The Concept of Mean Radiant Temperature Is Key to Understanding Comfort

Ten years ago, physicist and energy expert Allison Bailes III, Ph.D. published a blog post with a silly name ("Naked People Need Building Science") and a sillier illustration that has probably killed his Google ranking ever since. But it may be one of the most important posts on building science that I ever read because it was one of the first to clearly—and humorously—explain the concept of mean radiant temperature (MRT).

Bailes attempted to explain why, after running around the house naked, he sits down to update his Facebook page. He wrote: "After you’ve cooled down from your exercise on this cold December day, you start feeling the chill. Hmmmmm. The thermostat says it’s 70° F in the house, so why are you cold?"

The answer is building science and MRT. Understanding MRT changes the way you think about buildings. It's critically important but almost nobody understands it. Sometimes I think nobody wants to understand it because it would mean codes would have to change, the way buildings are designed would have to change, and the way mechanical engineers and contractors work would have to change. And in the 10 years since this article was written, it appears that nobody actually wants to change.

Many of my earlier posts on this subject have been archived so this 10th anniversary is a good time to look at the subject again, from the beginning.

As engineer Robert Bean wrote on his Healthy Heating website, "thermal comfort does not come in a furnace or air conditioner nor is it a thermostat reading of 72°F (22°C)…as much as consumers have been led to believe that you can buy thermal comfort - you can't." Thermal comfort is defined as "a condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation.”

Bean notes our bodies have 165,000 thermal sensors spread out over 16 square feet of skin, about the area of the hood of a car. These sensors send signals to the brain, which determines whether the body is losing heat, in which case we feel cold, or gaining it, in which case we feel hot. We can gain or lose heat through conduction (direct touching), convection (air carrying the heat away), or evaporation (sweating) but fully 60% of heat loss is through radiation—the transmission of infrared rays that go from warmer surfaces to cooler ones. Or, as Bailes graphically put it, describing a naked man jumping in front of a big cold window in a warm room:

"Every object radiates heat. The amount of radiant heat it gives off depends on its temperature (to the 4th power!), surface area, and emissivity. So our naked man jumping on the bed in front of the single pane window is giving off not only more views than he’s getting back but also more heat. The surface of the window is much colder and gives off far less heat, so the net flow of radiant heat is away from the man in his birthday suit. He’s cold!" 

Our level of comfort comes from a combination of air temperature and MRT, together being the operative temperature. You can crank up your thermostat or tell Alexa to adjust the smart vents, but if your walls and windows are cold, you are going to lose radiant heat to them and you will be cold.

This is why you can't just call up a contractor and ask for a furnace to keep you comfy: because the walls and windows matter as much—or more. As Bean notes:

"No matter what you read in sales literature, you simply cannot buy thermal comfort — you can only buy combinations of buildings and HVAC systems, which if selected and coordinated properly can create the necessary conditions for your body to perceive thermal comfort."

This is why our building codes, home designers, mechanical engineers, and contractors have to change the way they work. Because as Bean notes:

"I say, if building codes dropped the reference to controlling air temperatures and switched the requirements to controlling mean radiant temperature, building performance specifications would have to change overnight."

This is why I am such a fan of Passivhaus or Passive House: The walls are almost as warm as the air inside, and the windows are designed to be within 5 degrees of interior temperature. They have great MRT. I have written:

"Many architects don’t get it, mechanical designers don’t get it (they will just sell you more equipment), and the clients don’t get it. And since there is always someone who will talk up the comfort potential of a smart thermostat or a radiant floor, it is hard to convince people that it is really all about the quality of their wall or window."

This is why I have so much trouble with the "fist pumps for heat pumps" and the "electrify everything" crowd. Because they think putting in heat pumps will solve everything. But people care about comfort, not carbon and heat pumps deliver heat, not comfort. For that, you have to "fix the fabric first."

Bailes concludes that if you want to jump in front of the window naked without cranking up the heat to 90 degrees, "You just need to make sure your building envelope has a high enough mean radiant temperature by having good insulation and air sealing. Whether the neighbors will be happy with your level of comfort is another matter, though."

Bailes wrote his post 10 years ago and writes in his email newsletter: "Wow! A whole decade has gone by since I posted a photo of a naked man jumping on a bed (was it me?) and turned it into a lesson on thermal comfort." It is still worth reading and sharing.