A whole house, or attic, fan is an good way to cool a house during mild weather and can reduce the need for air conditioning. However, you don't want a whole house fan to rob your home of heat in winter.

Louvers allow heat to escape

A whole house fan is usually located in the ceiling of an interior hallway and has a metal louver that closes when the fan is not running. However, the louvers do not form an airtight seal and insulate poorly. As a result, during winter considerable heat escapes through the fan. Since the fan is often located near the thermostat for the heating system, it can affect your home's comfort more than you may think.

Seal and insulate the fan

To stop heat loss through the fan, you need to insulate the louvers and make them airtight. Simply throwing a blanket over the fan in the attic doesn't help much, as warm air will rise up through the louvers and around the blanket. You need a solid cover that seals tightly and insulates well.

Interior insulated cover

One simple way to cut heat loss through the fan is to fabricate a louver cover out of insulation board. Use at least a one-inch think piece of board cut to cover the louvers. You may want to make the insulation board more rigid, and therefore easier to handle, by gluing a thin piece of plywood or other sheet material to the inside surface. White contact paper or paint will blend the cover with the ceiling.

To seal the edges of the cover to the louvers, try Velcro or a similar fastener. You may also use a threaded bolt and nut to hold the cover tightly against the louver.

An interior cover can blend with the ceiling (many home owners find it less noticeable than the metal louvers) and is easy to remove when warm weather returns and you want to use the fan. Covering the louvers in summer when you are operating the air conditioner will also help save energy. Naturally, never operate the fan with the insulated cover in place.

Attic box

If you don't like the idea of a cover that is visible from inside your home, then consider an airtight, insulated box in the attic. Try using fiberglass ductboard to fabricate the box. The trick with this box is to ensure an airtight seal against the fan housing.

With an attic box, you will have to climb into the attic to remove or replace the box. Because it is out of sight, there is a greater risk of operating the fan with the box in place, which could damage the motor. Consider a label on the fan switch to remind people to remove either the box or louver cover.