ASK THE EXPERTS: Slab Insulation Strategies

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Adding insulation above a concrete slab.

I bought a 1970s-era house that I am retrofitting to be more energy efficient. I upgraded the wall and ceiling insulation and have sealed gaps and cracks. I have started to replace the windows, too. My biggest conundrum is the slab floor, which is undoubtedly uninsulated. What can I do to reduce the heat loss through the slab in the winter and help make the floor more comfortable for us? What should I do around the slab’s edges and footer?

James Sand • Fargo, North Dakota

You’ve started with your energy-efficiency improvements in the right order, James. The above-grade parts of the building enclosure will have more heat loss in winter and more heat gain in summer than the below-grade foundation walls and slab.

Since you’re in North Dakota, I’ve assumed that the slab is the floor in a basement, not a slab-on-grade foundation. You also didn’t mention condensation, but I wouldn’t be surprised if moisture accumulates on the basement walls and slab. A big benefit of insulating those surfaces is that you reduce the chance of condensation. My final assumption is that you have no bulk water problems. If you have water leaking in from the outside—either from the top of the foundation walls, through the foundation walls, or up through the slab—you must deal with that problem first.

When you set out to make a more efficient building enclosure—defined as the boundary between conditioned and unconditioned spaces—you need to consider the flows of heat, air, and moisture. If you insulate your basement walls, for example, but don’t stop indoor air from finding a path to the cool concrete foundation wall, condensation could develop there. That water could then run down and rot the wall or wet the insulation, creating mold and mildew problems.

You’ll need to use control layers—insulation for heat-loss control, an air barrier for infiltration control; a vapor retarder for water vapor mitigation; and a water control layer for bulk water (on the outside) restriction. You’ll want these layers to be continuous, so pay special attention to transition zones: floor to wall, wall to band joist, wall to window.

If you use air-permeable insulations like fiberglass or mineral wool, include an air barrier between the insulation and the conditioned space. In your climate, include a vapor retarder between the insulation and the foundation wall as well. Some types of insulation, such as closed-cell spray-foam or rigid foam-board, give you all the control layers you need.

The most commonly used rigid foam insulation for slabs is extruded polystyrene (XPS). You could also use expanded polystyrene (EPS), but should choose a high-density type. Mineral wool insulation also comes in higher-density rigid “boards” and can handle the compression required of floor insulation. Once you insulate on top of the slab, you can install a floating floor, which isn’t attached to the slab, or you can fasten the subflooring to the slab through the insulation.

As for how much insulation to install on the slab, an R-value of 10 would be adequate in a cold climate. With XPS, that would be about two inches. If you don’t have much headroom in the basement, using one inch of XPS at R-5 would give you almost as much benefit as R-10. If you can’t give up even that much headroom, you can cover the slab with cork flooring or another material that will provide a little bit of R-value and raise the mean radiant temperature.

If you have a slab-on-grade foundation, insulation is more important because the slab’s temperature is less influenced by steady-state temperatures, which occur deeper underground. In warm climates (IECC climate zones 1 through 3), insulating the perimeter should suffice. But be mindful about the details—don’t give termites an easy path through the insulation and into wood. In cold climates and for new construction, you can wrap the whole exterior of the slab in insulation or insulate on the interior; with existing homes, you’ll have to work from the top. R-10 would be enough in cold climates; R-5 in warmer places.

Allison Bailes • Energy Vanguard

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