Through early December, we stepped into the house and into a hole. It was like being a child in a giant’s house, standing on tip-toe or jumping up to see out, with ceilings extra high.

The extra depth was there because we’ve chosen to install a 13-inch insulated subfloor system directly on the ground, over a six-inch layer of gravel. Unlike most upper Midwest construction, where houses without basements are built on a concrete slab, our house rests on a concrete perimeter foundation with the insulated subfloor nested inside.

This photo shows the area the insulated subfloor fills—roughly from the gravel to the wood band above the blue piece of insulation on the foundation.

This floor system makes sense for us because we wanted a natural wood floor and minimal use of concrete. At our architect’s recommendation, the plan does not include a heat sink (material, like concrete, that captures and holds heat). Because the house will be sealed and insulated so well, investing in a heat sink would be overkill and an unnecessary use of materials and money. In the project overview Christi explains this further, including comparison to insulated concrete floors in Passive House design.

This is how the system is designed.

We didn’t install the floor earlier in the project because it has to stay dry. It’s part of the tightly sealed building envelope that will keep us warm in winter and cool in summer, and water between layers or above the waterproof sheeting layer of this floor would compromise its effectiveness. So unlike conventional building, we waited until the windows and roof were in place.

In the weeks leading up to December 5th I wondered every time I walked on that gravel how it could be leveled to within a quarter inch, as Jeff often said it had to be.

When I arrived early that morning, Jeff was ready. Using a length of standard floor truss, Jeff, Troy and JR had fashioned a wooden screed to use as a guide for leveling the gravel. In the next photo the long board against the central support posts runs the length of the house, the same height as the concrete ledges around the foundation perimeter.

The screed rests on the long board and the concrete foundation ledges and can be moved the length of the house. The blue chalk line on the wall behind was used as a guide for checking levels with laser instruments. Once tamped, the finishing was leveled by hand, together with the screed.

Here, Jeff and Troy remove Tyvek from the largest window opening which had been left uninstalled so we’d have a place to bring gravel in.

Gravel from Sorum quarry, just a few miles away, was dumped at the south side of the house near the window opening early on December 5. It was brought in at the last minute because it would have frozen solid overnight. Even at the last-minute, sitting on the sunny south-side of the house, you can see clumps of frozen rock in the pile.

Bundled against the cold, Stan moves gravel from the pile to wheelbarrows just inside the house.

Three workers from Koball’s, our foundation contractors, were on hand to move, tamp and level the gravel at Jeff’s direction.

Gravel work started in the northwest corner of the house, where the main bathroom will be—a tricky area to ‘get right’ around the previously-installed plumbing/drain system.

Then the southwest corner was filled, tamped, and leveled.

Once Jeff was satisfied with leveling in each section, the first of three 4-inch layers of 4′ x 8′ expanded polystyrene (EPS) panels was installed. EPS is a closed-cell, lightweight, rigid foam panel with extremely low thermal conductivity, amazing strength and shock absorption properties. These three layers of EPS provide a combined R-50 level of insulation under the wood substrate.

Fill and leveling continued through the rest of the house. It was COLD, even for people working hard.

A simple sealing tape was used to hold the panels in place as they were laid in. This tape has no insulating purpose, though contributes oh so slightly to air sealing.

The entire south side was filled first.

By the end of the day, the first layer of panels was in place.

The edges were then sealed. Overnight, the panels on the north side buckled slightly and parts had to be removed, reset and sealed. Unfortunately, the spray foam used to seal and insulate the edges would not fully set up in the cold, so Jeff eventually added a construction furnace.

We weren’t able to be on site to photograph installation of the second and third layers the next week, but here you can see the second layer has been added (along with the furnace).

A continuous layer of Tu-Tuff high-density plastic sheeting was added between the second and third layers of EPS insulation. Siga tape was used to secure the Tu-Tuff to the walls, as well as sealing all seams, to provide a continuous wall-to-wall air and moisture barrier, a critical part of the envelope that will eventually wrap around the entire house.

Two half-inch layers of plywood were added in a staggered manner over the EPS to provide a 1” substrate for the final flooring.

Here you can see the top foam and plywood layers, as well as the way the air barrier is sealed around plumbing with Siga tape.

And the floor as it looks now, up to a natural level! In this photo also notice the big living room window…installed and fantastically bright.