The True Impact of Sub-Par Insulation
Even if there is some fluffy material in your attic, there is most likely a lot of room for improvement. In fact, poor installation, even over a small area of a home, compromises the effectiveness of your attic insulation greatly. Even if your energy audit report says that you have an appropriate amount of insulation, if there are low spots in an area where items have been stored, or any places where insulation has been disturbed by construction work (even by a previous homeowner), the insulation isn’t actually working anywhere near as well as it is supposed to.
Insulation materials are rated by their ability to resist heat flow (R-value). Every time R-value doubles, heat transmission is cut in half. Conversely, when r-value drops, transmission increases exponentially. For example, going from R-38 (code required insulation for an attic, about 10-12 inches depending on material) to R-10 (about three inches of insulation) increases heat transfer by more than four times for that area. In other words, the same amount of heat will move through 25 square feet of bad insulation as 100 square feet of good insulation.
The chart above shows how insulation gaps affect the performance of the entire assembly. Even with perfect R-19 insulation (about 6 inches, half of current code requirement for attics), leaving gaps over six percent of the surface reduces the entire system’s effectiveness by 1/3! 35% of the heat loss from the entire attic comes from the six percent that has gaps. One common location where insulation gaps develop is near the attic ventilation, where air moving in through the vents washes insulation away if the ventilation air is not properly channeled.
The bottom line is that unless your attic has been recently insulated by a reputable contractor who performed air draft sealing, installed insulation vent dams, insulated and weatherstripped the attic hatch, and had the work inspected by the city, your insulation likely performs at less than half of what is required by code today.
A well-insulated attic should have complete and even coverage of the entire attic. When it’s done, a blown in insulation should like a nice blanket covering your entire attic.
Sources: “Building Analyst Training: Day II”, Building Performance Engineering, Boone, NC
Copyright September 2014 by Advanced Home Energy
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