Okay, I confess I gave this a thumbs up. Thank you. 🔥

You know, I hadn't noticed until you pointed it out. I can speculate that the 2x4 framing is comparatively cheap and the builder may have chosen to have his crew frame the building conventionally with king studs/trimmers around the exterior openings based on conditions in the moment. Sometimes things happen fast and the guys framing go with what they are familiar with. Or perhaps there's a wider window casing detail I'm not aware of. In any case, you are correct that the inner wall is not the structural wall and it could get by with single stud framing around the window openings as long as all aspects of the design are adequately backed and supported. I'd be cautious with doors since residential units tend to be inswing and need decent support on the hinge side.

Yes in theory. Caution in practice. I'd still lean on the poly over taping the foam board. The 6 mil polyethylene sheeting that Alaska's municipal codes mandate as the vapor retarder (and more importantly, the critical air barrier) is a sheet membrane that covers large areas and is sealed with a durable, flexible sealant (Tremco accoustical sealant typ.) at the overlaps and perimeter. We are now seeing 40 year old walls where that sealant is still going strong and tenacious as ever. The foam board represents a patchwork quilt of 4x8 sheets (or smaller) where every seam presents a potential vulnerability to air leakage. Foam board is more rigid, not always dimensionally stable, and the air seal is totally dependent on the long term adhesion of whatever tape was used to seal the joints to whatever brand and type foam board was used. In a production setting, this means the crew installing and taping all those foam panels has more opportunities to make mistakes that may go unnoticed. I also have to consider that for us in AK many homes are built on seasonally unstable soils and can experience envelope stresses, I'd say that while the foam-with-tape approach can work (and I've seen it work anecdotally in one-off cases where a conscientious builder did all the taping himself), for us the poly approach is the safer bet. Not arguing with you on the taping the seams issue - just giving you my perspective based on my experiences.

@@IlyaBenesch I see where you’re coming from. I just look at the cost of materials and labor these days and it makes me sick. I look at all these high end “RU-vid” homes and can only imagine what the final cost is.

Haha

Yeah, I suppose that's an option, too. But, with the climate changing as it has been, those "milder" climates are becoming *HOTTER* climates... and I run my air conditioners 5 months out of the year in zone 4 as it is. 🥵 Insulating provides energy savings whether it's keeping the house warmer or cooler.

No doubt eh! 😂

I run my ac year round.

Genius!! 🤣🤣🤣🤣

There are a lot of critical caveats to this statement that would need to be clarified in order for it to work. Then you will have to consider the economics. This builder chose this system as he felt it more cost effective over using the 6" min of exterior insulation required in zone 8 for this approach to work safely.

Hmmm. I'm not sure why the drywall would be needed between the two walls? Otherwise as described I think it would work fine HOWEVER I highly suggest you share a drawing detail or a site discussion with an energy professional in your region for a second set of eyes directly on it. The bathroom is a high moisture/humidity area and exterior walls are involved, so getting a localized sense of the enclosure from a professional in your area may be worthwhile. That's me being duly cautious with advice like this. :).

The rule-of-thumb is vapour barrier 2/3 of the R-value away from the cold side.

@@k.c.sunshine1934 that works well in most cases, but in a climate where there’s 90F difference between indoor/outdoor temperatures any indoor RH above 33% risks condensation in the wall. I would imagine there are lots of places like Alaska where the delta is higher than 90F for extended periods, which would require strict humidity control for the 2/3 rule to be effective. Moving to 3/4 from 2/3 gets you to 45% indoor RH @ 90F delta, a much safer solution. You may need to play around with the construction materials a bit, like add some R value to the exterior side, but it would ensure a more robust assembly.

Thank you for pointing that out. That is one we can roll out this winter.

Can you imagine having ceramic insulated walls?!?! That would be awesome!

Yes absolutely. Sounds like you get it. Unfortunately its common for people to have an HRV in the home but maintenance and understanding of operation is lacking. Over the years here in Alaska I've seen failures in walls and roof systems of all types -and if those failures were primarily winter time condensation related, then there generally exists a high humidity component that plays a big part in the scale of the failure. We can have holes in walls and roofs. And we inherently have pressure differences across those holes due to the cold outside temps.......so if we throw in high indoor humidity levels AND those holes are in the positive pressure zone, then the moisture transport via indoor air leakage can be tremendous. When I go into a house with good quality triple pane windows - or even doubles in here in Fairbanks and there's excessive condensation on the glass I start looking hard at the ventilation component. Lots more to this obviously, but point is we have to have the indoor humidity control in AK in winter time.

what are you proposing? No windows?

The windows only practically need to be good enough to avoid condensation. Keep them a reasonable size and it is what it is. Agreed though; once you’re into an R40 wall your next step is better windows and doors, not more wall insulation.

This would be absurd cost for little benefit. Triple pane windows (R7-8)are >40$ per sq ft. R21 insulation is 0.60$ per sq ft.

While a fantastic sounding idea, that also sounds like the most expensive residential home wall design ever.

