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Tue, Oct. 15

Column: Attic insulation, Part II – where no man has gone before

Last time I wrote about how just a small area of missing attic insulation can have a large effect on the overall attic insulation. A quick review: we measure insulation by its R value - the higher the R number the better. Walls in a house usually have R-11 (2x4 walls) or R-19 (2x6 walls) fiberglass batt insulation installed. Batt insulation comes on a roll.

Most attics in our areas have loose insulation, either cellulose or fiberglass. There are other materials I find in older homes, such as mineral wool. I have found carpeting, newspapers and Styrofoam (one attic was insulated with cut apart coolers). I found cinders as attic insulation a couple times. I’ve heard they used this in Flagstaff for a while, until they discovered how well cinders will burn.

We also discussed the U value, which we need for our equation later. The U value is the inverse of the R value- U=1/R and R-1/U. So R-30 would have a U value of 1/30 or .0333. In our example last time we used a 1000 sf attic with R-30 in half and R-10 in half. So the average insulation would be R-20, and one would assume the ‘actual’ insulation would also be R-20. But that’s not true. You will lose more heat through the low insulation levels. There is an equation to compute the actual R value for different levels of insulation in the same attic: sf x U + sf x U (repeat for all different insulation levels), divided by the total sf. Using the equation we found that attic actually had R-15, which is 25 percent less than R-20.

We used the equation to calculate the loss of insulation if an attic access cover is not insulated. The typical attic access is only about 4 sf. But that dropped a 1000 sf attic with R-30 insulation down to R-28.7, almost a 5 percent drop in actual insulation. And it could be as high as 10 percent, depending on other factors.

I have found all the following: A rear patio made into a family room with R-11 batt insulation installed: total insulation went from R-28 to R-22. A 1,620 square foot attic insulated to R-30, except no insulation over a 120 sf addition. This lowered the attic insulation to R-18, a 40 percent loss! A 2,000 square foot attic with a 500 square foot garage made into living space with no attic insulation: actual insulation went from R-30 to R-2! Yes, R-2 - I ran the calculation three times.

But even small areas of missing insulation can have a significant effect on the overall insulation value, and on your heating/cooling costs. And I find this in a lot of attics! I have found uninsulated areas at furnaces, and where some repair or improvement was made (e.g. fixing leak damage, repairing a bath exhaust fan duct). These areas are almost always larger than the uninsulated access cover, and that lowered our attic insulation R value by 5 to 10 percent.

Often a plywood “floor” has been installed in an attic for storage space. Every sheet of plywood is 32 sf. Just two sheets would be 64 sf, or half the size of that uninsulated room we used a minute ago. And remember - that 120 sf uninsulated room lowed the actual attic R-value by 40 percent.

I don’t think there’s a formula for the “footprints” that I see in attics. In some attics, there are footprints from one end to another. R-30 cellulose insulation is about 8 inches deep. R-30 fiberglass insulation is about 12 inches deep. Often these “footprints” are right down to the trusses, especially if they have been used more than once. And of course they’ve been used at least twice, or I’d be finding bodies in attics.

A size 12 shoeprint in insulation is probably a half sf, because the “holes” in the insulation are larger at the top. But to be conservative let’s use three footprints to make a square foot. Let’s use that same 1,000 sf attic with R-30 insulation. But now, there are 45 footprints. 45 divided by 3 would be 15 sf. And let’s say the insulation is only three-quarters compressed, which would make it @ R-7. It would actually be less than R-7; last time we learned that if you compress insulation it loses some of its R value. But we’ll use R-7. So for our formula we have 985 sf at R-30 and 15 sf at R-7. The actual R-value would be R-28.5. That’s a 5 percent loss of R value for some footprints. And we were conservative on both the sf and the R value in the footprints.

When I enter an attic and there are no footprints, I always think of Star Trek: “where no man has gone before.” And here’s a surprise for the editors - I’m actually going to tie all this in to home inspections. The Standards of Professional Practice for Arizona Home Inspectors state an inspector has to “…observe readily accessible installed systems and components listed in these Standards.” The Standards defines “readily accessible” as “Available for inspection without requiring moving of personal property, dismantling, destructive measures, or any action which will likely involve risk to persons or property.” So an attic would normally be “readily accessible.” However, the Standards also state “... inspectors are NOT required to disturb insulation, move personal items, furniture …”

So in my personal humble opinion, inspectors are required to enter attics unless it would disturb the insulation. The glossary does not define “disturb,” but I would think leaving footprints that lower the R value by 5 to 10 percent would fall into that category. Anyone entering an attic and “disturbing” the insulation should level it out as much as possible on their way back out.

Randy West owns Professional Building Consultants in Prescott. He is state-certified and has performed more than 7,000 home inspections in the Prescott area. West serves on the Home Inspector Rules and Standards Committee for the Arizona Board of Technical Registration. Contact him at or visit

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