A couple of years ago, we (Steve Baczek, architect, and Steve Demetrick, builder) joined forces on Rhode Island’s first certified Passive House. As successful as that affordable project was, we agreed that the best aspect of the project was the ongoing dialogue that started between the two of us. When we were recently asked to design and build another Passive House for a client, we began by questioning everything we did on the first project, from the foundation up—both successes and challenges.

“Do We Need The Slab?”

One of the first things we looked at was the basement-floor assembly, which in most Passive Houses is a heavily insulated, concrete-slab system. Typically, the slab is covered with carpet, laminated flooring, or tile. But this particular client wanted a hardwood floor in the finished basement, so the assembly below that floor needed closer scrutiny. With a concrete slab, there is usually some type of sleeper system that supports floor sheathing, and then the hardwood floor is installed on top of that. But we’ve seen these assemblies grow in section, which can restrict headroom in the basement, and all those built-up layers can equate to a lot of extra cost.

Hardwood floors are also very sensitive to moisture, and a concrete slab adds a tremendous amount of moisture that needs to dry off (a conservative estimate would be at least 500 gallons of water in the concrete for a 4-inch slab of this size). The fact that a Passive House is an extremely airtight structure would only complicate the drying process. In the face of all these challenges, we finally found ourselves asking, “Do we really need the slab?”

In the building industry, installing a basement slab is accepted as standard procedure. But in some projects, a slab can pose more challenges than benefits, especially in airtight homes. In fact, the only real advantage we could see was the thermal storage that a slab can provide. But a Passive House operates at such low heating and cooling loads that even that benefit would be negligible.

The basement of this extremely tight house was to be finished with a hardwood floor, so pouring a concrete slab posed many challenges, including the introduction of hundreds of gallons of water that would have to dry off as the concrete cured. Instead, the design team decided to put down two layers of OSB underlayment subflooring. The insulated base below the subfloor would be the same as for a slab, including a 4-mil cross-laminated polyethylene sheet that doubled as a moisture barrier and vapor barrier. The two overlapped layers of OSB would sit on top of the poly without being fastened to the structure below.
Tim Healey The basement of this extremely tight house was to be finished with a hardwood floor, so pouring a concrete slab posed many challenges, including the introduction of hundreds of gallons of water that would have to dry off as the concrete cured. Instead, the design team decided to put down two layers of OSB underlayment subflooring. The insulated base below the subfloor would be the same as for a slab, including a 4-mil cross-laminated polyethylene sheet that doubled as a moisture barrier and vapor barrier. The two overlapped layers of OSB would sit on top of the poly without being fastened to the structure below.

Designing Without A Slab

As our discussion continued, we speculated about what a basement floor system might look like if we eliminated the concrete slab. We realized that everything we would normally install below a slab would be essentially the same (see Floating a Basement Floor, above). We’d need the same stone base and perimeter drain, and we would install the same Type IX EPS foam insulation, which would satisfy our thermal requirements. And with a compressive rating of 25 psi, the rigid insulation would offer plenty of support for the wood flooring and the basement partitions.

The most important detail that would carry over from a sub-slab installation was the polyethylene vapor barrier that’s installed over the insulation to control ground moisture and vapor movement. The polyethylene sheet would also provide the necessary air barrier for the building envelope.

For a subfloor, we decided on using two layers of 3/4-inch AdvanTech OSB layered in a staggered pattern to ensure that joints would not align between the layers. Basically we would create a 1 1/2-inch OSB “raft” that would float on top of the assembly and provide a structure for installing the hardwood floor.

After compacting and raking the stone base perfectly flat, the crew put down 4-inch-thick insulation on top of the perimeter footing.
After compacting and raking the stone base perfectly flat, the crew put down 4-inch-thick insulation on top of the perimeter footing.

Stone and Foam

We began by installing the perimeter drain and radon vent in a 6-inch-deep bed of 3/4-inch stone, all inside the concrete footing. Over this, we added a 2-inch layer of pea stone. The smaller stone was easier to rake and level out. After an initial raking, the stone was compacted and checked for the proper elevation using a pre-set laser level and a story pole with a wide base. The final level of the stone was 2 inches below the top of the footing.

The rigid EPS foam insulation came next, and we started by fitting the 4-inch pieces around the perimeter of the basement on top of the concrete footing. We filled in the field of the basement floor with 6-inch EPS, staggering the ends of the 4-by-8-foot sheets to provide more uniform support for the layers of OSB.

There were two rows of columns in the basement that required some puzzle fitting of the EPS foam. We cut slots in the foam so that we could fit full sheets around the columns. We then cut rectangular plugs to fill in the slots, but left plenty of room to shoot in expanding foam. Straight cuts in the 6-inch foam (such as for the column slots) were done easily with a circular saw or reciprocating saw. For long rips, we cut the sheets on a table saw, flipping them to cut through from both sides. We made the rip cuts as precise as possible for tight joints between the sheets.

