Insulated Double-Stud Wall Assembly vs. Exterior Insulation
tdbaugha
| Posted in Energy Efficiency and Durability on
Is there a consensus yet on double stud vs exterior insulation? I’d like to build at near passive house levels, R30-R40 effective wall r value. I was initially thinking a standard 2×6 wall, BIBS, Zip, and ~3” exterior insulation was the way to go, but I’m seeing how a double stud wall might be less overall work and cost when you factor in doing vertical siding, stone, stucco, etc as your facade. Can anyone chime in on overall cost of one vs the other?
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Both wall construction methods can give similar R values, but they're vastly different otherwise, and I'm not sure they should be compared in the way you're thinking. They solve different problems, and create some new ones in different areas.
The double stud wall just eats into your square footage, doubles the lumber required for a house, isn't cheaper, and has challenges in filling the insulation correctly. On top of that the sheathing is cold, but I suppose it saves drilling the studs to run electrical.
I'm biased, obviously, but I think the exterior insulation route is more effective material wise, and is more constructible, and utilizes your space better buy moving things outwards instead of inwards. The problems it creates is that windows/doors are harder, though I personally think the recessed window looks better than a surface mounted window. It increased the durability of the structure by raising the temperature of the sheathing, and it's been shown to be easily capable of supporting those claddings with appropriate fasteners with little to no deflection. The only other downside is that it probably adds $1k to the build for all the fasteners.
No information cost wise, just food for thought.
tdbaugha,
I'm not sure how easy it would be to generalize about the relative costs of the two. It probably varies with the thickness of the foam, and how comfortable the local trades are with each method.
To add to what Kyle said, The benefit of double stud walls comes as they get thicker. The structure stays the same, all that changes is the gap between them. Conversely, exterior insulation becomes exponentially more complex as the thickness increases. So to me there is point (maybe beyond 2"?) when the scales tip in favour of double stud walls, while below that thickness exterior insulation may make more sense.
To add more context, I’m in climate zone 6 with a fairly dry climate and I would need 2.5-3” in order to forgo the interior VB. <2” exterior insulation seems like a no brainer. Beyond that the fasteners get expensive and more difficult to hit studs. Im torn. I have not posed the question to the contractor yet, but I’m 99% sure they haven’t done either assembly before so they won’t have a preference.
I wrestled with this question this last spring, when I was about to start construction on an ADU. Lumber prices have come down a bit in my area since then, and rigid foam has gone up, but at the time, the cost of the extra lumber alone for a double wall system was higher than the cost of 4" of EPS foam. So I went for 2x6 24" OC wall construction, 4" of exterior EPS, with rough-sawn 1x3 horizontal furring 24" OC for vertical siding, secured to studs with 8" FastenMaster screws. I paid $160/250 screws from Amazon (where they no longer available, unfortunately).
In the next couple weeks, I plan to blow dense pack cellulose in the stud bays. The project was dried in last fall, and I'm trying to wrap things up this summer.
I have some background in building trades, and have done most of the work myself, assisted by a few student laborers, mostly because finding contractors has been impossible. Hitting studs through 4" of foam isn't too bad, but we certainly missed from time to time. In general, being able to take my time and change my mind has made things easier; not a luxury I'd have if I was working with (and paying) contractors.
I did 'outie' casement windows and 'innie' doors; both were pretty easy to detail.
There is, of course, a world of difference between doing an owner-builder project over a couple summers, and trying to find contractors willing to tackle unfamiliar building techniques then complete a project with tight timelines and budgets. Where I am, contractors typically have more work available than they can handle, and tend to focus on doing what they know, and know they can make a profit on. Hard to blame them.
The framers I asked about double stud walls never got back to me with a bid after the first phone call; I doubt I would have found anyone willing to do thick exterior foam either. Perhaps you'll have better luck.
Compared to what it costs now, wood was essentially free 15 years ago. I think the decisive factor is whether you are doing the work or paying contractors. If the latter, then find the best contractor you can for each method and decide who you want to hire. Your house will perform great either way, though you should consider design aspects (like window sill depth) too.
Everyone’s response has me thinking my initial plan is the way to go. Hire a GC and have them frame as usual. Then I DIY the zip tape, exterior insulation, furring strips, and window extension jams. Then the contractors side the house essentially like they would otherwise. Probably the best combo considering constructibility, contractor hassle, and cost.
Both wall systems are sound if done correctly, and both should be high performing if a rain screen is added. I personally would only use rockwool as an exterior insulation, because of both breath ability and various pests do not find it delicious.
I design high-performance homes and renovations and have found that the answer depends entirely on contractor preference. Most of the builders I work with think it's faster, simpler and less expensive to do double stud walls, so that's what I usually do. But others think it's twice the work and material and would rather do exterior insulation.
In most cases the idea that thicker walls eat into your square footage doesn't mean anything, as the interior space is the same either way. Exceptions include tight building lots or tying into existing conditions.
From the standpoint of up-front carbon emissions, exterior insulation is usually foam of some sort, which unless it's recycled means that your insulation contributes to climate change. Double stud walls are often insulated with cellulose, which has very low carbon emissions. You can insulate the exterior with rigid wood fiber which has very low, or in some cases, negative carbon emissions.
I'd be curious to know if there has been any work on internal + external insulation outside of the studs.
E.g. 2" exterior + 2*6 wall with batts + 2" interior
This would let you avoid the challenges associated with going 4" on the exterior and the costs of doing a double-stud.
If you're using rigid foam, that's a double vapor barrier, isn't it? Bad idea... if rockwool, maybe you could get away with it, but now instead of just dealing with extension jambs on the exterior, your interior ones are deeper too, etc.
Yeah you wouldn't use rigid foam - at least not on both sides.
In Canada, we do a VB on the inside so doing rigid foam on the inside, and then 2*6 wall with Rockwool and then Rockwool external would not be much of a stretch.
It's worth remembering these aren't the only options for improving on the standard 2"x6" wall. You've got Bonfiglioni, 2"x8" framing, Larson trusses, Mooney walls - and countless other variants.
My current house has 1” polyiso on the interior of a 2x6 wall and it’s a total PIA to do interior trim. Definitely not recommended.
LamiDesign comes to mind..
http://blog.lamidesign.com/p/usa-new-wall-info.html
2" outside and inside insulation on an 2x6 core ?
also:
http://blog.lamidesign.com/2022/06/usa-new-wall-example-of-better-wall.html