Praxis House - Part 2

August 1, 2023

Underslab Insulation

Ever notice how standing on a tile or concrete floor with your bare feet feels cold?  Even if you are on a second floor, with warm living space below, your feet can feel chilled.  This doesn’t make sense: the tile is room temperature in this situation, so should feel the same temperature as a wood, or carpet floor, and yet they definitely do not.  Why is this?

The answer sounds complicated, but it’s quite simple: specific heat capacity.  This is the amount of “heat” required to raise the temperature of 1kg of a given substance one degree Celsius.  Something with a high specific heat capacity means it takes a great deal of energy to raise its temperature.  Low specific heat capacity, therefore is the obverse.  Tile can absorb a great deal of heat from one’s feet, because it has a high specific heat capacity.

In planning the design of our family’s new home, we had to carefully consider this concept as it applied to the energy demands of our home.  Because our lot has a great deal of bedrock, we chose to build a residence with a slab on grade, and no crawl space.  This means that the main floor will sit directly on the earth.  Like tile flooring, rock and earth have a very high heat capacity, meaning they take a great deal of energy to warm.  And because the earth is, well absolutely enormous, the ground on which the residence rests would quite literally suck the heat out of our house forever before we ever got it close to reaching a temperature equilibrium with the interior of our home.  Talk about an energy (and therefore money) pit.  To compound the fact, without a crawlspace, the house sits directly on the ground.  Objects in direct contact with one another transfer heat with terrific effectiveness through conduction.


Combatting this requires a varying amount of insulation to stop the flow of energy from the conditioned space of the home down into the ground.  The best, most cost effective way to do so is through the use of expanded polystyrene insulation (EPS).  Type 2 EPS is placed directly on the leveled, compacted subgrade below the concrete slab of the home.  With an R-value of roughly 3.9 per inch, it is a very effective insulator.  It is also undamaged by moisture, and practically unaffected thermally by the very small amount of water it will absorb.  It can also support 15 lbs per square inch, making it easily capable of supporting much more than the typical floor loads placed upon it by a residence.

For Praxis House, because we have such high goals in terms of reducing our energy consumption, we chose to use a full 9” of underslab insulation, providing an effective R-value of about R-36 (taking into account the concrete slab and air film above the floor). Using thermal modeling software, we know that by doing so, we are able to get the heat loss through the main floor slab down to 6% of the total home’s thermal losses.



This does three things:

  1. It reduces the energy lost from the home which reduces both our carbon footprint and the size of our photovoltaic array required to offset our energy needs.
  2. It reduces the cost it takes to heat the space each month.
  3. It increases the temperature of the main floor slab, making it nearly identical to the temperature of the air and other surfaces directly above it.

One of the shortcomings of a typical installation of underslab insulation is at the interface of the slab edge (perimeter) where it abuts the inside face of the foundation walls.

Photo Credit: US Department of Energy


This common detail allows what is know as thermal bridging, which is an area with very little, or no insulation interrupting building materials that run from the interior of an assembly (conditioned space) to the exterior (unconditioned space).  This allows the rapid transfer of heat across the assembly in these areas, which is obviously undesirable from an energy consumption standpoint.  To eliminate this common slab edge thermal bridge, Praxis House employs a unique detail, whereby the underslab insulation connects directly to the wall insulation.


This provides a continuous thermal layer to run down the walls, and fold directly under the slab.  While it does mean some extra work to “form” a border against which to place the underslab insulation and the concrete during pouring the slab, it makes for an excellent detail in terms of reducing heat loss and keeping the slab at a consistent temperature around the perimeter of the residence.


Equally important to a continuous thermal layer, is a continuous air tightness membrane.  An average home loses up to 30% of its heat from air leakage.  Not only does this waste money and energy, it also has two other detrimental effects.  First, it can create uncomfortable drafts as cool air can enter the home through a myriad of unsealed nooks and crannies.  This, almost above anything else, is a common complaint of the owners of older homes when it comes to the comfort of their residence.  Secondly, unhindered air leakage through the building envelope can have disastrous effects on the durability of a building’s structure.  With air movement comes an incredible amount of moisture in the form of airborne water vapour.  As that air moves through a building assembly (in either direction), many times of the year will necessitate that it crosses a temperature threshold known as the “dew point.”  At this critical spot in a wall, roof, or floor, the water vapour in moving air will condense into liquid water, clinging to whatever materials are at that location in the assembly.  As more air moves past this plane, more moisture will condense, ad infinitum if conditions persist.  At some point, the building materials will become completely saturated with water.  If they are not able to dry out in a very short span of time, they will begin to rot wood, grow unhealthy molds and fungi, attract ants and other pests, and eventually weaken the building materials to the point where they could cause structural failure.  So, in short, building an airtight building is extremely important from an energy consumption, occupant comfort, and building durability standpoint.  Anybody that argues otherwise has either a very unique building purpose, or a poor grasp of current building science.


