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
Recommendations:
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.


- Sheetrock rarely goes all the way
to the floor behind baseboards. A
common place for cold air entry.
- 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 Green-Energy-Efficient-Homes.com.
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