I knew early on in the design process for our (mostly) one-level home that I wanted a concrete slab-on-grade foundation. There are many good reasons to choose slab-on-grade. First, it’s less expensive than wood floor joists; second, the thermal mass works in conjunction with the passive solar design; third, I wanted the house to sit on grade rather than be elevated up a step or two; and fourth, I didn’t need nor want a crawlspace.
It has become very popular in recent years to install radiant in-floor heating into the slab. In cold climates, I’m betting this system is a delight. But my research suggests that in our climate, where the winters are relatively mild (and short), the hefty price tag of such a system (~ $20,000) would take decades to recover in terms of saved heating costs. Also, during the “shoulder seasons” we can have days where you need heat in the morning and evening but not during the day. In that case, a system that requires heating a large thermal mass may not be very efficient – it takes time to heat up all that mass, and time for it to cool down – so turning it off an on throughout the day is not an option. Finally, if the heat source is in the slab, you need to add extra insulation under your slab so that you aren’t paying to heat the earth below your home. For all these reasons, we decided not to go with in-floor heating, and instead spent a whole lot less money just insulating the slab properly.
Inspired by this article, my design called for 2 inches of rigid foam insulation under the slab and 2 inches around the vertical edges of the slab, as shown in this image:
In the diagram above, the walls are shown sitting partially on the vertical rigid foam insulation, which is not allowed under our building codes and I can see why – I would not be comfortable with that either. Our exterior walls must sit squarely on the foundation walls. However, since we were planning to have polished concrete flooring throughout most of the home, that 2 inches of vertical rigid insulation around the slab edge would be visible around the edge of our finished floors. I was willing to live with that to ensure a warm and comfortable slab, and we have a few ideas for how to cover it up when we’re done.
In the image above, there is a step built into the inside of the foundation wall for the foam to sit upon. Unfortunately, there was a miscommunication between myself and the framers, and our step ended up being too shallow. Instead of a 6-inch step (to accommodate 2″ of insulation plus 4″ of concrete slab), we ended up with a 4-inch step.
To remedy the situation, it was suggested that we lay the under-slab insulation alongside the bottom of the step, rather than having it sit on the step itself. But that would create a thermal break along the bottom edge of the slab, as shown in this fancy diagram I made:
I knew from my research that the edge of the slab, which sits aboveground, loses a significant amount of heat, because the outside air can get colder than the ground in winter. So this was not an acceptable solution.
Instead, our builder came up with this solution:
With this setup, there is at least 2″ of insulation all around the slab. The upside is that we were able to take advantage of a deal on EPS foam from our local building supply store and get 3-inch thick boards for the whole slab for only $500 more (with the original plan, we would not have been able to accommodate an extra inch). We used the more expensive XPS foam for the slab edge insulation, because it has a higher R-value per inch than EPS. The downside of this setup is that the slab itself will be only 2 inches thick around the edges, and that might make it more prone to cracks. I’m not too worried about cracking, because with polished concrete floors you can have cracks polished over and they add a nice patina to the flooring. But to be safe, the builders added a layer of wire mesh around the slab perimeter, as shown in the photo below.
The slab install thus proceeded as follows, after tamping down the drainage rock (as you would do with any slab installation), the EPS foam was laid down and sealed with spray foam (see this post for more photos of that process). Since the ground is not perfectly level, but the top of the foundation wall is, the next step was glueing the top 2″ of XPS insulation to be level with the top of the foundation wall. The middle layer of XPS was then sandwiched in-between the top layer of XPS and the bottom layer of EPS, and any gaps were filled with spray foam (which has a higher R-value than the rigid foam, by the way).
When all the rigid foam insulation was in place, a vapour barrier was laid over the entire thing. It seems that in some regions it has become common practice to place the vapour barrier underneath the insulation. This article explains why that is a very bad idea. In short, concrete holds a lot of moisture, even when dry enough to use, and that water will try to exit through both the top and bottom of the slab. Placing a vapour barrier below the insulation will result in the EPS foam sitting in a pool of condensation, and although EPS foam is pretty moisture resistant, it was not designed to be continuously wet. With the vapour barrier above the insulation, the moisture will end up going out the top of the slab (and since my house has breathable exterior walls, that won’t be a problem).
The vapour barrier was taped to house wrap, which was installed underneath the sill plate when the framing began. This creates a continuous air barrier that starts under the slab (poured concrete is effectively airtight), goes under the wood framed exterior walls, and up around the exterior insulation (which I’ll describe in a later post). [note: in building terms, being airtight is not the same thing as being vapour resistant; you can have a wall that is draft-free but still allows water vapour to pass through]
The foundation wall error ended up costing us a bit more in labour and insulation, but overall it was a relatively inexpensive mistake. I enjoyed the problem-solving process, and I’m happy with how it all turned out. We ended up with R=10 perimeter insulation and R=12 under the slab (recommendations for our climate zone are R=10 and R=5, respectively). We won’t know until we start living in the house whether all my research and planning will pay off, but I’m feeling pretty confident that my feet will be happy next winter!