- Q.To turn a small outbuilding
into a living space, I plan to pour a slab floor within
the existing 4-foot frostwall foundation. How much and
what kind of insulation should I use for the
foundation, and where should I place it -- under the
slab or next to the frostwall?
A.Moncef Krarti, professor
of architectural engineering at the University of
Colorado in Boulder, replies: I suggest that
you insulate the exterior surface of the foundation
wall using extruded polystyrene foam boards. You
can place R-5 rigid insulation (1-inch board) to
the full 4-foot depth, or use R-10 rigid insulation
(2-inch board) to a depth of 2 feet. These two
options have almost the same thermal performance.
One advantage of the full-depth treatment is that
the insulation board will be supported by the
footing. Insulating under the floor is not really
necessary.
Either choice will save energy and cut heating
bills. For a small building in New England, for
example, a simple calculation (using various
assumptions about energy costs and heating system
performance) shows that you can expect savings
ranging from $50 to $100 per year from that level
of insulation, compared with an uninsulated
foundation.