People in houses want to control the flow of heat. During the winter, we want to keep the heat indoors and limit its ability to move outdoors. During the summer, we also want to control the flow of heat, but in the opposite direction; we want to keeping the heat outdoors. How are homes able to tackle these seemingly opposing tasks? Remember, heat always flows from warm to cold, not the other way around.
Let’s look at two ways we can make a home comfortable. Consider two identical houses side by side. The first house has very little insulation and the second has a lot. Both houses have very good heating systems that keep their houses at 70oF no matter how cold it gets. How is that possible?
The uninsulated house has much more heat flowing to the cold outdoors. However, the heating system is able to replace the lost heat and maintain a comfortable temperature. The second house is well insulated and loses heat to the outdoors at a lower rate than the house next door. The heating system has to add less heat to keep its occupants comfortable. The uninsulated house uses more fuel to generate heat. The homeowner will spend more on gas, oil, or electricity. Both houses are comfortable, but one costs substantially more during extreme temperatures.
The key to slow the rate of heat flow is insulation. Nothing can stop heat flow, but some materials slow it down more than others do. Materials like steel, lead, concrete, copper, plaster, stone, glass, and clay have relatively high thermal conductivity. They aren’t good at slowing heat flow. Materials that have low thermal conductivity are good at slowing heat flow and are considered thermal insulators. These include cork, sawdust, and some plastics.
Good thermal conductors are dense, while good thermal insulators have low density. Low density materials have a lot of air in them, and the thermal insulators depend on trapped air to stop heat flow. We said glass was a thermal conductor, but fiberglass is an insulator. It is the same material, but its thermal properties are very different because of the air in the fiberglass. As a simple rule, the more air it has in it, the better insulator it is likely to be.
We have talked about how air is a good insulator if we keep it in small bundles. However, air can also be the enemy when we’re talking about heat flow. Air carries heat as it moves, so any air that leaks from the house is lost heat. If we didn’t replace the air that leaked out of the house, a vacuum would be created and the house would eventually collapse. As warm air leaks out of the house, it has to be replaced by cold, outside air. Air moving into houses can create cold drafts, reducing comfort.
If we lose too much warm air from the house, we lose a lot of heat and can create moisture problems. On the other hand, if we don’t change the air inside often enough, we end up with stale, polluted air and an unhealthy environment inside the house. Striking a balance between too many and too few air changes has received a lot of attention over the last 50 years, and remains a tricky issue.