Opinions on this Permanent Wood Foundation Assembly
Hello, first time poster here! Looking to see if there are any problem areas with this assembly.
I’m playing around with a wood foundation assembly based on Mike Guertin’s suggestions from his 2020 presentation. This would be for a self-build DADU a few years in the future. I’m planning to get an engineer to sign off on everything, because this is a lowish seismic area and I’ve never built a house, just done extensive remodeling and a Habitat build.
Some details:
• Zone 3A, mixed humid climate, bordering Zone 4.
• Carolina clay, gently sloped site.
• Termites, water, and radon intrusion are the biggest concerns here.
• I didn’t write it down, but the rain screen would be wrapped top and bottom with a screen to keep out pests.
• I threw the inner ZIP sheathing in there as an additional fire/seismic precaution, not certain if it will be necessary though.
• This shot is of the crawlspace; the main wall assembly is different, based on the Bonfiglioli design. The roof plans are a bit unusual, with large applied eaves, so if there’s any interest I could post them as well.
• I’m aiming for near-passive performance, but not currently planning on getting it certified.
If you have any questions about details I’ll happily post better drawings!
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Replies
I'll start by saying I am at least 100% against wood foundations, probably even more than that :-) This is NOT the place to skimp in any way. This is really a place you want to use masonry, as ANY issue with a failing foundation in the future is going to be a MAJOR expense.
That said, since you are mostly above grade here, you're in much better shape, but I'd at minimum use a poured concrete footing here. Concrete is really the best product for below grade things that need to last. Remember that your entire home depends on the integrity of your foundation. Critters won't chew up concrete, water won't hurt concrete, etc...
Bill
Thanks for the feedback! A concrete footing is a fallback if the inspector doesn’t care for all wood. All wood just has the advantage of being able to DIY it and greatly reduce embodied carbon.
Josh,
What are the relative depths of the crawlspace floor and the grade outside? Is it similar to your section or is the crawlspace deeper?
Malcolm, it would be as close as possible to the way I’ve drawn it, so basically level. I’d probably raise the exterior level of the clay to get a positive slope rather than excavate.
Josh,
If they are the close, I'd try to modify the detail so that the crawlspace's framed floor was above the exterior grade. That, and the addition of a perimeter (French) drain would make water damage much less likely.
The two most vulnerable parts of the assembly are the Zip and the fiberglass batts in the floor, which rely on the ground-seal to stay dry. I'd eliminate both of them. Detail your exterior sheathing as a shear-wall, and substitute rock-wool or foam for the batts.
Two other minor issues are the lack of continuity of the exterior sheathing for shear, and the lack of a termite shield near the floor level.
I have no problem with wood foundations but there's no way I'd let fiberglass batts touch them or even hang out in the same vicinity. Exterior insulaton only, and separated from the wood with waterproofing. It looks like you have Blueskin on the outside of the GPS... that can't meaningfully dry back to the interior. Wet foam doesn't insulate.
Wet floors also don't insulate, and the batt-stuffed wood framed 'slab' looks like trouble. The temperature of the waterproofing beneath it will be below dew point much of the time, with again, no meaningful way to dry back to the interior. Any plumbing leaks will pond and make the crawlspace smell musty for eternity.
Last thought--drain tile to daylight would be cheap insurance, i.e. a good idea. It reduces both bulk water risk and soil gas pressure.
I forgot to mention a key detail, the crawlspace would have either a dedicated dehumidifier or be conditioned/heated along with the interior. Looking at Minotair + small heat pump because I like that it can also handle dehumidification. I’m no expert, but I think that would mitigate condensation issues in the floor? But not the foam I suppose, the ZIP would prevent that.
Also would like to have a vented radon system beneath the Moistop.
I originally had an exterior drain, but I removed it from these drawings once I added the EPDM surround because I couldn’t foresee any bulk water getting in there. (Another detail I failed to mention: 5' roof overhangs.) But I hadn’t considered exterior soil gasses, thanks for pointing that out! Do you think the 3/4 gravel plus vented rain screen would be insufficient to vent them?
Always build so that the structure can survive on its own.
I wouldn't rely on any mechanical interventions. What I mean is that you may install a dehumidifier, but the next owner might not. Or it might not dehumidify enough, or it might not perform 100% after a couple years, or...just too many variables to be sure that it would always be there and that the structure could survive without it.
Yes, that's definitely the ideal approach.
Josh,
It could be my northern climate bias talking, but I'm always leery of exterior vapor barriers, in your case the Moistop and the WP200. With so much insulation inboard of those it seems risky. Wood foundations are already a risk most don't take. I've seen them done well, even as replacement for failing old unreinforced concrete walls (house I grew up in), but I would do everything else conservatively. Waterproofing on the wood, exterior insulation. I prefer to literally see the wood stay dry.
