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Soffits
Soffits

Ask the Expert: Soffits

How do I spray foam soffits in an unvented attic configuration

By Mason Knowles 

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Soffits can be tricky to insulate and air seal properly. But if you apply a few basic building science principles, you can be confident the soffits are done correctly.

First, consider that the role of the building envelope is to separate the inside environment from the outside. That includes air, moisture, and temperature. So our goal is to use a design and material in the soffit to separate the outside air, moisture, and temperature from the inside. We also want to minimize air infiltration from the interior of the building into the attic. In northern climates, ice damming can occur if warm moist air from the interior of the building travels into the attic space. The warm air melts snow, which then collects at the roof edge.

The water freezes causing icicles along the roof edge and backs up the melted snow that can travel back into the house, causing water damage to the interior. One way to accomplish our goals is to place a backer board in the soffit that is structural enough to stay in place when spraying foam to it. The material should be easy to cut and hold its shape, is resistant to water and moisture absorption. Common materials available at your local hardware store include expanded polystyrene or extruded polystyrene foam insulation and polyiso foam board insulation. The material should also be placed on the outside edge of the wall. This prevents air from the interior of the building traveling through the wall cavity into the attic space.

A poor soffit design (left) can result in ice dams compared to correct soffit detail (right). Note the backer board separating spray foam from soffit space.

A poor soffit design (bottom right) can result in ice dams compared to correct soffit detail (top). Note the backer board separating spray foam from soffit space.

Regardless of whether you are using open or closed-cell spray foam, the backer board provides a uniform surface that allows a better control of foam thickness. Remember also to limit the lift thickness of closed-cell spray foam to prevent excessive exothermic heat within the curing foam.

I would not use fiberglass or fill the whole cavity with open-cell foam since those materials can absorb water and moisture. I would also caution against filling the void with closed-cell foam since the large volume of foam can cause excessive exothermic heat reaction within the foam that can lead to poor cell structure, lingering odors, and in extreme cases, auto-ignition. .

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Finally, should the exterior side of the soffit itself be ventilated? I defer to the conservative approach and say yes

In northern climates, ice damming can occur if warm moist air from the interior of the building travels into the attic space. The warm air melts snow, which then collects at the roof edge.

Treat the area as if it were a mini-vented attic space. Having air move through the soffit area should prevent humid air in the summer from condensing in the soffit.•