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How-To

Insulating beneath a slab

By William A. Randall Issue 81

Q:

Standard preparation underneath a residential slab these days seems to include the use of rigid insulation. But I’ve heard that with the perimeter wall insulated to below the frost line, the temperature of the earth below the slab is about 50° F, so how important is it to insulate against such mild temperatures?

Brent Harold, Wellfleet, MA

A:

William A. Randall, an architect in Eugene, Oregon, replies: You are right that the earth below the frost line does maintain a relatively constant 50°. The primary goal of insulating floor slabs is to prevent the escape of heat at the perimeter, not at the center of the slab. Generally, heat loss through the center of the slab will merely warm the earth slightly and thereby reduce heat loss until equilibrium is reached. The net effect is virtually no heat loss through the center of the slab.

Most codes allow the option of insulating the perimeter of the slab to below frost line (i.e., insulating the foundation wall to the bottom of the footing) or returning insulation under the slab a specified distance (usually 24 in.). I generally choose the former to allow a sound base under the slab for bearing. This effectively prevents heat loss from the slab perimeter and maintains energy-conscious design.

Fine Homebuilding Recommended Products

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

Utility Knife

This utility knife is well-made and comes in handy when cutting rigid foam insulation. You can extend the snap blade to cut through a 2-inch foam board.
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Great Stuff Foam Cleaner

It’s important to clean and maintain a professional foam-dispensing gun, and this product fits the bill. It has a spray applicator for general cleaning, plus it screws onto the dispensing gun so you can clean the inside.
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Insulation Knife

If you like insulating with mineral wool, this knife makes cuts that are clean, neat, and easy — it's much better than a utility knife which can be slow and messy.
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