Thermal Comfort
What is Thermal Comfort? That is what we need to understand if we hope to achieve it. Interestingly, there is a standard, ANSI/ASHRAE Standard 55 – Thermal Environmental Condition for Human Occupancy, that defines it as “that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation”. In simpler words – it is a state of mind for each individual to whether they are comfortable. Thus, if you think you are hot, you are hot; if you think you are cold, you are cold; if you are not thinking about it, you are properly comfortable (or close to it). The good news here is that everyone gets to decide what makes them comfortable. This also explains how people can live comfortably in places like Anchorage, Atlanta, Minneapolis, and Phoenix with their vastly different climates. The bad news is that it is difficult to regulate the thermal environment in any one building to satisfy everyone’s needs. To see how bad it can get, just go to YouTube and search for “Thermostat Wars”. However, there is hope. By fixing our houses, we can narrow the differences to an acceptable level, such that we can live with each other. The first realization that we need to come to grips with is that Air Temperature is NOT the most important factor in comfort. In fact it is one of the worst indicators, by itself, on whether we are going to be comfortable.
Here are some scenarios:
#1. Let’s assume it is 72 degrees F outside (your idea temperature), you and your spouse are both wearing shorts, a T-shirt, and sandals. You just happen to get a brand new quick read thermometer that you want to play with and your spouse wants to go to the grocery store, so you bring it along. Upon arrival at the store to check the temperature and it is the same 72 degrees F. When you go into the store, you check it again and still 72 degrees F. You continue to randomly check it during your time in the store and it is always 72 degrees F. When you reach the freezer aisle, your spouse says that she is cold. When you check your thermometer it still says 72 degrees F. So how is it possible for her to be cold, she wasn’t complaining earlier, there is no air vent blowing down on her, the freezer doors are all closed. You check your thermometer by placing it into a freezer to check it against the thermometer in the freezer and it agrees perfectly. So what happened?
#2. New day, but still that 72 degrees F that you like. The skies have a high overcast and there is no wind. It is so nice that you decide to take a long walk along the Jordan River. About a half an hour out you notice that the clouds are starting to break up a bit, but it is still so nice you keep walking. At an hour out, you notice the clouds are mostly gone and you are starting to get hot, but when you check your trusty thermometer it say it is only 73 degrees F. Your thinking, really only one degree hotter and I’m get hot. So you decide to turn around and head back home. At about an hour and a half out, you notice the wind is picking up and dark clouds are coming into sight. Just before you get home, the sky is now filled with really dark clouds, the wind moderately strong with plenty of gusts, and you are starting to feel cold. Again you check your trusty thermometer and is says 74 degrees F. WTF?? It is now even warmer, but now I’m cold. Is it time to make a doctor’s appointment?
#3. Another new day, but still that 72 degrees F that you like. This time you are headed to Atlanta, the forecast there shows a high of (you guessed it) 72 degrees F and calm winds all day. Several hours later you arrive in Atlanta, but once you get outside you immediate feel hot and sticky. Once again you pull out your trusty thermometer and it says 70 degrees F. It is now colder then you like, but you are feeling hot, HOW IS THAT POSSIBLE????
Clearly something else is affecting how we feel (thermally). As it turns out, there are a number of other factors that affect our Thermal Comfort. Therefore, the first step is to understand the 10 factors that affect Thermal Comfort, as shown in the table below. But first we need to remember that heat energy ALWAYS moves from high (warm) to low (cool). Plus, there are three type of heat transfer: conduction, convection, and radiation. Combining this information will help figure out why we are often not comfortable and lead to necessary fixes to improve our comfort.
Dry Bulb Temperature – aka Sensible Temperature or Air Temperature – This is the temperature of the air in the space. It is what most thermostats display and what most people think of when asked about what makes them comfortable. It also the least reliable, by itself, factor for comfort. Note: This temperature is only valid at the height it was taken, particularly in your house.
Mean Radiant Temperature – It is the average of all the surface temperatures throughout a room. Mathematically, this is very complex to calculate, but there are special devices that can measure it reasonable well. This is often the MOST important factor for Thermal Comfort, but is seldom even considered. This involves Radiant Heat Transfer, meaning you might be trying to warm up (or cool down) the wall with your body heat if the walls are cool (or warm). A great example of Mean Radiant Temperature effects is to go into your garage in the summer time (sunny day high 90’s). The garage will likely feels even hotter than the outdoor, it is because the attic is really hot, there is no insulation in the garage ceiling, and the ceiling is radiating heat down onto you. It might be 95 outside, but a 160 in the attic. Therefore, if would feel like 130+ in the garage.
Relative Humidity – Which is the amount of moisture in the air compared to the maximum amount of moisture the air can hold at a given temperature. High relative humidity can accentuate our feeling of hot and cold.
Air Speed – Rate of air movement. The higher the air speed, the higher our convective losses. This results in us feeling cooler for a given air temperature. However, hot air blowing on us can actually heat us up, thus is ineffective if we want to be cooled.