To achieve the airtightness of the building slab in Praxis House, we will actually employ two planes of protection.  Because the bottom floor, and the top level ceiling suffer the most consistently powerful air leakage pressures, it makes sense to employ the “belt and suspenders” approach of two air barrier planes of protection.   The first is by employing the vapour barrier placed directly under the concrete slab (and on top of the EPS insulation).  For this we are using 10mil sheet polyethylene, which is about 65% heavier and more resistant to punctures and breakdown than typical 6 mil poly conventionally used.  All seams and penetrations (from electrical conduit, water lines, etc) in the poly are taped with a profoundly sticky and durable acrylic tape by 3M known as “8067 All-Weather Flashing Tape.”


Not only will all the seams in the underslab poly be sealed, but the entire perimeter of this vapour barrier will be sealed to the peel-and-stick product which was adhered to the tops of the foundation walls.  The second line of defense is the concrete slab itself.  While not an entirely reliable air barrier (because of potential cracks and control joints purposely placed in the floor), concrete is quite effective.  By simply taping the edges of the slab itself to the underslab poly, and caulking any penetrations through the slab with a high quality construction sealant, the slab itself makes a fairly robust air barrier.  A typical slab will usually have neither the concrete itself, or it’s underslab poly sealed at all.  Working in harmony, they may do a reasonably serviceable job of stopping most air leakage, for a very energy efficient, durable build, carefully employing either layer (or both) as part of a whole-house air tightness strategy is essential.

Another important, traditionally overlooked consideration during the underslab prep phase is how to manage radon gas.  Radon is a colourless, odourless, radioactive gas that naturally occurs during the slow decay of uranium and thorium in rocks and soils.  While not present everywhere, the long-term inhalation of radon is the second-most common cause of lung cancer (Health Canada).  By placing a test kit in one’s home, it is possible to determine whether radon gas is present at your home’s location, and therefore needs addressing.  Unfortunately, this test can only be done AFTER the home is complete, so it is wise to arrange for the management of the potential problem during the construction process.

There are two conventional ways to address radon in one’s home: either prevent it from entering the conditioned living space, or remove it from the air inside the home if it is present. The first method involves placing backfill material below one’s sub-slab insulation with free air movement (such as ¾” or larger washed rock), and then placing within that layer perforated PVC pipes that allow pathways for the sub-slab gases to travel up a mechanically ventilated pipe that runs up through the interior of a home, and out through the roof.  While this method is very effective, it is fairly expensive to institute (several thousand dollars), may not be necessary at all if no radon ends up being present at one’s location, and lastly, in a highly energy efficient envelope, installing what is essentially a “chimney” of cold air all the way through one’s building is less than ideal during the heating season.

The second method is to make provisions for what is known as a “radostat.”  This is a sensor that is connected to one’s ventilation system.  It functions by continually testing the air in the home, and if radon gas above a very low threshold is detected, the radostat will instruct your ventilation system to elevate its rate of air exchange, thereby washing contaminated air out of your home and replacing it with fresh air from the exterior until the radon levels are reduced.  This system has the advantage of only requiring a small control wire to be installed during construction at almost no cost whatsoever.  It also means no expense at all is incurred if, after construction, a test indicates that there is no radon gas present at the home’s location.  And lastly, if there is a radon problem, the radostat actively monitors the levels, and only runs exhaust equipment as often as required to mitigate the problem, not 24/7 as the first method may do.


For Praxis house, because we have extremely shallow foundations and backfill (+/- 20”), and because the earth upon which the foundation rests is very open to air movement (3” plus blastrock with a thin layer of 1” minus stone base on top), I am confident that most of the radon gas will migrate outwards of the foundation once it dams up against our very tight underslab detailing.  However, should this not be the case, we will be placing a control wire for the possibility of a future radostat that will control our heat recovery ventilator (HRV) to remove any radon gas should we find it present after completion of the home using a test kit.

By incorporating measures for the management of potential radon gas, installing two planes of protection to ensure effective air-tightness, and placing over three times the code required minimum underslab insulation, Praxis House will make for a safe, durable, comfortable home for generations to come.

Stay with us over the coming months as we dig into the details and reasons that Praxis House will help us achieve our goals for comfort, durability, and high-performance energy efficiency.  We hope you’ll find inspirational ideas and education insights into how you can “walk the talk” on your own energy efficient projects.

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