How would you compare Greg La Vardera’s New USA Wall assembly with a SIP wall supplemented with Comfortboard mineral wool?
Planning to build a new “Pretty Good Home” in coastal Massachusetts (Zone 5A) in the spring, and am wondering about performance, durability, cost, and ease of construction with the two envelope systems in the question; architect Greg La Vardera’s “New USA Wall” framed with (advanced framing) 2x6s, versus a 6″ EPS SIP with an exterior layer of 2″ Comfortboard rigid mineral wool (R-8) and an interior 1 1’2″ utility race insulated with 1 `1/2″ Comfortboard (R-6)? Both assemblies have similar overall R-value, but which might be more cost-effective, durable, and easier to build? Advantages and disadvantages of each? Thanks!
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A 2x6 AF wall with R23 batts without the exterior rigid board comes in at about R15 whole-wall, R16 as an absolute best-case. If you go with his "Better" 2x6 stackup (see: http://www.lamidesign.com/blog/imgs/better_2x6_2s.jpg ) you'd be looking at about R27-ish whole wall performance. In your climate zone that would not be sufficient exterior R for dew point control at the sheathing if you add the interior insulated chase, and you would need an interior side vapor retarder. (With vented siding on the exterior you could probably get away with it without a vapor retarder though, due to the high vapor permeance of the rigid rock wool.)
A 6" EPS-core SIP has 5" of EPS, for about R21 before factoring in the thermal bridging of the necessary top/bottom plates,. window & door headers and internal splines, etc. call it R18-R19 overall (the details actually matter.) But it's still substantially higher performance than an AF 2x6/R23 wall, even before you add the R8, and add the insulated interior chases (use split R15 batts, not ComfortBoard on the chases if you go that route- it's quite a bit cheaper, and the performance delta is negligible.)
If you add R8 exterior rigid board, and an R6 insulated chase (probably R4 after thermal bridging & gaps) to the 6" SIP you would be at about R30 "whole-wall", which is NOT "... similar overall R-value...", but something like TWICE the performance of the 2x6 AF/R23 wall, (but a heluva lot more expensive.)
Cost-wise the 2x6 wall will come in substantially less than just the 6" SIP. If you add R8 exterior rock wool to the exterior of the 2x6 wall you'd be in the R23 whole wall range, and it would probably STILL be cheaper than a 6" SIP (or at worst, comparable in cost), and would have sufficient exterior R for dew point control at the structural sheathing in a 5A climate to skip the interior vapor retarder (la Vardera is a fan of using smart vapor retarders). See:
Air sealing SIP walls would be easier, but it's neither hard nor expensive to air-seal a wood sheathed studwall assembly if you air seal it as you go.
From an overall green-cred point of view the 2x6 rock wool assembly is hands-down the winner, due to the high polymer content of the SIP, which has a much higher lifecycle environmental footprint than rock wool, which is mostly made from recycled waste by product (slag) from steel making.
There are far more contractors capable of executing framed construction than SIPs. Though some may have little experience with AF techniques, if it's spelled out in the architectual plan even newbies can figure it out.
To hit "pretty good house" performance in coastal 5A you'll need whole-wall performance of at least R30, not R23, and definitely not R15, like la Vardera's simplest wall. Going with 3-4" of exterior rock wool on the AF 2x6/R23 wall would get you there, with or without the 1.5" of interior side insulated wiring chase.
Though I am still not sure which direction I want to put my effort in to; 2x6 "Best New American Wall," or a 6" Sip with additional interior and exterior insulation. Though more costly, the building process seems to be more expedient to go with a SIP package erected on site in a short time frame. What would your choice be? And thanks again for your response.
If you enter "best wall" (in quotes) in the GBA search box, you can read articles and Q&A threads for hours. Everyone loves to debate this topic. There are lots of good ways to build a wall.
Choose a method that you can afford, and that is buildable by your contractor.
If you want a high-R wall, the two methods that seem to rise to the top of most builders' lists are double-stud walls insulated with dense-packed cellulose (a wall type that should have a ventilated rainscreen gap and almost any sheathing other than OSB), or a 2x6 wall with insulation on the exterior side of the sheathing (usually EPS or polyiso, but sometimes mineral wool).
SIPs are expensive.
For a zone 5A location shoot for at least R20-ish whole wall, which would be a 2x6 /R23 AF wall with 1" of rigid rock wool on the exterior.
A 2x6 wall with 4" of exterior rock wool is not difficult to build, and would get you to the R31 range, for much less cost than 6" SIP + 2" of exterior rock wool that delivers R29. That much exterior R also gives you huge dew-point margin at the sheathing, and that's the approach I'd be inclined to take.
The extra labor of the insulated interior 1.5" chase hardly seems "worth it" in a 2x6 framed wall, where the wiring can already be accommodated by split batts, which competent insulation installers can deal with. Alternatively you could use 1.8 lb density blown fiberglass (Optima, Spider, L77 etc) which would fill more completely/perfectly than any batt solution, and would clog any air-sealing errors at the sheathing with fiber.
Four inches of rock wool is easy to install, but it's the devil to get your strapping flat for the siding at that depth.
I've had nothing but bad experience with SIPS. Although they are quick to put up, they have several downfalls, especially when you don't put exterior insulation on.
SIPS shrink, depending on type of SIP insulation some more than others - PolyIso lots/EPS little.
Air leakage at the joints - especially when the panel shrinks
Wiring SIPS is a nightmare, unless carefully (very carefully engineered)
Why pay for an extra layer of plywood on the outside that you are just going to cover with more insulation? If you are going to just add more insulation, just use a "nailbase panel" (has only one side finished i.e. drywall, osb, v-groove, etc.)
With a thick EPS SIP and 3-4" of Mineral Wool/Comfortboard you may not need a vapor barrier (Dana / Martin thoughts on this?) and your WRB would have to be on the outside of the insulation.
Here is what I am doing, after having done SIP's, and conventional insulation. I've posted these articles/links so many times, on this forum and to many others, because at the end of day - the most durable, healthy, sustainable wall seems to be PERSIST. It just seems so much simpler than Larsen Truss (Double Walls), keeps the wall dry, and my sheathing dry too. But read on for yourself - many of the links are to articles found on this site.....good luck
These will give you some further understanding too: