Even a little activity can improve thermal comfort. Table 2 shows that switching from quietly sitting to standing can increase heat output 20%, which is often sufficient to re-establish thermal comfort. Strenuous exercise provides so much more heat that one can overheat even in cold conditions. (This explains why careful consideration of thermal comfort is needed in gymnasiums and swimming pools.)

Differing activity levels often explain different thermostat preferences. For example, it is commonly believed that older people need higher temperatures to keep comfortable. Research has shown that age does not significantly affect thermal comfort preferences. However, older people are generally less active, and the lower activity level requires a higher temperature to maintain comfort.

When evaluating the feasibility of a thermostat setback, it is essential to consider the activity levels of the occupants, especially if there are elderly persons in the house. Recommending periodic calisthenics may improve their health but you may also get you laughed out of the house. Still, people sometimes forget that even a little activity can vastly increase their comfort.

The next group of thermal comfort factors are externally, rather than personally, generated. These factors--air movement, mean radiant temperature, and humi- dity--offer many retrofit opportunities. Again, the primary goal is to reduce the air temperature (lower thermostat) while maintaining the level of thermal comfort by increasing other thermal comfort factors. In other cases, the goal is to increase thermal comfort without raising the thermostat setting.

Avoiding the Draft

Air moving over the skin (or clothes) robs the body of heat. The source of air movement may be natural convection, a fan or draft (forced convection), or body movement caused by walking or running. In these situations, higher air temperatures are necessary to maintain the same thermal comfort level (See Figure 3). People are very sensitive to changes in air speed, and as Figure 3 shows, air movement can change the comfort temperature as much as 5deg.F under typical domestic conditions. The peculiar elbows in the comfort curves in Figure 3 reflect the destruction of the body's surface air film. As air speeds increase above 0.4 feet per second, the surface air film collapses and ceases to provide insulation. Heat loss increases markedly, which is reflected in the rapid increase in the comfort temperature. Accurately measuring low air speeds is expensive, but rough measures are possible. A "draft" exceeds 0.5 feet per second. A hand-operated fan, held a few feet away, can create local air speeds exceeding 1.2 feet per second.

House-tightening measures can significantly reduce air movement in a house. The blower door is an effective tool in identifying leaks. Note that house-tightening (that is, weatherstripping, leak-plugging, and other infiltration reducing measures) saves energy two ways. Since it reduces air infiltration, less air needs to be heated. Of secondary importance, the slower air movement permits a lower thermostat setting. (Unfortunately, it is impossible to translate reduced infiltration into lower air movement rates because infiltration is so uneven.) Upon completion of a major house-tightening, the retrofitter should recommend a 1-3deg.F thermostat setback if the occupant wants to achieve maximum savings. On the other hand, if occupants want a combination of savings and increased comfort, the thermostat can stay at the original temperature.

Sometimes drafts are inevitable. For example, houses without vestibules suffer from a blast of cold air (over 6 feet per second) each time an outside door is opened. Even warm air moving at a high speed feels uncomfortable. Heat pumps need high air flow through the ducts, and this often causes drafts near their registers. In these situations, repositioning furniture, or possibly relocating the most frequent activities, can lead to greater comfort. Deflectors on heating registers are simple and effective. Sites of sedentary activities, such as television-watching or card-playing, should be carefully assessed. Keep the favorite television chair away from drafts!

Radiant Temperature

All objects lose heat through radiation. The sun's radiation is the most familiar example. A person immediately senses the radiation when walking out of the cool shade and into the sunlight. Many new bathrooms are equipped with infrared lights to provide quick radiative heating. Radiant temperatures in a building are an important, but subtle, factor in determining thermal comfort.

Radiative heat loss is totally unlike the familiar conductive heat loss. The rate of radiative heat loss depends on the object's temperature. At the same time, all the surfaces around an object are radiating heat towards the object. The object's net radiative loss is therefore the difference between the heat it loses (a function of the object's temperature) and the heat it gains (a function of the surrounding surfaces' temperatures). Radiation is subtle because radiative heat loss is essentially independent of the air temperature around the object. People radiate heat, too, although the rate is much less intense than the sun (because a human's temperature is only about 100deg.F).

The mean radiant temperature (MRT)--the area-weighted average radiant temperature of the surfaces surrounding the person--is one factor determining thermal comfort. The combination of a very hot radiant source having only a small area (such as a quartz radiant heater) and large, cool walls can yield an acceptable average radiant temperature. The relationship between the mean radiant temperature and the air temperature is shown in Figure 4. There is a direct tradeoff between MRT and air temperature. A constant comfort level can be approximately maintained by raising the MRT 1deg.F and setting back the thermostat 1deg.F. Many people sense that the room is more comfortable after increasing the radiant temperature, but they are unable to attribute it to a particular difference. A room with high radiant temperatures simply "feels" more comfortable. Others comment that the temperature seems more constant.

To increase the mean radiant temperature in a room, eliminate cold surfaces. The window is the first culprit (if no sun is shining though it). Windows make a disproportionately large contribution to a low mean radiant temperature because they are so cold. Somebody sitting near a window on a 0deg.F winter day will need a 9deg.F higher air temperature to maintain thermal comfort. (lf unable to turn up the heat, most people move, put on warmer clothes or complain.) The temperature of a single-glazed window's inside surface--that's the one that counts--will be less than halfway between the outside and inside temperature.

