More Best Practices
In June 2019, I began collaborating on a project the team called “The Concreteless Slab-on-Grade Home.” I pitched the project to the editors at Fine Homebuilding and GBA for potential coverage. It did get featured in Another Take on a Concrete-Free Slab. This year marks the five-year anniversary of that project’s beginning stages. I thought it would be valuable to revisit the house, discuss what worked/what I would change, and share an unexpected series of events that could have been a major problem.
A quick review
We started construction of this 2000-sq.-ft. custom home in July 2019. I worked primarily as a consultant, though I did put in a couple hundred hours of labor. The main goal was to keep energy consumption low through high insulation values and good airtightness numbers. Comfort and indoor air quality were also high on the priorities list.
For various reasons, we decided against a concrete slab. Instead, we built a wood-floor system installed on well-compacted soil—a method I learned about from Steve Baczek and Jake Bruton. There are two 2-in. layers of EPS foam below two layers of glued-and-screwed Advantech subfloor. A vapor retarder sits between the foam layers.
The above-grade wood-framed walls are on ICF stem walls that extend to the 5-ft. frost depth. The structure was dried-in before the floor system was built; the 2×6 exterior walls have 2 in. of mineral wool rigid insulation over Zip Sheathing with a vented rainscreen. Fiberglass batts fill the cavities (R-21).
The continuous insulation and cavity insulation hybrid roof ratios did not meet the requirements to move to a class III vapor retarder, so we installed Pro Clima’s Intello, detailed for…
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9 Comments
Thank you for the update and reflection. You're helping the rest of us stay ahead of unforeseen water and moisture issues.
Regarding the concrete-free floor, have you installed one over heave-prone soils? In much of Colorado, clay/bentonite soil can expand so much that slab-on-grade floors need special detailing such as floating walls with 3" vertical movement allowed at the sillplate. I've been hesitant to design a plywood slab-on-grade over expansive soils without getting more feedback from those who have designed and built them successfully.
Oldaltnew,
I have not installed this system on expansive soils, though how we built this specific home using a frost depth, stem wall made of ICF, then filling the interior with clean sand, I don't think there would be an issue if we had been in an area with expansive soil issues. FYI, here's a really good explanation of why basements can have issues with expansive soils: https://watersmiths.com/2023/01/26/buckling-basement-walls/
No matter where this foundation system is built, you have to understand the soil conditions. It's not worth the risk of a call back where you are pulling interior walls apart and removing the slab system to access the soils under the slab.
Thanks for the question,
Randy
Great info Randy! I think you're being tough on yourself - If I had plumbing fittings fail I'd be more likely to just throw up my hands. We all rely on things like that to just work. I suppose moving to a manifold system eliminates the low percentage risk, but it is often more costly, and aren't there still connections that could leak? (albeit in a better location).
Also - I would think in that cold dry climate an ERV would be a better choice for keeping some moisture inside?
Thanks Sean, you are correct, the manifold system is more expensive but knowing what I know now, I'd have paid the difference. Hindsight is 20/20, right?
You are absolutely correct with the ERV, and I've been specifying them in current projects. Prior to that, I wasn't confident enough for their use in my climate. I'm happy we had the HRV in this home, I think it helped reduce the moisture problems faster than if we had an installed an ERV.
Randy,
Thanks for the write up. It sounds like a very successful build.
One thing though: I'm just not sure you can draw enough negative inferences about pex fittings from one poor plumber's work to merit doing away with them. Pex has been used in a good 95% of all housing here in BC for several decades and you simply don't hear about failures in pressure tested systems. I've plumbed about 30 buildings myself - always without incident. Something went wrong., and it's probably worth figuring out what before condemning the fittings.
Hi Malcolm,
You are right, over the nearly 30 years I've been in the industry, I've only seen a small handful of Pex failures with the majority of them being on this project. My biggest issue was neither the plumbing contractor nor the manufacturer of the fittings is being forthcoming on what caused the issue, even though they've had the failed fittings from the first water leak for more than two years now. The problem could have been as simple as a worn-out crimping tool. The GC on this project had three other homes with similar issues, all built roughly the same time. He's since switched plumbing contractors. Everything is now in the hands of insurance companies; we may never know.
Randy
Thanks for the updates, Randy. Half the energy use--that's amazing!
Amazing project. If I do another build, a concrete-free slab on grade is on my list. I'm curious if you know the kind of PEX fittings used (I see in a comment a reference to crimp tool). Every plumber I know swears by crimp fittings and have never seen a leak.
It was a crimp style fitting used, which we also have not had problems with in the past. The issue could have been with the fitting or maybe the crimping tool was worn and not crimping properly.
As far as the concrete-free slab goes, I would also like to do another.
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