More BS* + Beer
This episode of the BS* + Beer show features guests Mike Guertin and Jake Bruton talking about Permanent Wood Foundations (PWFs), aka “All-Weather Wood Foundations”—their construction, conditions for use, insulating details, modern-day viability, and more. Interestingly, their first documented use was in 1937. When that building was lifted to be relocated, its wood foundation was in excellent shape; it is still standing today beneath a functioning office building—a testament to the system’s potential durability and longevity.
Having built a number of PWFs over the years, Mike presents an unbiased overview of the assembly method, describing a few of the projects he worked on. Jake shares his experience, saying, “Of the six basements I have lived in, only one of them was dry—and it had a wood foundation.” There’s some talk around the distinction between split-level- and raised-ranch-style houses, which Mike believes are ideal for PWFs. Michael Maines proposes his idea for a kitchen addition on his home; his goal is to keep the project’s embodied carbon as low as possible, which is why he plans to use a concrete-free slab and pressure-treated 6×6 timbers as a perimeter beam. Mason Lord hops on to ask for constructive feedback on his plans for three barn floors that will use a sleeper-system-like foundation—also in an effort to use less concrete. The takeaway? There is an argument to be made in favor of PWFs as an alternative to concrete.
To read more about PWFs, see Mike Guertin’s list of resources:
How to Insulate a Wood Foundation
International Code Council 2018 IRC
Guide to Pressure Treated Wood
Permanent Wood Foundation – HISTORY
Permanent Wood Foundations 2016
Videos:
https://www.youtube.com/watch?v=AwJlyd-g6X8
https://www.youtube.com/watch?v=4IFqbUAi53w
Enjoy the show!
Join us on Thursday, October 22, from 6 to 7:30 p.m. EST, when we—in partnership with Passivhaus Maine (PHME)—will welcome guests Kate Stephenson, Bob Kovacs, and Heather Thompson for a discussion around getting more people into the trades. We will talk about strategies for filling the labor-shortage gap, broadening building-science training opportunities, and recruiting under-represented/marginalized peoples. Those strategies include promoting the multipronged mission of: mitigating climate change, advocating for human and environmental health, and encouraging local investment. With such topics on the table, a rich conversation is sure to ensue.
Guest bios
Kate Stephenson, partner at Helm Construction Solutions, is an experienced leader in the fields of green building, professional education, sustainability, and business management. She’s worked with established and emerging businesses and non-profits to achieve triple bottom line metrics, develop business systems, and plan for a dynamic and resilient future. She has deep experience in post-secondary education, and led the Yestermorrow Design/Build School for over thirteen years. Kate helped to develop and is a facilitator for NESEA’s BuildingEnergy Bottom Lines program. Kate is a Senior Fellow of the Environmental Leadership Program, the Chair of the Montpelier Energy Advisory Committee, and serves on the Boards of the Vermont Council on Rural Development, NESEA and the Studio for High Performance Design and Construction. She is a Passive House U.S. Certified Builder and a QuickBooks Certified ProAdvisor. She holds a MS in Management from Antioch University New England and a BA in Anthropology and Environmental Science from Haverford College.
Bob Kovacs is a senior pre-construction executive with Gilbane Building Company in Atlanta. During his 30-year career, he’s worked both in the field and in the office on residential and commercial construction projects ranging from $5000 bathroom remodels to $4 billion airport expansions. He currently oversees the pre-construction activities for a $300 million division of a $6 billion per year commercial construction firm. Bob has been heavily involved in trying to address the skilled trade shortage, including teaching residential and commercial construction courses at a local technical college, judging the TeamWorks competition for SkillsUSA at both the state and national levels, and serving on the industry advisory board for the construction program at the high school in his hometown. He has written numerous articles for the Journal of Light Construction, has presented at JLC Live, and has been a LEED AP since 2003.
