Looking for wall design to achieve R40 & Roof at R60
Hello. My name is Matt. I am reaching out to the community looking for imformation to design a wall that achieves R40 and roof assembly to get R60. I am working with my builder who is new to the Building Science group. We are in the intial design phase of the house. I would like to follow the 5/10/20/40/60 rule. I would also like to utilize the ZIP R product in the building process.
I would like to get some opinions on what the best makeup of the exterior walls to achieve R40 and roof at R60. I am open for any suggestions and pointers. I would also like to have material to present to my builder as he is very willing to work towards these goals. He wants to learn these basics so that he can roll them into his normal building process.
House Background:
– 2200 sq ft (Single Story)
– ICF Basment
– Climate Zone 5 (Zip 17701)
– 2×6 16″ OC framing
I appreciate all the help that I can get and look forward to discussing the options.
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Replies
Hi Matt.
To build an R-40 wall with 2x6 framing and ZIP R-sheathing (assuming no other exterior sheathing), I think you'd need to use closed-cell spray foam in the cavities. That's one of the least environmentally friendly options.
Unless you are willing to build double-stud walls, I'd consider dense packed cellulose in the wall cavities with exterior continuous insulation, probably polyisocyanurate over (OSB, plywood, or uninsulated ZIP) sheathing.
The simplest approach, in my opinion, to a high-R roof is to use raised-heel trusses and blown in insulation.
Keep in mind that these insulation values are great, but air sealing is just as important and should be tested.
I'm sure you'll get some good ideas on this from GBA members with lots of experience. In the meantime, check out these articles:
Walls that Work
Working With ZIP R-Sheathing
A Case for Double Stud Walls
How to Insulate an Attic Floor
You won't hit R40 with 2x6+ZipR. You can hit it with 2x8 and ZipR. In colder climate this assembly would require an interior vapor barrier.
I would also do a bit of energy modeling/energy costs. In Zone5 R40 walls are not worth it unless you are building it with low cost materials such as dense packed double stud walls. Even then, it is hard to make a financial argument for an over R30 assembly.
Good for you, and your builder, to have basic discussions now in the design phase, where change orders are cheap.
You've asked some fairly general questions, so I'll give some general advice from my side of the fence.
1) I'd look up articles here on GBA about the Pretty Good House. It's a concept that's been reviewed here a couple of times, and there's now a .ORG website. They've resisted putting in specific recommendations in the past, because there are many designs that can work well, when properly executed. Having said that, they've come up with a number of recommendations for cost-effective, comfortable, healthier, durable, and efficient homes. As Akos has hinted (my interpretation), these are general guidelines, and many have concluded that R30 is pretty good for a wall and that air sealing (and details that allow good air-sealing) is more important than a high R-value.
Also, the articles for PGH will contain good links for reading.
2) I am not a builder, so two grains of salt here, but if I was and I wanted to build more efficient, comfortable houses, I would concentrate on learning about and then getting my crews and subcontractors focused on air-sealing, and I would learn some approaches to using exterior insulation. Those two things will be very common and useful in the future. And they will benefit every future client, including you.
Matt, Fine Homebuilding just published my new Pretty Good House article that shows a couple of approaches for cold climates using reduced-carbon materials: https://www.finehomebuilding.com/2020/04/07/the-evolution-of-a-pretty-good-house.
Thank You. Started reading up on the Pretty Good House information. A lot of great details on every aspect. One thing that I have stressed with my builder is the need to cut down and eliminate air movement ie Air Sealing. I have started to look into the aero barrier product and service. I think so far reading this information I have backed off the R40 and will be seeking an R30 with the Zip R product. One of the questions i keep running into is with the assembly being a vented or unvented system. A lot of varying opinions on this item.
The other aspect is dealing with the cathedral ceiling in the living room and the best insulation method there. I will be keeping lighting out of the ceiling.
I would like to run a ductless/ducted mini split system. I like the idea of this system as we will have a bonus room over the garage and i like the idea of being able to have a unit on the wall and run it when we need to.
Any other suggestions would be helpful as I continue to this process.
2x6s 24 OC with R23 Rockwool, Zip sheathing and then 2" Kool Therm K12 Framing Board outside that at R16.7. R40 Give or take, class 3 vapor barrier on the inside. Tape the Zip wall and tape the foam outside of that.
For the cathedral ceiling, you can use raised-heel scissor trusses so that you can still use cheap blown-in cellulose and have a vented roof. Vented roofs with cellulose on the floor are generally the most durable and cost-effective roof systems. If cellulose is not available locally, blown-in fiberglass is OK, too. Makes sure to use blocking between the trusses, and chutes above the insulation for the first few feet to keep the insulation where it belongs and prevent wind-washing at the eaves.