Bringing down the cost of PERSIST construction
I really like the idea of PERSIST houses. Keeping insulation on the exterior makes a lot of sense, and there are few more foolproof methods to build a strong air barrier. But the undeniable drawback to PERSIST construction is cost. Peel and stick membranes are significantly more expensive than standard housewrap, and there is a lot of OSB sheathing involved. REMOTE constructions address some of these concerns, though really only work with unconditioned attic spaces. I was trying to think of ways to bring down cost and reduce material use, and I had an idea I wanted feedback on.
In short, the idea was to eliminate the peel-and-stick membrane and use exclusively foam sheathing, then spray a layer of foam between the studs, directly onto the rigid exterior foam — similar to how you would do a flash and batt construction. The spray foam would seal nearly all the gaps between the rigid foam boards, providing a strong air barrier. The hope is that eliminating the peel and stick membrane and OSB on exterior walls will bring down cost relative to the additional spray foam.
Obviously this approach would require an alternate method of shear-bracing the walls, likely 1×4 let-in bracing. Further, one of the layers of exterior rigid foam (likely the first) would need to be tape sealed to qualify as a WRB. The tape would be nicely reinforced by the spray foam, reducing the chance of the WRB being compromised, along with the air barrier. As an added benefit, without OSB, the consequences of water leakage are reduced.
Either open or closed cell spray foam could hypothetically be used between the studs, though closed cell might be marginally safer. Since the spray foam isn’t being relied on for insulation as much as air sealing, studs could be filled part way, leaving a service cavity. Additional layers of exterior foam are likely cheaper anyway. As per standard PERSIST methods, 2×4 framing would be used.
From what I can tell, the main weakness of the air barrier would be between floors on a multi-story building. However, this is common in a lot of air barrier methods, and there exist solutions to this problem that aren’t too costly. Even just lining up the rigid foam to eliminate cracks over this area should reduce air leakage to well within acceptable levels.
My main questions are:
Does this approach have any glaring mistakes?
Would this approach likely perform similarly to traditional PERSIST construction?
Would this method actually reduce cost relative to PERSIST construction? Would it be competitive with 2x6s with cellulose insulation and exterior rigid foam?
Is it likely to run afoul of code?
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Replies
Unless you're getting rigid board insulation cheaper than batt or cellulose insulation, for a given R-value, getting 100% of that R-value from exterior rigid insulation is always going to be more expensive than getting that same R-value for a combination of exterior rigid insulation and cheap interior fluffy insulation right?
So regardless of if your air barrier is spray foam, peel and stick, taped osb, fluid applied membranes, mechanically fastened membranes, taped rigid insulation, drywall, or whatever else, it seems like if budget is a concern, you should insulate the wall cavity with cheap fluffy insulation in addition to the exterior insulation. You just have to make sure that you have an appropriate ratio of exterior continuous insulation to cavity insulation for your climate zone to avoid moisture issues.
There's a BSI-096 article at BSC that has a design that may be worth reviewing. It has continuous air, water, and vapor barrier ("OSB w/ integral facing, taped) up the walls and over the roof, with a unvented conditioned attic. This, to me, has a lot of similarities w/ REMOTE, with mineral wool exterior. In that particular example, there is additional interior insulation, but that's your choice, and of course depends on your climate. I like it because there's no expensive peel-and-stick, and no foam, but the author makes clear that there are a lot of options with this design for both interior and exterior insulation.
Aedi,
If you manage to make your assembly relatively airtight, it could work. Airtightness would need to be verified with a blower door.
It isn't the cheapest approach, and there are questions about the durability of the air barrier. (Rigid foam can shrink.) I'm not sure that the advantages outweigh the disadvantages.
Brendan,
That is a fair point. There is a small added cost to fluffy insulation, namely thicker lumber, but in terms of cost effectiveness, cellulose or fiberglass still probably wins out. I've been so caught up in maintaining a "perfect wall" without thermal bridging that I forgot the economics still work out :p
Andrew,
I hadn't seen that BSC article before, and he does mention at then end that the same method could be used with foam - a preference of mine, since it is cheaper. The roof construction, in particular, was helpful, as that is something I'm still working out the details to, as were the various pest control measures.
Martin,
Panel shrinkage was a concern of mine too, and one you mentioned in your article "Using Rigid Foam As a Water-Resistive Barrier". However, your description of AC71 testing standards did hearten me a bit, as they seem to put those panels through hell. Out of curiosity, do you think using a foil-faced polyiso might be a little safer? On one hand, the foil seems a more stable place to attach tape, but on the other, I have heard wet polyiso is bad. The larger cells might also prove to be more stable.
Thank you all for the feedback so far
Aedi,
Foil-faced polyiso is environmentally friendly and easy to tape, so it's often a good choice for this purpose.
That said, there are lots of reports from roofers of shrinking polyiso. So I have no particular confidence in polyiso's long-term stability.
Between Martin and Joe's informative articles, and a great consult with Balance Point's Gavin Healy on my current plans, I'm convinced that a simplified version of PERSIST could be the most cost effective way to build a durable and efficient home in CA zone 5 (Grass Valley). Looking at building very similar to the BSI-096 Fig 2 but with PolyIso instead of stone wool, 2x4 vs 2x6 in the walls and 2x12 vs 2x10 on the roof. Will lose the overall expensive peel and stick layer in favor of quality tape on the plywood sheathing as well as each offset layer of polyiso. Interior will be standard sheetrock. The biggest difference would be utilizing 2x4s laid on their side to secure the roof insulation which would provide more meat for building on an 18 inch overhang as well as a vent channel under the top roof sheathing layer, underlayment and metal roof. Looking for detail suggestions on how to best build that larger overhang structure, ventilate the lower edge of the roof vent in the fascia, and how to ventilate at the top of the wall rain screen. Wish we had more data on the long term durability of good tape, but with the data already available, as well as the each layer keeping the layer below it pressed flat and protected, I'm willing to bet on the durability of this method now. Will definitely back up that bet with gravity driven flashing details wherever practical though. Aware of the loss of R value in polyiso below 40, potential for some foam shrinkage over time, and the criticality of getting the window and door flashing details right - any other cautions on this budget PERSIST plan?