Published on June 13th, 2013 | by Henri


Insulating an older concrete-block foundation

Q.    I have a mid 1950s ranch house with a block foundation. About two feet of the foundation is above grade. What is the best way to insulate it? I see my options as:

  1. digging down about a foot or two and attaching rigid foam on the exterior,
  2. rigid foam or fiberglass on the interior or
  3. having liquid foam pumped into the blocks by drilling holes in the foundation.

Number 3 is what my parents had done in the mid 1970s to their house.

I get water in the basement during heavy rain storms a couple times a year along one wall. The water seems to come in at the base of the block wall and concrete floor. Don’t know if it matters but, I run a dehumidifier most of the summer too.

A.    I vote for 2, using extruded rigid foam insulation (XPS). But you should address the leakage first. It is telling you that the ground against the foundation is saturated. This could result in frost pressure on the walls that could cause them to buckle. Examine carefully the grade against your foundation. Any low spot that can collect water should be raised so the grade slopes away from the foundation. If you have gutters, check that the downspouts discharge away from the house — often there is a low spot at the discharge point where water pools.

Once the leakage has been stopped, you can insulate the inside of the block walls but do not go any deeper than two feet below grade. (The greatest heat loss is above grade and the two feet below.) This will allow some heat to keep the frost at bay. Make sure the walls are clean and free of dust, etc. Apply daubs of Styrobond to the walls every foot in all directions and press 2-inch thick XPS (blue or pink) rigid insulation onto the adhesive.

Dehumidifiers are always a good idea in summer to keep the basement air as dry as possible and prevent mildew.

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