Editor’s Note: Lyndon Than is a professional engineer and Certified Passive House Consultant who took a year off from work to design and build a home with his wife Phi in North York, a district of Toronto, Ontario. A list of Lyndon’s previous blogs at GBA appears below. For more, you can follow his blog, Passive House Toronto.
A basic decision early in the design of a superinsulated building is the strategic choice of interior and exterior insulation placement and thermal mass. This a strategic decision because it has far-reaching implications and ripple effects.
Think of the building as a shell on all sides, including the parts in the ground. If we are designing an airtight envelope without thermal bridging, then we want to avoid having some of the insulation inside, and some on the outside. It can be done, but this frequently leads to thermal bridges and air-sealing problems.
For example, if we have insulation under the footings (outside the structure of the shell), but then we want to have insulation inside the basement walls, how do we to connect the insulation under the footings to the insulation inside the basement? Making this more difficult, insulation materials are generally weak and soft, while structural materials are hard and conduct heat.
To simplify the design and construction greatly, and improve the effectiveness of the insulation system, work to have all the insulation either outside the shell, or inside the structural shell. Cross-overs are to be avoided. In our case, we decided to place all the insulation inside the shell, and forego the benefits of thermal mass. I believe thermal mass benefits are less well proven than insulation benefits, and that “thermal” mass can be achieved without “mass” (for example by the use of water — a very thermally massive material without much mass that can be moved around).
Flooring can be removed for access
Our basement floors are installed over our interior insulation (15 inches of Roxul, rated at approximately R-54), which is located just above the concrete slab. As posted earlier, the flooring is removable and is a common material — regular construction 2×12 lumber. That means we can remove and replace pieces, but we also can remove the flooring to look underneath.
The insulation consists of an R-32 batt under the 2×6 floor joists, plus R-22 batts in the spaces between the floor joists. Below the batts is landscape fabric that is held off the concrete by pressure-treated lattice and blocks. This assembly keeps the insulation away from the concrete while providing an unimpeded drainage path for any water that does get into the basement. Nothing in the assembly will retain water.
We were able to get the pressure-treated lattice super cheap — it was culled material, an entire load for $50 — and the landscape fabric was about $8 for 150 square feet.
We’re currently pretty happy with these floors, and the system feels very solid to walk on, as if the floors were resting directly on concrete. It turns out the wood has shrunk a little in the two months since we installed it, but only the pieces that were wetter. Some planks did not shrink at all.
Some astute observers have commented that the floors will allow moist interior air to go into the spaces below. What will happen to this moist air when it reaches the cold concrete some 17 inches below? Well, we have Tyvek under the floor boards in one area to prevent this bulk movement of air, but most of the floor is left without any kind of air barrier.
Since the flooring is removable, we can make a correction if this turns out to be an issue, but I have a feeling the issue is fairly minor for a couple of reasons. If we think of regular basements, many have no insulation under the concrete slabs, and they are perhaps a bit damp on muggy, hot summer days. In Toronto’s climate, this problem is short-lived. In our case, there is a floor assembly blocking the bulk movement of air to some degree, and in addition, the space beneath our floors may be warm for much of the summer due to our under-floor (sub-slab) heat storage strategy. This raises the temperature of the basement concrete slab right when the chances of hot moist air condensing on it may be highest, which should reduce that whole issue quite a bit.
Hedging our bets
However, just in case there is a problem we placed some sensors at the bottom of the floor insulation in three locations. The photo at left shows a small pump with tubing, a water level sensor, and a temperature/humidity sensor in the background.
The sensors are inexpensive — about $5 each. The pump is from Princess Auto and was about $20. We had some problems with our basement floor pour. There was not enough slope in some areas, and during the big Toronto flood in July 2013, we noticed a little water in three locations on the floor. We marked these spots and placed these little pumps to transfer any water that does collect there to the sump pit.
Later, as the systems become live, we will be able to report the fluctuations in temperature and humidity at the bottom of our basement floor assemblies. We will also probably place sub-slab soil temperature sensors as well, one day…
We screwed the 2×12 pine to the joists, burying the screw heads 1/8 inch so we could sand the floor and get a somewhat finished surface later. The main reason for using this kind of floor was low cost. We were able to purchase the material at a 25% discount from regular contractor pricing, about $1.25 a square foot. Air sealing is not required at this floor. This was determined from previous airtightness test on the building, so we know we are already down to Passive House levels of airtightness.
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