Best Practices for Insulating Slabs & Footings

By Stephen Magneron, CPHC, Advanced Housing Specialist

Insulation under the slab is one of the most overlooked strategies in most homes built today. Not only does it help create a warm, dry, comfortable basement, but it will also save on heating bills.

Slabs can be from 10% to 30% of the heat loss surface area of a home, with townhomes on the lower end to sprawling bungalows on the higher end. Admittedly, the heat loss is less than that of the roof or the above grade walls that experience higher temperature difference between the indoors and outdoors. However, the ground’s constant temperature is around 10oC in the Ottawa area. Imagine if the temperature outside was 10oC all year round, 24/7, and your house was uninsulated. You would be either very uncomfortable or paying 24/7 to keep the indoor temperature at a cozy 20oC. It truly makes little sense that we’ve been upgrading all our other thermal assemblies yet commonly leave such a large portion of our building envelope completely uninsulated!

So why is slab insulation consistently being overlooked?

The ENERGY STAR standard v12.3 does not require insulation under the slab as a core requirement. However, it does recognize the benefits of this strategy, and encourages builders by awarding Builder Option Package (BOP) points for R-5 (0.1) or R-10 (0.3) installed continuously under unheated slabs.

Under heated slabs, the core requirement introduces a minimum of R-13.2, which is not eligible for any BOP points. The reason for this is the slab is a heating source, therefore there’s a much higher temperature difference to the ground. Sure, some heat will radiate into the room, but a significant amount would be lost to the ground in the absence of insulation. Clearly not very energy efficient!

When modelling a home in HOT2000 for ENERGY STAR Performance Path or R-2000 homes, or in the Passive House Planning Package (PHPP) for Passive House projects, the amount of insulation required will be based on analyzing the effects of different levels of insulation. This can range anywhere from R-10 to R-50 for a Passive House that is trying to squeeze out as many kWh of space heat demand as possible. For a heated slab our recommendation is a bare minimum of R-30, and no less than half that under any unheated slab.

The installation of the insulation under the slab is most effective with multiple layers of foam so that their joints are staggered. It is also critical that it connects to the basement wall’s insulation. Typical strategies were discussed in the last issue of the newsletter “Basement Insulation: Myths and Realities”. The idea to keep in mind is that you want to create a thermal bridge free connection at the perimeter of the slab. For example, if the basement wall is insulated to the inside then the slab insulation should be installed above the footing and must include a thermal break at its perimeter with an equivalent R-value. If the basement wall is insulated to the exterior, then the slab’s insulation strategy must also include wrapping the footing with the same amount of insulation so that it connects with the exterior insulation. This is a very effective strategy, but a little more costly, and requires a bit more planning. The foam under the footing, for example, will need to be high density foam that is rated to take the load of the building. The beauty of this strategy is that the structure can be completely enclosed with insulation, keeping it warm and dry.

The benefits of slab insulation for homeowners are undeniable – comfortable, warm and dry, with no more of that “basement odour”. The benefit for builders is that it represents another option to ENERGY STAR and happier clients because they are more comfortable in their basements. When builders, buyers and code officials finally understand the numerous merits of slab insulation, you can be sure it will not be overlooked in the future!

Leave a Reply