Double Stud Wall
I am investigating wall assemblies for a future build and have been researching external insulation vs. thicker 2x assemblies vs. double (or staggered) walls. I understand that a minimum amount of exterior insulation is required if going that route to keep the sheathing warm, but it isn’t really mentioned when talking about a thicker wall such as a 2×8 wall.
Wouldn’t the moisture issues be the same? Same with a double stud wall. Wouldn’t the sheathing get just as cold as a wall with too little exterior insulation? Or is it that there is a specific point where it is an issue but below that threshold it isn’t worried about? Is it the fact that the exterior insulation keeps drying down but without it the wall can dry?
Was interested in doing the double stud wall assembly below or a 2×8 wall but every time I read something I question if it will work or cause my house to immediately crumble 🙂 Trying to understand how to eliminate the chance of moisture issues as much as possible.
– Vinyl Cladding
– Zip System
– 2×6 24 OC R-19 Unfaced batt insulation (no space between walls)
– 2×4 16 OC R-15 Faced insulation
– Drywall
I have read about adding something like Membrain between the double stud walls or the interior side of a 2×8 or thicker wall can eliminate issues but wonder about anyones real world experiences.
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Replies
jb_429,
You may find this article interesting: https://www.greenbuildingadvisor.com/article/the-mythical-threat-to-double-stud-walls
If a double stud wall doesn't have moisture issues, then conventional single-stud construction shouldn't either.
The issue is when you stick foam on the outside of a wall, you have inhibited it's ability to dry outwards. So if/when the sheathing get moisture accumulation, it can't dry out correctly.
In the case of the double stud wall, I believe many here who use double stud techniques prefer to put CDX as the cladding in order to facilitate drying a little better than OSB/Zip.
Tim_O
Some early double wall builders (like Lucas Durand) went further and moved the vulnerable sheathing in to the middle of the wall, replacing it with a permeable WRB on the exterior face.
Tom_K,
That's true. The sheathing on double walls should stay colder and be more vulnerable to damage than conventional ones.
We know double stud walls work with good air-sealing, cellulose insulation to buffer moisture, and a rain-screen. The question is which of those can be removed and the assembly still remain safe?
I do a lot of double-stud walls, including designing the one Ben Bogie monitored in the article you linked to in comment 1. I would not eliminate any of those elements, and I would prefer to have CDX and an interior variable permeance membrane.
Michael,
So to bring the discussion back around to the OP's question, you would find these potentially problematic?
- The absence of a variable-perm interior membrane.
- The use of batt insulation, rather than cellulose.
- Low perm Zip sheathing.
- Vinyl siding as opposed to a separate rain-screen gap.
Good questions, Malcolm.
Variable-permeance membrane: good insurance but may not be necessary if everything else is done the best it can be.
I recommend cellulose due to its hygric buffering ability (i.e., it redistributes moisture via capillary action) and its borate component, which (thanks to Kohta Ueno/BSC research) we know protects the framing members it touches from microbial action. With any batt product, the wood is the most absorbent component and I don't trust it to remain mold-free over the long term. It could be fine; I've seen inside the walls of a couple of double-stud walls built in the 1980s with fiberglass batts and saw no problems. But I still think it's riskier than using cellulose. Of everything on your list, this is the most important in my opinion.
I like Zip sheathing but it's not my first choice for double-stud walls; the more vapor-open the exterior the better, since moisture must move through the assembly or else it will cause problems. Ideally I'd like to see a sheathing-free wall using European WRBs and diagonal bracing; I've designed them but so far none have been built. The next best would be board sheathing with a good WRB. I usually spec CDX with a WRB. But if everything else is done well, I'm sure--and Ben's data loggers--say that Zip can work.
Vinyl siding is so leaky that it doesn't need a separate rainscreen. That does mean that it needs a good WRB, and ideally I'd rather have a longer-lasting siding material with a ventilated rainscreen cavity, but I'd be ok with vinyl in a pinch.
Thank you everyone for the discussion! So the consensus is that a double stud wall needs the cellulose to buffer moisture and CDX / interior membrane for better drying potential. Does the same apply to a standard 2x wall?
For example, I think most would say a 2x6 wall with r-21 batt insulation is considered safe but at what point does it become vulnerable to moisture issues? A 2x8 wall with r-30? etc. At what point does it make sense to go away from batt insulation due to moisture buffering?
For me specifically I am located in Zone 5 but curious how this applies to other locations as well.
This one is behind the paywall, but summarizes my opinion:
Double-Stud Wall vs. Exterior Insulation
A high-performance wall assembly is a given, but which one makes the most sense?https://www.greenbuildingadvisor.com/article/double-stud-wall-vs-exterior-insulation
This is a decent resource when looking at assemblies. They ran models on a lot of wall combinations with a pretty good blurb about each:
http://effectiver.ca/calculator/search-go-to-the-calculator.php
Oddly enough the 2x8 wall performs worse in very cold climate than a 2x4 double stud. I would be curious to see what the difference was.
2x8:
http://effectiver.ca/calculator/wall.php?id=3959
double stud with 3.5" gap:
http://effectiver.ca/calculator/wall.php?id=16776
For reference, Vancouver is zone4, Winnipeg is zone 7, Toronto zone 5 and Montreal zone 6.
If you want to play with various stack-ups and have some time to figure out the metric units, this is also a great tool to see how various options will perform:
https://www.ubakus.de/en-ca/r-value-calculator/