The design of a building’s foundation system involves selecting, sizing, and designing the elements to ensure their proper interaction with the subsurface strata in providing adequate support for the building. In addition to the other serviceability requirements, it’s time to consider the thermal performance of the foundation system that is within the purview of the structural engineer. All too frequently, no one else is considering this. Neglecting this aspect of foundation design could make it difficult – even impossible – for the design team to accomplish a building that complies with required energy code provisions.
This doesn’t apply to all buildings. An unconditioned space requires no foundation insulation, unless the building is in a cold climate and insulation is needed to reduce the depth of the foundation (which can save a lot in construction costs). And if the building’s climate zone is temperate – without extremely hot or cold design conditions – foundation insulation may not be cost effective, since it won’t save much energy. But much of the country experiences significant warm temperature extremes – or both. If your building will require significant heating in the wintertime, or cooling in the summertime, it likely makes financial sense to insulate the foundations – even if it is not currently required by the building code since, historically, energy costs have doubled about every 10 years, on average.
Foundation insulation basics
The key to a successful foundation insulation system is to maintain complete continuity of the insulation plane around the building. Discontinuities create a gap where heating or cooling can transfer at a high rate through the envelope – which is known as a thermal bridge. Try to minimize thermal bridging as much as possible. With this in mind, there are three main strategies for locating foundation insulation.
1) Inside face of exterior foundation walls (Figure 1)
Rigid insulation works quite well at this location, provided the insulation extends up to the top of the interior concrete slab and connects to the exterior wall insulation, so that it thermally isolates the slab from the exterior temperature. This can be challenging. There are proprietary materials that can be used to accomplish a thermal break while providing a solid floor surface. There is also a compromise detail – a 45-degree top chamfer on the rigid insulation – so that the top surface of the slab extends to the wall. Many people oppose this detail over the concern that the edge of the slab will break off. However, I have reviewed dozens of buildings with this detail, and has not seen this problem manifest itself. This is a perceived potential problem, rather than a real problem. The real problem here is that if the insulation is cut off below the top of the slab, the building may be subject to a lifetime of poor energy performance.
2) Outside face of exterior foundation walls (Figure 2)
There are specific advantages to this system, especially if the building’s superstructure has exterior insulation (such as EIFS). Placed on the outside face, the insulation can also contribute to the foundation’s frost protection and reduce the required footing depth in cold climates. A potential drawback is the concern that sitework, or subsequent landscaping, might damage the insulation, although we have never seen this cause a problem. However, the upper portion of the rigid insulation must be covered by a durable, UV-resistant material.
3) Within exterior foundation walls
A cast-in-place "sandwich" wall can be constructed using proprietary insulation tie systems – "suspending" the insulation inside. An alternative, especially useful when the building has exterior masonry, is to make the main foundation wall thinner and support the exterior masonry on a wythe of solid CMU, with rigid insulation in between, filling any gaps with grout. This is useful when the exterior wall has an aligned insulated cavity.
Independent of the location of the insulation plane, there will always be challenging details. Door thresholds, for example, create thermal bridges, unless a continuous plane of insulating material is extended up under the door threshold – which should itself be thermally broken.
Finally, let’s consider convection – the transport of heat via air flow. More and more energy codes are requiring the use of a continuous air barrier system in building envelopes. While concrete foundation walls, fortunately, have good air barrier properties – the interface between the foundation wall and the superstructure or envelope needs to be sealed, to prevent infiltration or exfiltration. For a traditional exterior stud wall for example, a sill sealing material should always be specified by the design team. This material can consist of a strip of expanded polyethylene – think bubble wrap.
Jim D’Aloisio, P.E., SECB, LEED AP, is a principal with Klepper, Hahn & Hyatt of East Syracuse, N.Y., chair of the SEI Sustainability Committee’s Thermal Bridging Working Group, and chair of the NY Upstate Chapter of the USGBC. He can be reached at firstname.lastname@example.org.