New construction, adv. framing, ext. insulation, zone 4 wall design
I am a novice owner-builder doing a simple 768 square foot small home on a slab. The design is hopefully such that when/if we build a larger home, we can use this space as a garage. But… in the meantime, it’s a home. I’m trying to meet code on the high end while trying out some of the newfangled building techniques such as advanced framing and continuous exterior foam (or rockwool) insulation.
I am located in Virginia, in climate zone 4a.
Probably not surprisingly the thing that has me most baffled / concerned is vapor control. If necessary, rather than adding more exterior insulation (since this certainly isn’t a forever home, but would likely at least end up a forever-garage with some guest quarters) I’d probably just thin out the cavity insulation.
Is the attached wall cross section suitable? Essentially the whole assembly is vapor open except the paint on the interior sheetrock, which is (as I understand it) a class III vapor retarder.
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Replies
You won't have a vapor issue with that stack up. That's a lot more exterior insulation than needed to keep your sheathing warm in cz4, and is also vapor open. https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
The comments that I have heard here and elsewhere is that rigid mineral wool is easier to install with beefier furring strips, such as a 2x on the flat. The slight "squishiness" of the mineral wool stock makes it somewhat fussier to end up with a coplanar surface using 1x furring strips.
Rigid mineral wool is not inexpensive, and that much exterior insulation in a cz4 climate almost certainly exceeds financial rationality. Eliminating a layer of it would still be a very high performing wall (with no vapor issues).
Tyler,
That is a heck of a wall for your climate zone. It will work pretty well.
Don't worry too much about vapor control for your walls. It's the ground that you have to worry about from a vapor perspective. Make sure your slab is well drained, has adequate insulation, and includes a class I vapor retarder. Focus on achieving a continuous air barrier for your walls.
Also, in a mixed climate, I might opt for polyiso over exterior mineral wool- especially if you are concerned about water vapor. 2" of polyiso is more than you need to keep the walls above dew point in winter. The foil facing creates a class I vapor retarder against warm humid air trying to get into the air-conditioned interior in the summer.
Not sure I want a vapor -open assembly in the south, especially with walls that thick. Your AC will barely work leaving little opportunity to remove moisture. Meanwhile, your vapor open assembly may drive additional vapor into the interior. If you use exterior mineral wool, then consider a permanent dehumidifyer. Choosing Zip sheathing over plywood may help.
Polyiso over rockwool will save you a few hundred bucks too.
I'm nitpicking here... Your wall is a damn good one.
For a building that will eventually be a garage/ADU, the walls you are proposing are way overkill. It will work, just not very cost effective.
In your milder climate, the 2x6 24"OC wall with batts is good enough. If you want to give exterior insulation a try, than I would just go for minimum for condensation control in your climate which is around 1" of foam. There is no real energy savings benefit by going over that unless your local energy costs are very high.
Thanks for the replies. Based on the feedback, I think I'll drop down to 1"of _something_ on the exterior.
It sounds like my original wall is overkill even by GBA standards. :-) That means it's probably a _little_ silly financially speaking, and even more so if this is going to become an uninhabited garage one day. And I doubt when that day comes that I'm going to find that garage door air sealing technology has advanced much!
Bryan: Would a little time and some string lines simplify getting those 1x's coplanar, or would you just skip that and go for 2x's, hands down, when using rockwool?
Rick: There will definitely be some future questions about the slab and roof. One of the appealing things about Rockwool versus foam on the exterior for me was that, should I screw up some detail in my amateurishness such as, oh, flashing a window... there's a better chance of the wall drying out more quickly, at least, I think. Though... polyiso is _kinda_ vapor permeable right? So subbing out for polyiso (so long as no foil) wouldn't change the design right? In this area the only thing available at the big box stores is foil faced... but.. the humidity point is very valid. One of the reasons I wanted to go a little overboard is because I'm sick of the noise of things starting and stopping - especially massive forced air blowers and compressors. Having to own/run a dehumidifier for the whole house would make me sad I suspect. Is there somewhere that _should_ be a vapor barrier in this wall?
Thinks Akos, I integrated your 1" suggestion into my current wall cross section.
Has someone published a "perfect wall" / "perfect roof" / "perfect slab" etc for the various climate zones? I can usually follow direction pretty well, and while I'm a real cheap bastard, I do believe in buy-once cry-once.
I've never installed exterior rigid wool, just rigid foam; I'm just relaying what I've heard from others. My understanding is that the tiny screw adjustments compressing and decompressing the batten tend to split 1x stock.
If using 1x stock, I would try to install battens and wool boards simultaneously, using squash blocks to maintain a coplanar surface as the insulation is installed, similar to this:
https://www.youtube.com/watch?v=fBb0qQoY7kM