Understanding R Value

The term “R-value” refers to the resistance to heat flow, with a higher R-value indicating better insulation power. While it’s assumed that insulation with the same R-value should perform equally, this isn’t always the case in practice. The effectiveness of insulation largely depends on its installation. According to the consumer.ftc.gov, perfectly installed insulation can retain most of its R-value, but commonly used materials like fiberglass and cellulose often see about a 50% reduction in effectiveness due to installation issues.

Consider a typical attic scenario: during hot days with temperatures ranging from 85-100°F, the attic can heat up significantly more due to radiant heat build-up and hot air infiltration. This heat can cause attic temperatures to soar to 120-140°F. If the living space below is at 75°F, the temperature difference (Delta T) can reach up to 60°F. This significant Delta T forces air conditioning systems to work harder to maintain comfortable temperatures below.

However, by applying spray foam insulation directly to the underside of the roof deck and between the rafters, homeowners can create an unvented attic that significantly reduces this temperature differential to about 10-15°F. This not only makes the living area more comfortable but also enhances the efficiency of air conditioning units by reducing the load and frequency of operation. This adjustment in insulation strategy can lead to significant energy savings and improved indoor comfort.