Importance of Proper Installation of Blown-In Cellulose
Flat or Lumpy – How Would You Like Your Insulation?
Posted by Allison Bailes
Let me put the question to you this way. If your attic is going to have 50 bags of insulation blown into it, does it make much of a difference if it goes in flat or lumpy?
Let’s look at an example. If the insulation goes in perfectly flat, let’s say we have a nice uniform R-value of 30 throughout the attic. (We’re going to ignore the complicating factor of the framing and assume it’s a continuous layer of blown insulation.) There are all kinds of ways it can go in lumpy, but let’s assume that 50% of the attic has lumps of R-50 insulation, and the other 50% is valleys with only R-10. We want to find the average R-value of this lumpy configuration. It’s the same amount of insulation, and the average of R-50 and R-10 is R-30. But, how does it really perform? Let’s do the math! Now, we can’t just average the R-values. If we did that here, we’d get R-30, and we’d be wrong. Heat will take the path of least resistance, and the less resistance you give it, the more heat will flow. If you’ve studied physics, engineering, or building science, you’ve probably seen the equation for heat flow by conduction:
If we calculated the amount of heat flowing through the lumps and valleys in this attic, we’d see that the R-10 half (red bar below) would allow way more heat to flow than the R-50 half (yellow bar below). In fact, the R-10 valleys, having one-third the R-value, would allow 3 times more heat to flow than if it had R-30. The R-50 lumps would allow only 3/5 the heat flow as R-30. Also, the total amount of heat flow is greater for the lumpy insulation than for the flat insulation. As you can see below, the red and yellow parts together add up to a lot more heat flow than the orange (actually 1.8 times as much). Because the lumpy insulation allows more heat flow, it’s got to have a worse average R-value. Clearly, the average R-value is going to be more influenced by the underinsulated part than by the overinsulated part. To find the average R-value the correct way, we first have to convert R-values to U-values. R stands for resistance, as in resistance to heat flow, so the higher the number the better when we’re talking about building materials. U-value, the heat transfer coefficient, is the reciprocal of the R-value, so the lower the number, the better. (For a better understanding, see this Wikipedia article about R-value and U-value.)
The equation for average U-value is: Taking the attic as 1000 square feet total and putting in 1/R for U, we get: Taking the reciprocal of 0.06, we get the average R-value of about 17, which is much lower than the R-30 we’d get by incorrectly averaging the R-values. Conclusion As I said above, heat takes the path of least resistance, so the amount of extra heat going through the R-10 half far exceeds the extra heat flow that’s stopped on the R-50 side. Instead of getting an R-30 average, the lumpy attic has an R-17 average. In practical terms, this means that if you see an attic with lumpy insulation, get in there with a rake and smooth it out. In the example I just worked out, you’d nearly double the R-value without adding any extra insulation! Another common example of the flat-or-lumpy conundrum is an attic that’s perfectly insulated except for one small area, say the pull-down attic stairs.
We can go through the same steps as above and show that an attic that has a uniform R-30 over 99% of the area and 1% at R-1 (the pull-down stairs) will have an average R-value of 23. That’s right, those pull-down stairs can decrease your overall R-value by 25%. One small uninsulated area reduces the R-value dramatically. And that is why we pay attention to detail—-every detail!!
Little Things Matter–Insulate Everything
Attic Stairs – A Mind-Blowing Hole in Your Building Envelope
Posted by Allison Bailes
Green Building Advisor, one of my favorite sources of good info on building science and green building, ran my Flat or Lumpy article as a guest blog yesterday. (They also gave the Energy Vanguard blog a nice review!)
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 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.
U(avg): (1/38 x 990 + 1/1 x 10) /1000 = .036 R(avg): 1/.036 = 27.8
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.