Designing for Thermal Comfort

by Rachel Preston Prinz, Pratik Zaveri, and Asha Stout
Thermal comfort has been one of the most difficult aspects of designing buildings since we started building them. Our earliest civilizations were more concerned about portability in regard to their buildings so they could move from hunting spot to gathering spot depending on the seasons. Once we started planting crops, we needed to build more permanent structures to keep our stores of agricultural products safe, at a temperature they could safely be stored at, and moist or dry depending on the product.

Using natural materials in their unworked forms makes achieving thermal comfort in cold climates more difficult, because natural forms are almost never perfectly square, which means they do not fit together in a way that can prevent air and thus cold from moving through the walls.

The stone buildings of our earliest major civilizations in Egypt, Mesopotamia, and Greece had sophisticated tools which allowed them to work stone. They also had slaves and mason/architects who could tell the slaves how to work the stone. They used stone, not only for its monumental qualities which we still get to enjoy to this day, but moreover because a massive stone building is a great insulator from the heat. The cliff dwellings at Mesa Verde similarly use their overhanging cave shelter to provide shade for the hot summer months, while allowing the low-angled winter sun to come in and warm the spaces naturally. The stone buildings there were able to be tightly fitted with minimal working because of the type of sedimentary rock that was used. It flaked off in linear chunks that were relatively easy to stack tall without mortar.

But stone was not always available, nor were the slaves that many civilizations used to get these buildings built. Pretty much everyone else, everywhere else, had to make do with less formalized massive structures made of natural, easily worked, materials. They just came up with simple solutions for filling in the gaps. In log cabins, they would add a chinking of mud between the logs to stop the biggest leaks. They would add lime if they had access to it. Then they would put blankets and animal skins over the insides of the walls to keep the heat in and act like insulation. Even the teepees of the Plains Indian are lined with skins and blankets to keep the heat in. Igloo builders would often build their entrance on a sloped ramp that accessed the inside space below the living level so the heat inside the space would rise and stay in the building instead of escaping out the access hallway. Using this technique, they could heat up a space with just body heat and the heat from a lantern or small fire.

Early farmhouses in the Midwest and Eastern U.S. had a double entry vestibule, which acted as a buffer to present the direct loss of heat.

Tropical buildings, like the amazing bamboo Green School by Ibuku in Bali, Indonesia, are often built in the treetops. Tropical architects design their buildings like this to get up into the areas that are shaded and have access to cool breezes. They want to stay away from the earth because in humid locations, being near the earth also means being near water. These raised buildings stay drier longer during seasonal and storm flooding, and they stay cooler in the heat too.

These are just a few of the many ways in which humans around the world have found a ways of achieving thermal comfort.

Some building scientists will build their research around the idea that comfortable temperature limits are around a low of 65°F (19°C) in winter and a high of 80°F (27°C) in summer. However, when people are asked what their idea of comfortable really is, they will say that it is 70°F (21°C) in winter and 75°F (24°C) in summer. Knowing what your own comfort limits are will help you to plan your home for what you really need.

To achieve temperatures in these ranges, we really need to understand what makes those temperatures comfortable, and why they might be different in the summer versus the winter. To get to that, we need to understand that human comfort is dependent on heat retention and heat loss, which is affected by:

CONVECTION – the circulation caused by temperature difference; when air temperature is lower than body temperature, we get cold. This can be exacerbated by air motion.

RADIATION – when heat transfers are caused by electromagnetic waves; we radiate heat to cooler surroundings and absorb heat from warmer surroundings. Glass in winter can be 25 degrees cooler than the inside temperature at windows. That is a form of radiation. This causes “cold spots”.

EVAPORATION – changes liquid to vapor; this is another way we dissipate heat from our bodies: through the breath and perspiration. We can get hot or cold based on how much water we are evaporating from our systems.

We also need to understand a little bit about the physics of heat. Heat flows from the upper temperatures towards cooler temperatures. Like everything in nature, heat is trying to find balance or equilibrium. Because of this, the greater the temperature difference between two spaces, the more quickly heat will flow through them.

Building materials and insulation slow the movement of heat though them at a rate that depends on the properties of the material. No two materials work the same way exactly.

While it may be tempting to build as massive of a wall as possible, it is also important to note that resistance to heat flow (insulation) and heat storage (thermal) capacity are not the same. Concrete, brick, and stone are poor insulators, but work great as thermal collectors because they hold the heat for hours until the air outside them starts to cool down below the temperature of the warmth stored, and thus the heat starts radiating into the space.

One of the biggest ways we can improve thermal performance of the building is to minimize the air volume in the home. This allows us to use small systems to control airflow. Minimizing your footprint is the first step, and we will cover how to do that in later sections. The other important thing to consider is ceiling height, as this is what gives you your volume. The higher your ceilings, the more volume of air you need to heat, cool, humidify, or dehumidify. So soaring ceilings might not be the best idea throughout the house, unless you live in a hot/humid climate where getting that heat UP and out of the living space is a great idea for thermal comfort. When we have clients who want the beautiful effect of a large space but not the hassle of the larger systems, we suggest that they choose one room that is most important and make that space tall. Leaving the other spaces at normal heights will save you money.