To fit the sheets of insulation around the numerous plumbing pipes and support columns, their locations were plotted on the sheets and corresponding holes were cut out.
To fit the sheets of insulation around the numerous plumbing pipes and support columns, their locations were plotted on the sheets and corresponding holes were cut out.

The finished basement would have two bathrooms, as well as a laundry room with a floor drain and a drain for the washing machine, so there were numerous plumbing pipes that we had to cut around for the installation of the EPS foam. One crew member worked ahead of the installers, taking measurements and plotting out the hole locations and sizes. Those coordinates were transferred to a sheet of foam and holes were cored out using the proper diameter hole saw. One of the tub-drain locations had not been finalized, so we simply cut out a large void (about 12 inches square) to be filled in later. One of the beauties of this system is that a hole like this can be easily filled and blended in with the rest of the system one layer at a time.

The level of the insulation was checked using a story pole and a laser level.
The level of the insulation was checked using a story pole and a laser level.
When all the insulation was in, low-expansion foam was injected in all the voids around the pipes and columns.
When all the insulation was in, low-expansion foam was injected in all the voids around the pipes and columns.

The holes were oversized slightly to provide room for the nozzle on the foam gun. The installers then fit the cored pieces into place. As each sheet of rigid insulation was installed, the elevation of the top of the foam was checked with the same laser level and story pole used before, with a second reference line for the foam. Wherever there were spots where the top plane of a foam sheet didn’t align with the rest of the floor, the crew lifted out the problem piece and tweaked the level of the pea stone below until the plane of the foam was perfect.

Blocks of insulation that had been removed for installing the sheets were foamed in as well.
Blocks of insulation that had been removed for installing the sheets were foamed in as well.

The final step to the foam layer was filling in any voids with low-expansion foam sealant. To provide better access for the foam gun at the columns, we removed the filler blocks. After sealing around the columns, we slipped the blocks back into place and sprayed foam in around them. To keep the expanding foam from lifting the blocks, we placed a wide scrap of OSB over each one and wedged a length of strapping against ceiling framing to hold the block in plane with the rest of the foam layer. After the expanding foam cured, we trimmed away the excess with a hand saw to complete the thermal layer of the assembly.

Polyethylene Origami

The next step in the assembly process was installing the polyethylene air/vapor barrier. This is the most crucial part of the assembly, so we paid attention to making it as close to perfect as possible. We did a quick sweep of the EPS to check for debris or sharp objects that could puncture or wear through the polyethylene. We snapped guidelines 6 inches up the foundation walls and unrolled lengths of the sheets long enough to extend up the walls to our snapped lines. The poly we used is called Tu-Tuf by Sto-Cote. It’s only 4 mil thick, but it is cross-laminated, making it very puncture resistant.

After snapping a line on the foundation wall, the crew tacked the poly to the wall with small pieces of tape.
After snapping a line on the foundation wall, the crew tacked the poly to the wall with small pieces of tape.
The sheets were carefully smoothed out and cut for each plumbing pipe.
The sheets were carefully smoothed out and cut for each plumbing pipe.

We stretched out the poly over the EPS foam and carefully tacked an edge to the line on one side with a piece of tape every few feet. Working from the snapped line ensured that the sheet would stay straight and square to the room. After tacking the sheet to the line, we carefully pushed it down against the corner between the foundation and the perimeter insulation. We slowly and deliberately worked our way across each sheet, smoothing and stretching it flat as we went.

A neoprene gasket with the appropriate size opening was fitted around each pipe.
A neoprene gasket with the appropriate size opening was fitted around each pipe.
The gasket was taped to the surrounding sheet.
The gasket was taped to the surrounding sheet.

Once again we had to deal with the plumbing pipes. We located and carefully cut each plumbing penetration, one by one, so that the poly lay perfectly flat around each one. We fit each pipe with a neoprene flanged gasket sized for the pipe’s diameter. We then taped the gasket to the polyethylene using 3-M All-Weather Flashing Tape. After taping each penetration, we went over the taped connection with a roller to ensure maximum adhesion.

We treated the structural posts much like the plumbing penetrations, minus the gasket. The base of each column had been wrapped in Vycor, so after meticulously cutting the poly to fit around each column, we sealed the poly to the Vycor wrap with flashing tape. The flashing tape came with a split-paper backing, which facilitated installation at each column.

Once the poly was down, it was sealed to the columns using waterproof tape.
Once the poly was down, it was sealed to the columns using waterproof tape.
The crew laid out the poly sheets so the seams broke near the lines of columns wherever possible; as they taped the seams, the crew carefully unrolled the tape with the backing split to keep the roll centered on the seam.
The crew laid out the poly sheets so the seams broke near the lines of columns wherever possible; as they taped the seams, the crew carefully unrolled the tape with the backing split to keep the roll centered on the seam.