A dehumidifier or space conditioning helps but doesn't eliminate the possibility of moisture accumulation within these assemblies. Once it's in there and behind a layer of zip, subflooring and fluff it can potentially take a long time to make its way back out. With the slab, you don't have the help of the sun to 'cook' the moisture back out for instance. You're relying on diffusion for anything that condenses on the Moistop. It's not within the conditioned space. Maybe I'm overly conservative, maybe it stays dry. Not a chance I would take, personally.
Vented radon system is great. Perforated drain tile is still cheap insurance for bulk water assuming the EPDM skirt fails and some water moves laterally into your crushed rock bathtub. If the water table is low maybe it's not needed.
Thanks for all suggestions! I updated my drawings to create an assembly that will hopefully be more robust. Of course it all depends on having an engineer give approval.
I'm also uploading a shot of the wall and roof assembly in case anyone else just likes seeing wall assemblies like I do.
- 5' overhangs (to protect 4' deck and calculated to shade first floor windows in the summer)
- Applied eaves with lots of insulation on top (Comfortboard protecting GPS from rapid thermal cycling)
- Custom 3x8 rafters so I can hit them with the 12" screws.
- Bonfiglioli wall with 2x8s. Probably the cheapest idiot-proof way to get a low-30s R value?
- Single top plate, seems like it would be fine since wall sheetrock can be attached to the furring.
Josh,
Kudos for thinking outside the box! I can't say I don't love a good experiment, but this seems like a bridge too far. It's not because I think it wouldn't hold up your house, but because of the constructability, and long term performance. There are a few obstacles you'll need to overcome -
1. Compacting gravel to be in a single plane over that large of a distance is unobtainable. Even with the best laser, time, plate compactor, and will power, you'll have places where even if perfect, will get kicked around during construction, scraped from boards being drug across it, etc. The ground will move ever so slightly, and while a 2x12 is stiff across the wide axis, it's flimsy (in comparison to concrete) in the orientation you have shown. That unevenness will telegraph upwards to your top plate, and you'll be fighting gaps everywhere, making it difficult to have walls above it that are flat and planar, making it hard to have a roof that is flat and planar. Air sealing would be another challenge this creates.
2. Lumber is not a rectangular prism that is produced with micrometer precision. Leave that PT 2x12 in the sun for a few hours and you'll end up with a board that is nearly impossible to straighten such that you have a a wall that is square, plum, and true. The same goes for the PT plywood. Down the road WHEN it gets wet, the pullout/shear resistance of the fasteners decreases, and things can shift.
3. This assembly will cost more than the same foundation constructed from CMU / concrete footing. A regular old 16" cmu is $2.00, and judging from the height of your wall, you'd need about 5 courses, give or take. If you compare the costs (at the moment), I think you'd be surprised. From a self-build perspective I'm sure it seems like lumber is more user friendly than block or concrete, but I'd argue against that. Laying a block wall requires patience, and a little strength, but it's not something that even a beginner can't do with a a good amount of reading, youtube, and attention to detail. The tools used for block aren't more numerous or expensive than those used for lumber.
3. The repairability is a concern. Replacing that 2x12, should you ever need to would be a job that costs tens of thousands of dollars, even in one 10' run. What happens to the blueskin membrane in that section? Does it get replaced too? The insulation? The finish applied to the boral board now has to be matched again, etc. It's a job.
4. The whole foundation has practically zero shear resistance if the plywood fails. The interior plywood can help, but it adds to the cost.
I don't like to rain on parades, these are things that I wouldn't have thought of before I had to build something from scratch. In the end it's your house, and if you want to try it, try to think of the construction in terms of "What if the conditions aren't ideal, and how will this effect the rest of the project?".
Josh,
A few ideas to mitigate Kyle's concerns:
1. A layer of pea gravel at the perimeter can help level the footing base.
2. Avoid lumber with highly irregular grain pattern if possible for the footing and foundation. Less likely to warp. Difficult to avoid in 2x12, however.
3. You could remove sections of subfloor in the crawlspace and jack the whole house up in the event repairs are needed. Do the waterproofing well and that need is unlikely. (Glad to see the drain tile)
4. Use stainless fasteners and UC4B grade PT.
What about the other #3? :)
Other #3. Wait til lumber is cheaper :)
A few comments in addition to Jason’s solutions.
Cost: I drew up these plans when lumber was more affordable. I want to have a conditioned crawlspace and use it for storage. That would mean at least five inches of rigid foam on CMU. Perfect Block will be an option if lumber prices are still sky high in a few years.
