I would advise against installing the Solar Sheet on top of your floor joists.
The Solar Sheet is what I assume to be a radiant barrier typically made of a thin sheet of aluminum.
- It is generally not recommended to install a radiant barrier in a cold climate where the heating season is much longer than the cooling season.
- Radiant barriers are used to reflect heat away on those hot sunny days. However over time they can collect dust which will reduce the effectiveness of the product.
The greatest concern for your suggested installation method has nothing to do with energy savings, but more in the lines of moisture damage. These radiant barriers unless perforated are vapor barriers and can trap any moisture migrating from the house and saturate the insulation.
It is far more cost effective to air seal your attic floor and bring the insulation levels up to what is recommended for your climate (Climate Zone 5 or 6) to increase energy efficiency and comfort.
For more information:
Check Energy Star's recommended insulation level for your geography here.
Also, read "Should I include a radiant barrier in my new roof installation in Northern California?" a Q&A answered by Tammy Schwolsky.