After receiving a Bachelor’s of Art (Sculpture) from UC Santa Cruz in 1995, Heather built custom furniture and established an art gallery. In 2001, Heather Thompson and her family moved to Peaks Island and started a residential construction company. In 2011 Heather became the sole owner of Thompson Johnson Woodworks. Since then, she has doubled the size of the company (now ten remarkably funny, smart, highly-skilled, conscientious people) and has expanded TJW’s area of service from Peaks Island into Greater Portland. Heather and everyone at Thompson Johnson Woodworks is committed to high-performance building through best building, carbon-sequestering practices.
Use this link to register for The BS* + Beer Show
BS* + Beer Book Club
Because the hosts of the BS* + Beer Show all love to read, we thought we would celebrate the authors in our industry by adding a book club to the show every few months. We’ll announce the book, give you a few months to get it and read it, and conclude with a BS* + Beer Show episode where we will invite the author to join us, present, and take questions.
We have selected our second book: Healthy Buildings: How Indoor Spaces Drive Performance and Productivity by Joseph G. Allen and John D. Macomber
We hope you will pick up a copy and join the discussion on Thursday, December 17, from 6 to 7:30 pm.
________________________________________________________________________
-You can contact Kiley Jacques at [email protected].
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
38 Comments
Nope, nope, and more nope. I wouldn't build one, and wouldn't ever dream of buying a house with one. Much rather have a dry stacked fieldstone from the 1700s.
Concrete use in residential construction is so far down the chain of environmental concerns, that its mind blowing to see it mentioned...
Trevor, we discussed many reasons to consider wood foundations other than as a way to mitigate climate change, and both guests and several audience members relayed stories about how well the ones they have seen have held up. I have seen a lot of 1700s and 1800s rubble foundations--some are in good condition, some (including mine) are ok, and some are totally scary. One size doesn't fit all.
Trevor,
What is it about wood foundations that you object to? I do know that they are controversial for a variety of reasons.
My intuition is Michael Maines in on the right track. The most promising way to use wood as a base for a house is some variation on FPSFs. The big problem with basements is where they are (down in the moist earth), not what they are made of. Reduce the depth of the foundation, don't plan on building finished living space well below grade, and a whole range of problems are eliminated or become easier to solve.
Just poking at this:
What if you took it further than at-grade? I've been looking at helical steel anchors, and it seems it might be easier to:
Take a house which is 1-3 feet off the ground on anchors and do perimeter airsealing with dropped CCA plywood (or CCA plywood + EPS) buried inside a raised mound, in order to seal the crawlspace for conditioning...
rather than doing an entire insulated & airsealed platform underneath the whole bottom floor of the house.
Burninate,
Aren't you essentially suggesting a house on piers with a skirt? That always seemed like a retrofit solution to me, leaving you with a poor version of a crawlspace. GBA is full of questions by posters trying that and they seem to face a lot of difficulties, without ending up with very good results.
Although you would be using a sort of hybrid construction, from a code perspective, once you enclose the area under the house, it becomes a crawlspace, with all the attendant requirements - ventilation, access, minimum heights etc.
Forgetting about the architectural implications, using mounded up earth on the outside seems like a odd choice of materials to cover the skirt with. What would be the advantage of doing that?
This article/discussion is very timely for me, as I am finishing up the plans for a ~800sqft ranch in zone 6a that will (if life goes to plan) be converted to a garage in a few years as we build an adjacent permanent house. I have been intending to use a similar foundation detail to what Mike G. showed, a slabless PWF on grade design, and its very nice to get confirmation that experienced builders think merging slabless and PWF is a good idea. In my case it is desirable so when it is time to convert this to a garage I can pull out the subfloor and 4" of foam and re-use it in the new house, replacing it with a gently sloped slab for the garage. I have a couple questions I hope someone can answer:
1) In the SFPA PWF manual, crushed stone for footings and backfill is required to be 1/2" or less (including sand down to 1/16"), but on GBA 3/4" or larger clear crushed stone is frequently recommended because its free draining, largely self compacting, allows radon to move to vents, and reduces surface area which reduces wicking. I'd like to stick to the prescriptive path on this project- are those benefits substantially reduced by using 1/2" clear crushed stone in this application?
