Operative Temperature as an Early Design Metric

This article originally appeared on www.sefaira.com, and later on the SketchUp Blog.

Energy Use Intensity (EUI) — a measure of a building’s energy use per unit area — has become the de facto standard for evaluating building performance in early design. While there are many benefits of EUI as a metric — for example, it provides a standard point of reference that helps a design team understand how well the design is performing overall — it can miss key opportunities and problem areas.

Designers can better understand their designs — and make a stronger case for performance — with Operative Temperature, a metric that is arguably more related to the basic goals of architecture: providing thermal comfort.

Thermal Comfort is More Than Air Temperature

The most common way to measure thermal comfort is air temperature — which is how most HVAC systems are controlled. However, air temperature is only one of six factors that affect the actual perception of comfort (Image 1 below).

Operative temperature takes into account both air temperature and radiant temperatures of surfaces. As a result, it’s a more accurate reflection of the temperature a person actually experiences in the space, and therefore a more accurate predictor of occupant comfort than air temperature alone. This is particularly true in spaces with glazing because the radiant temperature of the glass can be much colder or hotter than the surrounding air temperature.

Because of this, operative temperature has been gaining traction as an assessment of thermal comfort in buildings. CIBSE’s recent standards for assessing overheating risk (TM59 for residential buildings and TM52 for non-residential) both utilize operative temperature as a measure.

These same strengths also make operative design a really useful metric for early design. Incorporating radiant temperatures specically highlights the thermal comfort impacts of design choices like insulation levels, glazing ratios, shading devices, and HVAC setbacks.

Using Operative Temperature in Design

One of the weaknesses of whole-building metrics like EUI is that they can mask problems that occur at the zone or room level: a west-facing room that is too hot in the afternoon, or a space with lots of glazing in a cold climate. If left unaddressed during design, these issues can ultimately mean HVAC systems running with higher setpoints — resulting in higher energy use — or worse still, can result in uncomfortable, unhappy occupants.

Here are just a few ways in which operative temperature is useful for making early design decisions, for both naturally ventilated and fully-conditioned buildings:

1. Evaluate comfort in spaces that don’t have mechanical cooling and/or heating. This is particularly applicable in moderate climates, or for buildings types like schools that may not be air-conditioned.

2. Evaluate winter comfort in spaces with a lot of glazing. Areas with lots of glass can be particularly cold in the winter because of radiant heat loss to the glass. The radiant effect, in this case, is particularly important — but won’t show up in other types of analyses based on air temperature alone. (In fact, this is one reason why a conditioned building may fail a thermal comfort analysis!)

3. Evaluate (and make a case for) envelope decisions like glazing ratios and shading. Decisions about glazing ratio, glazing properties, and shading may not always have a big impact on overall metrics like EUI — but can nevertheless have a major impact on comfort (as well as peak loads) in perimeter spaces. Operative temperature analysis can show the real-world impact of those decisions and help build a case for elements like shading devices in early design.

Let’s take an example: The design team for an elementary school is looking at the impact of adding shading devices. As shown in the figure below, adding shading has a small impact on overall EUI and Annual Energy Cost. But it has a sizable impact on peak loads (a 24% reduction in the highest peak cooling load), and a huge impact on operative temperature (an 80% reduction in uncomfortable floor area).

In this way, Operative Temperature can help reveal potential problems early on and can help designers make a case for envelope interventions that might not seem to matter based upon EUI alone.

Operative temperature often tracks closely with energy use. The better job a building does at providing comfort naturally, the less energy needs to be expended for mechanical heating and cooling.

In this sense, looking at Thermal Comfort portends a return of architecture to its most essential functions: providing shelter from the elements. Architects have long believed that good architecture is closely linked to its locale, responsive to its climate and the lived experience of its occupants. Thermal comfort — and operative temperature — is a means for returning architecture to its roots — to the essential questions of people and place.

For those interested in how I created the graphics in this post, I outline the process step-by-step in the following log post on SketchUp's blog: "Studying the Impact of Shading on Thermal Comfort: A Step-by-Step Guide."