Warm, dry and easily-constructed basements are every builder’s dream, and even though pressure-treated wood foundation SIPs are a building material that can make this happen, both you and your client probably need convincing. And why wouldn’t you? On the surface it doesn’t seem reasonable to rely on wood and foam alone to support a building, especially after builders have been making masonry-based foundations for centuries. But the fact is, pressure-treated wood (PWF) SIPs are a mature and proven technology. PWFs have been code-approved in one form or another since 1967, and SIPs foundations make especially good sense given the rising expectations homeowners have for warm, dry, inviting basement spaces.

The acronym SIPs stands for structural insulated panels, and it’s a code-approved building system that replaces stud frame walls with factory-glued sandwiches made of foam (typically expanded polystyrene foam – EPS) in the middle and sheet goods front and back. Above-ground SIPs are usually made with 7/16”-thick OSB on both faces, while foundation SIPs are made with 1/2”-thick pressure-treated plywood on the exterior side, with OSB on the inside. In both cases it’s the strength and rigidity of the glue bond between foam and wood that makes SIP buildings very strong indeed – about 300% stronger than equivalent stud frame structures. A typical 4-foot wide SIP can support about 30,000 lbs of downward load. As good as a SIPs foundation sounds on the surface, it’s natural to expect answers to key technical questions before trusting any project to such a seemingly unconventional, masonry-free foundation system.

Question #1: Will a SIPs Foundation Last?

Pressure treated wood foundations have been code approved in Canada and the US for the last 44 years, and PWF SIPs foundations have been in continuous use since 1984 without issues of rot or structural failure in thousands of homes. Preservative treatment of the wood-based sheet goods is one reason why.

The substances used to treat foundation lumber are either chromated copper arsenate (CCA) or alkaline copper quaternary (ACQ), impregnated into the wood at rates much higher than regular pressure treated lumber for above ground use. Pressure treatment levels are expressed in pounds of preservative chemical infused per cubic foot of wood, and typical pressure treated lumber usually has a rating of 0.25 to 0.4 lbs. per cubic foot. Foundation-grade wood, by contrast, as a minimum treatment level of 0.60 lbs. per cubic foot.

The adhesive used to bond foam to plywood for a SIP assembly is a formaldehyde-free, structural urethane. And while it’s true that the adhesive used to bond the layers of wood for plywood used for all types of house construction do emit small amounts of formaldehyde (typically in the 0.1 parts per million range) it’s an insignificant amount. By comparison, the human body normally contains about 3 ppm formaldehyde. CSA, NFPA and the Southern Pine Council have all tested and approved pressure-treated wood foundations both for safety and strength.

Question #2: How Do You Work With PWF SIPs?

A SIPs foundation costs about as much as a poured concrete foundation, but SIPs never crack. SIPs can be assembled at any time of the year without special trades, though construction techniques are different than for wood framing.

The bottom ends of SIPs straddles pressure-treated 2x lumber that’s bolted to the top of the footing. Corrosion resistant fasteners driven through the sheet material on both sides of the SIPs sinks into the lumber, locking the wall plumb. It’s a surprisingly solid way to secure walls, with more strength added later as the floor structure is built. In fact, if you don’t get the panels perfectly plumb before anchoring the bottom ends, forget plumbing them afterwards.

SIPs panels come in 4 foot widths, and whatever length makes sense for your project; typically 8-, 9- and 10-foot panels are available for building basements, and 4-foot lengths for frost walls. Insulation values range from R29.5 for a 4 1/2” panel, up to R47 for 10 1/4”-thick panels. All factory edges of SIPs are recessed to allow pressure treated lumber to nestle into the grooves and make connections between neighboring panels. These connections are sealed internally with expanding polyurethane foam during assembly, with exterior seams sealed again with caulking, inside and out, after everything is up.

1. In practice, start by applying spray foam to the recess along the bottom of one panel.
2. Tilt this panel upright onto the bottom plate, plumb it, then lock the panel vertically by driving fasteners through the sheet material along the bottom and into the bottom plate fastened to the footing.
3. Install a pressure treated wooden spline into one recessed panel edge using more foam and fasteners, then foam the next panel and tilt it upright. Since panel edges interlock, you won’t be able to tilt this second panel up in exactly the final position. Use a 12 lbs. sledge hammer and scrap wood to pound the panel over sideways once it’s straddling the bottom plate, until the grooves interlock over the spline.
4. Fill the cavity in the top of the panels with a double top plate once all panels are up. Most panels will be used uncut, so work goes fast. It takes just one or two days to assemble all foundation panels on top of an existing footing.

Some panels will need to be cut to create windows and possibly door openings, and that’s where two specialized tools come in. A Prazi beam cutter is the power tool of choice for cutting SIPs to size. It’s a kind of chainsaw bar made to bolt to an electric circular saw. Depth of cut is sufficient for working with the thickest SIPs, and the tool can be set to cut angles, too.

It’s sometimes necessary to re-establish the factory recessed edges on SIPs after sawing, and an electric hot knife is the thing to use. It’s a hand-held tool with a rigid, high-resistance wire that melts foam, allowing a swath to be removed to recreate the recess. The slow operation of hot knives is the limiting factor in SIPs construction speeds these days, especially during cold or wet weather. You’ll get faster results if you pre-slice the joints along each side of an edge using a utility knife.

Complete the foundation walls by protecting outside surfaces with waterproof membrane leading all the way down to the footings. Unlike concrete foundation walls, SIPs foundations are back filled with clean crushed stone, a full 12” out from the wall from grade level leading directly down to weeping tiles below the footings. This channels surface water downwards, then away from the structure. The SIPs will probably never get wet at all.

Pressure-treated wood foundation SIPs do make it faster and easier to build basements and frost walls, but like most kinds of progress it takes time and information for people to see beyond the usual way things have always been done. And while this is especially true in the construction business, things are changing even here.

Sidebar: What Are the Drawbacks?

No building material is perfect, and this applies to pressure-treated foundation SIPs, too. In the event of a deep, catastrophic water leak that remains undrained for days inside the basement, the interior OSB will swell. Also, since all types of pressure-treated wood foundations are new to many homeowners, skepticism may affect resale housing values.