Insulation and your attic hatch

Since Autumn is fully upon us and it’s starting to get quite chilly out there, let’s talk about energy efficiency.  Over the years I’ve told thousands of clients that they should insulate their attic hatch.  I hope they’ve all taken my advice, but for anybody who still needs convincing let’s put some numbers to the problem.

First a quick insulation lesson.  Insulation is rated by its R-value, which is a measure of the insulation’s resistance to heat flow.  A higher R-value means better insulation because it provides more resistance to heat flow.  So we want a high R-value insulation at our exterior walls and ceilings.

In contrast, U-value is a measure of how well a material allows the flow of heat.  Windows and doors are rated by their U-value (sometimes called U-factor), and of course you want a low U-value which would mean very little heat flow through the door or window.

R-value and U-value are reciprocals: R=1/U  and U=1/R

The International Energy Conservation Code currently requires R-49 insulation in the attic in the Chicagoland area.  This is a fairly new requirement, however.  For many years only R-38 was required in the attic, so if that’s all you’ve got it’s still pretty good.

When you add layers of insulation you just add the R-values to determine what the new total insulating value is.  So if you have only R-20 insulation on your attic floor, and you blow in another R-29 of insulation, you now have R-49 insulation in your attic.

But when you have different areas of the attic with different levels of insulation, finding the total average R-value over the entire area is more complicated.  The formula for finding the total average R-value when different areas have different insulation levels is:

U_tA_t = U₁A₁ + U₂A₂ + U₃A₃ + . . . . You find U_t and then convert that to R_t with the formula above, R = 1/U.
Let’s do an example and see how important it is to insulate your attic hatch.

Let’s say your attic floor covers an area of 800 square feet, and you have R-38 insulation over that area.  But your attic hatch, which is 6 square feet, is only covered with a piece of plywood.  That plywood only has an R-value of about R-1.  But it’s only a small area, less than 1 percent of the total floor area of the attic, so it can’t make much of a difference, right?  Let’s see.

Here’s the formula again:
U_tA_t = U₁A₁ + U₂A₂ + U₃A₃ + . . .

In our example:

Total —  A_t = 800 square feet, and we’re trying to find U_t

Insulated area — A₁ = 794 square feet,  R₁ = 38, U₁ = 1/38

Uninsulated area — A₂ = 6 square feet, R₂ =1, U₂ =1

U_t*(800) = (1/38)*(794) + (1)*(6) = 26.8947
U_t = 26.8947/800 = 0.0336    and R_t=1/U_t, so
R_t = 29.7

Wow, a small little uninsulated area reduced the total average R-value of your attic insulation from R-38 to R-29.7, a reduction of 22%.  That’s a terrible waste.

If you put just a single layer of 1-inch rigid insulation on the hatch, with a value of R-5, the new number for total attic R-value would be R-36.5.  That’s a huge improvement.  Just two inches of rigid insulation and you’re at R-37.3.

So please insulate your attic hatch.  Save some energy, and save yourself some money.