Installing a unique insulated floor system

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.

DSC04952

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.

floor design

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.

DSC04955

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.

DSC04958

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.

DSC04970

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.

DSC04964

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

DSC04972

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

DSC04998

DSC04987

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.

DSC04994

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

DSC05014

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.

DSC05019

DSC05020

DSC05024

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

DSC05030

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.

DSC05032

The entire south side was filled first.

DSC05037

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

DSC05040

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.

DSC05045

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).

102_0989

102_0990

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.

DSC05048

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

DSC05051

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.

DSC05046

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.

102_1007

Comments
  1. Renee Bergstrom

    Wow. You two are to be commended for thoughtful planning and progress recording. You must be so pleased with your contribution to sustainability and education for others who might wish to follow your lead.

Location

Lanesboro, Minnesota
Climate Zone 6 (cold/moist)
Latitude: 43° 44' 18'' N
Longitude: 91° 54' 48'' W

House Size

Net Treated Floor Area: 1,514 SF
Gross Square Footage (House only): 2,210 SF

Building Envelope

Roof: R-99
Wall: R-61
Ground: R-53

Windows & Doors

Glazing: U-0.10 BTU / hour / sq. ft.
Solar Heat Gain Coefficient (SHGC): 0.48”
Frame: U-0.19 BTU / hour / sq. ft.

Modeled Performance

Specific Primary Energy Demand (Source Energy Demand): 12.1 kBTU / sq. ft. / year

Specific Space Heat Demand: 7.0 kBTU/sq. ft. / year

Peak Heating Load: 7,047 BTU / hour

Space Cooling Demand: 0.44 kBTU / sq. ft. / year

Peak Cooling Load: 3,625 BTU / hour

Pressure Test Goal: Whole House Air Changes Per Hour (ACH) = 0.4 ACH 50

×

Post a comment

Leave a Reply

Your email address will not be published.

×

Subscribe to Blog via Email

Enter your email address to subscribe to this blog and receive notifications of new posts by email.