As a self-employed, utility-sponsored, residential and commercial energy auditor, I think you should make note of an alarming recent discovery in truss floor construction. I inspected a 15-month-old home in southeastern Minnesota during December of 1992. The 18 in. tall Blandex waferboard at the perimeter of the basement ceiling was covered with 6 in. fiberglass insulation that, when removed and shaken, deposited a large amount of moisture on the floor. The 0.5 in. thick Blandex behind the insulation was covered with mold. The extremely cold surface behind the insulation with no vapor barrier appears to cause this problem, which may easily result in extreme structural damage in 2-3 years. Unfortunately, half of the perimeter was inaccessible because the ceiling had already been drywalled. If you are building truss floor construction, be sure to provide a tightly sealed vapor barrier on the warm side of the insulation in the basement ceiling perimeter in cold climates.

Jonathan Ask
Asktek Energy Services
Houston, MN

The article "Refrigerator Replacement in Florida: A Case Study" (HE Jan/Feb '93, p.20) states that the new refrigerator's "demand is less sensitive to ambient temperature." While true on an absolute basis, it is not clear that it is true on a relative one. The coincident peak demand of the old refrigerator was 126% of its yearly average demand, while the new refrigerator's peak is 138% of its average. (The article didn't give the minimum summertime demand.) From Figures 2 and 3, comparing the energy uses of the old and new refrigerators, the ratios of the summer peaks to the winter lows are about 2.7 and 4, respectively. This suggests that replacement of the older refrigerator reduces current summer peak demand, but it may paradoxically also reduce the average system load factor.

Robert Clear
Berkeley, CA

Author Danny Parker replies: "Dr. Clear is correct. The new refrigerator does have a lower relative load factor than the old unit. However, since the peak load of the new refrigerator is less than half that of the old one, the absolute peak demand (which is often important to utilities) is significantly reduced. [Editor's Note: Load factor is the average load divided by peak load for a given time period, an indication of how well the power system is utilized. Utilities strive for a high load factor.]

Incidentally, the reason that the new refrigerator has a lower load factor on a relative basis is that the load due to door openings is more significant for the new unit. About 7% of the energy use of the old guzzler was due to door openings; since the absolute load of te warm air entering during door openings is the same in the new unit, this accounts for about 19% of its (much lower) energy use. As refrigerators become more efficient, the effect of door openings, and the need to revise the DOE test procedure to reflect this factor, are becoming increasingly important.

By the way, the table on page 22 of the article incorrectly gives a figure of 0.0 kWh for refrigerator minimum daily use. It should be 1.0 kWh."