How to Insulate Sill plates, mud sills, timber sills, basement rim joists, boxed sills

Key considerations:

There’s a lot going on at the first connection between your foundation and the framed part of the house.  This is a common site for air entry because it is the lowest part of the house where cold dense air wants to enter.  It is also a difficult, if not forgotten, location to properly insulate.  See also basements and crawlspaces.

Possible downsides:

  • Premature rotting of wood framing
  • Wet insulation
  • Mold growth and poor indoor air quality
  • Cold air infiltration
  • Cold floors
  • Poor energy efficiency for the whole house
  • Frozen pipes
  • Reduced efficiency from duct runs and hot water lines 


Properly fit insulation between the floor joists.  Air seal the insulation in place and air seal the wood to concrete joint and any wood to wood joints.  Insulate the foundation walls with a moisture resilient insulation system (closed cell foam is the best choice here) to cover the areas exposed above the outside grade.   On old stone and block foundations beware of frozen ground water pushing in on the foundation if you insulate all the way to the basement floor.  Air leaks can enter at ground level and exit well below ground level.

Decide if your basement/ crawlspace is going to be indoor space or outdoor space and insulate accordingly.  Many homes are insulated in the basement ceiling even with the furnace, hot water heater, and the heating distribution system in them.  In those cases it may make more sense to insulate in the plane of the walls to “trap” the radiant losses from those appliances than to leave them on the cold side of your insulation.

  1. Sheetrock rarely goes all the way to the floor behind baseboards.  A common place for cold air entry.
  2. Insulation is often stuffed between floor joists along the exterior band joist; it is rarely cut to fit and then sealed in place.  

Timber sills are often seen in older (pre 1940) homes on top of dry stacked or mortared stone foundations. 

3. Plank flooring and sheathing is notoriously leaky

If you already have the sill plate well insulated you may want to actively heat your basement or crawlspace. For more on crawl space heaters see

Back to "How to Insulate" from "Insulate Sills"