Image Credit: All photos: David Goodyear Removing the topsoil, nearly 2 feet deep in some areas, is the first step in preparing the site for the foundation. It will need a significant amount of compacted fill. It took nearly three dozen tandem loads of blast rock to create a structural pad to support the footings, frost walls and slab.The rock was compacted with a roller and inspected by a structural engineer. With the footing forms set on a compacted rock base, the contractor uses a transit to establish lines marking the finished concrete height. Once the footing forms were established, placing the concrete was a breeze. The footings are stepped to accommodate a sloping lot. The completed footings at Flatrock Passive.
Editor’s Note: This is one of a series of blogs by David Goodyear describing the construction of his new home in Flatrock, Newfoundland, the first in the province built to the Passive House standard. The first installment of the GBA blog series was titled An Introduction to the Flatrock Passive House. For a list of Goodyear’s earlier blogs on this site, see the “Related Articles” sidebar below; you’ll find his complete blog here.
April has brought relatively good weather. This week has been above 0°C (32°F), and the temperatures are climbing. For the most part all the snow is gone and the ground has thawed. The switch in weather has made it easy to get some work done.
The excavation company, Wade’s Excavating, dug down to solid ground. There was an amazing amount of topsoil on the lot. In places, it was almost 2 feet thick. It has all been pushed out of the way for now, waiting to be moved around and used for final grading after backfilling. After the hole was dug, we hired Aubrey Burt, a land surveyor, to roughly mark out the foundation pad (see Image #2 below).
We knew we would require fill in places, and we will need a lot of it. We won’t be able to use a 4-foot frostwall everywhere; if we did, backfilling to the top of the wall at the back of the lot will turn the lot into a ski hill. Our plan is to make a stepped frostwall of varying depths — from 8 feet to 6 feet to 4 feet — from the low to high parts of the lot. We may do siding part way down the insulated concrete or just leave it with the concrete parging over the foam.
Behind the garage will be nice and sunny and I have been thinking about using that area for my solar kiln for drying green wood for woodworking and firewood. So leaving the foundation exposed there may not be a big deal.
Creating a compacted structural pad
Wades Excavating completed the structural pad in early May, a day ahead of schedule and a day quicker than the original time estimate. They trucked in 34 tandem loads of blast rock to fill the hole for the house and rolled it in with a vibrating roller to pack it down (see Image #3 below).
The site was inspected by a geotechnical engineer who will now certify it’s good to build on. The depth of the hole was very deceiving and I realize that, in such large quantities, fill cannot be estimated visually. So keep in mind that structural fill will really chew a hole in the excavation budget if you ever decide to build on a sloped lot. A 4-foot drop over almost 80 feet is enough to add significantly to the cost of your home.
With the pad in place, we now have a good solid foundation for the house. Parts of the pad will have to be dug down in the back for footings and to facilitate our grading plan. I’ll also be calling the well driller to have a look at the proposed position of the well. If he thinks he can fit the drilling rig between the house and the trees we’ll be ready to move ahead; otherwise, we’ll be waiting for several weeks for the well to be drilled before we start the foundation.
The build is now about two weeks ahead of my original proposed timeline, so we have some time to play with. But I know that other things will pop up. If we can keep the momentum going, we’ll be in better shape later.
Digging the foundation footings
Armed with our plan, we had the surveyor mark the foundation corners. We started digging and taking grades with the transit as we went. The digging went smoothly. All the rock that we brought in for the pad was excavated and placed in the center where it will be used to fill in the foundation before placing the slab.
After the footings were completely dug out, they were “tracked in” using the excavator. I followed up by tamping the footing areas for a couple of hours so we have a good stable base for the footings. With the footings dug, the next logical step was pouring them. We had the surveyor re-pin the foundation location and the footing contractor, Todd Brenton, set up the footing forms. The lot was pretty level so for the most part, we have an 8-inch footing everywhere except one small length where it’s about 6 3/4 inches. The whole foundation will be setting on a structural rock bed which was originally tamped using a large vibratory roller and now has been further tamped, to compress the loose rock after digging, using a large diesel plate tamper.
Although I was a little unsure about the leveling process, now that I have seen it, I realize it’s much simpler than I expected. The footing contractor lays out the forms according to the survey pins. Then he uses a transit to find the highest/lowest points (see Image #4 below). He goes to the corners and sights an elevation and pounds a nail into the inside of the form. Then on to another corner, or along the form, sighting elevations to match the previous. Then he snaps a chalk line between the nails on the inside of the forms and that gives them a measure of how much concrete to pour.
This work took about a day. The concrete got poured in less than 20 minutes (see Image #5 below). As the concrete was poured, one of the laborers followed behind leveling the footing to the lines with a shovel by moving the concrete around. Another guy followed, further flattening the concrete with the end of a rake.
What you are left with is a poured footing, ready to set up overnight to a nice set of hard concrete footings: a solid foundation for the Flatrock Passive House (see Image #6 below).
Adding the frostwalls
Since the home design has been completed, I’ve been studying the drawings over and over again. Every couple of days I come across an interesting conundrum. This one is a tale of two foundations.
One of the principles of Passive House design is building an efficient envelope that is superinsulated, airtight, and thermal-bridge-free. If a foundation in a cold space is joined to a foundation in a heated space there are several potential issues. First, it provides a short circuit for heat to travel outside of the thermal envelope during heating season and for heat to enter the envelope during cooling season. This also could lead to interstitial condensation in the walls, promoting mold issues inside the wall at the junction of the short circuit. To minimize this transfer, the designer wanted to separate the thermal envelope of the house from the porch using 2 inches of EPS foam.
Because of the way the forms are connected it would have been possible to insert a piece of foam at the connection between the two foundations. However, once the concrete was poured there would be no guarantee the foam would stay in place. Additionally, there would be no way for the two foundations to be connected. So is there a way to do this using fancy Passive House materials? Probably. However, I had to work with what I have.
Our solution was two pours. The house foundation was poured first. The following day, the forms were removed and a piece of foam 8 inches wide was glued to the foundation at the point of connection between the two walls. We drilled holes through the foam and into the newly hardened concrete, then glued rebar into the holes with epoxy. The forms could then be set up and the concrete for the second foundation poured. The rebar makes a semi-rigid connection between the two foundations.
Calculating energy losses of thermal bridging
Is there an energy penalty for using rebar? There definitely is! It is a thermal bridge directly into the foundation. But since the heat flow is proportional to the area, one would expect the cross-section for several pieces of rebar to be much less than the area connecting the two walls.
Using some highly simplified assumptions, I determined that using the rebar has an energy penalty about 14 times less then leaving the concrete connected; it’s safe to assume the energy lost is about an order of magnitude less when using the rebar. Using data found online, I calculated the power lost through the rebar could be as high as 0.5 W/ degree C. So a temperature differential between the outside foundation and the inner foundation of 15° would mean an energy loss of about 7.5 watts. With the concrete walls joined together, the power lost through the wall would be about 9.2 W/ degree, equating to about 140 watts. Assuming the temperature differential between the foundations is at least 15°C for one-third of the year, this equates to almost 400 kWh of energy, or about $40 on my electricity bill.
This is significant and has a short payback for the time invested. But keep in mind there are a lot of assumptions here. The total energy savings is really still a question left up to energy modeling. But for the cost of rebar, epoxy, a masonry drill bit (about $70), and some minor inconvenience associated with doing two pours, I figured it was worth it.