Concrete Free Foundation Using Wood at or Below Grade
I have read various articles on GBA and elsewhere regarding Concrete Free Foundations. As an example, the following great article by Michael Maines here:
https://www.greenbuildingadvisor.com/article/minimizing-concrete-in-a-slab-on-grade-home
highlights a concrete free slab on grade but maintains a CIP stem wall foundation to support the primary structure.
I have conceptualized a completely concrete free foundation using Mass Plywood and a structural beam all supported by helical piles. This would be no problem if it was above grade 18″ or more to allow for a crawl space; however, I want to eliminate the crawl space and even more so reduce the imposed lateral loads on the piles that results from having them 18″ above grade.
In knowing that code requires a minimum of 8″ clearance from soil and non-treated wood, how do code officials view the 6″ clearance in Michael’s article for the SOG example? And further, how would my detail below be perceived? Technically, my proposed approach, consisting of both treated and non-treated wood, doesn’t fall into the Permanent Wood Foundation code section. Are there any precedents that I can point to for this or a similar approach.
Thanks in advance, Eric
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Considering your project requirements, I personally would be more comfortable placing cellular glass insulation material below grade, rather than wood fiberboard or Rockwool. It also drains well after compacting, so you may not need as much gravel as shown in your detail. I only know of European brands (such as Misapor, Glavel). It looks like this -> https://www.youtube.com/watch?v=RQBe-7EymAI
You could try Foamglas, but my guess it's going to be more expensive. It's usually used to solve thermal bridge details at doors and windows, even though they sometimes advertise it as wall and slab covering (due to cost).
Pavatex and Blue Ridge are other brands of wood fibre board, besides Gutex. The US market is only just waking up to its potential, with pilot projects here and there.
Depending on where you are in the project, you could look at the potential of hemp blocks or hempcrete, as an alternative for your insulation material in the exterior wall. --> https://www.hempitecture.com/hempcrete
I wanted to share my thoughts, as I am going through the same conundrums.
I settled on EPS for below grade insulation, due to local trade standards and its durability testing history.
Thank you for turning me onto Hempitecture. I will definitely look into this further.
Eric, you have an interesting idea. If you haven't seen it already, Josh Salinger posted a somewhat similar idea in the comment section to my article, and we had him and a few others on an early episode of the BS + Beer Show to discuss it further: https://youtu.be/iGXoAN4znTI.
Code compliance could be complicated, since these are new ideas. On my project I spec'd 6" to 12" from grade to wood; 6" is the IRC requirement (https://codes.iccsafe.org/content/irc2015?site?type=public/chapter-3-building-planning#IRC2015_Pt03_Ch03_SecR317.1). When I have designed homes and additions on piers I follow the code requirement of 18" for access, though pressure-treated beams are allowed to extend into the 18" space.
To help get through code approval, I would call your design a concrete-free slab on grade. Wood fiber insulation solves a lot of problems but it should not be placed in contact with the soil; I would maintain at least 6" clearance as you would to other untreated wood products. Under the floor system it could work, as it would be protected from moisture, but mineral wool would be safer.
Thanks for the BS+Beer link, I hadn't come across that discussion yet but did see Josh's detail in your articles comments. Just finished watching the whole show and know I need to sink up with Josh Salinger for sure. I think the two of us together can convince Steve Baczek as Josh and I are conceptualizing nearly the same thing! I'll try calling him at Birdsmouth Construction or maybe you wouldn't mind connecting us. I am also a certified installer of GoliathTech Helical piles here in NorCal and can certainly add some value to Josh on that front.
As far as Gutex under slab: They make a product called Thermosafe WD that traditionally is shown above a Concrete SOG; my thought was to first use Rockwool Comfort Board above the Gravel/Pea Gravel sub base, then a minimum 6mil Vapor Barrier on top of the Comfort Board and then the Gutex Thermosafe WD on top of that. A safer solution would be to just use all Comfort Board below slab and near grade level as you suggest. Lot's to unpack here. Thanks again.
Oh, and has Mike Guertin published his article/ideas about Pressure Treated Stem Wall Foundation mentioned in the BS+Beer episode?
Eric, you can reach Josh through his website: https://birdsmouthpdx.com/who-we-are/contact-us.html.
I've been interested in the Gutex slab product, but I would only use it where it was sure to remain dry. I have not tried to order Gutex but I know it would be easier to get their standard products than their specialty products. Probably worth talking with a 475 rep.
I don't believe Mike Guertin has published his ideas, but he posted about an old wood foundation on his Instagram page: https://www.instagram.com/p/B4FWs0yghBE/. If that link doesn't work, find his post from October 2019: https://www.instagram.com/mike_guertin/