It may be mid-winter, but Scott Woodward is pressing ahead with construction of a two-car garage in the Lakes Region of New Hampshire, Climate Zone 6. The garage will have a 740-sq.-ft. apartment above, and Woodward is wondering whether he should insulate below the garage slab.

“After asking my foundation guy several times, he said insulating under the slab isn’t needed,” Woodward explains in this Q&A post. “But I’m questioning that advice.”

Woodward is already planning to insulate the perimeter of the foundation wall. Will sub-slab insulation really make much of a difference? And, he continues, should he choose extruded polystyrene (XPS) rather than expanded polystyrene (EPS) in the event he decides to go ahead with it. One wrinkle: the area inside the foundation has already been backfilled and compacted up to the level of the slab.

Those are some of the issues for this Q&A Spotlight.

Calculating energy savings

To Kyle Bentley, it’s a question of “energy optimization.” Bentley does some rough calculations to compare energy use with an insulated slab to a slab with no insulation. The variables in this hypothetical exercise include floor area, ground temperature, and intended indoor temperature of the building. He estimates the no-insulation option would yield a yearly energy cost of $3720, vs. the $372 for a building with an insulated slab.

With the cost of R-10 insulation at about $1 per sq. ft., insulating the slab would cost about $750, which would be recouped in roughly two years.

“The labor involved in installing it includes removing the dirt, grading, re-compacting, (hopefully gravel now too) compacting, laying down the insulation, and adding the vapor barrier,” Bentley says. Woodward should be able to get that done for about $1500, or another five years of energy payback.

Insulating the perimeter will cut down…