By the time outside temperatures have reached triple digits, most people are trying to figure out ways to keep their home cooler. Since attics are full of hot air, it seems like a natural place to invest a bit of time and money to reduce the amount of heat transferred into the rest of the house.
There are a couple of ways that we've seen people try to do that, but in most situations they aren't worthwhile investments.
Add more insulation
If you have more insulation in your attic than is standard, that should keep your home cooler than other homes, right? If you go in your attic in the peak of summer, it's probably 160 degrees up there.
You might guess that a lot of that heat would transfer to your house. But as a matter of fact, that r-30 or r-36 insulation in your attic is taking care of about 90% of that. The thickness and the r-value (the capacity of an insulating material to resist heat flow) of your insulation play into how much heat transfer the insulation allows into your home.
With an r-value of r-30, you're limiting the heat transfer about as much as you can. Adding more insulation or insulation with a higher r-value won't decrease the heat transfer by enough to make it cost-effective.
Radiant barrier in the attic
A radiant barrier blocks out a lot of the sun's radiance heat transfer to cool off the attic. The sun shining on your home's shingles makes those shingles hot, no matter what temperature the air is. That makes the wood underneath the shingles hot, and the air underneath that wood becomes hot as well.
It seems like a radiant barrier would make sense to cool down your home, but it's really only necessary if you don't have insulation between your attic and the rest of your home. The insulation doesn't transfer heat very well, so the middle part of the insulation is much cooler than the top, and the bottom of the insulation, close to your home, is cooler than that.
The bottom line is, you're getting some heat transfer from your attic, but it's not a lot. That heat transfer is almost completely stopped with a typical amount of insulation in your attic, so it's not usually worth the investment to get more insulation or install a radiant barrier.
Want to know the real culprit of an overly warm house? Your door.
You have to go in and out of your house sometime, even in the summer! And when you leave and enter your home, you let hot air in through those exterior doors.
If you want to keep your home cooler, you'd be better off keeping your doors shut as much as possible rather than spending money on "improvements" that don't give you much return on your investment.