XPS + 2×6 AF options MN
I’m currently designing a new house build for myself to live in. Looking to really make it energy efficient. I live in northern MN where it gets extremely cold during the winter. I have been looking at options for insulation on walls and commonly hear about foam placed on the sheathing and cladding attached to that. But I fear that will lead to issues of trapping moisture.
My thought is to do 2×6 advanced framing, highest fiberglass R-Rating to fit in the 2×6 that I can get (probably R-21) and on the interior side, fasten horizontal 2×3 strapping to the 2×6 studs. In between the strapping, I plan on 1.5” XPS foam to give the wall more R value, then top the assembly off with poly vapor barrier closest to the drywall. Anyone have any thoughts on this or other suggestions?
I want high insulation value, relatively ease of installation (building myself) and without a giant added cost. I am quite focused on air sealing already. I do like the idea of foam on the exterior however again I have fears of moisture trapping, especially in my cold climate. Windows and doors will be a little more difficult to waterproof but I’m okay with that. Thank you everyone!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The wall can dry to either or both sides. Foam on one side doesn't matter, aside from you wanting to avoid an interior side vapor barrier in that case. What I would do here would be to use exterior rigid foam, but use a smart vapor retarder on the interior (I like to use Certainteed's MemBrain, primarily because it's the cheapest commonly available smart vapor retarder). The exterior rigid foam keeps the sheathing warm enough -- assuming you use a thick enough layer of rigid foam as described elsewhere on GBA -- to avoid condensation within the wall assembly during the winter. The vapor RETARDER (NOT barrier) helps limit how much moisture can get into the wall assembly while still allowing it to dry towards the interior. If you use AT LEAST the miminum amount of rigid foam for your climate zone, your wall will actually be LESS prone to moisture problems than if you didn't use the rigid foam. Using the rigid foam on the interior doesn't improve things, it just makes interior details more complex.
You can probably get up to R23 in the walls, which is the commonly available mineral wool batt for 2x6 walls. You might be able to order in high density fiberglass batts to get R23 too, but it depends on how much money you want to put into that insulating layer.
I would put a rain screen on the exterior too, which will also help prevent moisture problems.
Bill
Thanks a lot for the reply bill! My only question to that is what thickness of rigid foam then would be the sufficient then? I believe it is climate zone 7 I’m in, the coldest in the US. Or if you have a resource to find that info, that would be appreciated as well!
Deleted
You can refer to Martin's classic article about rigid foam thickness here:
https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness. In CZ7, with 2x6 walls, you want AT LEAST R15 worth of exterior rigid foam. That would be 3" of XPS, or 2.5" of polyiso. I agree with Michael too, who suggested below to use EPS or polyiso instead of XPS. I only use XPS for underground applications (like subslab insulation). EPS is cheaper and greener, polyiso is more R per inch and greener.
A double studwall is another option here. If you stagger the studs, you cut down on thermal bridging. Offset 2x4 and 2x6 studs on a 2x10 sill/top plate allows for R34 even using cheap R21 and R13 batts. That's another option for you.
Bill
If your area has adopted the 2024 IRC, it allows you to have as little as R7.5 continuous insulation on the exterior of a 2x6 wall in CZ 7 provided you have a "responsive" vapor retarder on the interior. Membrain meets that requirement.
If the foam is on the interior, in your climate zone the thickness doesn't matter from a moisture accumulation viewpoint. It would only matter if there were strong, prolonged vapor drive toward the interior as there is in southern states. Any amount of interior-side insulation will prevent moisture accumulation coming from the exterior.
I recommend choosing a foam with lower carbon emissions than XPS, such as polyiso or EPS, or using another material such as rigid wood fiber, but that can be hard to find.
You can also use regular batts for the 2x3 section. Install the poly on the exterior stud wall as your vapor control layer and fill the strapping space with cheap fluffy. The cut and cobble rigid into that space doesn't increase your assembly R value all that much and makes wiring much harder. If the 2x3 is on flat, you end up with somewhere around R25 assembly which is not bad.
You can get similar R value by building with 2x8 studs 24" OC with R30 batts.
You can even bump up the stud wall to 2x10 which gets you around an R30 assembly. Nice part about thick walls is they are very simple to build, everything is standard construction and your upcharge for the extra lumber is minimal.
In either case, in very cold climate, it is good to use a rain screen for your siding. This significantly improves the drying ability of the wall as well as life of any painted siding.
EDIT: at 2x10, you are probably better off with a double stud wall.
Thank you all for all this great info!! I may consider a double stud wall more than at this point. I always see with a double stud wall, the vapor barrier is place between the two stud walls. Would it not be better to place the vapor barrier against the drywall for the best vapor control? 2.5” foam on the exterior to me is going to more than what I’d want to deal with so I may stay away from that now. Looking at either a 2x6/2x4 double stud wall or a 2x4/2x4 double stud wall. The house will have a “great room” type of room so instead of a double stud wall with really tall ceilings, that may have to be a 2x8 wall assembly of some sort.
I plan to do R60 cellulose blow in for the ceiling on top of a poly vapor barrier unless someone has a better option. I’ve heard fiberglass loses R value the colder it gets otherwise I’d like to use fiberglass blown.
This is a good overview of how to build double-stud walls: https://www.finehomebuilding.com/project-guides/insulation/a-case-for-double-stud-walls. The author is a friend and frequent collaborator; the project before the one featured we did together, and we're doing another one currently.
