Insulation for your home's attic is one of the most important aspects to ensuring that you save the most money and energy in regards to heating and air conditioning.  I briefly covered insulation in our insulation intro, but it was very basic.  While catching up on my energy conservation blogs I ran across Energy Vanguard and their blog; especially one of the features on insulation.  Energy Vanguard was started by Dr. Allison Bailes, who has his doctorate in Physics.  I like his explanation of why he decided to pursue a career in home efficiency from his quick biography:

What really started the ball rolling for my new career and the founding of Energy Vanguard, though, was when I built a house myself. In 2001, I bought a piece of land and spent the next two years building a green home that's beautiful, efficient, and comfortable. Having never built anything bigger than a bookcase prior to taking on this project, I had a steep learning curve in front of me—and I loved it!

After finishing the house, I became a home energy rater and left academia for my new career in the field of high performance homes.

It turns out that Allison only lives about 25 minutes from my house so I was able to find another sustainable blogger in my home town!

I asked if I could repost his great article on proper installation of blown insulation: Flat or Lumpy - How would you like your insulation?.  The majority of the post appears below, but visit Allison's site for the full post:

(From Energy Vanguard)

My dissertation in grad school was called "Flat or Lumpy." (Of course, it had the requisite incomprehensibility in the subtitle, with words like 'heteroepitaxy.') Those two words in the title, which cut to the heart of what my surface physics research was all about, also describe a property of insulation that's important in building science.

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!

[Warning: This section contains mathematics. If you feel nausea or dizziness, skip to the conclusion below. If you feel excited by this discussion, see your local physics department. If your arousal lasts longer than 4 hours, you should work through the problems in Classical Electrodynamics by J.D. Jackson.]

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:

(Mapawatt Note: Go to Energy Vanguard for section on Heat Transfer and R-Value calculations )

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.

In my graduate research, getting smooth, flat layers was our goal. In the building science of insulation, flat also wins out over lumpy.