Challenging Common Energy Misconceptions – Part 4: Cooling

There’s a bunch of misconceptions and mistaken assumptions about energy that often mislead people into doing things that wind up either causing more problems or costing them more money. The very vexing thing about these misconceptions is that at first glance, they seem to make sense. Let’s take a closer look to find out why these energy misconceptions are scientifically wrong, wasteful of your money, and just not cool at all.

Condensation or humidity is caused by too much insulation because your house needs to breathe.

This statement assumes that insulation blocks water vapor from exiting the house. While it’s true that fiber glass insulation often comes with a kraft faced or paper vapor barrier, when it coes to humidity having too much insulation isn’t the problem. And while your home needs to “breathe”, it should do so only in the right places.

Moisture problems (excessive condensation in winter; sticky, humid air in the summer) come primarily from two sources. One is from the normal activities of everyday living, such as showering, cooking, and drying clothes. Humans breath out a LOT of water vapor. The other source is moisture migrating from ground or outside air is finding its way through unsealed surfaces into your home. For homes with vented crawlspaces, moisture from bare earth as well as outside air can penetrate into your home. In either case, moist, humid air holds more heat and this leads to higher summer cooling bills.

The trick about using vapor barriers is how to use them correctly. Vapor barriers are effective when they keep moist air away from cool surfaces. Broadly speaking, you want the vapor barrier on the warm, moist side and a thick layer of insulation keeping the cool side well away.

There are ways you can control humidity in your home. First, make sure your attic is venting air properly. This is where your home needs to “breathe”. In most instances, there is no vapor barrier between between the living area and the attic. This allows moisture to move into the attic. Keeping eaves soffits and vents clear of debris will allow excess moisture to escape. Another tip is to run ventilation fans in bathrooms and laundry areas and to make sure that clothing dryers are properly vented to the outside. If you have a basement, check that downspouts from rain gutters move water away from your foundation effectively. Run a dehumidifier, too (use the water for plants, flushing toilets, and washing clothes). Also, air seal your home to keep the humid summer air outside.

“It’s more energy-efficient to keep the AC running all day.”

This is similar to the “Keeping your home at a steady temperature uses less energy…” argument about heating. In the winter, the problem was about slowing heat transfer from the inside to the outside. That’s when your home loses heat energy more quickly at 72°F than it does at 62°F because heat transfer slows down as the difference in temperatures gets closer to thermal equilibrium.

It’s the same in the summer only here we’re trying to slow down the transfer of heat from the outside to the inside. Say it’s 85°F outside and your thermostat is set to 76°F. Your home will heat up slower the closer the inside temperature is to the outside temperature. That’s why you use more energy trying to maintain 72°F temperature versus 76°F.

The other thing affecting your energy useage is that when your central AC fires up, the motors pull more power when they start up. Obviously, you don’t want your system turning on and off in short bursts all day. But by the same reason, you don’t want it running all day either. The best practice is to use a programmamble thermostat to operate the system at a higher (set back) temperature setting while you’re away during the day. This way, it can keep the humidity low in your home and you can set it to run at a lower, comfortable temperature a half hour or so before you return.

Ceiling fans cool your home.

Thermally speaking, the answer is, “Nope”. Ceiling fans circulate air in a closed or confined space. Their breeze makes it feel cooler due to evaporation of sweat from your skin. Ceiling fans can also help even-out the distribution of cool air (heated air as well in the winter) but they don’t thermally increase or decrease affect the air in your home. If you’re wondering if you can save energy by turning off ceiling fans when you leave a room or leave your home, the answer is YES. However, if you are using a fan to blow hot, humid air out of your home through a window in an upper floor or in your attic, then you can leave that running because it’s ventilating your home.

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