How much insulation does a basement need?

The importance of properly insulating basements cannot be stressed enough. Uninsulated basements and poorly insulated basements can be responsible for as much as 1/3 of the total heat loss of a home.

Basement wall insulation
Basement insulation © Ecohome

Technical edit and thermal images by Denis Boyer

Before we began insulating foundation walls and below slabs (see slab on grade), it was generally estimated that basement heat loss accounted for about 1/3 of the total heat loss of a house. It is likely that such an abysmal starting place made the addition of any insulation at all seem sufficient, so basements are still chronically under-insulated.

Current building codes vary by region, but what is consistent is that if you insulate to the minimum requirements of code, your basement will still account for a significant and unnecessary portion of the heat loss in your home. Somewhere between R5 and R10 under a slab tends to be what we typically see now, yet energy modeling shows that additional insulation will have a relatively quick payback period.

Compare the following thermal images of figure 1 - a slab floor insulated to R64, and figure 2 below, one insulated to R5. In figure 1, note that the colour of the concrete floor is almost white and the ground beneath is green. This indicates a comfortable slab temperature of approximately 20°C and very little heat lost to the ground, as the red to yellow to green temperature drop happens within the insulation.

Thermal image of concrete slab floor
Figure 1: slab floor insulated to R64 © Denis Boyer, Ecohome


In figure 2 below, the slab is a reddish colour, indicating a temperature of 18°C, dropping to 16°C closer to the walls. The orange blending to yellow seen beneath the slab indicates a much higher ground temperature due to heat loss. Limited insulation such as in figure 2 will result in a less comfortable floor that will need a much higher ambient room temperature or in-floor radiant heating to maintain the slab at a comfortable temperature.

Thermal image of concrete slab floor
Figure 2: slab floor insulated to R5 © Denis Boyer, Ecohome

Despite the constant temperature of the earth just below frost depth (approximately 2 or 3°C in winter), the type of ground on which you are building will have a dramatic effect on the rate of heat loss through your foundation walls, as heat moves more easily through some materials than others.

Heat will be drawn from your home much quicker through rock than clay, and quicker through clay than soil. Heat will also move quicker through wet clay than dry. We would recommend at least R20 under slab floors in cold climates, but depending on the type of ground on which you are building, you might need to double that in order to maintain the same level of energy efficiency.

The rate of heat loss through foundation walls also varies between below-grade and the parts above. Over a heating season, there will be much more heat loss through above-grade walls than those below.

Therefore, with one wall and two rates of heat transfer, this leaves the dilemma of insulating more than you need below, or less than you need above. There is the option of beefing up insulation at the level above grade; however, this is a less common building technique. To be truly effective, it would need to be done on the exterior of the wall to prevent thermal bridging.

Building code requirements for foundation walls vary across Canada, and range between R19 and R24.5. This is a minimum and, depending on location and climate we would recommend at least doubling that in the interest of saving money and energy.

Insulating concrete slab floors

Aside from the obvious benefits of energy efficiency, a well insulated concrete slab floor (as shown above) will be warmer and more comfortable. Radiant floor heating systems are one way to combat the cold, but they require a much greater initial investment, and an endless investment in energy for operation.

A more affordable option would be increasing sub-slab insulation and installing a baseboard heater or two. You would accomplish effectively the same thing for a fraction of the cost, for the following reasons:

Insulation below basement slab floor
Insulation below basement slab floor © Alain Hamel for Ecohome

1) Insulation will need no maintenance or repair; eventually the moving parts in a radiant system will.

2) You will pay much less for heat every month to keep a well insulated concrete floor at a comfortable temperature. 

3) The less heat you lose, the less heat you need to add.  You could save thousands of dollars in initial building costs (spending a few hundred in baseboard heaters instead of $10,000 or so on a radiant floor system), and perhaps save hundreds a year in operational costs.

Not only that, you will still need to insulate properly under a radiant floor. In fact, as that floor is now warmer, you will have increased the rate of heat loss to the ground. So, in the name of energy efficiency, you would need an even greater amount of insulation to retain the additional heat in a warm floor compared to a floor kept at ambient room temperature.

In-floor radiant heat is certainly enjoyable in winter, and it is arguably one of the more efficient methods of heat distribution in a home. It is however, not a cheap system to install, but no heat generation system is. For this reason we advocate investing whenever possible in heat retention rather than generation.

Through energy modeling you can get a good idea of much insulation would be required to keep a concrete floor at a desired temperature (19 or 20°C). There is the possibility here of adding enough insulation  that may render a radiant heat system completely unnecessary.

By insulating your floor instead of heating it, you won't have a hot floor, but you won't have a cold floor. It will just be a floor like any other, except it will save you a lot of money in construction costs and operation.