The North American Insulation Manufacturers Association (NAIMA) frequently receives questions from insulation professionals, consumers and others looking for general information on the performance of various fiber glass, rock and slag wool insulation products. One question that has been asked repeatedly over the years relates to the possible loss of R-value of loose fill (blown in) fiberglass insulation in colder temperatures, a misconception often perpetuated by competing insulation products to try and discredit the performance of loose-fill fiber glass insulation.
Several years ago, a series of tests were completed on fiber glass loose-fill insulation that was intentionally installed improperly at a lighter density than the manufacturers specify in their instructions. Because the insulation tested was “light,” when the temperature in the attic dropped below 12°F the R-value began to degrade because of a phenomena called convection, which occurs when air is heated it rises and then when it cools as it falls. Still air is an excellent insulator and fiber glass and rock wool slow heat transfer by preventing or slowing air movement. This convective air movement can increase the heat loss through the attic insulation if it is not at the proper density. This is because loose-fill insulation with a lighter (improper) density does not have enough closely packed fibers to adequately slow air movement at very low temperatures — it still slows the air movement, just not as well as loose-fill fibers at the proper density.
Subsequent to the aforementioned analysis, NAIMA members now formulate products and installation guidance specifically for colder climates. As a result, the fiber glass loose-fill insulations on the market today are not representative of what happened during the testing conducted a few years ago. In addition, further analysis showed two things – first when loose-fill insulation is installed at the proper density the R-value does not decrease (in fact, the R-value increases at lower temperature) and even in the case where the insulation is not installed properly, the increased heat loss for a typical attic is fairly small. Researchers estimate that even at the lighter/improper density the added heating cost for a home in the northern United States would only be $20 (1992 dollars) per year.
While the cost of not doing the job properly and in accordance with manufacturer’s specifications may be minimal, the optimal solution is installing loose fill insulation at the proper density to achieve maximum thermal benefit. When it comes to insulation jobs, like most other home improvement or building projects, the devil is in the details. And getting it right the first time will result in a more comfortable home and a happier homeowner.
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 Energy Design Update article “Good News on Attic Fiberglass Performance,” (January 1992).