# What is the rationale for a base temperature for degree-days of 12 degrees?

When evaluating the energy demand of a building, the weather conditions are supposed to be controlled for by calculating degree-days (sometimes also known as accumulated temperature difference).

In Europe, these are defined by ISO 15927-6:2007 and in Switzerland by SIA 380/1.

Each of these norms recommend the use of a base temperature of 12 C, but neither of them explain the rationale behind this value. And no references are provided to any research that may explain it.

Does anyone know where this value comes from? Or does anyone know whom to ask?

• I'm guessing from "In some countries, a threshold temperature different from the base temperature is used" that it represents the annual mean temperature in Europe and the reason "some countries" use other values is that 12 degrees isn't relevant to them. 12 degrees Celsius would be a horribly cold midwinter day in Sydney, for example, but a terrifyingly hot summer solstice at Scott Base.
– Móż
Mar 30, 2016 at 2:40
• @Moz not quite. A building's base temperature (en.wikipedia.org/wiki/Balance_point_temperature) depends on the physics of the building only. It's the outdoor temperature above which no heating is required. Apr 1, 2016 at 18:41
• what is your question, then? On the one hand that page makes no sense, because the heat generated by occupants is not "building physics", but then they also assume that factor away in the definition. But on the other, you link to the definition and description then say "how is this defined". I just find it amusing that a mechanism that is defined as independent of outside temperature relies on outside temperature. I also don't believe that it is actually independent, and that is why different base temperatures are used.
– Móż
Apr 1, 2016 at 21:59
• The simple specific case is heating climates vs cooling climates. Most of Europe was traditionally a heating climate, where the dominant energy input is heating (the outside is cooler). But most of the population live in cooling climates, so the European assumption, insofar as it is still valid in Europe, does not work. If your mean annual temperature is, say, 310K, you will likely want ventilation rather than enclosure, and it's unlikely that you will ever need heating. But without cooling, you will die.
– Móż
Apr 1, 2016 at 22:05
• @Moz my question is "Why do these norms assume a base temperature of 12 degrees," when it is well known that the base temperature can widely vary from building to building. Apr 5, 2016 at 10:57

## 2 Answers

It depends on the typical insulation level of a dwelling, the size of incidental gains, and residents' preferences on internal temperatures. Thus, it varies by country, and possibly even by household.

In the UK, the base temperature for heating is 15.5°C, which reflects local conditions. And for cooling, the UK Met Office uses 22°C

A lower base temperature suggests much better-insulated stock, combined with higher incidental gains - probably solar gains.

The base temperature is typically calculated from empirical data - looking at patterns of energy consumption at different temperatures; sometimes supplemented with model results, from running dynamic simulations of buildings, and stocks of buildings, in a package such as energyplus.

Here's an example estimation, for South Korea, which uses piecewise linear regression of empirical data.

It is the result of thermodynamics and statistics. Here's what we know for sure.

1. Homes are essentially fixed, enclosed structures when the windows and doors are closed and the HVAC is off.
2. The Second Law of Thermodynamics indicates that the functioning of human beings and machines that humans make will necessarily throw heat into the structure.
3. The only way the heat can escape with the windows and doors closed and the HVAC off is through the components of the structure.
4. When the humans in an enclosure feel too warm, they want cooling.
5. When the humans in an enclosure feel too cold, they want warming.
6. Humans may open doors and windows to heat or cool naturally, but cannot expect to arrive at a better temperature if they are cold and it is colder outside or if they are too warm and it is warmer outside.

To minimize the energy consumed it is simply a matter of determining what the perceive ideal room temperature is for most humans and then subtracting the common thermal drop is (indoor to outdoor) for typical levels of human activity.

By careful study, by trial and error, or by educated guess, they decided indoor temperatures are approximately X degrees Celsius above outdoor temperatures when the doors and windows are closed and the HVAC system has been off. They decided that Y degrees Celsius is the perceived ideal temperature of the average occupant.

Y - X is apparently 12.

• This doesn't really answer the question... what specific values are used for X and Y? Where do they come from? Are they universal? Why wouldn't they vary from building to building and place to place?
– LShaver
Dec 29, 2016 at 4:04
• @LShaver, to answer the question better one would probably have to read the ISO standards committee meeting minutes for whatever technical publication they used that contained X and Y. I could find not find the exact calculation anywhere, but the ISO standard itself indicated that they applied classic thermodynamics and attempted to minimize resource utilization across the spectrum of outdoor temperatures. Dec 31, 2016 at 2:08