Zone 7 wall insulation
Hello,
I live in rural Alaska (Zone 7/Borderline Zone 8) and I’m in the process of building a new construction home. Livable square footage is approximately 2,600 SF with a 750 SF garage. It is a slab on grade construction, in floor gas radiant heat, with 9 ft ceilings and a vaulted living room (scissor trussss in approximately 1,000 SF of living space). I recently read the article discussing the use of Insulated rigid foam on the exterior wall sheathing. I discussed this with several local general contractors and apparently none of them have done this, so I wanted to come here for some input and to help better understand this. I am hoping to achieve a 5* to 5*+ energy rating once completed. I am not sure how local contractors have achieved high energy ratings if they have not used rigid foam on the exterior of the home.
The attic of my home is unconditioned with soffit and ridge vents. It will be blown-in pink fiberglass to an R-60 level. My current plan with the 2×6 walls (24″ OC) is to put R-21 batts in the walls with a 6 mil vapor barrier on the interior. I currently have 1/2″ OSB on the exterior side of the walls with Tyvek. I am planning to use spray foam on all roof penetrations and use acoustical sealant on all seams of the vapor barrier.
Based on the article, it appears I would need an R-15 product to be installed over my Tyvek to help comply with the current energy standards. R8.8 R-Tech is readily available near me, so would it be advisable to use double R8.8 R-Tech on the exterior walls and then side directly over this? This is the highest R Value I can buy without special ordering something else. Siding will be LP Smart Side which I was (previously) planning on installing Directly over the Tyvek. I would imagine this would be installed directly over the rigid foam?
Can someone please help clarify this and let me know if the double R8.8 would be sufficient or if there’s something else I should do? I have just stumbled upon this article and would truly appreciate any advice on the matter.
Also, any other input or feedback on the building aspect would be greatly appreciated.
Thank you!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
AKbuilding20,
I'm not sure that this is the best wall for your climate if your local builders aren't familiar with using exterior foam (and 4 inches in two layers is a lot of foam). Exterior foam with an interior class 1 vapor barrier would also keep me up at night since I would worry about trapped moisture rotting the structure. You could mitigate this risk by using exterior mineral wool, but you'd still have the challenge of getting it built.
Have you considered a double-stud wall? This approach is a lot more framer friendly and delivers good performance with less risk. There are a bunch of articles on GBA on this topic. See https://www.greenbuildingadvisor.com/article/how-to-design-a-wall for example.
In the attic space, I assume you are using raised trusses, correct? Do you have a ventilation plan? If not, see https://www.greenbuildingadvisor.com/article/how-much-fresh-air-does-your-home-need for more information.
I'll be curious to see what the experts advise. Alaska presents some unique challenges.
Hi AKbuilding20.
I agree with everything Steve said. Just going to offer some more resources.
If you do install the exterior rigid foam, you won't be able to, or want to, install siding right over the foam. You'll need furring strips to hold the foam to the building and to fasten the siding to, which will also give you the benefit of a rainscreen. See this article: All About Rainscreens.
I'd be cautious pushing a builder to do something they are not familiar with unless they are the type to want to research and learn and make sure they get the details right. In the case of exterior rigid foam, the flashing details are complicated.
For a look at how double stud walls are built, read this article: A Case for Double Stud Walls.
And even though double stud walls are perhaps less of a challenge for good carpenters to build, I'd want a local expert in your climate to spec R-values, the need for vapor control, etc.
In your vapor barrier case, there is no code requirement for R15 of exterior foam and doing so would create a moisture trap. To increase whole wall R value, you can use foam on the interior side, where it won't interfere with drying to the exterior. Code doesn't require any continuous insulation - so more fiber is also fine.
Do verify your air sealing with testing.
AK Building,
It appears that Alaska uses the 2012 Building Code. It states that you need R20+5 walls or R13+10. So, your idea of two layers of the R-tech EPS is sound from a code and dew point perspective. Code also states that when exterior rigid foam is used, no interior vapor retarder is needed.
Alaska is a big and varied place from a climate Zone perspective. Sitka, for instance, is milder than Boston but both are in climate zone 5. Climate zones can be misleading as they are based up on Heating Degree Days rather than temperature extremes.
Are you in a place that gets really cold (like Fairbanks)? Or do you live in a part of Alaska that is never cold but chilly all year round? Also, are you in a place that is dry (think Fairbanks again) or wet (think Juneau)? Finally, what are the transportation challenges where you live? Many Alaskans appear to travel by air or boat to get home. Cellulose can be a challenge due to its weight.
If you are in a cloudy, constantly chilly climate, then I would opt for the exterior foam option as planned. I would also use a bomber peel n' stick under the foam. I would make the roof unvented with the same assembly. I think this is the one climate where a conventional [exterior-air barrier] double stud wall would fail. Remember, it's the warmer summers that dry out the exterior sheathing of a double stud wall. Wet, cloudy, and cool parts of Alaska rarely see this long enough to dry out a wall. Especially, a north-facing wall.
If you are in a dry/ bitterly cold yet accessible part of Alaska, then go double stud and pack in the cellulose.
Since it is pretty late in the game to think about major insulation changes, your best bet is probably something like:
https://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm
In your case, I would go with batts in the existing studs with vapor barrier over them, install 2x4 on edge, run your services than install 2x4 batts inside this cavity before the drywall goes up.
Exterior rigid insulation is great but you need somebody experienced with it to get all the details right.
Very interesting perspectives and input. Thank you everyone for all the info.
Rick ~ I am in Soldotna, a city inside the Kenai Peninsula Borough. Our summers are dry with highs typically around 60-80 degrees. Winters are also dry and cold. We experience much less rainfall than southeast.
My dad's in Fairbanks, I'm in Anchorage. He has 2x6 walls on the shop that several years ago added 6" of 8.8. the walls have interior 6mil. No concerns with moisture, it'll dry out in the summer if any is trapped.
In Soldotna it's a little more humid, but we don't see -40F, or 85F (except last summer.....). Add exterior foam that moves the dew point beyond the sheathing, and don't worry. The white foam is mildly vapor open, so some moisture can escape. Keep water out of you walls and you won't have moisture to worry about.
Craig ~ Thanks for your input. Do you suggest adding R8.8 R-Tech over the Tyvek that is currently on the exterior of the sheathing now and then put the LP siding directly over that? Also, would I still put 6 mil poly on the interior side prior to hanging drywall? Thanks again !
That's what my dad did. 3 layers of 2" foil faced, and 1x4 furring strips on top of that installed in 2018. Then vinyl siding was installed last year. But prior to the insulation, it was just 2x6 plywood sheathed walls, fiber glass Batts, poly, sheetrock, since 1992. Never had any rot, mold, condensation, or other long term damage even without gutters. Fairbanks is much dryer, so that helps.
I wouldn't recommend foil faced for our area with poly inside, it's 62% RH inside right now, just a touch more humid... But if you don't have poly up, go with the intellimembrane inside, as it's vapor open and allows your wood and framed insulation to dry inward. Lots of good resources to design that wall assembly on GBA.
We have major challenges that most members in this site don't. Climate Zone 7/8 and seismic are the big ones. That requires a little bit extra studying of our codes and best practices from GBA.