Gary Smith - Home Inspector - Stairway

ATTIC STAIRS = Energy Wasting Hole in Your Ceiling

I’ve lost count of the number of homes I’ve inspected that have folding stairway built into the hallway or laundry room ceiling. More often than not the folding legs are not trimmed to the correct length or properly secured to the framing. That condition has the potential to create a fall hazard.  Additionally, the stair’s energy loss factor is one of the most convincing reasons to either relocate them to your garage or create a better way to prevent them wasting your hard earned cash. I’ve devised several ways to help combat the issue…contact me for more info.

Here’s an article by Dr Allison Bailes, PHD. It’s one of the best I’ve read that addresses the impact of this energy hog!

With the subject of insulation and R-value fresh in my mind again, I figured now is a good time to take it a little further. In the first article, I showed how to calculate the average R-value for lumpy insulation, and I assumed in that case that an attic had 50% coverage of R-50 and 50% coverage of R-10. The result was that the average R-value was an amazingly low R-17, not the R-30 you might guess.

So, let’s extrapolate. (Don’t you love that word! I think it does for verbs what ‘fenestration’ does for nouns, but maybe that’s just me.) Instead of an attic that’s 50/50, let’s look at one that’s 99% good and only 1% bad. In this case, the 1% is from the attic pull-down stairs, which typically have no insulation.

If we have 1000 square feet total of ceiling area, and we put R-38 everywhere but the 10 square feet of the attic pull-down stairs, you may be surprised when you see the answer. (For ease of calculation, I’m going to ignore the effect of the framing in the attic.)

Are you with me? We’ve got 990 sf at R-38 and 10 sf at R-1. (I’m being generous by assuming that quarter inch of luann plywood plus the air films give it a full R-1.) When you plug those numbers into the equation for average U-value and then convert to average R-value, the answer is R-28. (See the Flat or Lumpy article for details on the math – but be careful!)

No, I am not kidding! Because of that 1% of the attic that’s uninsulated, the average R-value for the whole attic drops by 27%. I told you it was amazing, didn’t I?

The reason for this is that, although the attic stairs account for only 1% of the area, the rate that heat flows through them by conduction (per square foot) is 38 times higher than in the insulated part of the attic. In other words, the amount of heat that flows through the 10 sf of attic stairs is the same as what flows through 380 sf of the insulated attic. Wow!

Also, what I’m talking about here is just the heat that flows through the solid material, not all the extra heat that leaks through the gaps around the edge of the attic stairs. Remember, the building envelope has both insulation (to limit heat flow by conduction) and an air barrier. I’m just talking about the former here.

So, what’re you going to do? Add more insulation to the rest of the attic and bring it up to R-49? Or find a way to insulate the attic stairs? The answer’s obvious, isn’t it?

Home Inspector – Home Builder and Building Consultant/Coach

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One Response to “ATTIC STAIRS = Energy Wasting Hole in Your Ceiling”

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