2) Due to cost and local availability I'd like to use reclaimed roofing polyiso as the subfloor insulation, which isn't typically recommended due to moisture uptake. To keep it dry I plan to place a layer (or 2) of 6 or 10 mil poly on top of the pea gravel screed layer, then the 4" thick polyiso, then the subfloor. With clear crushed rock under all of this going to a gravity footing drain, this seems very low risk. Is it still a bad idea?
Thanks in advance for your time.
John, I'm not sure why they would require 1/2" crushed stone, other than it's easier to screed flat than 3/4". 3/4" is readily available in most markets that I'm aware of; 1/2" might be harder to find. When it comes to being able to drain freely and allow radon gas to move, the aggregate size is not as important as that the pieces are of similar size. A weird rule of physics is that regardless of the objects' size, when packed together they will leave the same amount of clear space. That's one reason to specify washed stone, so you don't get "fines"--small particles--clogging up the open pores.
You will probably be ok with the polyiso as long as you don't get leaks from above, and your site is not in danger of flooding. I don't think I'd risk it for a client but might on my own house.
Thank you Michael, that's very helpful.
Wisonsin John - did you go through with this design? Did you look into considerations for uplift? Per the PWF Design manual: "Design for uplift and overturning shall be in accordance with applicable loads and load combinations... Anchorage shall be provided to resist that portion of the uplift and overturning that is not resisted by the dead load of the structure"
Another great episode of my favorite TV show.
Question - Can a PWF be used in termite-prone regions (I'm in Colorado, CZ 5B)? Termites were mentioned briefly, and Mike Guertin showed a metal termite shield in his proposed FPSF, but it wasn't specifically addressed in much detail. Do the treatments used to achieve UC 4B spec. provide any termite protection?
Curious why Steve Baczec recommended omitting the exterior foundation insulation. I saw a recent video by Jake Bruton in Build Show Network where he (quite persuasively) described the problems with sub-grade exterior foam, but Mike M specified mineral wool here. Would that be safer? Safe enough? I view both these designs as concrete-free variations of a turn-down slab. I thought some kind of exterior perimeter insulation was necessary for a monolithic slab in all but very mild climates.
Chris Laumer-Giddens wrote a recent article in JLC about his (concrete) slab, which had mineral wool all around and under the slab except directly underneath the thickened edge, where he used high density EPS.
Donald, I too was perplexed by Steve's scoffing at exterior rigid foam on a foundation. He is a really smart guy so I would be curious to hear his reasoning behind it as well.
It does suck to have to cover exterior foam in a cementitious coating... but you would have to do this anyway for a PWF frost wall covered in a SA membrane.
Also, attaching the rigid foam to the wall can be tricky especially with thick foam. Long fasteners designed for concrete are hard to find (Tapcons are only 5" long, max). For PWF, this might be even more challenging despite the substrate being wood as Mike mentioned that all below-grade fasteners must be stainless steel. These may be hard to find.
Finally had a chance to watch this morning... Another Great Episode. Massive Thanks to Mike Guertin for his excellent presentation. You would think he was an expert in PWFs but- nope- just Mike being Mike: smart, prepared and a great educator.
The PWF approach seems extremely simple and has a lot going for it. If you are committed to building a dry basement/foundation (as you should be) then these should really pose no risk at all. I absolutely love the idea of a shallow PWF with great drainage. I've already started some sketches... :-)
Couple of observations:
1. For a PWF, you don't need expensive, high density foam under your footings. The bottom plate of the assembly also doesn't need high density foam. A vertical PT stud that is 30" tall will have an R -Value of around R-40! Add some mineral wool batts and you have a whole-wall R-Value that is quite enormous.
2. I wonder how difficult it would be to keep the PWF frost walls plumb when back filling- especially if you compact the stone. I imagine that it would be best to add stone in equal parts on both side of the stem wall to keep it from sliding out to one side.
