Fact Checked

What is Sill Sealer?

B. Turner
B. Turner

A sill sealer is a type of insulation used at the base of exterior walls. During the home-building process, builders first construct the floors out of wood or concrete. As they build the walls, they use a piece of lumber or steel as a base to support each wall. This base, or sill, is fastened to the floor structure with screws or bolts to secure the wall in place. When a sill sealer is used, it is installed between the floor and the sill plate around the perimeter of the structure.

Often, the material used to construct the floor in a new building is not perfectly level. This may be attributed to the material itself, or to sloppy construction techniques. The use of a sill sealer makes up for these imperfections and helps to ensure the walls will be level. This not only results in a better-looking building, but also makes the building process easier and reduces the risk of costly errors.

Some builders also use the same sill sealers around window frames.
Some builders also use the same sill sealers around window frames.

Like all types of insulation, sill sealers also block air infiltration between the sill and the floor. This improves the overall energy efficiency of the home, which often results in lower heating and cooling cost. A more efficient house also tends to be more comfortable, as there are fewer cold drafts and less need to continually adjust the thermostat. By filling the gap at the base of each wall, the sill sealer also helps to prevent insect infestation, and may also reduce the amount of unwanted moisture that enters the home.

Sill sealers are typically made from foam or fiberglass. The sill sealer is typically less than one inch (2.54 cm) thick, and should be cut to the same width as the walls. Many feature a built-in adhesive that makes it easy for installers to fasten the sill sealer to the floor, though some others may require glue or other fasteners. Many are treated with chemicals or borate to enhance fire-resistance and prohibit mold growth if the sealer gets wet.

Some builders also use the same sill sealers around door and window frames. The sealer can be placed between floor framing and the sill of a door, or around the entire perimeter of a window frame. When used in this manner, these products help to improve energy efficiency and minimize air infiltration into and out of the home. They also level out the door or window in openings that are slightly uneven or out-of-plumb.

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Discussion Comments


@Charred - You raise an excellent point. I’ve known of people who were excited about the type of energy efficient windows they were about to install, like thermal windows for example.

They installed the windows but didn’t use a proper sealant and had problems later on. The sealant is not that expensive either. You can get inexpensive weather sealing tape from any home improvement store and immediately save yourself a lot of time and money later on.


A foam sill sealer is practically a necessity for wooden window frames, in my opinion, for another reason that is not mentioned in the article. They help prevent water damage.

Your window is constantly exposed to a variety of weather conditions, including, cold, heat, rain and snow. All of these conditions cause the frame to expand and contract and could result in water leaks.

You should apply the sill sealer to the frame to prevent this from happening. Take your time applying the sealant, too; you don’t want to cut corners here only to have to deal with water damage and leaks later on.

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    • Some builders also use the same sill sealers around window frames.
      By: Iriana Shiyan
      Some builders also use the same sill sealers around window frames.