Keeping your home comfortable is all about reducing the interaction between the home's inside temperature and that of outside influences such as air temperature and sunlight. Thermal, or "blackout," curtains are one of the older solutions for reducing thermal transfer, and remain as effective today as they ever were. Granted, some designs are better than others, but generally anything is better than nothing.
Thermal transfer happens in one of three basic ways: by conduction, which is direct contact between two objects of different temperatures; convection, or the heat transfer caused by the movement of air; or by infrared radiation. The sun sends most of its energy to Earth via infrared radiation, which is the lowest spectrum of light. Infrared radiation corresponds directly to heat; the more of it that an object can block, the better the object's ability to maintain a temperature differential.
Windows and Heat
Thermal curtains work by inhibiting all three forms of thermal transfer. Without intercession from an air conditioner, the temperature of the windowpane will eventually equalize with that of the air outside. The warm windowpane will then transfer energy directly into the room as if it weren't even there. Add in the infrared energy of direct sunlight or infrared bouncing off outside objects, and the window will actually begin to radiate heat into the room. Even a double-paned window with inert gas between the panes will still allow infrared energy in the form of light to penetrate the room.
How Thermal Curtains Work
Thermal curtains work by blocking energy transfer in all three parameters. The material's ability to seal against the wall forms an air pocket between the windowpane and the room, thus inhibiting thermal transfer through conduction or convection. The curtain's dark material absorbs infrared light, which reduces the amount of thermal transfer.
An insulator's R-value refers to its ability to inhibit energy transfer in British Thermal Units (BTUs). Uncovered windows can easily add approximately 1,000 BTUs' worth of energy each to a room. The home's air conditioner has to offset this gain in BTU in order to maintain the room's temperature, which equates to higher electric bills and a less comfortable room. A good commercial thermal curtain has an R-value of between three and five, which is better than your roof and about the same as the fiberglass insulation in your walls.
Any thermal curtain is better than nothing, but certain factors can make them more efficient. Most thermal curtains are composed of cotton or synthetic fibers, which by themselves don't have a fantastically high R-value. However, very thick layers of material will decrease the amount of convective or conductive heat transfer. Look for a curtain that's thick enough that you can't distinctly make out an object on the other side. If it's transparent, it's probably too thin.
Dark-colored curtains would seem like the ideal solution, but this isn't always the case. Dark curtains will absorb infrared energy from the outside; once they're saturated with heat, the curtains will release it into the room. The best solution is to use two curtains: a white one on the window side to reflect infrared energy back outside, and a thick, dark one on the inside to catch the light that the white curtain doesn't reflect. A layer of tinfoil over the windows would be even better, but may look unattractive from the outside.
- "The Mechanics of Materials"; Ferdinand Beer; 2005
- "The Mechanical Engineer's Handbook"; Myer Kutz; 1998
- "Fundamentals of Fluid Mechanics"; Bruce R. Munson; 2005
- Thermal Insulated Curtains; How Thermal Curtains Affect R-Value; Apr. 2009
- Green Knowe; The Thermal Envelope; Dec. 2009
- Photo Credit Thomas Northcut/Photodisc/Getty Images
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