Basement Box Sill

Preventing Cold Moist Air Infiltration

The area of a home where the exterior foundation wall meets the perimeter or rim joist is called a box sill or sill plate. This is the second-most common area for air leaks in a home following attic spaces. Where accessible, we recommend insulating these boxes with a spray foam that is both a vapor barrier and a provides valuable additional R-value for the box sill space.

Because this area is typically in very near plane to the ground, it is where cold air migrates into the home. It is a very common place for condensation to occur as it is a junction of cold and warm air. Historically, homes were built solid 2”x10” joists, but construction standards have changed and it is more likely to now see engineered lumber in these areas. The engineered wood is typically thinner and provides a lower R-value resulting in more exposure to exterior temperature swings. Because of this, they are more susceptible to sweating and eventually rot. However, protecting your box sills with a vapor barrier forming expanding foam can increase the long-term durability of these products.

In some cases, the rim joist is already insulated with fiberglass or it may not be insulated at all. Either way, air is leaking in because the fiberglass is likely doing a better job filtering air and dust than stopping it. We pull out any existing insulation and vacuum out the box sill for a clean surface to adhere the spray foam insulation to. We then spray the box sills to a thickness that provides enough R-value coverage to protect the basement.

Benefits From Insulating Box Sills

Reduce Cold Floor – Increase the comfort of your basement space by reducing the amount of cold air infiltration. Cold air can fall directly to the floor and cause the floor to be much colder than if you plugged up the space.

Lower Heating Bills – Prevents heat loss from your basement space by limiting air travel

Durability – Protecting the box sill from temperature-induced condensation can help to extend the life of these products.

Insect and Rodent Control – Air sealing also aids in preventing insects from getting in behind the siding and at the rim joist.  Not to mention it aids in preventing mice from entering into the home and traveling along the box sills. They have been known to chew through electrical cords and access other areas of the house from just the box sill area.