Spray Foam Substrate Moisture and RH Testing

Posted by Randy Isaac on

Substrate Preparation SPF can be sprayed on wood, concrete, metal, asphalt, foam sheathing, and other substrates. But the substrate needs to be clean and dry — “paintable” is a reasonable benchmark — for the foam to adhere well. SPF shouldn’t be installed when temperature and humidity levels fall outside the range recommended by the SPF manufacturer. Moisture. When a surface is wet or damp, the moisture acts as a blowing agent that reacts with the “A” side of the SPF system, resulting in off-ratio foam with poor physical properties and poor adhesion. When the foam is sprayed on wet framing or sheathing with greater than 19 percent moisture content, for example, there’s a risk that the foam will have poor cell structure and won’t bond well when the framing dries, leading to cracks between the foam and framing. For this reason, the spray-foam industry recommends that installers measure substrate moisture content with a moisture probe before applying foam — but many of them don’t. Safe moisture content is relatively easy to determine in wood, but trickier to do in masonry. Concrete may appear to be dry yet still hold quite a bit of moisture that will be brought to the surface during foam’s exothermic reaction. Currently, the SPF industry doesn’t have any specific moisture content recommendations for concrete, beyond allowing for a 28-day curing time if it’s green. Some coating manufacturers consider MCs as high as 85 percent acceptable for paint, but I think this number is too high for foam. To be safe, tape an 18-inch by 18-inch sheet of clear plastic to the concrete, making sure it’s sealed on all four sides. If condensation appears on the plastic or if the surface of the concrete darkens after 16 hours, the concrete is too wet for foam. An experienced installer will often just spray a small section of concrete to see if the surface becomes damp and reacts with the rising foam. If it does, he’ll wait until the concrete dries out before continuing to spray. Temperature. While there is some variation among manufacturers, most SPF systems are designed to be installed when substrate temperatures are higher than 55°F. A few low-temperature foams can be used in colder conditions, but they’re the exception. With most foams, spraying when temperatures dip below 55°F (or below the manufacturer’s recommendation) can lead to the formation of a high-density shellac-like coating on the surface of the substrate that reduces the adhesive quality of the foam. In some cases, the foam can actually separate from the substrate, because of the difference in temperature between the two materials.


When relative humidity levels are high, moisture can combine with the liquid components of rising foam and affect the foam’s cell structure. To avoid problems like low density, low compressive strength, and too many open cells, the foam shouldn’t be sprayed when the ambient temperature is within 5°F of dewpoint. Since dewpoint is based on relative humidity and air temperature and can vary widely over the course of a day, these conditions should be measured and recorded a few times daily with a humidity and temperature gauge, such as a Tramex Data Logger. 

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