@@ProleDaddy It would be more certainly cost more to have such a heavily insulated wall, but having an R-80 wall means you can significantly size down both heating and if installed cooling equipment and there would be significant savings over the life of the home in heat fuel costs and electricity if air con is installed. I would rather pay more upfront and have a short payback period and then have minimal energy costs for the life of the home. Personally I would have a micohydro or solar system and again because of the high R value the hydro or solar system does not to be as large as a lesser insulated home saving money. I would also have a wood stove as a redundant heat source incase of a power loss or generator failure, but also because firewood is not traded on Wall St and can be cut and split off ones own property thus no government or corporate BS artificially inflating prices

The 2x12s would be a thermal bridge as well as being very expensive. You could get the same R value with a double 2x4 wall with a 4" gap in between and it would be cheaper!

Exactly.

This is very case dependent based on region, builder, etc. If you want to start a frenzy among piranhas, toss a salami in the water. If you want to start a frenzy among builders, have one builder stand up in front of a nation of his peers and say that he's got the best wall. Yes, I am very familiar with Joe's perfect wall and that building science has set a standard. Love it. Case in point, we have a variation of that wall here in AK called REMOTE wall. cchrc.org/media/REMOTE_Manual.pdf Uses all the principles as brought forth by Joe. Exterior insulation is a great approach, but has a comparatively steep learning curve and doesn't lend itself well to complex/custom structures with out a solid understanding of how every detail needs to be air sealed and adequately insulated. Think grade changes, decks, roof lines, intersecting exterior walls, bay windows, split levels, often limited residential budgets, etc. Keep in mind that my perspective has a spectrum that tries to include builders of all scales and skill levels. And as I wrote that manual over a decade ago, I have had plenty of time to see some of the challenges of the system (and builder feedback) on jobsites in zones 6,7, & 8 here in AK. And it continues to evolve. At this moment in time, this particular builder used the system in the video as he'd need 6" of exterior foam board (in two layers) here in Zone 8/Fairbanks. The foam board is currently as expensive as it has ever been. In addition, this home is two stories tall on a hillside so every exterior layer represents a trip around the building and more labor. Yes, I agree that with practice and planning much of the foam can be applied in advance before the walls are raised.......but not all of it and the attention to air sealing is critical. I hope that makes sense. These types of discussions have to have clearly defined common ground as there are so many considerations - otherwise the water starts to froth around the salami.

My guess is they don't need to connect the two walls, other than where they meet the windows and possibly at the corners. Looks like a pretty well thought out system to me. If it didn't work you'd find out pretty quickly in a place like Alaska.

This builder made sure the exterior walls were straight and then used long structural screws about every 2 -3 feet to tie the inner wall to the outer wall through the foam board. This had the added advantage of helping clamp the sealed overlap where the ceiling vapor retarder meets the wall vapor retarder. I have seen another builder not fasten the walls together but nail the inner wall to the trusses. Like all wall systems, this system continues to evolve in small but significant ways with different builders taking slightly different approaches in the details.

I believe insulation works everywhere. Maybe your heating bill will be too low, though. ;)

I believe that it would work anywhere; insulation can keep heat in or out.

🤭 Of course, silly 😉

One reason we design walls differently for different climate zones is that we want to prevent moisture from rotting out our walls. A wall that works well in Miami may not work well in Chicago. There is a whole science to designing walls and complex computer programs to model the movement of moisture and vapor and the potential for condensation. How you layer the different parts of the wall (insulation, vapor barrier, sheathing, etc.) is important. How much insulation and where it is placed is also important. The wall design in this video is new to me and I don't know how well it would work in my climate zone.

Typically the foam is installed tight to the subfloor. No sealant. At the bottom, the 6 mil poly vapor retarder is sealed to the subfloor with a 3/8” continuous bead of Tremco acoustical sealant. At the top it gets interesting. Different builders are trying different things. I have seen one builder who drove plastic capped nails through the 2” foam board and into the double top plate of the exterior wall at about 12” OC. He then used the plastic washers as fastening points for the staples that hold up the wall vapor retarder. The ceiling vapor retarder then laps and is sealed over the wall vapor retarder at the double top plate line. The 2x4 inner wall is fastened through the top plate, through the foam, and into the double top plate of the exterior wall with structural screws every 2’ - 3’ -which clamps the vapor retarder joint at this juncture. So that’s the best outside the box thinking on this I’ve seen so far. And all ideas on this are welcome. The traditional way to seal the ceiling vapor retarder to the wall vapor retarder would be to rip a 2” furring strip of framing lumber and fasten that to the double top plate. The wall vapor retarder would then be securely fastened to this strip at the intersection of exterior wall and truss. You do however, now have a small area of thermal bridging rather than continuous foam board all the way to the trusses. This area of solid wood would would be about R-11 for an 11” thick wood wall section. That is still double the r-value of the windows but purists may cringe at this strip of solid wood while others like myself might be conservative and use this approach while the system evolves at this point.