Effective insulating window film
(Forest Hills, NY)
I am interested in installing window film on interior residential windows to mainly reduce electricity use in winter due to oil/electricity-dependent baseboards. Although there are a number of films available that help reduce damage due to light and heat through sunlight in the summer, I have heard that those that claim to provide benefits for cold months, are not as effective quite yet.
My goal is to reduce cold air coming in and interior heat going out due to convection in Fall and Winter, especially given utility rate hikes that are about to go into effect where I live. Do you know how effective the winter benefit is offered by the film products you mention on your site?
Answer from Green Energy Efficient Homes
It's important to distinguish between energy efficient window film and plastic window insulation.
Window film is a self-adhering film you apply directly to the inside of the window glazing, and it has the effect of filtering out infrared radiation, which is the same thing as radiant heat. (It also filters out ultraviolet radiation but this has no effect on heat transfer). In summer this is a definite plus as it prevents the infrared heat from sunlight from entering your home, but still allows the visible spectrum sunlight in; as a result, you reduce the cooling load on your air conditioning (or keep your house cooler if you don't use air conditioning).
Energy efficient window film performs the same function in winter, but since there is infrared radiation on both sides of the glass, it can help as well as hinder. It can help prevent radiant heat from passing through the glass to the outside, but it can also prevent solar radiation from entering your house, so you wouldn't want to apply it to south- or southwest-facing windows that get a lot of sunlight. Instead, use an energy saving window covering such as an insulated roman shade or curtain or blind when it's dark or the window is shaded, and leave it exposed when the sun is shining in. If, as with most windows, you don't get a lot of solar gain from the window in winter, it will definitely provide an overall benefit by reducing the amount of radiant heat escaping through the glass.
Window film does not do anything to stop heat transfer via convection - warm air blowing around the window pane will cool down as it comes in contact with the colder glass - or conduction - warmth in the glass from that warm air will be conducted to the cold outside. It only affects radiation.
Plastic window insulation kits, on the other hand, are practical but somewhat unattractive kits of plastic sheeting that you install using double sided tape on the window frame, trapping an airspace between the window glass and the plastic. You can then blowdry the plastic to make it smooth. This makes it harder to notice the plastic but it's not invisible in the same way window film is. However, because it traps a pocket of still air, it improves energy savings significantly by (A) reducing convection currents, since the air against the cold glass is not constantly being recirculated into the room, and (B) cutting down on heat loss through drafts directly to the outside.
It's difficult to answer your precise question about how effective these window films are, but bear in mind that the window films essentially provide the same benefit as buying low-E windows, which are recommended for cold climates as well as hot ones, because they reduce heat transfer due to radiation. I don't know of any expected breakthroughs in this technology in coming years that would make me want to hold off on buying the film for windows that aren't already low-E windows. On the other hand I haven't found any studies that provide details of what the energy savings are in winter and summer.
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