Insulating The Basement

The basement is usually the last place people think to insulate, yet it can account for up to 1/3 of heat loss in a home. Upgrading basement insulation can significantly cut down on energy use. It also creates a comfortable space - a boon to people who wish to extend their recreation and living space into the basement. The perfect time to consider insulating the basement is during a renovation.

Where Should the Insulation Go?
From a building-science point-of-view, it is better to insulate the outside of the foundation. From a practicality point-of-view, however, insulation is easier to apply from the inside, especially if done while renovating.

Here are the pros and cons of exterior versus interior insulation:

Exterior Pros

  • You can address any foundation water issues at the same time.
  • Insulation will not take up interior basement space.
  • The foundation itself will be warm (at the interior temperature), reducing condensation issues.
  • The foundation stays dry inside, reducing or eliminating mold and mildew.

Exterior Cons

  • Disruptive and expensive: you have to dig soil to get at the foundation.
  • The above-grade portion of the foundation insulation is difficult to finish and protect. Rigid-foam insulation is the most common exterior insulation material, most often finished with stucco, a fragile finish that gets damaged from impact of lawn tools, etc. Possible fix: use exterior insulation for the below-grade part of the exterior and then insulate the above-grade part from the inside.

Interior Pros

  • Much easier to install and less expensive than exterior insulation.
  • Does not disrupt landscaping.
  • Can be done while renovating the basement.

                                                                         

Interior Cons

  • Foundation wall will be at exterior temperature (cold), making it prone to condensation. Possible fix: foam-based insulation that is tightly sealed to the foundation wall is a good way to control air leakage from the house to the foundation wall surface.
  • Likely migration of moisture into the finished wall due to seepage from outside and from capillary action that draws moisture up from the ground through the footing.
  • Any moisture that gets into the wall system from the two mechanisms above does not dry readily, making the wall prone to mold.

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