Wall assembly/house wrap for zone 5
Hello,
We’re in a zone 5 where we get 90-100°F high-humidity summers and -10ish winters.
We have walls framed with 2×6 studs and Advantech sheathing. We will be using RockWool in the walls and either drywall or 3/4″ tongue and groove inside. I’m also leaning toward using RockWool Comfort Board on the exterior, then rain screen and Hardie board.
I’m struggling to figure out what the best house wrap/air barrier should be on the exterior and whether I need an air barrier or vapor retarder inside. And should the exterior air barrier go between Comfort Board and sheathing or on top of Comfort Board?
My wife has environmental illness from a moldy house so we’re paranoid of condensation and mold. Thanks for any advice.
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I was in the middle of leaving you a lengthy reply but I think there is a better resource for you than I am.
I would suggest you go to https://www.buildingsciencefightclub.com/ and buy access to Christine Williamson’s “Quit Worrying about Dew Point!” module. It costs $200 but after you have purchased it you have unlimited and indefinite access to it. You can watch it as many times as you want. I probably paid for it over a year ago and I have watched it quite a few times since then.
It will answer your questions and I have never seen a more thorough answer to your question in one location.
In my opinion “buildsciencefightclub” is kindof a dumb name but Christine Williamson is anything but dumb. She was originally from Canada and is now in the USA. She has experience in a wide variety of climate zones. She looks at failed structures and figures out why they failed in order to better learn how to prevent other structures from failing. She has a remarkable level of knowledge and seems to be extremely good at sharing that knowledge.
But if you want the short answer:
1. Your advantech (I assume you’re talking ZIP and tape) properly installed is your air and vapor barrier
2. You do not want multiple air and vapor barriers. You can end up with trapped moisture in your wall assembly and that is bad news.
3. To remove the chance of mold, you need to eliminate both water leaks and problematic condensation. I’ll assume you’ve got the water leaks under control so we’ll talk about the eliminating problematic condensation. This basically means keeping your air/vapor barrier warm enough that condensation doesn’t form very often. It also means making it so that your air and vapor barrier has the ability to dry for the few times that it does go below the dew point and creates a little condensation
Basically, in climate zone 5, ideally you’ll have at least 30% of your insulation exterior to your air and vapor barrier.
In terms of allowing it to dry out the few times that you have condensation in your wall assembly, you don’t want an internal vapor barrier. You want the wall to be able to dry to the interior through the drywall. You also want it to be able to dry to the exterior which means outside of your external insulation you really want a rain screen to allow airflow between the exterior insulation and the cladding.
I would also recommend that you deal with someone like Emu Passive to help you make a really solid plan about the durability and performance of your future home. Their website is https://emupassive.com/
I’ll attach a couple of recommendations from them for the home we’re building in Climate Zone 5. They have probably 60 or 70 pages of stuff in their report so this is an extremely small sample of what all is provided but I have found it to be quite valuable. Our wall assembly will be very slightly different than what is shown in the attachments in that we’re planning on using ZIP like you instead of the taped OSB.
I’m realizing you said, “we have walls framed with advantech” meaning you’ve already started your build.
If so I think it’s going to be hard to get exterior insulation done at this point. Honestly, I think you’re going to be fine unless you’re in one of the harsher regions of climate zone 5. There is a lot of variability in climate zone 5. I prefer the attached map to provide a little more clarity.
Even then, if you’re on the edge of climate zone 6 I feel the importance of exterior insulation does increase significantly.
This kind of thing didn’t used to matter as much. Houses were very leaky and even if they were poorly designed, they dried out because they were so leaky. But that’s not the case anymore. Many houses are built pretty tight and some are built extremely tight. Things just can’t dry like they used to and so reducing the frequency with which condensation forms and the quantity that is produced when it does form is important.
External insulation is a key to that as shown by the 2021 IECC codes for insulation (also attached). That’s what the “20 + 5” or “13 + 10” or “0 + 15” means on the 2021 IECC code. It means an R value of 20 internal and an R value of 5 external or an R value of 13 internal and an R value of 10 external or an R value of 0 internal and an R value of 15 external.
