Double Walls

This video details a variation of double wall construction which has evolved in Fairbanks, Alaska to increase home energy efficiency.

In all three of Alaska’s climate zones, residential building codes mandate the use of a 6 mil polyethylene vapor retarder. Since this vapor retarder also serves as a home’s primary air barrier/pressure boundary, it should be installed as airtight as possible to prevent excessive air leakage and moisture movement into exterior wall and roof assemblies during Alaska’s extended heating seasons. In a generic double wall system, however, wiring and electrical outlets still penetrate the vapor retarder, and thus require additional air sealing measures to reduce air leakage through and around these penetrations. The double wall system described here utilizes an outer 2x6 wall and an inner 2x4 wall, with a continuous layer of 2” foam board separating the two walls. The vapor retarder is located behind the inner (2x4) wall and is located directly over the layer of 2” foam board, where it remains intact and protected.

The layer of foam board serves as backing for the vapor retarder and also provides a robust thermal break for the 2x6 exterior wall, elevating the r-value of the outer (cold-in-winter) side of the vapor retarder to a minimum of R-30. This allows the inner 2x4 wall on the inner (warm-in-winter) side to be fully insulated with fibrous insulations and remain safely above the dew point. Electrical outlets, ducting, plumbing, etc. can all be installed conventionally inside the 2x4 wall and remain protected from freezing. This insulation technique substantially increases thermal efficiency, bringing the total wall R-value over 40.

CCHRC YouTube channel: @coldclimatehousing

[matthewambler]

Built my house exterior walls exactly like this 14 years ago. Along with R60 in the ceiling I'm very happy with the cold weather performance. An added bonus is the walls are almost totally soundproof.

[anthonyhitchings]

Double walls are real gold if you live by a freeway.

[XXbeyond]

I used this exact system on my studio remodel 5 years ago and have been very satisfied with the performance.

[calsbarn]

We built a super insulated home based on the Saskatchewan house in Fine Homebuilding issue 11(?) back in 1984. It’s similar to this concept. That was two 2x4 walls separated to 1 foot wide by using 1’ wide strips of 3/4” plywood top and bottom. Visqueen was used at the time and placed as in this example behind the inner wall.
Without going into the rest of the construction details I can confirm that the energy efficiency has been excellent. The house is ALL electric, in Michigan, and 2500 sq ft with another 1500 in the basement. Until 5 years ago we never exceeded an average $175/mo for utilities (well, utility - no gas, propane, wood stove, etc.). Then we added solar panels and now we average about $30/mo. (The service fee is about $10 of that.)
In addition, our well pump died in the middle of one winter and took a week to fix. This put our ground water heat pump out of commission and we heated the house with 3 electric wall heaters that were only used when we were home and awake. Temp never dropped below 62 even though outside temps were in the teens to 20s at night. So, yes, this kind of insulation has proven very worthwhile over the last 40 years.

[jasonbroom]

We just had a pole barn built in northern Michigan. It has R7.5 foam board insulating all of the bents, then a 2x4 wall built inside of the barn walls, with an inch and a half off-set between them, allowing conventional R19 insulation to be installed. This created R26 walls and made running electrical wires quite easy, as shown in this video.

[rod]

A well insulated, tight system is great. Adequately addressing indoor air quality and moisture levels on the inside are essential to completing this system. Recently inspected a very tight building with an HRV that had failed years prior and it was pretty destructive.

[superspeeder]

Glad to see you mention keeping the vapor retarder above the dewpoint. If you haven’t already, a dedicated video on how to determine that is in order. 😉

[LongWalkerActual]

This is a wise decision when building anywhere.

[edmiller]

Does no one offset the studs from the outer wall to inner wall anymore? That's how I learned to build double walls so insulation overlaps, rather than have insulation aps wherleaving insulation gaps where studs are.

[NikeMikey]

Moving to a mild climate fixed 99% of my insulation problems

[guytech]

Better option is exterior rigid foam insulation which solves most of the condensation issues, cheaper & easier to install than double wall framing. I live in the south east US, & my walls are about R-33 with 2x6 24 oc framing & exterior foam insulation. I don't like to use vapor retarders as I believe it does not really work & leads to moisture build up. Joe Lstiburek (Building Science Corp) has several video presentations on the topic with double wall construction as well as moisture build up with vapor retarders. I recommend watching his "perfect Wall" presentations.

Vapor barriers don't really work well as they get full of holes for utilties (plumbing, electrical, HVAC) as well as drywall screws. The best option for air sealing is the ZIP system that is fully taped & nail holes caulked on the exterior side, and caulking on the inside of the stud bays as well as sealing any wall pentrations with caulk, or other sealing products. Even with the 2/4 wall for utilities, you can bet a contractor is going to rip a hole in the vapor barrier whiling doing his installation work.

[offgridwanabe]

Wall insulation and a air tight seal is still the best in my book and the wiring is way easier.

[stephenmanning]

I like watching videos like this. Ruth and I live on the edge of the Western Desert of Western Australia where the temperatures are sort of +50C rather than your -50C. The problems are the same. Keep the home comfortable to live in without owning your private power station. Great video. Thank you. (I am stealing some of your clever ideas)

[filster]

Feller looks like a real builder. His left thumb has the tell-tale black nail from a framing hammer. 🙂

[wetrock]

On 1984 I built my house exactly like this. Also, my inner division walls are double with inter woven sound proofing rock wool batts. My ceiling is R55 and my basement concrete wall is insulated outside and inside to a value of R35.
I live in Montréal.

[Craig-qxuy]

If it’s good enough to keep my coffee warm it’s good enough for my house.

[byronn.]

My understanding was that the rigid foam is a vapor barrier providing that all seams/edges are taped properly. This all seems very redundant. Having done a similar construction on my pole barn shop if I could do it over I would have used more foam board or a few inches of closed cell spray foam and skipped the fiberglass bats.

[clivewilliams]

One point that wasn't mentioned is that the original 2x6 framing with insulation between the studs creates a cold bridge at every stud that reduces the overall thermal effectiveness of the wall. Although these cold bridges are still there in this construction they have been mitigated by the PU board insulation. However, greater benefit would have accrued had the PU board been on the outside of the 2x6 stud and then infilling with PU board between the studs with a vapour barrier over the surface of the internal face of the 2x6. The finish board can then be applied. This would obviate the need for an additional stud frame, saving money and space.

[marvineid]

The walls are great but what about those ginormous? Windows?

[bukboefidun]

My home in northern New Hampshire was 2×10 on 16" centers... accomplished the same thing but in not near as cold a climate.