February 2007

Floor insulation is a very important part in the complete energy conservative package for a home. Floor insulation is sometimes overlooked when people are building their house.

Why Floor Insulation Works:
Heat doesn’t rise. The myth that heat rises was used for many years to justify not insulating floors at all. That led to some very cold feet. It’s hot air that rises. An air mass that is warmer than the air around it will rise in relation to that cooler air. The process of hot air rising is called convection. It’s only one way that heat can escape a building. Two others are conduction and radiation. Floor insulation limits all three modes of heat loss. A warmer floor reduces the temperature difference that drives convection. Floor insulation also directly resists conduction and radiation to the colder air below the floor.

Lift The Batt: 
Like walls, floor cavities should be completely filled with insulation—without gaps or voids. To get your money’s worth from floor insulation it must contact the sub floor and both joists. In many cases, it’s worth the extra cost to buy enough insulation to fill the cavity, even if you end up with R-38. To avoid a gap in this situation, the batt must be pushed up into the cavity. With the proper support, that can be done. Springy metal rods are commonly used to hold insulation up in the top of the floor cavity. Plastic straps are another option.

Fill The Cavity:
Buying a thicker batt may be a better option than trying to lift a thinner batt into the proper position. Material costs will climb slightly but labor should be the same. Attaching the insulation support to the bottom of the floor joist will be easier. It could also lead to a higher quality job because there is less chance for compression or gaps.

For example, upgrading floor insulation from R-19 to R-30 or R38 can save several times more money in heating costs over the life of the house than the initial cost to install it. Of course, colder climates will benefit most, but even in moderate climates the economics are generally positive