The Corner of the Envelope:

Know the Limits of Spray Foam Insulation –

A Quick Fix Can Be A Total Miss

By Jim D’Aloisio, P.E., LEED AP |Summer 2024

Some people think of polyurethane spray foam insulation as a go-to solution – it has a high R-value (conductive thermal insulation) per inch and air barrier properties. However, in our recent forensic engineering work, we have come upon several cases where the use of spray foam insulation has caused thermal envelope, as well as structural, problems.

Case One – A contractor in the North Country installed spray foam between and around existing wood roof rafters in an old commercial building. A few years later, a portion of the roof collapsed, and the joists were found to be severely rotted. What had happened was the roof leaked, but the properties of the spray foam prevented detection of the leak (closed cell spray foam is vapor impermeable). The wood roof framing absorbed the moisture and allowed it to spread across the entire roof bay – the spray foam acted like a watertight bathtub.

Case Two – A similar situation occurred in a single-family residence in the Finger Lakes region – a contractor applied spray foam between and around the floor joists, visible from the basement. When a bathroom fixture developed a water leak, the adjacent floor joist became saturated, and the moisture spread throughout the nearby floor joists and beams, trapped by the spray foam. The floor became completely structurally compromised by wood rot.

Case Three – During construction of a single-family residence in the Mohawk Valley, to make up for the lack of a raised heel (sometimes called “energy truss”), a contractor spray foamed in the attic around the end bearing of a series of metal plate connected wood roof trusses. Within a year, concentrated areas of dark-colored mold appeared on the roof soffit above the spray foam. Gaps and cracks in the spray foam created openings that allowed concentrated jets of warm, humid air to exfiltration up and out of the conditioned space below, resulting in condensation on the roof deck during cold weather.

These are not isolated cases – our office has seen several variations on these themes. Spray foam insulation is a vapor barrier, that is, it does not allow water vapor to pass through, preventing any drying from the sprayed surface. Also, while it is very flexible during installation, it dries to form rigid insulation with very different thermal and moisture expansion properties than the wood or metal substrate to which it is applied. This can lead to cracking of the spray foam when the base material undergoes changes to temperature and, in the case of wood, moisture content. The gaps that this can cause can allow streams of air to flow through that is unlike air that migrates through other types of material. Since spray foam has virtually no thermal or moisture storage, the warm, humid air that enters into a crack or gap comes out the other side with the exact same hygrothermal properties.

While our office has become adept at identifying and offering solutions to mitigate the structural damages described in the case studies, we would rather see fewer of these cases if it meant that structures were being maintained properly. Contact KHH if you need help identifying the cause and extent of damage to your thermal envelope or structure.



Envelope Systems

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