You should not have a vapor BARRIER anywhere in a double-stud wall. You should have a vapor retarder; I prefer a variable-permeance membrane on the interior, whereas Kolbert (the author) prefers painted drywall only. The exterior sheathing should be as airtight as possible. I recommend only doing double-stud walls with dense-packed cellulose because there will be some moisture accumulation near the sheathing in late winter, and cellulose can handle it more safely than other types of insulation.
I was able to read through this guide a bit. One thing I would like to change is the thickness of the wall, I would only want about a half an inch between the two stud walls to not take up tons of interior square footage and have extremely deep windows/doors. Would this change the design aspects then of the wall or would there still be enough R value to construct it the same other than thickness? I was hoping to use batts just to save some labor on my part and probably some money but I know I could rig up a dense pack machine for cellulose. I hear lots of issues around cellulose so I am hesitant to use it, essentially being paper which absorbs moisture. A simple google search will tell you all about how moisture is sucked up by it. My goal is to take it up a notch above 2x6 walls with regular fiberglass batts and make the place last a very long time. Most houses around here are 2x4 with fiberglass or 2x6 with fiberglass only and very poor vapor barrier. Thoughts on this?
You don't need a gap if the studs are not lined up.
For a DIY friendly build, here is what I would do.
Build an exterior 2x6 or 2x8 24"OC load bearing wall. Use either CDX or densglass for sheathing. Once the roof is on, insulate with cheap fiberglass batts and cover with a smart vapor retarder of some kind. This will get you dried in and with some insulation to take the edge off in colder weather.
Once dried in, you can now build an internal 2x4 wall right beside the exterior wall. Since the on center spacing of the outside is for sheathing and the inside would be for drywall, the studs would be offset by default.
Inside this 2x4 wall run all your services, once done insulate with batts and cover with drywall.
This puts the vapor retarder about 2/3 of the way through the wall which works in cold climate.
The most important part is to get a solid air barrier. The best is taped sheathing which is pretty easy to do.
As for deep windows, I love mine, they are great place for plants and decorations.
A 2x6+2x4 would get you about an r30 assembly, 2x8 would bump that up to about R35.
Simple to build with cheap materials. If you have a local lumber mill you can even save a bit more cost as the 2x4s don't need to be graded.
>"I’ve heard fiberglass loses R value the colder it gets otherwise I’d like to use fiberglass blown."
That used to be the case with blown fiberglass made the "old way", where they basically cut up batts into little cubes. The new stuff is more evenly shredded and no longer suffers from that "loses R value when it's cold" problem. It's easy to tell the difference between the old (decades old, BTW) stuff, that looks like lots of chunks, and the new stuff, that makes a much more even-looking layer.
Note that fiberglass batts never suffered from the "reduced R value when cold" problem, it was ONLY loose fill (blown) fiberglass with that issue, and the new blown fiberglass no longer has this problem. I'd still go with blown cellulose though, as it's cheaper and tends to perform better since it helps reduce any air leaks that may exist. You still want to put some effort into air sealing prior to insulating though.
Bill
Ar1976,
One small point. Pick and choose among the features of AF. Some like increased stud spacing may make sense, others add complication without much appreciable benefit.
Ar1976 , you could consider using the method described by builder Stephen Bonfigliloi in this article: https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge
He adds strips of foam and strapping to the inside of a 2x6 wall, bumping it up to a 7-1/4" cavity depth, which you could fill with an R28+ batt. It sounds pretty easy to do and minimizes the amount of relatively expensive foam.
Not a bad aritcle, thanks for sending. Id like to hear the long term reviews of this type of building, Im curious if the strapping ends up feeling "wiggly" being separated from the studs by 1" of off the shelf EPS foam.
Why not just put up a SIP house?
https://extremepanel.com/
They are right there in Cottonwood, MN!
begreener,
SIPs and spray foam, the two solutions you invariably suggest as solutions to every question, also are the only two building technologies I know that are associated with irremediable problems when they go wrong - and they do often enough that the risk has to be factored in to the decision to use them.
Malcom-
Yes, pick a good one ...
Extreme Panel seems to have a good reputation - no?
begreener,
Or accept that although SIPs have been around since the 1930s they remain a niche product for a number of reasons.
I think SIPS are a little out of my skill set for building. Plus I can guarantee the cost is very great compared to many different ways to insulate well. I do appreciate the recommendation.
Does anyone have any comments, experience or any feedback on this? @FrankD brought this to my attention.
https://www.finehomebuilding.com/membership/pdf/9750/021250059.pdf
This seems like a fantastic thing for the budget and straight forward with minimal added materials. I'd think this type of assembly would outperform a standard 2x6 wall by a long shot. On this, would a vapor BARRIER be recommended on the interior or would a vapor RETARDER be recommended on the interior?
Why not just buy a "warm stud"?
https://www.thermalstuds.com/warmstud
Price. I'm not a builder, just someone who wants to build their own home on already owned land. Warm stud seems like a good idea for the volume efficiency builder though.
Ar1976,
It works very well. The main reason you don't see more widespread adoption is it is very labour-intensive, so it only makes sense in DIY situations. Like Mooney Walls (another alternative you may consider) you do have to adapt your framing at inside corners and where the wall intersects with interior partitions, to provide backing for drywall.
You want a class 2, or variable perm vapour-retarder on the inside.
Thank you for the insight! I am going to heavily look into this method, I am not volume building so this may be something I implement. I like the idea of the CertainTeed vapor retarder as it seems readily available.