3. Regarding the fear of using a permanent wood foundation over concrete. Let's not forget that as great as concrete is, it sometimes has its own issues. Remember this?
https://www.greenbuildingadvisor.com/article/a-glimmer-of-hope-for-connecticut-homeowners-with-crumbling-foundations
4. There is a builder in NH that I believe uses SIPS below grade as part of a PWF system. Here is a company that seems to specialize in them:
https://www.thermapan.com/product/foundation-panels/
It would have been interesting to learn more about them from builders that have used them. (Kiley, perhaps this could be a GBA article?)
Rick,
- I'm having trouble visualizing how you end up with an an R-40 assembly by using a 30" pt stud vertically. How is it incorporated so that the separation between inside and out is a 30" piece of wood?
- I understand why you wouldn't need foam under the pt wood walls, but how do you deal wth point loads? What are the wood equivalents of the concrete pads usually used?
Malcolm,
I wasn't referring to a post and beam type assembly but rather a buried PWF stud wall (perhaps one with some mineral wool between the studs).
If there is no foam under a concrete footing, then the r-value from the bottom of the footing to the bottom of the sill plate, is maybe R-1. I was just stating that this is not the case with a PWF stud wall used in lieu of a concrete thickened slab edge. This is because, unlike concrete, wood has a measurable r-value.
In my example, rather than measuring the R Value from side to side, as we often do, I was measuring it from bottom to top. I was assuming that the PWF studs have an R value of 1.4/ inch like any other lumber. So a 30" vertical stud in the buried wall would have an r value of 1.4 x 30". Adding insulation between the studs obviously increase the whole-wall R-Value of this wall- especially when measured from bottom to top (which is all that is relevant when contemplating the need for foam footings).
I would still want rigid foam on both sides of this stud wall (for side-to side R-Value). But, given that wood has a meaningful R-value (unlike concrete), you really don't need foam under footing like you do with concrete.
Based upon the video, it sounds like PWF lumber still uses the old Arsenic- based preservative. I assume this will not affect r-value. I wonder though if modern conventional PT lumber has a lower r-value/inch given its copper content?
Rick,
Many houses, especially those with one storey, still have p0int loads to pick up from interior posts, or concentrated exterior loads. It's quite common for me to get back my drawing from an engineer with areas where the footings has been widened from the standard 18". The loads don't disappear simply because the material used changes from concrete to wood.
The R value of any assembly is measured from inside to out. It simply makes no sense to measure the R value of a wall vertically. You could do this with any wall in a house and come up with very high numbers. It doesn't give you any useful information.
Malcolm,
You wrote: "The R Value of an assembly is measured from inside to out".
I couldn't agree more. The exterior (bottom) of the bottom plate of a buried stud wall is outside. The same is true of the bottoms of thickened slab edges/footings. If they were not exterior surfaces, then people wouldn't feel the need to insulate them in cold climates.
Given that the bottom of the bottom plate of the PWF stud wall is outside, the question for me was, does this constitute a weakness in the thermal layer, like concrete? The answer is no, because the stud wall itself has some meaningful R-value unlike concrete.
Im not sure why you are brining up point loads. As I understand it, the PWF studs serve only as a stem wall. The crushed stone beneath the stem wall serves as the footing. I presume you could widen it to as much as you like and still use the PWF stem wall?
Rick,
You don't see calculations of the the value of an assembly of pieces of lumber which are is exposed to the outside on 0ne side and the conditioned interior on the other by adding up the R-value over it's length, because it can't be done. The fact that the piece of wood is 30" or 300" high is immaterial, and it's R value over it's length yields no useful data. That goes to your point that you can end up with "whole wall R-value that is quite enormous" You really can't. Not without thickening the wall - just as you have to with the wood walls above.
I was musing about point loads because dealing with them isn't just as simple as adding more compacted stone (see page 22 in this link) https://www.awc.org/pdf/codes-standards/publications/pwf/AWC-PWF2007-Commentary-ViewOnly-0903.pdf
What I wondered was whether a hybrid foundation might not make more sense, using limited amounts of concrete for pads, or even the whole footings?
