When tested in simulated attic conditions, the material showed much worse thermal performance than that measured in small-scale tests. Scientists then observed in the field that convection caused a significant reduction in thermal performance. Since laboratory testing did not simulate the interactions between heat and air flows, it did not predict the decline in field performance. Since convection depends on air permeability which, in turn, depends on the structure of the finished product, improved thermal resistivity also increases resistance to air flows. Manufacturers in the United States did not follow suit.
Air movements can compromise the thermal performance of glass-fibre in wall applications as well.
Windwashing, which occurs when wind enters and exits at different locations of the building facade, may also reduce thermal performance. High-performance batts of glass, slag and rock fibres demonstrate good field performance, provided they are protected from air and moisture. Although it did not have today's chemicals, it performed well. Clearly, the leaky walls of older homes permitted rapid drying of the CFI and eliminated the problem associated with moisture entrapment. But the popu- rarity of CFI in walls has declined.