Why is Closed Cell Spray Foam Different for Air Sealing?

Closed cell spray foam insulation has revolutionized the way building science professionals approach air sealing. Unlike traditional fiberglass or cellulose materials, which primarily resist heat transfer, spray foam uniquely combines insulation with air barrier capabilities. This dual functionality addresses both thermal resistance and air infiltration, leading to enhanced energy efficiency and indoor comfort.

Beyond energy efficiency, spray foam contributes to improved indoor air quality. By sealing gaps and preventing air infiltration, it reduces the entry of outdoor pollutants, allergens, and moisture. This is particularly beneficial for individuals with respiratory conditions or allergies, as it minimizes exposure to airborne irritants.

Benefits of Closed Cell Spray Foam

Closed cell spray foam is particularly effective for air sealing due to its unique physical and chemical properties.

Here’s why:

High Density and Rigidity

Closed cell spray foam has a dense structure (typically around 2 lb/ft³).
When applied, it cures into a hard, rigid barrier to resists air movement.
Unlike open blown or batt insulation, it doesn’t shift or compress over time, maintaining a tight seal and the integrity of the insulation.

Closed Cell Structure

The foam consists of tiny, completely enclosed bubbles (cells) filled with a gas to enhance insulation.
These cells prevent both air and moisture from passing through, making it an excellent air and vapor barrier.
Open cell spray foam, by contrast, has interconnected cells which allow air and moisture through.

Adhesion and Expansion

It adheres tightly to most surfaces (wood, metal, masonry), sealing even irregular or hard-to-reach areas.
During application, it expands slightly before hardening, allowing it to fill gaps and cracks, stopping air leaks at the source.

Dual Purpose: Insulation + Air Barrier

Closed cell foam provides both thermal insulation (high R-value, around R-6 to R-7 per inch) and a continuous air barrier.
This dual role simplifies construction, especially in energy-efficient homes or buildings aiming for airtightness (e.g., Passive House standards).

Considerations for Application

While spray foam offers numerous advantages, proper installation is crucial. Incorrect application can lead to issues such as incomplete sealing or moisture entrapment. It’s essential to ensure the building envelope is appropriately prepared and the spray foam is applied under suitable conditions to achieve optimal performance.

Mitigation and Best Practices

Building-science authorities and manufacturers offer clear guidance to avoid shrinkage gaps:

Layered installation: Apply ccSPF in multiple thin passes rather than one thick layer. This allows each pass to fully cure before the next. For example, many contractors spray 1–2″ lifts per pass.
Manage temperatures: Follow manufacturer’s temperature guidelines. Avoid spraying onto extremely cold, hot, or wet surfaces.
On-ratio spray: A properly mixed foam will cure to its full adhesion strength; an off-ratio mix will not. If in doubt, do test sprays and inspect sample cores or adhesion pulls.
Surface preparation: Ensure all substrates are clean, dry, and dust-free. Remove frost, standing water or any contaminant. Some substrates (e.g. vinyl, glossy metal) may need primers or scratch coats. Proper prep is critical to bond strengths.
Edge detailing (air barrier continuity): At edges of cavities, apply a “picture-frame” or fillet of foam all around before spraying the field. Small pull-away gaps often first appear at edges, so pre-spraying a bead at edges helps maintain continuity. After spraying, inspect for any voids or cracks and reseal them.
Post-spray inspection: After each application day, inspect for any signs of pull-away or cracks before covering the foam. Repair any gaps by re-spraying or caulking. Detecting and fixing them immediately is much easier than after finishing.

Following these practices will normally prevent noticeable shrinkage. Indeed, many foam system guidelines emphasize with good technique, post-installation shrinkage is negligible.

Not all air leaks are in the wall cavity

Even if all the wall cavities are sealed with spray foam, there will be air leaks that cannot be sealed with spray foam. These air leaks come from small gaps between framing lumber found at the top and bottom plates of the wall, headers, jack studs and a lot of other places that are not inside the wall cavity. To seal these areas, caulk is required. This can be done with a traditional caulk tube, but the quality of air seal with this method is largely dependent on the workmanship of the installer. The best way to seal off these pesky air leaks is with spray applied caulk, and a lot of spray foam installers also offer this service.

Spray Jones specializes in providing cutting-edge spray foam solutions and supports the industry with education and advisement. This video reviews the Journal of Building Physics article on “Air Filtration Through Building Envelopes” and discusses the importance of proper air sealing. Since diffused air can be detrimental, close cell spray foam provides a better air seal than batted or fibrous insulation in the wall assembly. Our first line of defense can be compromised if the outside of the structure isn’t protected from wind and water with a primary air seal product in the walls which air and water do not go through.

Journal of building physics: (PDF) Air infiltration through building envelopes: A review

“The tightness of the construction depends, largely, on workmanship. Typically, residential construction falls in the low end of the ‘medium’ category of the table below.”

The Way Around All This...

If you don’t want to have to worry about caulking all the small air leaks and shrinkage gaps along the thousands of feet of studs in your home, you can simply move the spray foam to the outside of the home. The X-Bracket System by InSoFast allows you to do just that. When the foam is moved to the outside, there are no seams that can open up and spray foam sticks better to itself than anything else, and the spray foam will completely seal all the small air leaks that would need to be caulked if the foam was used on the inside. This is in addition to the other benefits of exterior insulation.

Conclusion

Spray foam insulation stands out in the realm of air sealing due to its unique ability to simultaneously provide thermal insulation and airtightness. Its application addresses multiple facets of building performance, from energy efficiency to indoor air quality. For building science professionals, understanding the properties and proper application of spray foam is essential in designing and constructing high-performance buildings.

Closed cell spray foam is different for air sealing because it creates a dense, rigid, and continuous barrier to stop air infiltration far better than other insulation types. It’s not just insulation—it’s an integrated part of the building envelope to help control heat, air, and moisture movement.

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