7 Minutes of BS: Heat Flow

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heat flow
hēt flō (n).

“Heat flow is the movement of heat energy between objects from a hot temperature to a cold temperature.”
We know that heat moves from hot to cold, but it's not always clear on how it moves.
So, it moves in one of three ways, either by conduction, convection, or radiation…
Conduction is direct heat transfer from one object to another by touching
More often than not, though, heat is moving in more than one way at once.

Conduction is probably the best understood of the three and is simply the transfer of heat through molecular contact, which is just a geeky way of saying two things of different temperatures that are touching and heat transfers from hot to cold.

For example, If you pick up a hot pan, the heat transfers to your fingers from the pan and you get burned.

If you stick your tongue against a cold metal pole in the winter, then heat from the surface of your tongue transfers very quickly, and your tongue freezes to the pole.

If you learn nothing else from this podcast, learn this: always, no wait, NEVER touch your tongue to a cold flagpole.

So that’s conduction, and as I said, it’s usually the easiest to understand.

Convection is a fluid process
Convection is a process of moving heat through fluids. In construction, those fluids are usually air or water.

In a lot of cases, we have pumps that move heated fluid into radiators or radiant floors.

Another example is using fan-coil units, or an air handler unit to push warm or cool air around the building to transfer heat that way.

So that’s the majority of the water part, let’s look at the air: the majority of convective activity through the building enclosure.

Even for what might seem like small airflows, especially in a well-insulated enclosure, this convection, this air movement across the building will dominate the heat loss across the enclosure.


Dominate the heat loss across the enclosure. That’s an important point that answers the Why it matters question, but we’ll come back to that.

The fluid convection talked about earlier is what’s called forced convection, because there were pumps, fans, or some kind of power behind the movement.

Another example of convection would be what we call natural convection. So because warm air is less dense and cooler air is denser, we often get stratification of air and movement against surfaces.

Natural convection is when air moves based on the temperature difference. As the air cools, it will become denser, and it will fall and will be replaced with warmer less dense air.

For example, if you’re standing against a large window, the air in the room will cool as it comes in contact with that window and it will fall down the surface of the window, often leading to condensation in the corners…

This cold air movement can also make a room feel drafty. There is no actual air leaking through the window, but the cold air rushing past it makes the room feel drafty and the people feel cold.

Also within wall assemblies, within enclosures with air-permeable insulation, if you don’t do a good job installing insulation in your stud cavities, you can get convective looping in your stud cavities.

Just like the convective current that rolls past your window and drops condensation at the bottom, little convective loops in the stud cavities can mean little lumps of water in your walls. Puddles might be a bit over the top.

OK, that’s the high and low on convective heat flow: fluids carrying heat.

Radiant heat flows through space or clear objects
The last form of heat transfer is radiation.

Radiation heat transfer is a bit more complicated to understand because you can’t observe it in ways that you could observe conduction or convection.

Radiation can go through clear surfaces, such as windows, it can go through vacuums like space, and vacuum insulated panels, but it can not go through solid objects.

The most obvious, important, and probably easy to understand example of radiant heat flow is the sun. In the thermal-envelope-world, the heat can be either wanted or unwanted, depending on the season and where you live.

When it comes to radiation heat transfer and the opaque enclosure, in general people don’t understand radiative heat transfer as well, there can be confusion in how radiant barriers, foils, low e coatings on surfaces can work.

I happen to know that there is a whole podcast on the topic of radiant barriers, so dig into that if you’re still hungry.

Radiation heat transfer is just two surfaces of different temperatures that are ...

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