Contractors' Guide and Energy Design Update both published reports of a field study by Union Electric that compared blower door results from two sets of three similar houses insulated with either high-density fiberglass batts or sprayed cellulose. All houses were caulked and sealed. A control house had batt insulation, but no air sealing. Improved performance was found in the caulked and sealed homes regardless of the insulation used.

Knight indicates that a cellulose manufacturer's recommendation to omit vapor retarders is a supposed benefit. There are no data supporting recommendations that omitting vapor retarders is technically sound in Chicago's cold climate. It also violates the building code. The real reason for omitting a poly vapor retarder is to permit the wet cellulose to dry out after drywall is applied. This has little to do with building science. It is about not wasting time while waiting for the stuff to dry.

Robert A. Sullivan
Rock Wool Manufacturing Company

Paul Knight replies: We don't depend upon cellulose as an air barrier. As I described in the article, the interior drywall serves as the air barrier in our buildings.

We like cellulose because it completely fills wall cavities with no gaps and spaces, ensuring minimal convective degradation of R-value. Nevertheless, Energy Design Update (EDU) states, Cellulose fiber insulation suppresses air leakage to a much greater extent than other types of insulation (see EDUCellulose Insulation and Air Tightness, Dec. '86). The EDU report was based upon a number of studies measuring air leakage in homes with various types of insulation. Also, according to the Builder's Field Guide, (p. 182) Where blown or sprayed in-place cavity insulations are installed at sufficient density, they can significantly reduce air flow across the assembly.

A vapor retarder prevents the movement of moisture into a wall assembly by diffusion. However, air-transported moisture is 100 times greater than moisture movement by diffusion. According to the Builder's Field Guide, In most climates, if the movement of moisture-laden air into a wall or building assembly is eliminated, movement of moisture by vapor diffusion is not likely to be significant. We seal the drywall to prevent this air movement. Furthermore, should moisture accumulate in a wall cavity, we want that wall to dry to the inside. This drying can't happen if a vapor retarder is present.

From our experience, we have found that once building code officials understand these principles they are willing to grant waivers. I would concur that not wasting time is part of the picture; however, I believe our practices are based upon sound building science principles.

Finally, I believe fiberglass insulation is a fine product when installed properly. But given our building conditions and design, cellulose is the better choice.

Donald J. Schmidt

Editor's Reply: According to the U.S. Department of Energy, 200 million tons is the amount of straw that gets burned annually in fields across the United States.

Heat Pumps and Temperature Controls

A good thermostatic control is key to a heat pump's energy savings. We have a Chronotherm III--which I love--because it allows multiple and flexible setbacks. It remembers recent usage and determines a target time when heat will be needed. It turns on the heat pump earlier if necessary to reach the target temperature at the target time, minimizing the use of the electric resistance backup.

Since outside temperatures fluctuate, the intelligent control isn't perfect, and the programming favors comfort over cost. If the chip doesn't think the house will be warm enough by target time, it calls for auxiliary heat--which I hate. A little green light tells you when auxilliary heat is being used. When it lights, it means that your meter is spinning three times faster than normal and your expensive investment is operating near the efficiency of baseboard heaters.

My HVAC person showed me the dip-switch on the system's main board that controls auxiliary heat. I now leave it off. Sometimes, target temperature is a few degrees low at target time, a minor inconvenience that is more than compensated by energy savings. The little light still comes on, but now it means the thermostat is calling for, but not getting, auxiliary heat. I only turn the switch on when outside temperatures get below the system design temperature of 15°F.

Andy Kerr
Joseph, Oregon

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