How to Insulate Windows with Curtains or Other Window Treatments

When you draw the curtains across your windows, you probably feel as if you are keeping out the cold. However, are you really? You probably are not, unless you have taken care of the radiant, conductive, and convective heat losses that most drapes exhibit.

Usually drapes in people’s homes are not really effective at insulating windows, because they are not properly sealed around the edges, and they are not thick enough. However, if you are hoping you can use your drapes or curtains to insulate your windows, read on; it is possible to do so. You simply need to make sure your drapes are set up correctly to do the job.

Window treatments (curtains, blinds etc.) need to address heat losses in the following aspects:

• -convective heat losses from around the edges of the curtains, drapes, or blinds,
• -convective heat losses through the fabric of the curtain, drape, or blind
• -radiant heat losses through the fabric of the curtain drape or blind
• -conductive heat losses through the fabric of the curtain drape or blind.

Now, let's look at a step by step approach to cutting out heat losses by window treatments.

If a window covering does not fit closely to the casement of the window it is covering, air that has been cooled at the surface of the window glass tends to flow into the room from the edges of the curtain or blind. The article How to Get Your Drapes to Insulate Your Windows: The Importance of a Good Seal explains how to prevent convective heat losses from occurring around the the edges of window treatments. This form of heat loss from window treatments is very important. No matter how well constructed drapes or blinds may be otherwise, if this form of heat loss is not addressed, the insulating value of the window treatment is seriously compromised. Besides this, the cold air that rushes out from behind a curtain or drape feels drafty and reduces comfort in the room.

Curtains or drapes also need to be tightly woven in order to prevent air from moving through the fabric itself. If the fabric of a window treatment is not tightly woven, a vapor proof layer can be added to the drape or blind which would provide the same function. In other words, the main curtain or drape fabric does not need to be a tightly woven fabric if a liner is used.

A significant amount of heat is simply radiated through a curtain from the warm room, and then through the window glass into the great outdoors. To prevent this loss of heat, a curtain might include a reflective layer which reflects radiant heat back into the room, rather than simply allowing it to radiate outwards through the window glass. You can assemble fabrics and materials into a quilted type of curtain yourself, or you can purchase ready made insulating liners that provide both insulating and radiantly reflective properties with R-4 values.

If you want to save some money and make such a curtain yourself, you can purchase a product from the hardware store called “Reflectix”. You find this in the insulation department. This material (also known as Astro Foil) is nothing more than aluminized bubble wrap with a few layers of bubbles sandwiched between aluminumized outer surfaces. It is used to insulate difficult spots, and as a heat barrier in the attic among other things. I am sure you have seen bits of this stuff around. It would make a most excellent addition to a liner.

The best way to use Reflectix is by sewing it in between two layers of fabric (any material would do,) so that it could be easily hung and fastened. Reflectix is light, flexible, very inexpensive, highly reflective, and has an R value of over 6, and quite possibly could perform at an R value of something more like 10 in this type of situation. Something with an R value of 10 is on par with the insulating value of the usual insulated 2x4 framed wall.

The more slowly heat travels through the material of the window treatment the better. Heat is conducted through the fabric of the curtain or blind quicky or slowly depending upon how the window covering is constructed. Ideally, a curtain or blind should at least include several layers. These layers are needed so that air can be trapped between them. It is a dead air space which provides the greatest insulating capacity. So, the more layers the better.

Watch out for floor registers beneath the drapes. Unfortunately, heating vents are often placed in front of windows. So, it is likely you will find them blowing up into the window/curtain space. If this is the case, you need to purchase a plastic or metal diverter, and hen you can blow the warm air out into the room where it belongs.

By applying this information, and turning your draperies and curtains effectively into removable insulation, you not only reduce your heating requirements and increase the efficiency of solar inputs that your home is able to achieve, but you also make your living space much more comfortable by reducing draughts and temperature stratifications in your environment.