Question: Why is perimeter slab insulation important? Where and how should perimeter slab insulation be applied? Is there any way to eliminate perimeter slab insulation completely?
Answer: The impact of slab edge insulation varies with the basic level of thermal performance of a home and with climate conditions. Homes built to pre-1993 Model Energy Code (MEC) standards had such large thermal losses through the rest of the building enclosure (the walls, windows, roof, etc.) that slab edge losses were a relatively insignificant portion of the total loss.
With improved standards (MEC 95, IEC, Energy Star®, and Building America), the relative importance of slab edge thermal losses has grown because other building enclosure losses have been greatly reduced. Thus, slab edge insulation is desirable in all but the mildest of climates. Slab edge insulation also helps keep the edge of the floor slab warm in winter and helps control the potential for condensation.
The thickness and configuration of slab edge insulation varies by climate. Generally, R-5 insulation is considered a minimum in the climate regions where slab-on-grade construction is common (mixed and hot-dry climates). A higher R-value may be required to meet specific building codes. It's desirable to have a two-foot depth of insulation, if possible.
The configuration of slab edge insulation is highly dependent on the nature of the slab construction. It can be located on the outside or inside of a slab edge or grade beam. It can be projected horizontally out from the slab edge or under the slab. For instance, in mixed and hot climates, insulation shouldn't be more than 2' under the slab, because the cool ground acts as an aid during air-conditioning periods. Details of many configurations are available in the Energy and Environmental Building Association (EEBA) Builders Guides.
Protection of exterior slab edge insulation and its role as a pathway for termite movement to the frame of the home are major concerns. It's important to protect the insulation with rigid materials. There are several protection systems on the market that use panel materials or troweled-on coatings. You can use termite aversive insulations, such as fiberglass board stock combined with a termite shield at the sill plate, to stop termite movement. Placing the insulation inside the foundation, either vertically or horizontally, will help you avoid both termite and protection issues. However, this method isn't as thermally protective as installing the insulation on the exterior.
While not often easy to do, good slab edge insulation is an important part of a high performance home.
- Brad Oberg
Chief Technology Officer, IBACOS
