I want to add some boards in the attic for storage. Should I use 1/2" or 3/4"? I also need to add insulation since it is about 4" below the top of the joists. I can get 6" r-19 unfaced and add it on top but it will be 2" above the joists. I figured no big deal and I could just lay my storage boards on top and it will just compress it. The guy at Home Depot told me not to go above the top of the joists or it is just a waste and he said I should not compress the insulation either. I am having a hard time understanding his information. My boards are only going to cover a part of the attic, I was only going to use 3 4x8 boards.

 

Before you think about adding storage up there, you might want to consider just how much insulation is needed in your attic. See current recommendations by the Department of Energy at: http://www1.eere.energy.gov/consumer/tips/insulation.html

They have both a Zipcode link and a map/graph to help you figure out what is needed.

You would want to add the proper amount of insulation before you consider adding your sheets of plywood. The person you talked to at HD was correct- you do not want to compress fiberglass, and if it extends above your joists, it will be somewhat less effective. Blown in loose insulation, either fiberglass or cellulose might be the best method for you to add r-value to the attic.

Depending on how much insulation is needed, that will determine the level at which your plywood must be installed. If it is only 2" above your rafters, you might be able to add some sleepers on top of your existing joists, and lay the plywood on top. But if you need more than that, you would probably be wise to sister larger floor joists onto your existing joists, ensuring that the ends of these new joist would be resting on load bearing walls. Then the top of those new joists could be covered with your plywood to create your storage area. This might be wise regardless, because attic space is generally not rated for storage. If you put 75 lbs / sq ft. in an area that was only built to handle 20 lbs / sq ft. you could have problems.

At any rate, if your joists are 16" OC, 1/2" would be fine for storage. If your joists are 24" OC, 3/4" would be better.

 

Why is it so bad to compress the insulation?

 

Insulation only works if it is fluffy and has voids of air that slow the transfer of heat and cold. If you compress 5 1/2" thick R-19 insulation it compresses the air spaces, and it would be no more effective than if you had used a batt of 3 1/2" R-13.