Designing Spaces for Exceptional Comfort
Designing indoor living spaces that are exceptionally comfortable demands attention to six factors that impact the rate of heat loss from the body into the surrounding space. When these elements are properly controlled, excess heat from metabolism is balanced with the heat dissipated into the space. Such thermal harmony gives the mind the greatest opportunity to express satisfaction with the environment and enjoy comfort and wellness.
The relevant factors can be divided into two groups; environmental and personal factors.
Environmental factors include:
Air Temperature is generally accepted as the primary determinant of comfort in a home. Ironically though, the human body is incapable of sensing the temperature of surrounding air. The influence and impact of the the other five factors make prescribing an ideal air temperature for year round comfort largely impossible. Consequently, a wall mounted thermostat, sensing room air temperature is a poor surogate for delivering human comfort.
Radiant Surface Temperatures - Since the body looses about 60% of its heat through radiation, the surface temperature of surrounding walls, windows, floors and ceilings greatly affects the rate of heat disipation from our skin. As a matter of fact, these surface temtemperature have a greater influence on the body's heat loss than air temperature.
The body continually looses heat by radiation to any object that is colder than our skin temperature. Its the same phenomenon you experience when walking in the frozen food isle at the supermarket.
Even though the air temperature is the same as in the cereal isle, a stroll amoung the frozen foods gives a sensation of cold. That's because the body's heat is being radiated into the cold glass doors of the freezers.
Similarily, the rate of heat loss in a home is influenced by the surface temperatures of windows, walls, ceilings and floors.
The rate of heat loss is directly proportional to the difference of the body's skin temperature and the surface temperature of the building assemblies the body sees. The greater the difference, the more heat is disipated to that surface. In other words, the colder the surface temperature, the more heat the body radiates to that surface. Of course anyone who has sit next to a large window on a cold winter day knows the feeling.
Let's take a simple example from the diagram below where you see various surface temperatures of the windows, walls, ceiling and floor. Radiant heat loss from the body is influenced by our skin temperature and the difference it sees in the cummlative or average temperatures of those surfaces. This average or cummlative effect is referred to as the "mean radiant temperature" or "MRT". Iin reality, there are other factors that add complexity, but our example will relate to just this fundamental principle.
MRT = 63.25°F
Windows = 56°F
Walls = 65°F
Ceiling = 68°F
Floor = 64°F
Total = 253°F
The example below shows each assembly's temperature as
Since there are four assemblies represented, simply divide the total by 4 which equals 63.25°F. This describes the average or mean radiant temperature (MRT) of those surfaces at 63.25°F. Even with an air temperature of 68°F, this room will be uncomfortable to the vast majority of homeowners.
As mentioned before, this is a grossly simplistic example of how radiant heat transfer affects comfort. The physics include a number of other factors that bear significanlty on mean radiant temperature the body sees. In general, three factors dominate. They include the area of each assembly and its distance and angle from the body. Although we will not discuss these elements in depth, it is benefical to know how they affect our comfort. For example, windows are usually responsible for the coldest surface temperatures in a home. Consequently, the more windows, the more radiant transfter from the body and the more difficult it will be to design the space for exceptional comfort.
The same scenario controls when a home has poorly insulated walls, ceilings and floors. There is a direct correlation between the insulation performance of any building assembly and its resultant indoor surface temperature. In other words, well insulated homes pay dividends not only in energy savings, but also in providing mean radiant temperatures that support exceptional comfort.
The principle above cannot be overstated. In order to create an environment with an “effective comfort temperature” of 70°F, for every one degree Fahrenheit that the average surface temperature or MRT is below 70°, we will need to raise the air temperature 1.4°F to compensate for the radiant cooling of those cooler surfaces.
Similarly, for every degree above 70°F in MRT, we could reduce the air temperature by 1.4°F to arrive at the effective comfort temperature of 70°F. Using the chart below, our simplistic example room with an MRT of 63.25°F would need an air temperature of around 80°F to provide an equivalent comfort range of 70°F.
This demonstrates why it is always less expensive to invest in better performing windows and insulation as opposed to setting the thermostat higher.
The example below shows an example of summer conditions. In this incidence, the difference between skin temperature and MRT is not as dramatic as winter temperatures. Consequently, the heat disippated by the body will be reduced.
