©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
[Editor's note: Roger and Lynn Normand are building a Passivhaus in Maine. This is the 22nd article in a series that will follow their project from planning through construction.]
Great news: We’ve begun site preparation and excavation!
Shawn Woods of Woods Home & Land showed up on site with his tools of the trade: a Cat 3115 excavator, a John Deere tracked skid-steer, a dump truck, and his assistant Isaac.
The first order of business was to put up some high quality silt fencing along two-thirds of the perimeter of the lot to prevent soil from eroding into the river. I was very impressed with how he deftly used the excavator to dig a 6-inch-deep trench along the south, east, and west sides of the lot.
He installed the fencing while carefully leaving a bottom flap to be buried with soil to pin the fence in place, and then created a berm of wood chips retained from when we cut the pine trees down last fall as an additional erosion barrier. He left a band of undisturbed ground to further filter any silt before it reaches the fence.
This multi-layer soil erosion defense worked exceptionally well. We had a weekend storm that dumped over 7 inches of rain after the topsoil was stripped off, and I saw no evidence then or since of any silt breaching the silt fencing. Nice job, Shawn!
Shawn then pulled up some 35 large pine tree stumps and stripped the topsoil. He created several categories of material, all to be recycled in some manner:
It took Shawn two days to do the site preparation and another two days to excavate the foundation. He dug down about 9 feet below original grade, and about 3 feet wider than the foundation wall to allow plenty of room for working on the outside of the foundation.
Shawn set up a laser transit, and while he operated the excavator, Isaac remained in the excavation area holding a rod with laser receiver checking the depth. The goal was to dig the basement area to achieve a uniform, level bottom within 1/4 inch of the design elevation. The only way to achieve such a tight tolerance is to use a laser transit and continuously check the depth.
I was surprised to see the bottom consistently fill with several inches of water while excavation proceeded. The lot and surrounding area is flat, except where it drops down some 35 feet on the south side to the river below, and the drainage area on the east side. While the water accumulated, the bottom of the hole remained solid and never got “soupy.” Shawn periodically used a pump to remove the water. I am sure that the interior and exterior drain tiles will keep our basement dry.
He’s a short video of the site preparation and excavation process.
The next video shows the placement of concrete for foundation footings, the installation of footing drain pipes, and the installation of a layer of crushed stone inside the foundation.
[Editor's note: OSHA regulations limit workers' access to excavations that are subject to trench collapse. The practices shown in this video appear to be dangerous. Be sure to follow OSHA requirements when digging trenches. Trench walls can collapse suddenly; when this happens, any workers in the trench can be killed.]
We were very pleased to see a big semi-trailer lumber up Edgewater Lane to deliver our Logix brand Platinum series insulated concrete form (ICF) blocks.
ICFs are open-ended, hollow-core blocks made from expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) and plastic internal webbing; they will be used to build the foundation. They stack together like the Lego blocks you played with as a kid. Once assembled, linked with plastic zip ties and rebar, then filled with concrete, they offer a one-step foundation sandwich with a concrete core and a continuous layer of EPS foam on the outside and inside that provides insulation, a thermal break, and sound attenuation.
We received about 640 blocks in a mix of straight, taper tops, right and left 90-degree corners. (Corner block are not reversible – like most Logix blocks, they have protruding “male” nubs on the top and receiving “female” recesses on the bottom.)
Our block has a 6-inch-wide opening that will be filled with rebar and concrete, and 2 3/4-inch thick foam sides. The basic straight block is 16 inches tall and 48 inches long.
The blocks mechanically lock together vertically and horizontally; adjacent block are further joined with long plastic zip ties. Rebar is placed into the open center of the block before the concrete is poured. The rebar snaps into horizontal channels in plastic webbing inside the block, and is also placed vertically.
