Frequently Asked Questions

Question:

Where should the insulation be located for my basement — the walls or the ceiling?

Background:

A builder in Luzerne County, Pennsylvania is building a walkout basement with radiant heat installed in the concrete floor (slab). He is not sure where to locate the insulation.

Answer:

The Pennsylvania Uniform Construction Code applies to new construction, and has three options for energy code compliance: the International Energy Conservation Code (IECC), the International Residential Code (IRC) Chapter 11, or Pennsylvania's Alternative Residential Energy Provisions (PA-Alt). All three of these paths are the same with regard to where the insulation should be located in a basement. The key to this question is whether or not the basement is a conditioned space. A conditioned space is defined as a space that either: 1) is being heated or cooled, 2) contains uninsulated ducts, 3) or has a direct opening to an adjacent conditioned space.

The heating elements in the concrete floor of the basement would qualify the basement as a conditioned space. That means the building thermal envelope requirements apply, and insulation should be installed to separate the conditioned space from the outdoors. That means there should be insulation located at the basement walls. This insulation may be installed on either the inside or outside of the basement walls, and, in Luzerne County's climate zone (Zone 5), should have an R-value of 13 when installed within a framing cavity, or R-10 if the insulation is continuous and not interrupted by framing.

In addition, because it is a walkout basement, it is very likely that slab insulation is also required. The perimeter of slab-on-grade floors, with conditioned space above, are required to be insulated anywhere the surface of the floor is less than 12 inches below grade. The required R-value is R-10 with the insulation extending down from the top of the slab, and then two feet below grade. Further, the code states that R-5 shall be added to the required slab edge R-values for heated slabs (heating elements , hydronic tubing, or hot air distribution system is in contact with, or placed within or under the slab). So in this case, the slab should have R-15 insulation, which equates to roughly three inches of extruded polystyrene (XPS) foam. Note that the PA-Alt requires only a half-inch thermal break, provided high-efficiency HVAC equipment, or other energy enhancement option, has been installed.)

No insulation is required in floor/ceilings that separate two conditioned spaces, so in this case, no insulation is required in the floor above the basement. If the basement was not a conditioned space, the insulation should be located in the floor above.