In our metro Atlanta area, these whole-house fans are rarely beneficial except for 1 month in the fall, and 1 month during spring. Although there are some who absolutely love these ventilation marvels, despite the uncontrolled heat gain/loss, the noise, and the dust, etc sucked into the house through the open windows.
Bottom line: If you have one of these in your house, protect yourself from the summer heat gain and winter heat loss. And we haven't even talked about the uncontrolled air infiltration that trespasses into and out of your home through the louvers. Just take a look at all that dust on the shutter! I've seen sleeping bags and blankets thrown over the housing, as well as elaborate wood-framed, insulated boxes with rope and pulley.
We offer a hook & loop insulated cover that attaches from inside the house. Color is white. We also take them out, and re-sheetrock the opening in your ceiling. One last thought... disable the fan by removing the drive belt or de-energizing the electric, if you plan to throw a blanket over yours!