Poor installation of insulation matters. Even in cases where you might think the R-value won’t be affected, it is. Eleven years ago this month, I wrote an article about one of the most important and misunderstood principles in building science. The article is titled “Flat or Lumpy: How Would You Like Your Insulation?” In it, I showed what happens to the R-value when a poor installation of insulation is particularly bad. The attic in question was supposed to be a uniform R-30 but instead got only R-10 over 50% of the area and the other half was R-50. It does not average to R-30. In fact, it averages to a mere R-17.
Having that big a disparity in the insulation distribution, however, isn’t the most realistic situation. More likely is that you’ll get a couple of inches less in some areas and a couple of inches more in other areas with blown or sprayed insulation. Or maybe the installers shorted you on the insulation in one part of the attic but did not make it up elsewhere. Let’s look at those scenarios today.
Right amount, poorly distributed
The graphic below shows what happens to the R-value when the installers put in the right amount of insulation. The top line shows perfect installation. The other six show various degrees of poor installation of insulation where half of the attic gets less than it’s supposed to and the other half gets more. On the right in black is the average R-value for each installation. On the right in red is the percent decrease in R-value from the target of R-38.
As you can see, when the insulation is shorted only an inch or two over half the attic and increased by that amount on the other side, the hit to R-value isn’t too bad (-1% and -3%). A 3-in. short on the left side knocks 6% off the R-value, and beyond that you’re in double digits, which I would find unacceptable.
I didn’t model all of the scenarios above when less than 50% of the attic has the poor distribution. But for 9 in. and 15 in. over half of the attic and 12 in. over the other half, the reduction in R-value is 3%. Compare that to 6% (line 4 above). For 6 in. and 18 in. over half the attic and 12 in. over the other half, the reduction is 14%. Compare that to 25% (line 7 above).
Wrong amount, poorly distributed
If the installers haven’t predetermined how much insulation material they’re going to install, another way poor installation of insulation can reduce R-value is by getting the correct amount over part of the attic and shorting the other part. The two graphics below show this scenario. In the first one, the shorted side gets 2 in. less insulation than it was supposed to. The top line shows perfect installation of a uniform 12 in. depth of insulation. The next five lines show the R-value and percent reduction in R-value when various portions of the attic are shorted those 2 in.
Getting 2 in. less insulation than you pay for over part of the attic knocks the R-value down only 2% if only 10% of the attic is affected. If, however, they short you those 2 in. over 50% of the attic, you’ve lost 9% of the R-value you paid for. That’s for a 2 in. short.
If the installers short you 4 in. over part of the attic, here are the resultant R-values and percent reductions in R-value:
As you would suspect, the 4 in. short becomes painful more quickly. Even 10% of the attic shorted 4 in. knocks your R-value down by 5%. With 20% of the attic insulation reduced by 4 in., the average R-value is down 9%.
Contracts and quality assurance
Understanding the tolerance of blown or sprayed insulation can help you get the insulation R-value you pay for. Insulation contractors usually specify the average thickness of insulation you’ll get. As you can see from the analysis here, average thickness isn’t good enough.
A better way to ensure you get what you’re paying for is to specify the average thickness of insulation as well as a minimum thickness. The tool that determines that for blown insulation is the depth marker, as shown above. Make sure your contractor puts in a lot of them. It’s easy for installers to pile it up to the required depth at the markers and install less between the markers. With more markers in the attic, it’s easier to see problems. With spray foam insulation, the tool is a depth gauge (shown below).
After the installers finish blowing or spraying the insulation, look at those markers or check the depth in enough places to know you’ve got enough insulation. Average depth alone is not a sufficient metric to determine the quality of installation of blown or sprayed insulation.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.