I'm not really interested in measuring the r value. My point is simply that there is *some* r value unlike concrete. (Given the nature of the assembly, 'u factor' is probably a more accurate term). And therefore no foam under the wall would be needed. I threw out numbers simply to illustrate my point.
It's a moot point anyway as adding foam under a PWF wall would be akin to adding foam between a stem wall and footing which is obviously absurd. I only mentioned it because I feel that avoiding expensive, high density rigid foam may be a positive attribute of PWF when compared to concrete foundations.
I'd like to know where to find CCA lumber in the Northeast. Looks like BB&S in Rhode Island treats some marine products with CCA - pilings and 3x8, 3x10, and 8x8. Menards seems to stock CCA 2x lumber, but the closest one is in Ohio...
I'm not sure I would try a PWF or even a pole barn without CCA PT. I've heard too many poor reviews of MCA and ACQ.
Tim,
Neither MCA or ACQ are rated for ground contact.
I do not believe this to be true, at least for "normal" ground contact, UC4A. The "ground contact" PT at my local suppliers in VT are MCA treated.
Tim,
Sorry I used the wrong term. MCA is rated for ground contact, but not burial.
I did a lot of work for a resort over a two decade period and used all three types of PT lumber as posts and landscaping, so it became my own wing-nut testing lab. All of the CCA still seems fine, but the ACQ almost all shows deterioration after being in place for a much shorter period of time. The MCA is too recent for me to know how it will do, but I don't have much faith. Sure a lot nicer to work with. Much dryer, and not having to worry about it eating fasteners is a plus.
Tim,
You need CCA that is treated to .60 and rated for foundation grade specifications. I believe that spec. also includes the lumber species and grade. The 6 x 6 posts I have seen used for pole barns is rated for that specification, and mostly, if not always does not include the heart of the tree, which often causes extreme bowing, twisting, and splitting. Southern Yellow Pine is commonly used for this grade. Typically, the material features fine and closely spaced growth rings which provide excellent stability. I am not sure if Menards CCA meets the foundation grade specs.
Also, the term "ground contact" comes with conditions, and it does not mean that it is okay to bury it or use it for foundations.
A lot of people say that a wood foundation is undesirable because treated lumber rots in 15-20 years. That seems to be true with various species and treatments. But foundation grade treated lumber has an indefinite lifespan when buried, and testing has proven the lifespan to be at least 80 years, with no deterioration whatsoever.
The Menards stuff is actually 0.60 CCA / UC4B foundation grade lumber.
Answering my own question, I've found that my semi local supplier Allen Lumber in Montpelier is a Lonza Wolmanized wood dealer and I would guess they could order me some of the "industrial" UC4B 0.60 CCA lumber that Lonza makes. I haven't been offered anything other that just 2x "PT" at the local lumberyard which is MCA treated.
Couple more questions:
1) When you're essentially burying a framed PWF ( as with FPSF / slab on grade), would you sheath both sides? Insulate the cavity? Mike Guertin's 'slab on grade infill apartment' slide shows something in there (looks like EPS, but hard to say), but I don't see any sheathing on the inside face.
2) Both Mikes' drawings show the sub-floor going in before the above-grade walls and roof. How would you keep that floor dry during construction? Maybe a sacrificial sheet of poly that you cut out afterwards, except for the bit that's under the wall plate? I recall that Steve Demetrick, Jake Bruton and Randy Williams built the walls first (on concrete frost walls), then put in a concrete-free slab on grade (or basement) floor later, after the shell was dried in. Maybe you could do something similar with PWF frost walls or FPSF?
Thanks, Don Christensen
Don, I'll post my proposed section for others to see. It would be easy to just run a rip of subfloor around the perimeter and fill in the field after the roof is on. I've done that on many projects for various reasons.
Well yeah, why didn't I think of that? Thanks Michael.
