Insulated Window Coverings

Last updated 2008.03.26

What are insulated window coverings?

In simple terms, it's a blanket to reduce heat transfer through your windows. In cold weather, windows are usually the single largest source of heat loss from our homes. While walls vary in their insulating capacity from R12 (common building code value in cold climates) to R40 (typically considered super-insulated), windows may be as poor as R2. Low-Energy, argon-filled, double pane, vinyl-cased windows are typically R3 to R4. Very high quality, high efficiency windows may achieve R6 or R7. I don't have any scientific data, but I'm guessing the quilt we use on our window (as illustrated here) has an effective R-value of about 4 to 5. That's about the same as an inch of styrofoam board. It certainly made a huge difference in the perceived warmth in the room.

There are 3 ways windows lose heat from your house: conduction, convection and radiance. Conduction is the simple transfer of heat energy by contact from the warm air in the room, to the glass, to the cold air outside. (In the case of double glazed and triple glazed units, conduction losses are slowed by creating more layers to be crossed, including insulating layers of air or inert gas.) Convection is the loss caused by moving air. Radiance is the loss of infrared energy simply transiting the window barrier as light energy, to which the windows are fairly transparent. The quilt covering our window addresses all three heat-loss modes. It creates an additional boundary layer between the warm air in the room and the cold air outside – in fact, 4 layers of boundary, to reduce conduction loss. By hanging against the window frame, it prevents air from flowing from the room to the window surface, reducing convection loss. Finally, by including a radiant barrier within the quilt, it reduces radiant heat loss.

By hanging the quilt on our window when the sun goes down each day, we reduce the heat loss from our windows when we want to keep the heat in. By taking it down when the sun is shining, we get the maximum benefit from passive solar heat gain from our sun-facing windows when we want it. The insulated window covering (and a bit of discipline) let's us have the best of both worlds from our windows.

Quilt from inside, with vertical blinds open (248 KB - wall is dark blue-grey, window frame is white, blinds are grey, fabric strips in quilt are various blues and complementary colours)
Quilt from inside, with vertical blinds closed (111 KB - not visible)
Quilt from outside (183 KB - unobtrusive to neighbours, especially at night – item at bottom is our solar water pre-heater – the rings at the corners of the window glass are for attaching shade cloths)

Our window covering is based on conventional quilting techniques. It uses the warmest batting (highest insulating value) we could find available commercially, and a layer of aluminized mylar as a radiant barrier. Cotton fabric is used on both sides for strength and aesthetics.

By using a light coloured fabric on the reverse side, there is also some reflective benefit against solar gain in the summer, helping keep the house cooler. Not as effective as the shade cloth, but the two can be used together.

That should provide enough information for you to make your own. We recommend machine washing on a gentle cycle, and hang to dry. The mylar sheet does not respond well to clothes dryer temperatures.

Another option for many windows is to buy polystyrene insulating board (either the extruded board which is typically pink or blue, or the pebble board which is usually white; we prefer the extruded board) and cut it to fit the opening, just snug enough to stay in place. Insert in the evening and remove again in the morning. You can glue fabric to the inward side of the panels for aesthetics, and aluminized mylar to the window facing side to improve reflectivity against summer solar gain.

We still prefer the quilt combined with clear plastic in the winter to reduce drafts and improve the insulation, but it's likely a matter of taste as to which you will prefer.

Note: "Window Quilt" is a registered trademark of The Warm Company.

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