MRT = 73.25°F
i.e. Relative Humidity (RH) is defined as the amount of moisture in the air, described as a percentage, compared to what the air can "hold" before saturation. Saturation simply means the air at that temperature can't hold any more moisture.
The ideal humidity for human comfort is around 45%. But RH between 30% to 60% makes little difference in our perception of comfort. Low humidity means the air is very dry and perspiration will be more effective in cooling down the body. When humidity is high, the air is damp and clammy. So perspiration is no longer very effective in cooling down the body. As the RH falls outside the 30% to 60% range, a host of undesirable consequences are likely. Some of these increase the risk of health complications as the chart below demonstrates.
ASHRAE Standard 55 - The cummlative effect of air temperature, mean radiant temperature and humidity is demonstrated in the chart below. The chart is a graphical representation of a standard for comfort created by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). The standard is referred to as ASHRAE 55.
The bottom line or X axis is represents the "operative temperature". Operative temperature is best described as the
Operative Temperature = Air temperature + Mean Radiant Temperature
The vertical or Y axis defines the relative humidity. The green area represents the sweet spot that will satisfy the largest number of people. Conditions that fall more on center in the green tend to satisfy more people.
ASHRAE 55 Comfort Standard
Air Speed or Drafts - Air movement has a significant effect on the body's heat loss. It can be one of the most influential factors in thermal comfort because of our sensitivity to it.
Air speed or air velocity defines how fast the air moving. It is usually expressed as the rate of meters per second (m/s). The impact of air movement on comfort is somewhat fickle. It can support comfort or in some cases produce discomfort.
Everyone recognizes that a draft in cold weather can create extreme discomfort. Even cold air moving slowly is uncomfortable to many people. On the other hand, if there is no air change or air movement, a room may feel stuffy and uncomfortable as well. The chart below gives an idea of the relationship.
When operative temperature and humidity fall within the green on the ASHRAE 55 chart, there is generally no requirement of minimal air movement to be comfortable. But when indoor conditions rise toward the fringes of the green, natural air movement my no longer be adequate for comfort. This is when a ceiling fan can provide relief. Air velocities between 0.10 m/s and 0.45 m/s are generally acceptable, but this depends on conditions position on the chart. When indoor conditions rise drop to the lower fringes of the green, even slight air movement can be objectionable.
Older buildings tend to have areas such as cracks around doors and windows where air leaks cause uncomfortable drafts. On the other hand, new homes can be so well sealed that little air movement is the result. Badly designed heating and cooling systems can also be the cause of high air velocities in rooms. Extreme care should be taken when designing air diffuser systems with location and air speed leaving the diffuser.
Activity represents our metabolic rate. The excess heat our body's need to disippate depends on muscular activity. A person sitting and reading a book will generate less heat than someone doing sit ups.
Engineers use a term called a Met to measure the body's metabolism. One Met equals 58.15 watts of heat being disippated per square meter of skin (1Met=58.15 m²). The average person has a little less than 2 square meters or about 19 to 21 square ft. The table below illustrates Met rates for common activities.
Clothing is perhaps the most important of all comfort factors, as it directly affects the body's ability to disippate heat. What you wear has a significant impact on your comfort. Of course this is intuitive since our moms taught us not to go out in the cold without a coat, hat and gloves. And everyone knows you dont wear that same outfit on a 90°F day.
Engineers must have some idea of how occupants intend to dress indoors to create conditions to meet their expectations. The term defining the insulating value of clothing is called a "Clo" value. One (1) Clo corresponds to the insulating value of trousers, a long sleeved shirt, and a jacket. The higher the Clo value, the greater the insulation value. The table below gives the Clo value of common garments and apparel.
One challenging task in designing spaces for exceptional comfort involves the master suite and the adjoining bathroom. Your Clo value when stepping out of the shower is zero. Moreoever, you're wet. Typically, under these conditions, comfort demands a much warmer operative temperature. However, studies show for the best sleep quality, our sleep zone should be maintained around 65°F. Best practice requires the sleep zone include its own temperature control, while the master bath needs supplemental heating. A perfect solution is radiant floor heating which produces a warm floor. There's nothing quite like stepping out of the shower on to a warm tile floor.