There are many ways to increase a window's radiant temperature. The cheapest retrofit is a heavy curtain or a piece of rigid foam insulation (which is more effective). A heavy curtain shields the cold window and presents a new interior surface that is much closer to room temperature. Installing a double-glazed or storm window will raise the surface temperature so that it is about 5/8 of the outside-inside temperature difference. Triple-glazed windows present an even warmer inside surface. Of course, one should make the north-facing windows the first target of any retrofit.

An invisible benefit of insulating the ceiling and walls is an increase in radiant temperature. The increase may be relatively small compared to window improvements, but the large area affected results in a significant increase in mean radiant temperature--sometimes worth a few degrees of air temperature. Again, occupants often indicate that the house feels warmer, or more evenly heated, after insulating.

Installing radiant heating ceiling panels is another way of increasing a room's MRT. These panels contain wires heated by electrical resistance. The manufacturers argue that lower inside air temperatures are possible with radiant heating, so radiant panels should should be considered a conservation measure. (They typically contend that the higher MRT permits a 5deg.F lower air temperature.) The argument has some merit but the heat may still be more expensive because it is electric resistance and some heat may be lost upwards, into the ceiling. Another problem is that the untrained consumer may still put the thermostat at 70deg.F, so no energy is saved. If, however, radiant panels are installed in conjunction with a "comfortstat," that is, a device that measures thermal comfort based on both air and radiant temperature, energy savings would be more certain.

It is difficult to calculate the savings, but any single action (wall insulation, ceiling insulation, or window upgrade) should permit a 2deg.F setback while maintaining constant thermal comfort.

Recommendations

Thermal comfort means much more than just a high thermostat setting. Comfort depends on the occupants' clothing and activity levels, and on a home's construction features, including air flow and mean radiant temperature. These factors can be modified through retrofits and careful recommendations to permit a lower thermostat setting and energy savings, while keeping occupants just as comfortable. Major retrofits, such as insulation, window treatments, and house-tightening, should be accompanied by thermostat setbacks of 1-5deg.F. Also, the location of activities inside the home should be examined from the perspective of drafts and low radiant temperatures. Even a modest repositioning of furniture or small retrofits can can vastly enhance thermal comfort. n

Table 1. Clo-Values for Different Items of Clothing and Ensembles Clothing Clo-value ___________________________________________________________ Naked 0.0 Briefs 0.06 T-shirt 0.09 Bra and panties 0.05 Long underwear upper 0.35 lower 0.35 Shirt light, short sleeve 0.14 heavy, long sleeve 0.29 Add 5% for tie or turtleneck Skirt 0.22-0.70 Trousers 0.26-0.32 Sweater 0.20-0.37 Socks 0.04-0.10 0.02-0.08 Light summer outfit 0.3 Working clothes 0.8 Typical indoor winter clothing combination 1.0 Heavy business suit 1.5 __________________________________________________________________________ Note: The clo-value is a measure of insulation provided by articles of clothing. A clo-value of 1 roughly equals R-0.88. Clo-values are additive, so one can calculate the clo-value for a person wearing a T-shirt and light socks (0.09 + 0.04) = 0.13. (Adapted from ASHRAE Fundamentals and "Technical Review of Thermal Comfort," Bruel and Kjaer, No. 2, 1982.)

Figure 1. The potential for offsetting reductions in air temperature with increases in other parameters. Major parameters include insulation value of clothing, activity level, air movement, and mean radiant temperature of surfaces (although in this diagram they have been graphed on an arbitrary scale). Thermal comfort measures can also make a home more comfortable without increasing heating energy use. Such measures correspond to shifting to a higher line of thermal comfort.

Figure 2. The relationship between clo-value and thermal comfort. A person can maintain a constant level of thermal comfort at colder air temperatures by putting on more clothes. (An obvious concept, but for once it is quantified.) A nude person has clo-0.0 and a standard business suit is about clo-1.0.

Table 2. Met-values for selected activities

Activity Mets ________________________________________________________________ Sleeping 0.7 Seated, quiet 1.0 Reading Home Energy articles 1.2 Standing 1.2 Walking, level 3 feet/sec 2.0 6 feet/sec 3.8 House cleaning 2.0-3.4 Cooking 1.6-2.0 Typing 1.2-1.4 Driving 1.5 Writing Home Energy articles 1-5 Dancing 2.4-4.4 Calisthenics 3.0-4.0 Carpentry, sawing 2.0 Basketball 5.0-7.6 Wrestling 7.0-8.7 ________________________________________________________________ Note: A met corresponds to 58 W per square meter of body surface area (18.4 Btu per hour per square foot). For the average man, one met corresponds to roughly 100 W. These numbers are only approximate but indicate the range of typical metabolic rates. (Adapted from ASHRAE Fundamentals.)

Figure 3. The effect of air movement on comfort. A person needs higher air temperatures to maintain comfort when air moves past them. Air movement can change the comfort temperature as much as 5deg.F under typical domestic conditions. The elbows in the comfort curves reflect the destruction of the body's surface air film. As air speeds increase above 0.3 feet per second, the surface air film collapses and ceases to provide insulation. Heat loss increases markedly, which causes increased discomfort.

Figure 4. The relationship between the mean radiant temperature and air temperature. There is a direct tradeoff between MRT and air temperature. An approximately constant comfort level can be maintained by raising the MRT 1deg.F and setting back the thermostat 1deg.F.

Related Articles
"Profiles of Multifamily Weatherization Projects: A Tale of Five Cities" (Kinney, Wilson, and MacDonald)
"Selecting Windows for Energy Efficiency" (Warner)
"Shade Trees as a Demand-Side Resource" (McPherson and Simpson)
"Sizing Up Skylights" (Warner)
"Weatherization Assistance: The Fuel Oil Study" (Ternes and Levins)

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