One of the key components of a successful dense pack installation is a powerful enough blower. Bill Hulstrunk, technical manager of National Fiber Cellulose, recommends a pressure of no less than 80 water column inches (80 inches WC, or 2.9 psi), measured at the blower outlet while blowing air and not material. These numbers seem to be appropriate, based on follow-up tests that Hulstrunk and I conducted with a calibrated cavity test box. Packing this test box of known weight and volume can reveal the net added weight of cellulose per cubic foot. Inspectors have yet to see a blowing machine that can reach 80 inches WC and can’t achieve a standard 3.5 lb of cellulose per cubic foot. The blowers that can’t generate the necessary pressure can’t pack the cellulose to the optimal density. Also, blowing equipment that blows with enough pressure to dense pack the cellulose is easier and faster to work with. The cavity fills quickly and to the proper density.
There are a number of reasons for the lackluster densities. They include
- old, undersized, or otherwise under performing blower motors;
- failure to maintain equipment, especially worn seals (when air can bypass worn seals, less pressure is available on the outlet side);
- inadequate power due to low voltage or small-gauge wire;
- a flawed air-material ratio (decreasing material feed and/or increasing blower air setting results in higher density); and
- poor installer technique—including withdrawing the wall tube too quickly or not using a wall tube at all.
Consistent dense-pack is a readily obtainable and worthy goal, but ongoing testing indicates that it doesn’t happen by accident. The following recommendations will greatly increase the likelihood of getting consistent dense-pack results:
- Use an infrared thermal imager for quality control inspection.
- Conduct equipment pressure-testing on a regular basis, supplemented by the appropriate ongoing maintenance program.
- Train field crews with a calibrated test box to ensure that they have the right air-material ratio and installation technique.
Improvements in cellulose manufacture, advances in blowing equipment, and access to infrared cameras are moving the weatherization industry in a positive direction. For years we had to endure stories from homeowners who had remodeled their bathroom, only to find that their blown-in insulation had “settled.” This led to the notion that cellulose settles more and more as time goes on. We now know that much of this cellulose was never dense-packed to begin with, and much of the “settling” had already taken place before the contractor’s truck pulled out of the driveway. Fortunately, we now know how to avoid it.