A lot of the reason they call for external insulation is to reduce the odds of harmful condensation forming in the wall assembly and not being able to dry out in todays tighter houses.
Thank you for the detailed reply.
I actually used just Advantech (not Zip) on my walls. In hind sight, I wish I had used Zip. That being said, I want to put some kind of membrane over that, but I'm not sure what to use. Then wouldn't it make sense to put exterior insulation on top of that? Why would it be hard to do that at this point?
Sorry, I guess I’m not very familiar with Huber Wood’s Advantech product. You should know before taking what I’ve said too seriously, I’m not an expert. I’m just a guy in climate zone 5 that’s trying to have a high quality and energy effficent house built and I’ve done a lot of studying on what is important to me.
But that puts me in positions to miss obvious things like, “Just because Advantech is a Huber Woods product does not mean it’s ZIP”….
Now that I realize what you are talking about, I believe I would want a peel and stick product to serve as your air and vapor barrier although taped Advantech probably does quite well for an air barrier and probably isn’t incredibly horrible as a vapor barrier. I would have to study that more.
The reason it would be hard to install external insulation at this point is dependent upon your builder and the available contractors in the area. In my area I have never seen or heard of anyone willing to do external insulation. I have been getting my builder gently warmed up to the idea for the entire planning phase of our house and he still is just barely on board.
Hopefully people who are willing to do external insulation are more common in your area.
I do think you might be waiting some for Comfortboard if that is the way you want to go. I got a quote on August 1st and was told lead times were 35 days. I haven’t ordered yet but I plan to soon.
Wow, I didn't event think about lead time. I do see 2 inch Comfortboard online that's available now with a hefty shipping cost. Fortunately, this is just a 400 sq. ft. addition so we don't need big quantities. Our builder doesn't have experience with exterior insulation either but he's really open to new stuff and has been great to work with in that respect. Thanks again for your input.
All walls need a solid air barrier somewhere. Generally the simplest and most robust approach is to tape the seams of your sheathing with a quality tape. Not all tape sticks to OSB, this is a good read:
https://www.finehomebuilding.com/2013/11/07/backyard-tape-test-10-months-later
Walls also need a WRB so this can be standard housewrap. You can also skip the taping and combine the WRB and taping into one by using a peel and stick membrane. Important part here is the self adhered membrane has to be permeable (ie Henry VP100).
In cold climates walls always need a warm side vapor retarder and a reasonable warm side air barrier. If you have lots of exterior insulation, painted drywall (a class III vapor retarder) will work, if you have little or no exterior insulation you need at least a class II (ie faced batts or one of the smart vapor retarders).
So for the drywall section, you are fine with just drywall as long as you match the values from Table 2 here:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
So that is R7.5 exterior insulation for a 2x6 wall. About 2" of rigid mineral wool, so not too bad but you are into screws for attaching strapping. It is also pretty squishy so it takes a lot of finesse. You can drop to 1" of rigid MW where the strapping can be nailed up with a framing nailer so much quicker and flatter, but in this case you need to add a class II vapor retarder behind the drywall.
T&G is neither an air barrier or a vapor retarder. So for that section you need to put something under the wall. Generally the simplest is to build it same as the rest of the walls and nail the T&G up over the drywall afterwards.
There is one way to skip any drywall and vapor retarders by putting all your insulation on the exterior. This is called a Remote assembly but it is overkill for a house, it is something you need for places like an indoor pool.
That building science link you shared is awesome. Thanks!
I agree with dpilot that if you use a sufficient amount of exterior insulation you don’t need to worry about the dew point in your wall cavity and therefore don’t need an interior side vapor retarder or barrier. I am not a fan of Rockwool but it will work fine. Put your WRB on the outside of your sheathing and include a rain screen by using battens on the outside of your exterior insulation. Peel and stick or fluid applied would be my recommended WRB’s.
Great info, thanks, everyone!