In my last article, I described the way heat moves in and out of a home through a fluid or gas such as the air. This process is called convection.
With all the hot weather we have had, it's a great time to discuss how heat moving by "radiation" or electromagnetic waves can get into and out of your home. We call this type of radiation "infrared." Learning how infrared radiation works is important in understanding how heat is transferred into your home during summer as well as out of your home during winter.
To understand heat movement, we need to know a little about infrared radiation. The sun is our source for infrared radiation. Of all the sunlight which hits the earth, over half of it is infrared radiation. The rest is mostly visible light and a little bit of ultra violet. When infrared radiation hits a surface, that surface absorbs the energy and becomes warm.
As with most things in nature, heat energy moves from areas of higher concentration to areas of lower concentration. In summer, when the outdoor temperature is hot, the heat wants to move from the hot outside to the cooler inside of your home. Our job is to stop this heat movement, to fight against the physics of the natural world in an effort to reduce our utility bills, be green, and save energy.
You may have seen television shows where criminals, ghosts, or monsters such as Bigfoot are tracked from a helicopter using an infrared camera so you can "see" their heat as they glow in those funny infrared colors.
Here is a question to think about when you watch these shows: Do you think that the amount of heat being given off from a human body (or a Bigfoot!) is hot enough to seen by a camera, after having traveled across a field, or up from the ground through the air while helicopter blades are blowing that air back down? Most people I ask this question to reply, "Probably not," and they are right! So, what is the camera really seeing?
The camera is seeing electromagnetic energy radiated by the surface of an object in the infrared range, just like a regular camera sees the electromagnetic energy reflected off the surface of an object in the visible light range. It's not "heat" as we think of it in conventional terms rather its heat energy traveling by electromagnetic waves. It is invisible to the naked eye but like visible light, it can travel across a field, or up through the air to a helicopter or even up through space to a satellite.
For the purpose of saving energy in our homes, we need to use the infrared coming from the sun wisely. If we fight against it, then we pay the utility companies more money as we try to either cool or heat our homes to a comfortable level.
In summer, the most important thing to do is to prevent the primary source of infrared energy, sunlight, from ever reaching the building. You can protect the home and keep it cooler by shading it from the outside. Trees with large canopies do an excellent job of shading a home in summer. Then in winter, when the leaves are gone, all that infrared energy can stream in through the windows and warm the walls to help reduce the heating bill.
If planting trees is not an option (or you can't wait 20 years for them to grow), exterior awnings or shades are a very effective way to shield the high summer sun, but still allow the lower winter sun in. Skylights can be covered with perforated landscape fabric which allows some light in but will block a significant amount of infrared from entering the home. The next best option to stop the sunlight would be to place a shade or drape on the inside of the window with a reflective or light color surface to reflect infrared back out and prevent it from being absorbed by the home's interior.
(I'll discuss windows, replacement windows, and window films in a future article.)
Here is why your house gets warm on a hot day: Recently, Rockland has been quite warm with daily temperatures approaching 100 degrees and strong sunshine. The temperatures in your attic probably reached 120 or even 130 degrees. If you don't have enough insulation, and most people do not, that heat was in direct contact with the exposed ceiling beams. (Insulation is just as important in summer to keep your home cool as it is in winter to keep it warm.)
The hot air heated up the ceiling beams, then the heat traveled through the wood beams and other spaces that were poorly insulated to the back of the sheetrock or wood of your ceiling. It then traveled through the ceiling material to the ceiling surface. As the ceiling became hotter than the room below, it started emitting its heat energy in the form of infrared radiation into the room, causing it to be warmer
When direct sunlight hits the side of your house, the exterior siding gets warm. This heat then moves through the wall to the cooler inside. While the ceiling and walls are radiating heat into the home, infrared is also pouring in through the windows. Add in a few outside air leaks as I discussed in my last article and you have a hot house.
Now that all this building material is saturated with heat from the day, it takes a lot of time for it to cool off after the sun goes down. The heat energy eventually needs to make its way back to the exterior surface of the home to be radiated out into space. Some of it can be removed more quickly if we open windows to provide natural air circulation, or if we mechanically remove it by using air conditioning. But no matter which way the heat is removed, the house remains warmer until all the energy received and absorbed from the sun during the day is gone.
Only good insulation in the walls, attic and basement and shading of the home or at least the windows can minimize the amount of infrared radiation your house receives in summer from making it into the inside of the home.. Closing shades and drapes in winter will help to keep heat from radiating back out through the window glass in winter. Blocking sunlight in summer and letting it in during winter is a very simple, but very effective strategy to save energy year round.
Wayne Swirnow writes about home energy issues for New City Patch.