Insulating Windows: Coverings & More to Prevent Infiltration, Conductive, Convection & Radiant Heat Loss

Insulating Your Windows

Insulating windows might include switching from you present windows to new ones which are insulated by nature, or by adding insulating window coverings to the windows you presently have.

Windows loose heat in four different ways.

  • Infiltration
  • Conduction
  • Convection
  • Radiance

Infiltration heat losses of windows are due to the actual leaking of air into or out of the building from around the edges of the windows. These type of heat losses can be prevented by weather-stripping and caulking. Seee article: Window Weather Stripping for Your Passive Solar House by this author.

Conduction heat losses through windows are related to the actual R value of the window panes themselves. Heat conduction involves the direct movement of heat through a material, or through a series of materials such as is the case with a double or triple glazed window, or with the layers of materials from which a building’s walls are constructed.

Convective heat losses at the surface of windows refers not to the infiltration heat losses around windows (which could actually correctly be described as convective) but to the convective movement of air which cools at the surface of the window. The cooling of this air causes it to becomes more dense, and to drop down to the floor. As the cool air drops away it draws more room air in from behind it. This air is now cooled by the glass and so ad infinitum. The conductive heat loss from the otherwise still (motionless) room air to the window glass is tremendously accelerated by this process.

Radiant heat losses are those that take place when a warmer object radiates toward a cooler object. The heat inside a warm house radiates through the windows in just the same way that heat from a fire radiates from the flames out past the hearth to your skin. Up to 65% of the heat lost through a window can be in this form. Modern window coatings address this, in part, but since we need to allow radiant gains from the sun, the situation is not so simple.

Additional panes of glass in a window do serve to increase the insulating value of the window but the insulating value of glass is so low that the improvements brought about by simply adding additional panes of glass are only modest. The R-Value of a pane of glass is about 1. Up until now, adding a completely new, hermetically sealed triple glazed window would have only been able to increase the R-Value of your window up to close to R-3 with most manufacturers. This is the approximately the same as insulating a wall with just one inch of fiberglass insulation. However, a brand new company, “Serious Materials” at is producing windows with R values of over 11. A window like this would be providing the same insulating value as a regular 2×4 framed wall with 3 ½ inches of fiberglass insulation between the studs.


Now we are cooking with gas! This R value for these windows is in the same ball park as the average R value offered by the walls of an insulated 2×4 frame built house. In some cases, exchanging current, low R value windows to these high R-11 value windows could eliminate over one third of the heating energy losses of the home.

Unfortunately this improvement will come at a significant cost. But, do not despair. If you are not yet in the market for a new set of these remarkable windows there are some quicker, easier and less expensive ways to increase the insulating value of your windows.

The cheapest way to increase the insulating value of your windows is by adding removable insulation to them – insulating window coverings.

Alternatives to Retrofitting Your Windows: Insulating Window Coverings

Articles on insulating window coverings are linked below. If you are not in the ball park for a window retrofit you still can take some tips from these articles to reduce window heat losses and to increase the comfort of your home.

How to Use Drapes and Curtains for Window Insulation & High R ValuesHow to Get Your Drapes to Insulate Your Windows: The Importance of a Good Seal

This post is part of the series: Insulating Windows

Windows loose a great deal of a homes heating of cooling input. There are many ways to insulate windows. If you are working on cutting energy costs, or preparing for solar heating or cooling one of these approaches may be useful to you.
  1. Insulating Your Windows
  2. How to Get Your Drapes to Insulate Your Windows: The Importance of a Good Seal
  3. How to Insulate Windows with Curtains or Other Window Treatments