seems to take a lot of fuss out of the equation.
I like the double stud wall approach too. I know Bob Irving in NH builds homes with double stud walls all the time with little if any markup compared to a normal 2x6 house. Prudent Living in Windsor Vermont is doing the same.
PERSIST walls are the least risky but you need really long, expensive screws to get a meaningful level of insulation on the exterior. (I like the REMOTE wall better for that reason... ) The best approach of all is when you can sheath the interior wall of your double stud wall. Then you the get the economy of a double stud wall with the safety of the REMOTE wall. (google: Rochester Passivehouse)
If you want to use a wood stove and you have really good southern exposure then a normal 2x6 wall with good windows (for comfort) and lots of attic insulation may not be a bad choice. I suspect you are already familiar with Unity Homes in Walpole/Keene: They make a lovely wall for great price.
For local EPS, check out avilitecorp.com out of Manchester.
Michael Maines- Thanks for the insight on the Borate-EPS supplier! Very helpful!
There is a better way to build double stud walls - the following description is stud face to stud face from outside to in: 2x6 outside wall your choice of insulation between studs - polyiso (you pick thickness) on inside face of 2x6 exterior wall seems taped and/or caulked plus taped and/or caulked to both floor and ceiling note that you must have something on the ceiling to tape or caulk to - gap (you pick width - adjust for desired R value).
2x4 inside wall your choice of insulation between studs. This allows you to put any desired material on
exterior of house for structural and siding and protects the polyiso from damage (it's in the center of the wall structure), from insects, and from R rating drop in really cold weather in addition there is no conceivable outside temperature which will lead to condensation on either the polyiso or the outside
sheathing. Example-lets assume you went with blown in cellulose in both with 2 inches of polyiso and a 1 inch gap (which ends up packed with cellulose of course) then the total wall thickness (stud face to
stud face) becomes 12 inches and the actual (real) R rating will be approx. 40. The other advantage to this is that the 2x4 interior wall is treated as just another interior wall by the builder after the 2x6 is installed and insulated. Easy-peasy!
MICHAEL MAINES- Great Info!!! and another hat tip for the borate treated EPS source! as far as the interior vapor retarder (and you stated preferably variable permeance) do you have a product you would recommend in particular?
RICK EVANS- Thanks for the great info! I actually have a couple of the designers at Unity (Bensonwood) helping me on this project as I know them from around town/racing mountain bikes. Their walls are great but not all of their tech translates well to site built houses. (its amazing what they can do in a shop with giant CNC's and vacuum tables and all the other cool gadgets)
I do keep spinning back to the idea of a 2x6 wall... I really need to make a decision on this soon!
Hi tech123, I'm in southern NH looking for professionals to help us design an energy efficient (remote or otherwise, I'm learning from this website currently) home which we aim to build ourselves at least partially. With Unity being all pre-fab, are the individuals you worked with available to hire for design / drafting?
Tech, my personal go-to is Pro Clima Intello Plus, only available here: https://foursevenfive.com/product/intello-plus/. I also like Siga Majrex, which isn't really variable permeance but mono-directional; the inward drying ability is lower than Intello: https://sigatapes.com/product/siga-majrex/. Both are very durable. Much flimsier, but easier to get and with good performance numbers is Certainteed Membrain. As far as I know those are the only three options on the market right now.
MICHAEL MAINES- Thank you. That is a huge help.