The Corner of the Envelope: What’s Your Foundation Insulation Situation?
By Jim D’Aloisio | Summer 2023
Foundation and slab edge insulation is the true “corner of the envelope.” It’s one of the least understood, and frequently one of the least well-detailed, parts of the thermal barrier. Yet the Energy Code conveys clear requirements, and it can be a source of building energy loss and interior occupant discomfort (e.g., cold feet!) if not handled properly.
The E-Code codifies the thermal properties of slab-on-grade edge insulation as either a minimum insulation R-value or a maximum assembly F-Factor for prescriptive code compliance paths. An F-factor is similar to a U-factor, except the unit is Btu per linear foot of slab edge, as opposed to Btu per square foot. F-factors for various insulation configurations are listed in ASHRAE 90.1 Appendix A. Paradoxically, the most effective location for slab edge insulation, through the center of the foundation wall to align with the above-grade perimeter wall insulation, is not explicitly described in the code. Rather, the code allows two locations for the insulation: Exterior and interior.
Insulation along the exterior face of a foundation wall
The Energy Code prescriptive requirement is for a minimum of two feet down the face of the wall from the elevation of the top of the slab-on-grade. This is where the insulation will be most effective. However, by extending the insulation down to the top of the footing, which is typically how it’s installed, additional thermal losses are avoided and the resulting F-factor is reduced. Exterior face insulation requires protection, primarily from UV radiation, on all portions of the exposed insulation and extending down six inches below grade. For building walls supporting exterior masonry, a useful variant is the use of a bottom course of insulative structural load-bearing material such as foam glass, to minimize thermal bridging losses at the top of the wall. Proprietary material that suits these requirements is readily available, is heat and chemical resistant, and has been successfully used in buildings for decades.
An interesting aspect of exterior face insulation is that it can be incorporated into a frost-protected shallow foundation system. Assuming proper attention is paid to site drainage and subgrade material, this approach results in the required depth of the foundation as roughly half that of a conventional footing that would need to be set below the local extreme frost line depth.
Insulation on the inside of a foundation wall, along the exterior edge of the slab
This variant is currently more common. There are two alternatives to this: Insulation that runs from the top of the slab down the inside face of the foundation wall, and insulation that runs along the exterior edge of the slab continuously with insulation that runs underneath the outer two feet or so of the slab. For higher thermal performance a combination of both schemes can be used. In both cases, the challenge is the top edge: If the insulation does not extend up to the top of the slab, it does not comply with the prescriptive E-code requirements. KHH and other engineering firms have relied on detailing a 45-degree top chamfer, so that the slab surface is continuous to the face of the wall, while most of the insulation value remains. Despite frequent concerns expressed about this detail, we have not seen it cause any constructability or durability problems. In any case, interior slab edge insulation frequently results in a small, code-allowed but distinct amount of continuous concrete thermal bridging between the slab edge insulation and the superstructure wall insulation.
What type of insulation is used?
The most common types of perimeter foundation insulation are rigid expanded polystyrene (EPS), rigid extruded polystyrene (XPS), and rigid rock wool, or mineral fiber. In Climate Zones 4, 5, and 6, the prescriptive thermal requirements are R-10 minimum. One caution – when using XPS on commercial projects, low-GWP material must be used in New York. The standard, high-GWP XPS is still readily available and legally allowed for residential construction. Not all designers and contractors are aware!
Basement wall insulation
Below-grade basement walls require continuous insulation. The Energy Code calls for a minimum R-value for the insulation or a maximum C-factor for the wall assembly. The only difference between a U-factor and a C-factor is that the latter is a measure of the heat flow from surface to surface, that is, no air films or backfill material can be included in the calculation.
For all of the above conditions, every building has foundation insulation detailing challenges. These include the conditions at door thresholds, exterior columns, and loading docks, to name a few. In addition, on some design teams the architect shows all foundation insulation details, and on others the structural engineer takes the lead. Ideally both sets of drawings convey the same insulation geometry. We consider it an opportunity to discuss and coordinate building details with the project architect, to achieve successful and code-compliant construction.