Don Christensen
Has anyone designed/built a PWF crawl space on high PVR and expansive soils? I'm working on a large TX house (10k sf) that the Owner's kids are asking if we could use "less" concrete. Also, termite comes in play. Most houses I researched with PWFs are on the small side. Any thoughts?
“[Deleted]”
Any carbon reduction effort should be compared to alternatives. For example, say a house uses 25 yards @ 400 lbs CO2 per yard. Donate $500 to a good carbon reduction cause and you will be doing more for the environment than using a wood foundation.
Great episode and discussion. Learned a lot. One item that felt a bit brushed off was the use of harmful elements in the treating, the indoor air quality and any possible future contamination of the soils, water table, etc. Does anyone know of any studies that have been performed in this regard? Have we considered the effects of mining copper vs the ghg emissions of concrete?
And how would one deal with the indoor air quality, other than simply suggesting keeping a positive air pressure in the house at all times?
Many thanks for these insightful shows.
Jason, good questions. As far as I know, the current, commonly used formulations for pressure treated lumber do not impose any IAQ or soil contamination risks. I have heard that the humic acid in soil could leach arsenic from older formulations (CCA) but I believe the newer formulation (ACQ, etc.) are just micronized copper. Not that copper is benign, but it's pretty common and not airborne, and I don't believe there are VOCs other than what is naturally in the wood.
I have not studied the carbon emissions associated with mining and smelting the copper used in pressure treated wood. I imagine it's comparable in some ways to creating Portland cement or steel, but without the chemical reaction in Portland cement that releases almost as much CO2 as the rest of the process combined.
With indoor air quality, using low-VOC materials and proper ventilation takes care of most issues that aren't moisture-driven.
Perhaps someone with deeper knowledge of pressure treated lumber will weigh in. I'll add researching it to my to-do list. And maybe we should consider a BS + Beer about rot-resistant lumber.
Jason,
As Michael said, it depends on what type of treatment was used. My understanding is that CCA was never prohibited in Canada, it just lost sufficient market share that it largely disappeared. There is definitely cause for concern using arsenic-based products like it, but the risk from copper-based woods like ACQ and MPS seems at best minimal - especially as where they are being used is outside the air-barrier.
You may find this article useful: https://www.researchgate.net/publication/248945037_Human_Health_Risk_Evaluation_of_ACQ-Treated_Wood
Appreciate the quick responses. I'll have to dig deeper into the use of CCA in Canada as I thought it had been prohibited. I'm nervous to use any of the new treatment systems in a buried, structural foundation - I haven't been impressed with their ability to fend off rot in exposed + ground contact situations.
I'm not as concerned about touch transfer (outside of workers/construction phase) as it would all be encapsulated anyhow.
Jason,
A quick search seems to show it wasn't entirely banned, but just effectively taken out of residential use:
"Environment Canada and Health Canada, specifically the Pesticide Management Regulatory Agency (PMRA), decided to institute a labelling program for each piece of treated lumber. Fortunately, in April 2002, the manufacturers of CCA informed the PMRA that they would voluntarily cancel their Canadian registrations for residential uses of CCA-treated wood. Since 2003, wood for playground equipment, decks, fences, walkways and landscape timbers cannot be treated with CCA."
It was nasty stuff, but very long-lasting. Like you I don't have much faith in longevity of the newer formulations.
For those that have experience such as Mike G.... what is the key to getting the gravel footings level? How would you set a first time PWF builder up for success? Joe states that this is no easy task... but suspect Mike has a trick up his sleeve...
"When I did my first wood basement I went with concrete footings. Made things a lot easier. You have no idea how difficult it is to get a crushed stone drainage pad level. For real level. Not approximately level. Level. Level enough to build a building on top of."
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-020-wood-foundations-picasso-does-foundations
Davin, thanks so much for linking to that article! Figure 3 showing the wood slab on grade looks amazingly simple (other than aforementioned leveling). What am I missing? This does not seem to be excavated even as much as a concrete frost protected slab. I'm trying to figure out uplift/overturning concerns as well as whether this detail is really a serious proposal.
Log in or create an account to post a comment.
Sign up Log in