Metabolic Rate – The rate of transformation of calories into heat and mechanical activity (exercise or work) within our body. This is directly affected by the activity, general physical fitness, age, and sex. More physical the activity means a higher metabolic rate and thus more heat that is generated within our body.
Clothing – This acts as an insulator for our body, meaning it increases the resistance to heat transfer either into or out of our body. By increasing the amount of clothing (insulation) we wear, we can still be comfortable with decreasing air temperatures.
Vertical Air Temperature Differences – with the normal air based HVAC systems, it is common for the air temperature near the ceiling to be noticeably above the thermostats temperature and the temperature near the floor to be lower than the thermostat’s temperature. Therefore, if we are laying on the floor, we will feel much cooler than if we are standing up. If the HVAC system does not properly mix the air within the room, then stratification can occur. This means the temperature in the upper part of the room can be a lot hotter than the air in the lower part of the room. In fact, this can show up as a well-defined line on the wall to a thermal camera, sort of like oil and water.
Radiant Temperature Asymmetry – Typically near a window with sun shining through, sitting in the sunshine if we feel cool – it will warm us up, but if we are comfortable or warm – it will make us hot. On cold cloudy days, being near the window will likely make us feel cool/cold. We can eliminate this effect by simple moving away from the window. Note: low-E glass will reduce significantly reduce your feeling cold effects, but not the feeling hot effect in Salt Lake City. Places like Phoenix would have the opposite reaction because of where the low-E coating is applied.
Floor Temperature – This has the greatest affect when we are bare footed. On hot days a cool floor can make us feel nice by cooling us, but on cold days it will make us feel colder. Conductive heat transfer is what is happening.
Drafts – Seemingly random air movements. Sometimes they are from house leakage, such as from doors and windows, but often times they are convective currents that naturally develop within the house. These convective currents are caused temperature differences within the house. For example, in the winter time, the cold glass panes of a window will cool the adjacent air. As the air cools, it gets denser and descends to the floor. Warmer air from above replaces it near the window and gets cooled. The cooled air reaches the floor, spreads out, starts to warm up a bit, and starts to rise. The result is a vertical circular pattern of air descending near the window that can be felt, and thought of as a draft.
So how does this all affect Thermal Comfort?
It is really a simple equation:
Metabolic Rate + heat gains – heat losses = net gain/loss from our body
If there is a net gain – we are likely warm or hot.
If there is a net loss – we are likely cool or cold.
If there is no significant net gain or loss – we are likely comfortable.
It really is that simple!
But how do we apply this? If you look at the Table of factors affecting Thermal Comfort, you will notice a number of the items have asterisks. These asterisked items are the ones that are directly affected by home performance. By doing Home Performance upgrades, we can ensure that these affects are minimized and gain more control over our house’s environment. Thus, we can gain more comfort for everyone.
For Example:
Let’s assume it is wintertime and the heating system is running. Air leaks (particularly near the ceiling) let out our warm moist air. As a result, cold air comes in (normally down low – i.e. the basement). This can feel like drafts or a significant vertical air temperature difference.
Insulation defects in the wall let heat energy out resulting lower surface temperature on that wall. This means that the Mean Radiant Temperature within the room is also lower and you end up with a net loss to your body.
Improper HVAC installations can produce significant Vertical Air Temperature Differences, which can lead to lower (basement) and higher (upper floor) Mean Radiant Temperature in different rooms, as well as significant Air Temperature variations throughout the house.
Exterior doors can cause drafts as well as localized lower Floor Temperature.
Windows can also cause drafts (both real and convective types).
Clear glass windows can cause Asymmetrical Radiant heating or cooling.
A Home Performance upgrade is always to air seal first, starting at the top. Continue on to the bottom and finally the middle. Next up is Insulation, again starting at the top and working our way downward. HVAC is next, properly sized and actually performing correctly, make such it is tested. Finally, doors and windows as necessary. There is some latitude in this, for example, with a limited budget you would still want to air seal the attic first, but then you could just add insulation there next. That will generally get you biggest bang for your buck. However, there are still likely to be large variations in comfort throughout the house. Final thought, don’t forget to account for ventilation if you significantly air seal the house.
Scenario Solutions
#1. What is occurring is that the freezers are sucking the heat out of your bodies (radiant heat transfer). Women are more sensitive to this loss, plus Men's metabolism kick in quickly to compensate for the heat loss.
#2. When you are getting hot, the Sun's radiant energy is to blame. You are in direct sun light and it is warming you. Later when you start feeling cold, the wind, higher humidity, and no direct sun light are to blame.
#3. The high humidity is to blame. You are acclimatized to Utah's lower humidity and going to Atlanta subjects you to a much higher amount of humidity. Our body attempts to compensate in the usual fashion, but the high humidity accentuates the feeling of heat.
Interesting Links
Healthy Heating -Lots of info about heating and indoor environment quality, although quite a bit is written in a technical manner. This is the website that inspired this article.
Dept of Energy -Retrofit Techniques and Technology: Airsealing
Energy Star -Airsealing Info