The polyethylene came in 9-foot widths, so we had to overlap successive pieces and tape those seams. We cut the strips of poly lengthwise so that the seams landed close to each line of columns. This minimized waste and made the cutting, fitting, and taping around the columns go much more quickly.

The waterproof tape was thoroughly rolled to ensure maximum adhesion.
The waterproof tape was thoroughly rolled to ensure maximum adhesion.

Taping the seam was a bit of an art form. The super-tacky tape had to unroll without wrinkles and with the center of the roll tracking over the seam. The quickest method was to split the backing over the seam while slowly unrolling the tape. After the tape was stuck down, the seams were properly rolled, again to provide the best adhesion.

When the poly sheet was completely installed and sealed, the crew snapped a line 54 inches from the back wall of the foundation.
When the poly sheet was completely installed and sealed, the crew snapped a line 54 inches from the back wall of the foundation.
The first course of subfloor sheets was then set on the snapped line.
The first course of subfloor sheets was then set on the snapped line.

Floating The OSB Floor

Before installing the first layer of 3/4-inch AdvanTech, we snapped a chalk line 54 inches out from the back foundation wall. Then we cut the sheets for the first course and set them in place with the groove edge on the line.

Each sheet of AdvanTech is clearly labeled with the instruction that a 1/8-inch gap must be maintained along all edges, so we ripped a pile of 1/8-inch-thick strips to use as spacers between the end seams of the sheet. Maintaining the 1/8-inch gap along the sides requires the sheets to be driven together—albeit gently—to engage the T&G edges.

Because the subsequent courses had to be driven together to engage the tongue-and-groove edges, solid backing was created by putting a 2x4 against the back of the first course and then screwing down scrap blocks that rested against the foundation.
Because the subsequent courses had to be driven together to engage the tongue-and-groove edges, solid backing was created by putting a 2x4 against the back of the first course and then screwing down scrap blocks that rested against the foundation.
The rest of the courses then went in, with the crew measuring carefully to maintain proper spacing between sheets.
The rest of the courses then went in, with the crew measuring carefully to maintain proper spacing between sheets.

With the first layer “floating” and unattached to the layers below, we needed something to drive against. First we placed lengths of 2x4 in the gap between the edge of the sheets and the foundation. With the 2x4 touching the edge of the sheet, we placed scraps of OSB against the foundation every couple of feet and screwed them to the 2x4. That gave us a solid edge to drive against.

The second layer of subflooring was installed perpendicular to the first in a pattern that maintained double overlapping seams.
The second layer of subflooring was installed perpendicular to the first in a pattern that maintained double overlapping seams.

For each successive course, we staggered the end seams much like we do when framing a first-floor deck. The crew meticulously measured and adjusted each sheet to maintain the proper gaps and alignment. As with the previous layers, the sheets had to be cut to fit around the columns and the plumbing.

This layer was screwed to the first with 1 1/4-inch screws that would not penetrate to the poly sheet below. Screws went in every 6 inches at the edges and every 12 inches in the field. When both layers were finished, the poly was secured to the wall with two rows of waterproof tape.Photo by Steve Demetrick
This layer was screwed to the first with 1 1/4-inch screws that would not penetrate to the poly sheet below. Screws went in every 6 inches at the edges and every 12 inches in the field. When both layers were finished, the poly was secured to the wall with two rows of waterproof tape.

Photo by Steve Demetrick

We ran the second layer of OSB perpendicular to the first layer and screwed it down with 1 1/4-inch screws. We positioned the sheets so they would overlap the seams on the first layer by at least 2 feet in both directions. When the second layer was completed, we went back and double-taped the polyethylene permanently to the foundation wall.

The walls for the finished basement were framed directly on top of the OSB sheathing. Plates were screwed to the sheathing with screws that did not penetrate through to the poly barrier. Photo by Steve Demetrick
The walls for the finished basement were framed directly on top of the OSB sheathing. Plates were screwed to the sheathing with screws that did not penetrate through to the poly barrier.

Photo by Steve Demetrick

The floating OSB subfloor was completely solid with no give at all, and the basement partition walls went in easily. We screwed the plates to the OSB with fasteners that would not penetrate through to the polyethylene barrier. All in all, the installation went pretty much as planned with no unforeseen challenges, and we came away assured that this type of basement-floor system would make sense for any kind of energy-conscious building. It was also cost-effective. Installing the AdvanTech cost a little more than pouring a slab, but was much less expensive than prepping for a finished hardwood floor over concrete.

Photos by Roe Osborn except where noted