The Cost of Building Enclosure Heat Loss

The rate of heat transmittance—known as the U-factor—is a basic and essential metric for determining the thermal performance of any building enclosure assembly. Take the U-factor for every assembly of a building's enclosure, along with each component's surface area, and one could determine the rate of heat transfer for an entire building.

The heat loss coefficient ("UA value") is just that. It adds up the U-factor (U) × area (A) of every assembly comprising a building enclosure to determine the rate of heat flow through the building when a temperature difference exists between the indoor air and the outdoor air under steady-state conditions. You could think of the heat loss coefficient as the average U-factor for the entire building.

In the U.S., the most common unit to measure the heat loss coefficient is in British thermal units per hour per degree difference between the indoor and the outdoor temperature (Btu/h degrees Fahrenheit). When you break it down, this unit tells a specific story: 

How many units of heat are being transferred over the course of an hour (Btu/h) if there is a one degree difference between the indoors and outdoors?

The latter part of that question seem innocuous, but it is actually everything when it comes to determining how much heat transfers through your building's enclosure. This is because the rate of heat transfer increases as the temperature differential increases. In a steady-state scenario, whatever that heat loss coefficient (UA) is, that entire quantity of heat (Btu/h °F) is transferred through the enclosure.

Let's play out an example.

If the heat loss coefficient is 10,000 Btu/h °F, that is roughly 3 kilowatt-hours (kWh) of heat moving through your building enclosure. For many of us, that is approaching $0.50 per hour—per degree!

So, what is the difference between the indoor and outdoor temperature? Once your building is designed and constructed, this becomes the key question. You want to design for summer and winter conditions.

Again, let's play out an example.

Assume the heat loss coefficient is 10,000 Btu/h °F per above. Also assume the thermostat is set at 68F all year long. If the project is located in Indianapolis, the summer design temperature is 87F. That is a 19F difference. For that "worst case" hourly condition, we should anticipate:

(UA) × (Δt)

or 

(10,000 Btu/h °F) × (19F) = 190,000 Btu/h 

That is about 56 kWh, or around $9.33 per hour, during the design summer temperature.

Now, let us consider winter. The winter design temperature is 8F. That is a 60F difference. For that "worst case" hourly condition, we should anticipate:

(10,000 Btu/h °F) × (60F) = 600,000 Btu/h 

That is about 176 kWh or around $29.33 per hour during the design summer temperature. This difference is a huge reason why many of our winter electricity bills are so high (assuming your facility is all-electric and that you are located in a climate zone with considerably cold winters).

Figure: The greater the difference between the indoor and outdoor temperature, the greater the heat transfer. In much of the U.S., this means winters will require more energy to maintain thermal comfort. Figure by Daniel Overbey.

What can be done?

There are a number of strategies that can be implemented to reduce the heat loss through a building enclosure. Solutions come with a range of costs and could include:

• Changing thermostat setpoints. This is an easy, no-cost way to reduce the Δt value if you can take the less comfortable temperatures. Every degree counts.
• Reducing air leakage through weather-sealing. Infiltration can account for at least a third of a building enclosure's heat exchange. In older buildings, that percentage can be significantly greater.
• Increasing insulation values. When opportunities arise, investing in increased thermal resistance values for certain portions of a building (perhaps in an attic or maybe through high-performance window replacements) can make economic sense.