Ask the Building Scientists

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?

-Pam Cole and Diana Shankle,
Building Energy Codes Program
Pacific Northwest National Laboratory

Answer:

The impact of slab edge insulation varies with the basic level of thermal performance of a house, and with climate conditions. Houses built to pre-1993 Model Energy Code (MEC) standards had such large thermal losses through the rest of the building enclosure (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 most mild of climates (San Diego, Hawaii, Miami, etc.)

Slab edge insulation, in addition to reducing energy use, helps keep the edge of the floor slab warm in winter, a comfort issue, and will help to control condensation potential.

The thickness and configuration of slab edge insulation varies by climatic location. Generally, R-5 insulation is considered a minimum in the climate regions where slab-on-grade construction is common (mixed and hot-dry climates.) Higher levels may be required for specific building codes. It is desirable to have a two-foot depth of insulation, if possible. The configuration of this insulation is highly dependent on the nature of the slab construction and may be on the outside or inside of a slab edge or grade beam. It may be projected horizontally out from the slab edge or under the slab. Note that in mixed and hot climates, insulation more than two feet under the slab is not desirable because the cool ground (but not cold) is an aid during air-conditioning periods. Details of many configurations are available in the Energy and Environmental Building Association (EEBA) Builder Guides (visit www.eeba.org for more information).

Protection of exterior slab edge insulation and its role as a pathway for termite movement to the frame of the house are major concerns. The use of termite aversive insulations, such as fiberglass board stock combined with a termite shield at the sill plate are helpful. Physical protection with rigid materials is still necessary. There are several protection systems on the market using panel materials or trowled-on coatings. Placing the insulation inside the foundation, either vertically or horizontally, though not as thermally protective as the exterior installation, will avoid termite and physical protection problems.

Details have been developed that, though not eliminating slab edge insulation, have replaced it with a downward extension of exterior wall insulation in a brick veneer wall. While often not easy to do, good slab edge insulation is an important part of the high performance home.

-John Holton
Research Project Manager, IBACOS