If they are insulated at all, floors over crawlspaces or unconditioned spaces are insulated with fiberglass blankets or batts. The ideal installation includes a uniform, uncompressed layer of insulation in contact with the subfloor and the sides of the floor joists (see Figure 1A). How does reality differ from this ideal?

Insulating underneath floors is complicated by the fact that, unlike inside walls and above ceilings, there is no surface below the insulation to hold it in place. Two main methods are used to hold up insulation. In new construction, plastic netting is draped over the floor joists before the subfloor is installed; the netting is typically stapled to the top of every second or third joist. Once the netting is installed, the insulation is placed in the hammock-like support baskets, and then the subfloor is installed (see Figure 1B).

In retrofits, springy metal rods placed every couple of feet are typically used to hold the insulation. These rods, sometimes called "tiger teeth," have sharp ends and are longer than the space between joists, so they bow upward when they are pushed into place (see Figure 1C). Other techniques used in retrofits (and sometimes in new construction), include wire mesh and steel wire criss-crossed under the joists, but the rods are by far the fastest method--installers often refer to them as "lightning rods." Note that plastic straps are typically used in mobile homes (see "Checking Out HUD's Proposed Mobile Home Performance Standards," HE Nov/Dec '93).

Performance and Code Issues

How well do these standard installation techniques work? Note in Figures 2 and 3 that some ideal characteristics are compromised by the methods used: both the uniformity of thickness and the completeness of fill are reduced.

With plastic netting, the bowl-shaped undersurface of the insulation leaves gaps next to the joists. These gaps increase unwanted heat transfer between the joists and the surrounding air. Even under ideal conditions, joists act as heat-transfer fins, accelerating heat gains in summer and heat losses in winter; these gaps just make the problem worse.

With rods, insulation bows the other way (high in the center, low at the edges) and is compressed at every rod. When insulation is installed on wires or mesh attached to joist bottoms, there will be a large gap at the top unless the insulation is as thick as the joists.

So how much derating of the insulation's R-value is needed to accurately reflect typical conditions as installed? According to Harold Lorsch, editor of the Drexel Insulation Report, and Jeff Christian at the U.S. Department of Energy's Oak Ridge National Laboratory, no one seems to have measured or estimated the effects of these imperfections. If there are no air leaks into or out of the gaps (cavity) between the bottom of the subfloor and the top of the insulation, the space actually improves insulation performance slightly. However, any openings into the cavity will allow air to flow through the space, which can significantly reduce insulation effectiveness. Gaps between joists and other framing, plumbing or wiring penetrations, insulation cut too small, damage from humans and animals, and offsets in the joists at support beams are just a few examples. The effectiveness of all insulation is highly dependent on the quality of the installation.

According to Mike Lacher of fiberglass insulation manufacturer Certainteed Corporation, not only is the gap a performance issue, but the Model Building Code calls for all unrated vapor barriers to be in contact with the adjacent building surface to suppress fire.

The most important thing to remember about insulating floors over unconditioned spaces is that it should be done. Too many floors are left uninsulated. The second thing to remember is that the vapor barrier should face the warm side, usually toward the subfloor. Then concentrate on how much insulation and on installation quality: uniform thickness, complete coverage from joist to joist, and full contact with the subfloor. If full-thickness insulation is used, wire or plastic mesh can be used along the bottoms of the joists to hold the insulation in place. If thinner insulation is used, the spring metal support rods are probably the best retention scheme.

-- Steve Greenberg

Figure 1. Three conditions for insulation installation.