Energy Source Builder

Filling a Floor with Batt Insulation

If you have ever participated in a utility energy-efficiency program or built under a strong energy code, you have probably found yourself searching a set of plans for someplace to get just a little bit more insulation. Builders that have been through this often find some help under the floor.

It costs less to put more insulation under the floor than to build a thicker wall or add foam sheathing. Attics also can start to pinch insulation after about R-50 to R-60. Next, look in a floor over an unconditioned basement or crawlspace. The main advantage of boosting floor insulation is that extra R-value can be added with little additional cost.


Floor framing varies from place to place. Some builders use a pier and post system with beams 4 ft. on center. Floors framed with conventional lumber use 2-inch material that's 8, 10 or 12 inches deep. Engineered products such as I-joists are growing in popularity. They tend to be lighter, but create a deeper floor cavity. All this variation creates a challenge for floor insulators. One of the options pictured here should work no matter what kind of framing you use.

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 subfloor 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.

The amount of floor insulation required by some codes can be less than the space available. For example, an R-19 batt is 6-1/4 inches thick. A floor framed with 2x8s is about 7-1/2 inches deep, while a 2x10 floor is 9-1/2 inches. If you decide to stick to the code minimum insulation level, you'll have extra space. 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.

In some areas, it's common to hang plastic mesh over floor joists. Installers drop the insulation onto the mesh before the subfloor is installed. However, hanging the mesh creates sagging bellies. Insulation compresses near the framing and sags in the middle. Mesh should be attached to the bottom of the floor framing as shown on page 5.

Batt Thicknesses

When selecting a floor system, it helps to think about the thicknesses of typical fiberglass batts. Here is a list of batt thicknesses to help get you started.

Individual brands can vary by as much as one inch. It's a good idea to check with your supplier for the thickness of the brand they use.

R-19 6 1/4 inches
R-22 HD 5 1/2 inches
R-22 7 1/2 inches
R-25 8 1/2 inches
R-30 10 inches
R-30 HD 8 1/2 inches
R-38 12 inches
R-38 HD 10 inches

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 R-38 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.

Blown-in Insulation

Blown-in insulation systems offer another option. They fill the cavity completely, even around pipes and wires. Just as with batts, a high-quality blown-in job requires well-trained installers and attention to detail.

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 impedes conduction and radiation to the colder air below the floor.

This article appeared in Energy Source Builder #38 April 1995
©Copyright 1995 Iris Communications, Inc.

Common Flaws

compressed insulation

Wood Lath

wood lath support

Lath provides a sturdy support for insulation.

Spring Rods

spring rod support

Metal rods are available through insulation distributors. They're easy to use, but compress insulation in the middle.

Plastic Mesh

plastic mesh support

Mesh should be attached to the bottom of the framing. Draping the mesh over the joists leads to compression that reduces insulating value.


twine support

Polypropylene twine resists rot, mildew, rodents and other dangers. Staple every 12 to 18 inches.

Plastic Straps

plastic strap support

This method requires a product called Insul-Strap. It's a thin strap made of recycled plastic. The ends are wider for easy stapling. The strap can attach to the joist bottom or lift a batt up into the cavity without compressing it.