Lumber issues: I would panelize the foundation walls so they could be set as quickly as possible and be built square. Depending on what an inspector wants, I think I could have the Advantec done within a few days of laying the foundation plates, to minimize weathering. Just spitballing there of course tho :)
Shear strength: I’ll be working with an engineer. I would certainly prefer a belt and suspenders approach. What it may look like, not certain yet.
Thanks for your comments!
I would look at ICF. Simple to DIY and it minimizes the amount of concrete you need. It avoids all the issues with wood foundation and simplifies most of your details. Take a look at this article:
https://www.greenbuildingadvisor.com/article/minimizing-concrete-in-a-slab-on-grade-home
I’ll look into it more, thanks. I’ve overlooked it to this point because it would still need careful application of a barrier to keep termites out of the foam. That’s a major concern in this area. I’m more interested in a solution such as Perfect Block, but I’ll give ICFs a look.
Skeptics and proponents alike will appreciate this episode of the BS*+ Beer Show on the topic of Permanent Wood Foundations with Mike Guertin, a builder and Fine Homebuilding/GBA contributor who has been using them for decades.
Have you evaluated you plans for conformity with the Permanent Wood Foundation System Manual from the National Forest Products Association?
This doesn’t follow their exact guidelines. Mike Guertin spoke of their guidelines in the presentation Kylie linked above, but he suggested a few improvements.
Josh,
If you want something that could be DIYed while minimizing carbon, a pier and beam foundation might be more practical and durable.
That’s highly appealing, and something like the Diamond Pier looks very manageable. A crawlspace just has the allure of being able to store stuff (drying slabs in my case) and run wires, ducts, and plumbing with impunity. Definitely giving piers a good look tho.
See https://www.greenbuildingadvisor.com/article/all-about-helical-piles
I think helical piles are a lot better than Diamond Pier.
UPDATE
Beth Williams is a PassivHaus consultant based out of England, and she'll be giving a talk about concrete-free foundations this Tuesday at 7am Eastern time (if I've done my math right). Free on Zoom! She's also a great follow on Twitter, @BethWilliamsSE.
You can sign up at this link:
https://www.aecb.net/event/concrete-free-foundations-aecb-expert-advisor-beth-williams/
(Also, here are updates to my design based on Mike Guertin's advice. Now I start shaking the nickel bush :)
Josh,
Thanks for the update! I won't dish out any advice, but I would make a request, so that I'll sleep better tonight. Could you add that perimeter drain back in? That bowl shaped dirt section is begging for an undiscovered pipe burst to fill it to the top.
Building Science Corp (Joe Lstiburek) has a good article on wood foundations. He does not hate them and he is pretty smart.
https://www.buildingscience.com/documents/building-science-insights/bsi-020-wood-foundations-picasso-does-foundations
If I remember correctly from the article he still thinks the footings should be concrete for the reasons mentioned above.
I came across this and wanted to share a couple of thoughts in case you're still working on this. I think the resistance to wood foundations is based more on emotion than fact.
- I would use a structural concrete panel instead of exterior plywood. It will be much better protection from termintes and failure. https://www.usg.com/content/usgcom/en/products/roofing-building-envelope/concrete-foundation-wall-panels/structural-panel-concrete-foundation-wall.html
- Instead of Blueskin, use an exterior barrier suited for below grade use such as Delta MS or Grace Bituthene.
- I have no idea what that Zip System panel does for you on the inside. If you're putting insulation in there, you don't want any vapor retarder, just drywall. Your climate is so mild that I would not worry about fibrous insulation in that cavity.
Hi Andrew, thanks for the reply! I've changed my foundation details and (reluctantly) won't be using a PWF for two reasons, neither of which anyone listed above I believe.
- One, soil bearing capacity. Carolina clay apparently doesn't have great bearing capacity, and while I could work with a soil engineer to do the math, that's an extra step I don't want for what will be my first build. Thick, two story walls weigh a lot, especially if I opt for cellulose (but leaning toward fiberglass at the moment).
- Two, roots of nearby trees. While other foundation or slab types could also be vulnerable to root pressure or ingress, PWFs are likely more vulnerable. I have some nearby hemlocks I'd like to keep if I can.
So what am I planning now? Basically, a version of Michael Maines' shallow frost-protected foundation. Except no exterior frost protection because we don't have a frost line here in Zone 3A. It will allow me to minimize concrete, and still have an insulated floor. I doubt I could get any concrete contractor to pour a raft slab over foam here.
I may still switch to a hybrid foundation with concrete footing and pressure treated stem wall, but in trying to value engineer my design, it looks like the "slabless slab" will win out.