Another reason why opossums shouldn’t be allowed to root around in your attic
As a home inspector, I often find that the insulation in the attic is not even. Sometimes there is a path that someone has walked. Maybe a bathroom fan needed to be installed or serviced, I don’t know. But for whatever reason, the insulation is thicker in some places than in others.
This causes a HUGE heat loss.
You might think that if you measured the thick places and the thin ones, you could simply find an average, and decide not to fuss with making it smooth or adding more (and making THAT smooth).
But lumps of insulation are like rocks in a river bottom – the water just goes around the rocks.
We can use math to make my point. Green Training USA has a great course on energy conservation.
Imagine an attic space that is 40 by 25, and has R30 cellulose insulation, but the attic hatch is a pull-down ladder of 2×5, and has no insulation. The R value of the hatch itself is 1.
Total attic floor space = 40 x 25 = 1000
Attic hatch = 2×5 = 10sf
Area A = 1000 – 10 = 990
Area B = 10
Then we convert R value to U value. U value is the inverse of R. Where R shows resistance to heat transfer, U value shows how fast heat goes through something. The higher the R value, the lower the U value. R=1/U, and U=1/R
So R30= 1/ 30 = 0.0334 R1 = 1/1 = 1
Uavg = (A1 x U1) + (A2 x U2) / Total area
Uavg = (990 sf x .033) x (10 sf x 1) /1000
Uavg = (32.67 + 10) /1000 sf
Uavg = 42.67 / 1000 sf = .0.43
Ravg = 1/U Ravg = 1/ 0.013
Ravg = 23.24
So you can see that a relatively small area of low insulation can reduce the overall R value of the roof by a lot!