We have to talk the same language. Like any other field of endeavor, building science has its own terms, and if we’re going to be working together, we have to speak the same language. The “potato/potahto” thing is a matter of pronunciation. But what if I started talking about the lower chord in the attic and you thought I was talking about music, and I thought I was talking about a roof truss?
Before we start down this path, I have to make it very clear that a house is a system. The parts of the system work together, sometimes in mysterious ways. But just know that if you change one part of the system, you are going to change something else even if you don’t know it, even if you never see that change.
Conduction, Convection and Radiation: So let’s start with the three forms of heat movement: conduction, convection, and radiation. When we talk about conduction we are talking about the way heat moves through a material, molecule by molecule. As you know, heat moves to cold. So something that’s hotter will conduct its heat into something that’s colder if the two of them are touching. Some materials are better at conducting heat than others. The iron handle of an iron skillet will get hotter more quickly than a skillet with a wooden handle. When they put those little paper collars on paper coffee cups, they aren’t very thick, but they are thick enough to keep your hand from touching the hot coffee cup which is touching the hot coffee. The heat migrates through the wall of the cup, but it has a hard time leaping across the air barrier or thermal break to your hand.
When we refer to conduction, we are referring to materials that are touching. For insulation to do a good job insulating, it has to be touching the surface that it is insulating. Lying in bed on a cold winter night, the pile of blankets touch each other. The thermal path from the body in the bed to the cold bedroom air is through all of those layers of materials.
Where this gets difficult is that the insulation in the ceiling has to be touching the topside of the ceiling surface. Where it’s not touching, there is an air pocket. And if there is an air pocket, air can move. And if air is moving its moving heat like blowing on a spoonful of hot soup to cool it down.
Oh, and moving air is called convection. It’s the next form of heat movement. Sometimes we want convection, but if we’re trying to stop heat movement, we want the air to be still. In fact, it is the still air pockets in the insulation that cause it to insulate! Glass fibers are not good insulators at all. Glass is a pretty good conductor of heat. But all those little air pockets in the layers of fiberglass act like that sleeve around the coffee cup. Heat has a hard time moving through the material because the material to material surface contact is very limited.
Moving air, on the other hand, carries the heat with it, cooling the surface. Fans moving air around a room make it feel cooler. Fans running in a computer carry the heat away from the electronics to prevent them from melting down. As the electronic components heat the air around themselves, the fan carries the heat away, replacing it with cooler, ambient air.
The big fan in the air handler that heats and cools the house does the same thing. The heat exchanger component in the system gets hot. The fan blows the heat into the ducting, pushing it out to the rooms in the house. The heat exchanger cools down with the airflow and then is reheated.
That fan is a big airflow. The airflows in insulation are very small airflows, but very small airflows will reduce the effectiveness of the insulation. Air blowing through leaks and cracks in the house, cooling the house are convective losses. Cold, unconditioned air is drawn in from outside while warm conditioned air leaves. You’ve paid to condition the air that’s leaving. You have to pay again to condition the air that’s coming in. The same thing is true for air that has been cooled when it is hot and humid outside. Air leaks are a huge percentage of the heat loss in a house. We’ll find out what percent for your house, but it can easily be 30% or more.
Resvent 