Tuesday, February 23, 2010

Mysterious Winter Coziness

Careful readers may recall my concerns that my radiant barrier is going to cost me some much wanted solar heating during the winter. While this is undoubtedly true during the seemingly rare sunny days this season, as this winter progresses I get the feeling that the effect may be pretty well balanced out by other effects of the improvements during the cold nights. While not easily definable, the house does seem to have more "coziness" than in previous years, especially given the long runs of cold temperatures we've had this year.

Given that my thermostat is set at the same temperatures that it was before I finished my attic improvements, how is it possible that the house would feel different now? Isn't 68 degrees 68 degrees? Well, yes and no. Your skin senses more than just the air temperature, although that is a major factor. What your skin really senses is energy: specifically, the amount of energy being absorbed by the skin or leaving the skin. The balance of that energy is what matters.

If you have read this blog for any length of time, you know that there are three modes of heat transfer: convection, conduction, and radiation. Convection matters only for your breathing, which unless you're in a sauna or Texas in the summer, acts to cool the body as the exhaled air carries heat from your lungs out into the environment. One might argue that this is really conduction, in that the heat conducted out of your lungs into the air, but we will not delve further into metaphysical hairsplitting than that. Suffice it to say that the rate heat flow from breathing is proportional to the difference between the temperature of your lungs (your body temperature) and the air temperature. Since my thermostat setting hasn't changed from pre-improvement house until now, convection cannot account for the difference that I feel in the warmth of the house.

That leaves two modes of heat transfer in and/or out of the body to discuss. First, the one everyone knows about is conduction. Your skin's contact with the air, or any other surface, that is cooler than your skin temperature causes heat to conduct out of the skin and into the air. The heat flow from conduction will be a rate affected only by the amount of clothing around the skin (which acts just like insulation and slows the heat flow) and the temperature difference between the body and the air. Since I still tend to wear sweatshirts around the house, and my air temperature (a.k.a. thermostat setting) hasn't changed, conduction can't be the difference in coziness that I'm feeling.

The final mode of heat transfer is one near and dear to my heart: radiation. The fact is that your skin absorbs radiated energy very well and warms nicely because of it. Why do you stand in the sun on a cold winter day? "Well, the air is warmer in the sun", you might reply. Is it indeed? What if the wind is blowing? Suppose that air that surrounds you in the sun just blew in from under the trees, where it's shady. In fact, the air in the sun tends to be the same temperature as the air in the shade (at least as a first approximation; over time, if the air isn't moving, it will warm, of course). Instead, _you_ are warmer in the sun, but the air is not. You are warmer because your body is a solid object, which makes a nice "brick wall" for all those energy-carrying photons from the sun to run into. Your body, in fact, will absorb a lot more of the sun's energy than the air around you, because that air is mostly empty space and most of the photons "miss" the air molecules as they pass through.

What does all this have to do with my house feeling cozier? Well, you've probably figured out that it has to do with radiant energy, since we've eliminated the other two forms of heat transfer. This is indeed the case. But what mysterious object inside my house is radiating that energy, and why has it changed from previous winters to this one?

Before I answer that, we all need to remember that every object that contains heat radiates that heat in the form of energetic photons. Some objects are better at it than others, depending on their surface characteristics and their temperature, but stated scientifically, any object with a temperature greater than 0 degrees Kelvin ("absolute zero" or -273C) will lose energy or "cool itself" by radiation of energetic photons, typically in all directions. The wavelength ("color" if it falls in the visible spectrum) of those photons depends on how much energy they contain: the longer the wavelength, the less energy. Most objects around us every day radiate heat in the infrared spectrum; that is, at an energy level below what the naked eye can detect. However, given some nifty infrared "night vision"-type goggles, one could easily see everyday objects obeying the laws of physics, dumping out energy from themselves in the form of photons. Some particularly hot objects like a hot coil on an electric stove might get energetic enough to put out photons that cross from the infrared (Latin for "below the red") up into the visible red spectrum, which, of course, is why that hot coil would glow red to the naked eye.

Once again, let us return to the house being cozier this winter. Some fraction of the energy balance entering and leaving my body in a given instant must be from these infrared photons slamming into my skin, warming it up ever so slightly. (Or, perhaps more precisely, hitting my clothes and warming them slightly, causing less of a temperature difference between my clothes and skin, thereby reducing the heat conduction). Those photons must be coming from all the objects around me inside the house, even as I sit here typing this. But what photon-radiating objects would be radiating MORE photons this year than they were last year, making me feel cozier?

Well, since we're tracking down a change in behavior, let's see what circumstances changed that might cause it. The improvements I made to the house were all in the attic. And the surface of the room I'm in that is connected to the attic is... the ceiling! Is it possible that the ceiling is radiating more photons down into the room than it did last year, warming me and making me feel cozy? If so, what would make it do that?

Indeed, the energy imparted by energy radiated from an object like my ceiling is greatly affected by its temperature. In fact, the exponent in the relationship is 4, meaning that a doubling of the absolute temperature gives you 16 times as much radiated energy (2 to the 4th power). Now, is it possible that my ceiling is staying warmer than it was last year?

It is certainly possible and indeed likely. However, I did not have the foresight to measure the temperature along the ceiling before I made the improvements, so even if I devised a way now, I wouldn't have two numbers to compare.

But research, logic, and the above reasoning do tell me that the increased coziness has to come from there. The fact that I added significant insulation above the ceiling in question, doubling it at least, should tend to keep it warmer in the same way adding a blanket on top of your body on a cold night keeps it warmer. Add to that the fact that the top of the insulation, facing up into the attic airspace, will now radiate its own energy directly upwards into a newly completed reflective radiant barrier, which will direct about 97% of those photons back down into the insulation, keeping the insulation warmer and thereby reducing conduction from the ceiling into the insulation above it. In other words, that ceiling really should be warmer.

So, although the jury is still out for another month or so as to whether the dramatic savings I've seen in summertime will appear in winter, or whether my hot-season-targeted changes will hurt my wintertime bills, I can say that the experience of the first cold season under the new attic has led me to notice a definite improvement in comfort.

While we wait for the end of winter and my post-winter efficiency analysis, please enjoy this picture of me in attic regalia assisting a friend in some radiant barrier installation. 'Tis the season for attic work, after all, and as I am sure we never tire of hearing, Energy Efficiency waits for no man!

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