We chose the Logix Platinum series ICFs , which are made from Neopor EPS (a type of EPS that includes embedded graphite particles; the graphite boosts the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the EPS by 23 percent). For the crew assembling the block on a bright, sunny day, the graphite particles also lend a much appreciated eye-friendly soft gray hue to the product, rather than the blinding white color on typical ICF blocks.
Before beginning to assemble the block, the design called for a capillary break on the top of the footers. Concrete readily absorbs moisture. When the footers are exposed to groundwater, this moisture can “wick” upwards into the foundation, creating a potential mold, odor, or moisture concern in the basement. Applying a capillary break is a simple, effective and inexpensive preventative measure. We used Drylok, a readily available basement waterproofing product made by UGL.
Our general contractor, Caleb Johnson Architects, also took the additional precaution to book the surveyors for a return engagement to pin the outer corners of the ICFs on the footers. Yeah, the foundation crew could have done the same, but it was a easy task for the surveyors to precisely set the outer boundaries of the ICFs on the footers.
We are having the double 2x4 wall panels with attached sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. built in a factory rather than “stick built” on site. Much planning has gone into making sure the dimensions of these wall panel are accurate. Having an equally accurate and level foundation should avoid having to tweak how the wall panels fit onto the floor deck and foundation – and avoid additional opportunities for air infiltration points in the building shell.
The foundation crew snapped chalk lines between the survey points on the footer and nailed lengths of 2x4 along the chalk lines for the entire outer perimeter. This gave them a hard, straight edge to quickly align the first course of block.
After the first course was installed, the crew nailed lengths of 2x4 along the entire inner perimeter of the block. (Note: this method does make it more difficult to adjust the height of the first course of block if the footers are not absolutely level. Our footers were set by transit, so that should not be an issue.) This inner and outer 2x4 blocking will prevent blowing out the bottom course of block when the concrete is poured. The 2x4s will be removed before backfilling soil against the foundation to eliminate a subterranean food source for termites.
Achieving Passivhaus certification requires not only installing very robust levels of insulation, using very high-performance windows, and eliminating air leakage, but also avoiding thermal breaks that allow heat transfer through materials. Our architect, Chris Briley, cleverly added a 2-inch thick layer of high-density XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. installed at the bottom of the first course of block. The XPS creates a thermal break between the footer and the foundation.
And this became the first of several rapid fire foundation snafus.
The foundation crew mistakenly installed 2 inches of regular XPS with a compression resistance of 25 psi rather than the required high-density XPS. Oops. No big deal, though, as only the first course had been loosely laid on the footers.
But we also needed clarification on which of three available types of high density XPS was required. Owens-Corning makes Foamular 400, 600, and 1000 versions, with compression strengths of 40, 60, and 100 psi. The answer was we needed the Foamular 600. But high-density XPS is a commercial, not a residential, product. The nearest vendor was some 50 miles away in Portsmouth, New Hampshire.
Then we needed clarification on the spacing of rebar dowels that will be drilled through the high-density XPS into the footers, tying the footers with the foundation. This will lock the foundation in place so that soil pressure from freeze/thaw cycles doesn’t move the foundation wall out of plumb. (As an aside, this actually occurred in our home in Virgina, where a below-grade wall moved over 1 inch out of plumb.)
This question led our Logix supplier, Advanced Building Solutions, to review the spacing of the vertical rebar within the ICFs, which led him to review the size and spacing of all the rebar in the ICF wall. The result (based on Logix design tables): we needed thicker rebar, and lots more or it – some $2,000 in additional rebar costs! The dowels would be spaced with every other vertical rebar in the foundation, except along the narrow stairwell from the basement to the garage where it will match up with each vertical rebar.
How did that much rebar get missed? The design has not changed since January. It took several work days to re-engineering the rebar in the foundation. Meanwhile, the foundation crew moved on to other jobs.
We’ve been at a complete stop for one full week of ideal, construction-friendly Maine weather. Ugh.
But we’re glad to have caught the error while it could still easily be fixed.