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Sun, Nov. 17

Column: Attic insulation and brain damage, Part I

Attic insulation
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Attic insulation

When I see an attic access in a heated area, such as a hallway or laundry room, I always recommend insulating the cover. If it’s not already, of course. I have heard of sellers and handymen arguing that this is “silly,” a waste of time, or just the inspector trying to sound important. I know buyers have ignored this recommendation (I find out when I inspect the home again five years later). So I decided to do a little research and see just how much one small area of missing insulation would affect the overall insulation.

I found formulas to calculate this. I even used them, they’re not that difficult (although it’s been a long time since college algebra!)

First a few basics. We refer to insulation as R-11, R-30, etc. So what is R exactly? This is from Wikipedia: “The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions, it is the ratio of the temperature difference across an insulator and the heat flux (heat transfer per unit area per unit time) through it. Thermal resistance varies with temperature but it is common practice in construction to treat it as a constant value.”

I deleted some formulas from that quote. The formulas were very complicated and had strange symbols that made my brain hurt. Simply put, R is the insulating value of a material. The higher the number the better the insulation. Let’s use fiberglass batt insulation for an example. Three common ratings are R-11 (3.5 inches thick), R-19 (5.5 inches thick) and R-30 (10 inches thick). Note the thicknesses are approximate, depending on the insulation. Owens Corning has R-19 insulation that is 6.25 inches thick and R-21 insulation that is 5.5 inches thick. The 3.5 and 5.5 inch insulation is made to fit inside 2-by-4 and 2-by-6 walls.

Insulation has some funny properties. If you add layers of insulation, you can add the R values. If you put R-11 insulation over R-19 in your attic, you now have R-30. But that’s only true if you don’t compress the insulation. If you put 6 inch R-19 in a 2-by-4 wall, it is no longer R-19. Compressing any type of insulation will lessen its R value.

Most attics in our area have loose insulation, either fiberglass or cellulose. This is where the math gets really brain-hurting. (Note: for this discussion we will disregard the framing members and drywall, which also have R ratings). If you have a 1000 square foot attic, and half is insulated with R-30 and the other half is insulated with R-10, we should have an average insulation of R-20, right?

Wrong! Because heat will find the path of least resistance. R-20 is the average of the “thickness,” but you will lose much more heat through the lower insulated areas. There is a formula to determine the average R factor. First you have to convert to the U factor. The U factor is how much heat can get through something, so a lower number is better. Windows are rated by the U factor. The U and R factors are inverses of each other- U=1/R and R=1/U.

The formula is sf x U + sf x U (+ as many sf x U as needed for the total sf) divided by total square foot. So in our example we have 500 sf of R-10 and 500 sf of R-30. After converting to U, our formula would be 16.6 + 50 divided by 1000, or 66.6 divided by 1000, which equals .066. This is our U value- divide into 1 for the R value of 15.15. So our R value is not R 20, it’s R 15. That’s a big difference - 25 percent less than the “average” thickness.

Now let’s figure out that missing insulation on an attic access cover, which is at 2 feet by 2 feet or 4 sf. We will assume the rest of the 1000 sf attic is R-30. We can use the same formula. We will count the uninsulated attic access as R- .5, which is the approximate R value of drywall (and we can’t use 0 in our equation). So we have 996 sf at R-30, and 4 sf at R-.5. Which is .033 x 996 (32.8) and .5 x 4 (2). 32.8 + 2 equals 34.8, divided by 1000 = .0348, which computes to R-28.7. This does not seem like that big a difference, only a 5 percent loss. But that 5 percent loss is over the entire attic.

And actually we need to think of something else regarding that access cover. You can transfer heat by conduction, convection or radiation. Conduction is heat traveling through a material - a good insulator is a very poor conductor. Convection is air movement- heat will rise and cool air will settle. Radiation is the heat hitting and warming an object-, the sun heats by radiation (only warms you if you stand in the sunlight). With no insulation an attic access cover will also have radiation. When heating the home you will radiate (lose) heat into the attic, when cooling the home the uninsulated cover will radiate (gain) heat into the home.

I have read where a small uninsulated attic access cover can reduce your total attic insulation by 10 percent, and a pull-down stairs reduces the insulation by 20 percent. Since we know we lose 5 percent from the lack of insulation, it’s possible the radiation (and possible air leakage around the cover) can account for another 5 percent. But even if your home has only one small attic access it would be cost effective to insulate the access cover.

And if you didn’t enjoy this column, you definitely won’t enjoy the next one: Attic Insulation and Brain Damage, Part II.

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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