Preferred New England wall assembly in 2025?
Howdy,
We are building a new “Pretty Good” house at some point in the near future. I hope to meet or exceed the current energy codes while not over complicating the build. I came here to pose the age old dilemma of what type of wall assembly to use. The house will be a simple story box shape with energy heel roof trusses with plenty of space for loose fill insulation so we are covered there. The question is what to do for the wall assembly…
I really dislike the ZipR system and prefer the sheathing nailed directly to the studs. We plan to build this as our forever home. The two options that we keep coming back to:
1. Exterior mineral wool or wood fiber with batts in the stud bays. This gets more complicated with the thicker exterior insulation required for our climate zone 6A(deep window bucks, etc)
2. Go double stud. Pretty easy to frame, install windows, etc. But a tight interior air barrier can get a bit more difficult.
3. Should I consider the R-12 ZipR system with batts on the inside?
Long story short if you were going to build a “Pretty Good” house in New Hampshire what would you do for a wall assembly and why.
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Re double stud walls, we had no problems achieving a tight interior air barrier using Intello on the interior of the inner wall and ceiling with Contega sealant and Tescon Vana tape, gasketed electrical boxes or add-on gaskets for electrical boxes in "inner" exterior walls (e.g., Southwire MDSK1G 1-Gang Draft Seal Kit) with the Intello sealed to the gasket, and putty pads where necessary to wrap electrical boxes that didn't already have good seals. A EDPM gasket on the bottom plate of the "inner" exterior helps too.
Tip: You can get Intello in narrow rolls (~12 inches); get your framer to place that between interior partition walls where they adjoin the "inner" exterior wall and above interior partition walls where they meet the ceiling. Our framer sandwiched the Intello strips between the top plate of the partition wall and the 1x6 or 2x6 (or whatever width is needed) that serves as a backer/attachment point for the ceiling drywall.
sb1616ne,
If your primary air-barrier is the exterior sheathing, the tightness of the secondary interior one is not very important.
Any thermal break you can provide your wall studs and ceiling rafters are a bonus. If I were to do it again I'd wrap the house in styrofoam of some sort. Zip R maybe.
Attention to air leaks/infiltration is everything. All the insulation in the world won't help a leaky or drafty house.
You don't need thick exterior rigid. You can use less but you need a warm side vapor barrier. If you exterior rigid is permeable like with mineral wool or wood fiber, the wall can still dry to the exterior.
What you build lot of times comes down to what your builder is comfortable with. A double stud tends to be the easiest. If you are not in an area where exterior insulation is common a mineral wool or wood fiber wall will be very hard to build.
I know you mentioned not liking ZipR, but you can get a pretty good wall with 2x8 24" OC covered in either R6 or R9 zip R. The thinner ZipR can be nailed up with a regular framing nailer and if you use let in bracing, you the nail pattern is much less onerous. This wall does require a warm side vapor retarder (faced batts or smart membrane). The nice part is that it is still pretty close to standard build and you don't have to deal with any windows bucks or flashing tape origami around opening.
Dan Kolbert came up with the Pretty Good House idea in 2011 and we both use double stud walls on most projects here in Maine. He wrote about his system here: https://www.greenbuildingadvisor.com/article/a-case-for-double-stud-walls. I prefer to add a variable permeance membrane at the interior but Ben Bogie has been doing testing that implies it might not be necessary. In any case, locating the primary air control layer at the sheathing is the fastest, simplest approach.
Another system I like (but so far have only used on roofs) is the Swinburne truss, in the first image here: https://www.finehomebuilding.com/2024/01/10/understanding-larsen-trusses.
Michael,
Good info and I apricate the reply! If I go with the exterior as the air barrier and use a variable permeance membrane on the interior does the interior membrane need to super sealed for all penetrations? Sealing all the electrical can be tricky as you know.
Also, how do you feel about plywood and other sheet products as the air barrier and/or vapor control layer between living space and roof assembly. We plan to use roof trusses and ideally have the air barrier on the exterior for the walls then transition at the top plate to the interior surface of the trusses. I have seen this done with CDX and OSB. I was thinking CDX for a higher per rating. The CDX will also double as a sturdy surface for the loose fill cellulose to sit on.
Thanks!!
You only need one really good air barrier/air control layer in an assembly. Having a second one is beneficial but it doesn't need to be as carefully detailed as the primary. I don't worry about things like sealing around electrical boxes or taping all transitions.
Randy Williams recently shared an excellent article that includes how to transition the air control layer from the wall exterior to the truss interior: https://www.greenbuildingadvisor.com/article/ceiling-air-sealing. Zip and other OSB on the exterior of some assemblies can be a bit too vapor-closed for comfort, but on the interior of walls or ceilings, it works just fine. CDX also works if the quality is good, but it's spotty these days.
My 2 cents - A Mooney wall with the exterior sheathing taped as an air barrier. R20 between the studs and r6 continuous inside.
This is a good lower cost option. At that point instead of more wall insulation upgrade other elements. Probably the biggest bang for buck is putting the effort into air tightness.
The advantage of putting the air barrier at the exterior face of the exterior sheathing is that you don't have to do all that fussing about the interior wall penetrations. If you do an unvented attic you can make the air barrier continuous from the exterior of the wall sheathing to the exterior of the roof sheathing as well, and avoid the issue of ceiling penetrations. But that means a different approach to attic insulation, involving either rigid foam above the roof sheathing or closed cell spray foam on the underside of the roof sheathing. Some people don't like going the foam route.
I realize that it's not one of your considered options, but wonder if you've read about the Furred Continuous Insulation wall assembly that was highlighted here back in August '24?
https://www.greenbuildingadvisor.com/article/furred-continuous-insulation-wall-assembly-to-meet-2021-codes
I think Michael Maines was one of the authors. It avoids Zip-R and new-to-many exterior insulation detailing and flashing issues. Make the exterior sheathing your main air barrier.