How to estimate benefits of a radiant barrier?

[JGB]

After reading some of the responses to this thread, I was left pondering the benefits of adding a radiant barrier to our attic. What is a good way to judge how much this would help us?

I appreciate general "do this to judge" tips, so everyone can benefit, but I'll also give the specifics of my house, so that you can give tailored opinions as well:

• 2-story, 3000 sq ft home; ~1800 sq ft are downstairs, and ~1200 sq ft are upstairs (this does not include the attic space)
• 2nd story is divided as a bit of air/empty space over the foyer + game room + 2 bedrooms + 1 bath + attic
• The attic is about as large as the combined size of both upstairs bedrooms, or the same size as the game room. The attic is mostly over the garage. It has some loose insulation, but is otherwise just open wood with a handful of boxes sitting around, and the HVAC equipment.
• I live in Oklahoma, where we probably have 45 days that reach 100 degrees during the summer, and another 50 days that are in the 90s. Our average electric bill during the 3-hottest summer months is $170 with the thermostat set at 78. During the rest of the year, it's about half that.

So, my estimate is that the attic accounts for 500-700 sq ft, above the insulated garage. Will a radiant barrier make a meaningful difference to our cooling costs?

[Greg]

If I understand right, you have A/C that you're using in the summer.  Anything like the radiant barrier and also more insulation (and maybe an insulated wall in the attic between the attic over the living area and the attic over the garage) would likely help reduce costs, both for A/C and heat. Also look at air sealing the ceiling over the living spaces, meaning sealing around pipes, wires, fixture boxes etc. where they pass through the drywall or framing.