Thompson Johnson Woodworking completes a superinsulated basement for a custom home on Peaks Island, Maine.
Foundation piers for the raised-floor portion of the foundation, after forms are stripped.
Carpenters Dale Cunningham and Chris Byron attach a 3-inch layer of expanded polystyrene (EPS) insulation to the outside of the basement wall, using 4-inch GRK screws and plastic washers. Including the ICFs and this additional EPS, the total R-value of the wall insulation is about R33, according to Mark Pollard (8 inches of EPS at R4.2 per inch).
Carpenters Dale Cunningham and Abe Bates attach Soprema Colphene ICF self-adhering waterproofing membrane to the face of the EPS foundation insulation. From the Soprema website: "COLPHENE® ICF is composed of a high quality elastomeric styrene-butadiene-styrene (SBS) polymer-modified bitumen blend and a polyethylene woven composite facer... The underside consists of a release film and self-adhesive bitumen layer for adhering to approved substrates."
"The waterproofing membrane was run vertically from about 6 inches below the top of the footing, over the top of the ICF foundation, and about 2 inches down the inner face of the ICF," Mark Pollard explained. "This seals the joint between the footing and ICF, and acts as a capillary break between the concrete at the top of the wall and the 2x8 treated-wood mud-sill. The additional exterior-applied layer of 3-inch EPS extends up past the top of the ICF by an inch and five eighths, to account for the thickness of the mud sill and EPDM air-sealing gasket (Conservation Technology BG65)."
In preparation for placing the basement floor slab, the crew installed PVC pipes next to the footings for drainage and for radon control, and placed a sub-base of compacted gravel.
Here's a view of the gravel sub-base for the basement floor slab, with interior drainage and radon-control perimeter pipes installed. The outlet pipe for the perimeter pipes is equipped with two backflow preventers, to help keep the system relatively airtight with respect to the outside, according to Mark Pollard. If water level beneath the slab ever got high enough to create a pressure, the backflow flaps would open to let water out.
Here's a view of the exterior perimeter drainage system during construction. Visible here are the perforated PVC drain tile, gravel backfill, and filter fabric to protect the gravel from silt infiltration.
The basement half of the house foundation is equipped with a door that opens into the area under the raised-floor portion of the house. Here is a view of the insulated door under construction. Says Pollard: "The foundation is about R33, so to reduce thermal bridging, I wanted to match the door R-value if possible. I chose to use 5/4x6 treated wood for the door frame, and in-fill with 5.5 inches of polyiso. The 2x4 diagonal brace runs up from the bottom hinge toward the top of the latch side, and is sandwiched on both sides by 2 inches of the rigid polyiso."
Here's a view of the basement access door with rigid insulation fully installed. "The door slab is air-sealed with a combination of sealant and tape," says Pollard. "The hinges are 4.5-inch heavy-duty stainless steel with ball bearings, to account for the weight and wider than normal door dimensions (48 inches). The rest of the hardware is also stainless: handles, sliding latch, and heavy-duty roller catch."
Here's a view of the basement access door in place. Visible through the door is sub-slab waterproofing and airtight membrane. Beveling and gasketing the door was a challenge, explains Mark Pollard: "This was the first 6.5-inch-thick basement door I had ever built. The bevel on the latch side needed to be in the 6-degree range, so the door wouldn't bind on the jamb. But by doing so, the door stop on this side would have to be extremely wide (projecting into the door opening) in order for the double gaskets to make contact with the door slab. I ended up splitting the difference: I beveled the door to 3 degrees, and I angled the extension jamb of the door buck to 3 degrees as well. This allowed the door stops to all be made from 5/4 stock. The door stops are double gasketed, with both gaskets kerfed into the stops for easy servicing in the future. The primary gasket is an extra thick Q-lon, and the secondary gasket is a 5/16-inch silicone bulb."
"The 2x8 mud sill is air-sealed to the foundation waterproofing membrane with 4-inch-wide 3M All Weather Flashing Tape," says Mark Pollard. "An extra filler piece of EPS insulation was installed prior to the 3M tape, to fill the gap between the mud sill and the additional exterior-applied layer of 3-inch EPS. The mud sill anchor bolts were also taped at this time, to aid in future air-sealing of the floor and wall framing."
The crew installed a layer of 16-mil Stego Wrap vapor-barrier material before placing the basement slab. "It is fully taped," Mark Pollard points out, "and the portion that extends up the wall will be trimmed flush to the top of the 4-inch concrete slab. The joint between the slab and the ICF will then be sealed with high-quality polyurethane caulking."
The home's ICF floor frame is inset from the outboard face of the insulated foundation. House walls will be framed with a 2x4 inner frame and an outer wall of vertically oriented wood I-joists. The inset floor frame allows for the insulated I-joist wall to align with the insulated basement wall. The floor frame is air-sealed with a combination of silicone caulking and tape.