Basement Box Sill Insulation

What are Box Sills?

An area of the home that usually gets neglected when it comes to insulation improvements is the basement.  The space around the perimeter of the basement ceiling where the joist meet the wall is called the box sills / sill boxes. Most homes have either no insulation in this space or at best fiberglass batt insulation. In the box sill area all that separates your home from the outside elements is a 1 to 3 inch board.  Most air leakage in the basement occurs around the perimeter of the box sill and any pipes or wires going out of the house through that area . An easy do it yourself test to see if you might have air leakage in this area  is to pull out a piece of the fiberglass and inspect the back side of the insulation to see if it is very dirty or discolored. If your insulation is dirty it is a sign of air leakage and your fiberglass batt is acting like a filter.

How can I fix it

What we do to seal and insulate this area is we pull out any existing insulation and install a 2 part closed cell spray foam. This will seal and insulate the box sills to an acceptable level.

Benefits of spray foaming box sills

  • Seals out bugs and rodents
  • Prevents condensation that sometimes occurs in this area that leads to mold or damage to your home
  • Helps keep the home more comfortable
  • Lower utility bills
  • Better indoor air quality