Sheathless Double Stud Wall
Hi all,
I’m planning out a new build in the south west corner of Western Australia – climate zone similar to 4C (think Seattle but with warmer winters and summers and no snow). Looking at building a double stud wall with fibreglass batts – exterior insulation is another option but I don’t want to use foam and alternative insulation options are expensive here.
The wall I am thinking of building goes: cladding -> rain screen -> WRB -> framing -> air barrier (Intello) -> battens -> plasterboard.
In Australia it is not common practice to sheath a house with plywood/osb (we use noggins and metal straps for bracing). One of the main aims of building science methods seems to be to keep sheathing from getting wet, but was happens where there is no sheathing to get wet?
Thanks,
Declan
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
There will always be the first cool condensing surface no matter its function or name you need it to stay dry so all the rules still apply if you add exterior insulation.
The plan looks good with 100% of the insulation in between the framing.
What R value will you be able to fit in the space available and is it enough?
Walta
Thanks Walta. So in this case the framing timber/insulation would get wet (assuming things go poorly).
I'm aiming for somewhere around R35 for the batts (not sure what the total wall R would be). Code here is around R16 so it's a bit overkill given how mild the climate is but should be comfortable. Hopefully my imperial to metric calcs are correct!
Declan,
Many of the early double stud walls (like those built by Thorsten Chlupp) had no exterior sheathing and performed very well. As you say, no sheathing means no first condensing surface to accumulate moisture.
The wall you are proposing will perform very well. I'm not sure any energy modelling could support using a double-stud wall in a climate warmer than Seattle, but there is certainly no harm in it.
Thanks Malcolm. The wall is definitely overkill, but it this approach is actually equivalent (or cheaper) to just going with thicker framing (e.g., 6 or 8 inch wall) - in material costs at least.
It does get hot over here so I'm hoping the thicker walls will make a difference.
Declan,
There are certainly good arguments to be made for future-proofing buildings the way you are.
You might find this article interesting.
https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
Your plan a preposed is safe. What gets risky is when someone adds a small amount exterior insulation in a cold climate like zone 5 or colder.
My guess is you live in a desert climate and it is so warm and dry that nothing could possibly get wet moldy and rot away but I could be wrong.
I do not think once you get over R10 more insulation changes the comfort level. Now it is about reducing fuel usage and return on investment, that is a math equation. Given that R14 is recommended R35 seems unlikely to ever recover its costs.
Walta
Walta,
It brings up an interesting question as to whether ROI is the only metric we should use to assess how much insulation makes sense? - especially as ROI calculations rest on future guesses about conditions, both economic and climate based, and ignore resilience as a benefit.
Malcolm,
Declan said "I’m aiming for somewhere around R35 for the batts (not sure what the total wall R would be). Code here is around R16 so it's a bit overkill given how mild the climate is but should be comfortable. "
My point is above about R10 I do not think a human is able perceive the difference in a walls R value if everything else about the walls are the same. My guess is the surface temp would be less than .5°F. Spending 3 or 5 times as much building a high R value wall is not making one more comfortable, then you are doing so for some other reason and the reason is not likely to be rational in my mind but passive house stuff does not make sense to me.
I am old enough that that I don’t believe everything chicken little says. Peak oil from a few years ago come to mind. Sure, fuels cost more than it used to but so does everything else so that is mostly inflation. My guess is R35 walls ROI when oil costs more than 7X its record high price.
Walta