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Q&A Spotlight

Does My Garage Need Sub-Slab Insulation?

A builder wonders whether there is any point to insulating the slab when the garage will be just above freezing

The owner/builder putting up a garage in New Hampshire is trying to decide whether it's worth adding insulation under the slab. The garage bays will be maintained above freezing during the winter, but there will be an occupied apartment over the garage. Photo courtesy Scott Woodward.

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…

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One Comment

  1. charlie_sullivan | | #1

    One reason to consider sub-slab insulation, not mentioned here, is that during hot, humid weather an uninsulated slab is cold. It will cool the air in the garage without removing much moisture from the air, leading to high relative humidity in the garage. That will accelerate rusting of anything in the garage and can even lead to mold problems.

    But if we just consider the goal of staying above freezing in the winter, Mike's expert answer is right on.

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