I've had good luck doing the following:
I build a plywood (or rigid foam) box around the fan. In the winter
months I cover the box with a sheet of rigid foam insulation. I put a
piece of 2x4 wood on top (as a weight), else a gust of wind may create
enough pressure to move the top cover. I also put a few sheets of
newspaper over the fan blades, creating more dead air space.
I also make sure there is an extra kill switch in the attic, so no one
accidentally turns on the fan.
If you can't easily reach the fan from the attic, you could rig up some
type of (light weight (use rigid foam)) hinged doors (as the top cover,
which could be opened/closed from underneath).

You can also build the same type "box" over pulldown attic stairs and
save energy costs.



Amanda Robin wrote: