We’re building a new home. Should we insulate both sides of the foundation?

Asked by Marci Hess, Madison, WI

We are building a new home and in researching insulation have come across arguments for and against insulating both sides of the foundation. The debate seems to focus on whether insulation on both sides allows moisture to be adequately removed. Can you please provide further thoughts on this and explain the science behind both sides of the argument?


John Messerschmidt

Answered by John Messerschmidt

New York, NY

Four Points Consulting Group

July 6, 2009

There is no reason you can't insulate both sides of the foundation, as long as you only have one vapor barrier on the outside of the foundation.

The foundation's concrete floor and walls absorb moisture from the earth. A vapor barrier such as roofing membrane or dampproofing will keep this moisture out of the structure.

Insulation is needed to prevent another type of moisture—condensation—from occurring on the warmer side of the vapor barrier, inside the concrete block wall (similar to what happens on a cooler full of ice). The aim is to reduce the amount of condensation and allow the walls and floor to dry to the inside.

Here's how to do it:

  • install 2 inches of rock-wool insulation on the outside of the vapor barrier and foundation walls to reduce the amount of condensation.
  • Then install insulation that does not have a vapor barrier on the inside of the foundation walls.
  • For the floor, I recommend putting 4 inches of insulation under the slab, because you can't put insulation on top of the floor.
  • Frame the walls with 1-5/8" steel studs one inch from the wall and install Bonded Logic cotton insulation. (Leave a 2-inch gap above the floor in case of flooding.) This will further increase your R-value, reduce condensation, and make the walls sound solid and sturdy (not hollow).
  • As long as there is no vapor barrier on the inside, your walls will be able to breathe and dry out.

If you're on a budget, I suppose you could use fiberglass insulation, but just know that fiberglass is not an attractive choice environmentally. It is a probable carcinogen and extremely energy-intensive to manufacture.


For more information:

Read Mike Binder's Q&A "How far below grade should I insulate my house's exterior? What material should I use?"

Tagged In: insulation, residential foundation

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