How to Insulate a basement or Crawlspace


Key considerations:

1. Basements and crawlspaces are often the lowest location of air entry in a home.  Cold dense outside air pours into any opening to replace warm house air that leaves in the high parts of the house.  The foundation, sill plate, and rim joist are all locations for air entry.  Poured concrete foundations are relatively airtight, brick and block foundations less so, and stone foundations, mortared or dry-stacked, are often quite open to outside air.  The insulation system must be able to address air leakage above grade. 

2. Basements and crawlspaces are subject to moisture from a variety of sources, high water table, poor drainage and condensation.  Since foundation walls are permeable to moisture and can store moisture, basements can influence the relative humidity of the entire house.  At a minimum, the insulation system must be moisture tolerant.  Ideally, the insulation system is also a vapor barrier and helps reduce the moisture load in the basement.    

3. Concrete conducts heat much faster than wood. The insulation system needs to create a “thermal break” to reduce the heat loss through the foundation and through the slab.

Possible downsides:

  • Premature rotting of wood framing
  • Wet insulation
  • Mold growth and poor indoor air quality
  • Air infiltration
  • Cold floors
  • Poor energy efficiency
  • High relative humidity in the basement and the house

Recommendations:

Do everything within your means to keep your basement or crawlspace dry.  Install drainage, install vapor barriers over any exposed soil, install a sump pump, and waterproof your foundation.

Look at the insulation in your basement rim joist and sill area.  Is the insulation sealed in place or just stuffed in place?  Is there an air leak between the sill plate and the foundation?

Assess your foundation insulation especially where the foundation is exposed to the outside.  Make sure that you have insulation that can handle moisture and is air tight.  The best insulation for basement sills and the top of the foundation is closed cell spray foam.  It’s an air barrier, vapor barrier, high R-value thermal barrier and it adheres to uneven surfaces. 

If you have a stone or cement block foundation be careful how far down you insulate the foundation walls.  Many basements heat the ground immediately around the house and keep the soil from freezing.  Insulating the whole foundation right down to the floor can cause the soil to freeze and damage the foundation walls. Reinforced concrete foundations in good repair should be able to handle frost action without any trouble. 

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