R-values tell how well-insulated a component of construction happens to be, right?? Bulloney!
It is one number and it is, quite frankly, meaningless. Heat moves many ways. Let’s consider an attic on a cold winter evening. Let’s imagine a typical ceiling insulation job with 12 inches of blown fiberglass insulation and a vented attic space on a 20 degree night when the sky is clear and winds out of the North are whipping along at 10 mph..
Let’s assume the family is home and has the interior temperature controlled to a toasty 70 degrees. The pile of insulation is held down on the ceiling by good ole gravity and the furnace is cycling on and off to maintain comfort. Simple, right?
NOT!! Above the attic, the roof is cooling fast as it radiates what little heat it has to the night sky. The air in the attic is affected by several actions: the wind whipping through the soffit vents and wind-washing gobs of heat from the fluffy fiberglass, the even colder air falling from the cold roof and sinking into the fluffy fiberglass, and the slightly warmed up air drifting up from the ceiling with tiny remnants of heat nabbed from resting on the sheetrock. All these actions are robbing the house of heat, and the R-value of the fiberglass is one meaningless number as it fails to stop the flow of heat by convection, conduction through exposed framing members and radiation to the night sky
Contrast this typical attic with an attic insulated with 2-3 inches of sprayed urethane foam, with an R-value of around 8 per inch. The R-value of the foam would come in at around 20 and the R-value of the fiberglass is an oft-quoted 38. But here’s the rub. The heat loss is twice as much through the R-38 as through the R-20 because of convective currents. Wind is a factor; so is the convective current created by cold night skies and warm homes.
Sidewalls, knee walls, slopes and floors experience a similar fate. When builders use two inches of sprayed urethane foam, the houses are tight, easy to heat and cool and have lower bills than when side-by-side identical houses have standard fiberglass insulation filling the entire cavity.
Recent innovations have concluded the significant benefit of encapsulating the blown insulation, the exposed knee wall insulation and open under-floor cavity insulation with plywood, foam board or even sheetrock or scrim paper. By simply covering the cellulose or fiberglass or rock wool, energy efficiency experts have discovered surprising improvements in efficacy of the products. Voila! The R-values return when they stop the flow of cold air from robbing the pockets of warmed air inside the insulation.