Concrete Free Slab on Grade House – Five Years Later

This post first appeared on the Green Building Advisor website.

In June of 2019, we began a project we called “The Concreteless Slab on Grade Home”.  Kiley Jacques, (now the editor of Green Building Advisor), covered the project in an article she wrote back in early 2020.   Another Take on a Concrete-Free Slab – GreenBuildingAdvisor   I wrote a piece on the project for this blog, Construction Design-Concrete-less Slab on Grade – Northern Built.  This year is the Five-year anniversary of the start of that project.  I thought it would be a good time to revisit, discuss what worked, what I would change, and talk about a very unexpected series of events that could have been a major problem.

A Quick Review

We started construction of this custom home back in July of 2019.  I worked primarily as a consultant, though I did have a couple hundred hours of labor I put into the project.  The main goal of the home was to reduce energy consumption by increased insulation values and an increased level of tightness.  Comfort and indoor air quality were also high on the list.

The home is just under 2,000 square feet.  We eliminated the concrete slab and instead, built a wood floor system installed directly on well compacted earth (an idea I stole from Steve Baczek and Jake Bruton, who had just finished their own concrete-free slab home). Below two layers of Advantech subfloor glued and screwed together are two, 2” layers of EPS foam with a vapor retarder between the foam layers.  The above grade, wood framed walls are constructed on an ICF stem wall that extends to the frost depth (5-foot in our climate).  The 2×6 exterior walls (we had the structure dried-in before we built the floor system) have 2” of rigid mineral wool (Rockwool Comfortboard 80) over Zip Sheathing with a vented rainscreen using 1×4 boards.  The wall cavity is insulated using R-21 fiberglass batts.  The exterior CI and interior cavity insulation ratios did not meet the requirements for us to move to a class III vapor retarder, so we installed Pro Clima’s Intello, detailed for both air and vapor control.

The attic is insulated to R-60 using blown fiberglass.  A plenum truss was utilized to keep all our ductwork inside the building envelope and the ceiling is strapped using 2x material, also keeping the majority of the electrical inside.

The home has a 2-ton cold climate air source heat pump using a central ducted distribution system.  Seeing as the location of the home is in a very cold climate (around 10,000 heating degree days and a 99% heating design temperature of -22°F), we paired the heat pump with an electric plenum heater and also installed a thermostatically controlled propane sealed combustion fireplace as an emergency source of heat.  The home also features balanced mechanical ventilation in the form of a heat recovery ventilator (HRV).

The Problems

The air source heat pump is the primary heating system for this home.  This was the first time I had specified this type of heating as a primary system.  I’ve used ASHPs as part of a dual fuel system, pairing with a central ducted gas forced air furnace, or for use during shoulder month heating with a heated slab as the primary heat source.  As you can see in the photos, we were surprised by the amount of condensate that comes off the outdoor condenser.

We located the condenser on the north side of the home, away from outdoor spaces that were planned to be used during mild weather, unfortunately there is no wintertime sun helping to control the ice buildup that occurs.  We realized this may be a problem after the first year and decided to raise the unit higher off the ground.  Lesson one, try to locate ASHP condensers on the south and/or west sides of a home and be aware that there will be icing in a very cold climate.

A second, more serious issue that occurred was with the plumbing supply lines.  The plumber installed PEX piping throughout the home, the system was supposedly pressure tested and passed.  We’ve now had three different plumbing leaks, all from failed fittings.  All were discovered because of a wet floor, or the homeowner was able to hear a drip inside a wall.  One of the leaks was discovered just before winter, causing a high humidity event inside the home.  We had excessive condensation and frost on triple paned windows.  We are unsure how much water moved into the floor system, and if there was a large quantity, where it went.  The home seems to have recovered, no musty smell, no visible mold, and no shifting or cracked drywall.  Humidity levels are normal.  To this point, the floor system seems to be unaffected by the water leaks, but it is something we are monitoring.

Two things I would change if I were building this floor system again.  One, homerun all the plumbing lines to a manifold, no in-wall splices, couplings or tees.  Second, I would install a water monitoring system that checks for leaks.  The homeowner has installed a Flo by Moen water pressure monitoring system since we started experiencing the water leaks.

What caused the fitting to fail?  We are not sure; the fittings were sent in to be tested.  Both the manufacturer and plumbing contractor, so far, have been tight lipped about the cause, even though we asked for test results.

What I, and the Homeowner Are Impressed With

1. How comfortable the floor system is to walk on.

I’ve built and worked on many concrete slab-on-grade homes.  If the floor isn’t heated, it’s cool to walk on, and, because of the hard concrete, they can be uncomfortable to stand on for any length of time.  Even though this floor is not heated, the homeowner claims the floor is never cold, it can be cool near the exterior doors, but never uncomfortable.  Something I noticed when working at the home, I had my laser setup, it would slightly bounce when someone walked by.  There’s a little give making the floor more comfortable if you’re standing for any length of time.

2. The home is quiet.

The first summer the homeowner moved in, a strong storm moved through uprooting several trees in their yard.  The homeowners slept through it.  This is a plus and a negative aspect of the home.  The triple paned windows, extra insulation and air sealing make it hard to hear anything going on outside.  They may want to invest in a weather radio.

3. The home is efficient.

The homeowner has been tracking energy use since they moved in.  The average Minnesota home uses 102 MMbtu worth the energy per heating season.  This home has averaged 52.6 MMbtu, about half the Minnesota average.

4. So far, the home has been durable and resilient.

All the interior walls were built after the roof was complete and the flooring system constructed.  So far, we have had two minor drywall cracks, even with the plumbing leaks.  Once the leaks dried, we’ve been able to maintain a constant humidity level.

Would I use this system again.  I would, with the few changes I mentioned earlier.  I’ve also got a few variations of the assembly, one using a frost protected shallow foundation system and a second implementing an in-floor heating system using WarmBoard subfloor.  Both are waiting for a customer.

I’m aware of a few other builders that have used or adopted this concrete free slab on grade system.  Can any of those builders share their experiences?