Thermal Modelling Service

What is energy modelling?

Energy modelling is when we analyse a building to find out the approximate heating or cooling costs for it or how comfortable the building will be. Energy modelling is done for several different reasons:

  • To find out how much money you could save on heating and cooling energy costs by changing, for example, the insulation level of a floor, wall or roof; changing the way a home is heated; the sizes of a window or by changing the building layout

  • To find out the approximate annual or quarterly heating or cooling costs

  • To find out how comfortable your home or place of work is at different times of the day and year

  • To find out where and how big shading needs to be for a room to be comfortable year round

  • To find out what we could do better in our design to have the house performing in each area the way we want it to

What different types of energy modelling are there and what are the differences between them?

There are broadly two ways of energy modelling:

  • Static modelling: This in essence is when we fill out an excel worksheet for each component (e.g. wall with a certain insulation layer facing in a certain direction with an approximate shading). Static modelling usually assumes several things:

    That there are no internal walls. That means that, for example, the toilet facing away from the sun has the same heat gain as the kitchen you cook in facing the sun. This further assumes that the heat from any window is immediately distributed around the building evenly as no thermal mass or walls are in between rooms.

    As each component is calculated separately, it was not built to consider how the whole system works perfectly together in terms of heat and air flow around your home. While it can give us a fast and good estimate in terms of approximate heating cost for the entire home, it is not made to consider the payback rates for if we change insulation values. For example, if we put more insulation in a wall, then the wall has less heat loss to the outside. However, heat goes from hot to cold and therefore could now increase the heat loss through the windows, floor or ceiling as the heat movement around the building has changed by changing the wall insulation. By looking only at isolated components, we can make adjustments to thermal bridges (junctions in between the window and wall, for example) but cannot predict how the air- and heat flow will now change around the building in this method. Therefore we cannot give you a realistic estimate in terms of saving with different products with this method.

    A building is only considered overheating if the air temperature of the entire building is above 25 degrees Celsius. If for example, your living area is 30 degrees but the rest of the building is lower to give you an average of under 25 degrees, it is not considered to be overheating with this method. Some independent real-life studies have shown the accuracy of occupant comfort to sit between 40 and 50% with static modelling for spaces like, the living room or bedroom. We use static modelling for when it works best: if you are only after approximately annual heating cost for the entire home, or if your home is a large open space without internal walls, such as many smaller and tiny homes. Static models are usually used for certifying Passive Houses or in New Zealand also Homestar homes.

The image shows the same house as in the static model, but with interior walls where each space can be viewed and analysed separately.

  • Dynamic modelling: In a dynamic model your home will be recreated in 3D including interior walls and the shading around your building. We can say here that, for example, you will cook in your kitchen and spend the night in your bedroom. The thermal mass of each room is calculated in to find out the heat loss from, let’s say, your bedroom to the hallway. This model now calculates the heat gains, losses and thermal comfort for each area of your home to help you find out how comfortable each space will be.

    In a dynamic model the model can calculate the airflow around the home more realistically, giving us a better chance to find the hot and cold spots. This further means, that a dynamic model has a higher accuracy in calculating the payback of insulation upgrades such as if you want to put more insulation into the walls.

    Dynamic modelling is more time intensive than static modelling as it takes more time to create and then also run the model and the analysis. The accuracy in terms of predicting occupant comfort has been shown in independent real-life studies to sit between 70-80%. A dynamic model is based on past weather, it cannot predict when a dinner party with 20 people will happen and what the weather on this day will be like, or the extreme person that just always sleeps with an open window, no matter how cold it is outside. A dynamic model focuses on the standard day at home.

    Thermal comfort depends on different factors and if we change the parameters of comfort (i.e. a ceiling fan for use in summer or opening windows), we change at which temperature we feel comfortable. A dynamic model can take those things into consideration as it operates for each room separately.

When should I use which one?

Both modelling approaches have their advantages; if a quick estimate on annual heating costs is all you are after and you do not use a lot of high thermal mass materials (i.e. a log home), then static modelling is a great and fast way to do that. If you are after finding out how comfortable a space will actually likely be, how much money you can save on a certain product upgrade or design change, then dynamic modelling is the way to go. While we use both modelling approaches, most people are after cost savings and comfort and therefore most projects we work on are done with a dynamic model.

What is comfort modelling?

Comfort is more than just air temperature. It is about radiant temperature (the temperature of your surfaces, such as the floors and so forth), air movement, relative humidity, clothing level, activity in a space, architectural feeling (safety, inspiring and many more) and natural light. In our dynamic models we analyse air temperature, radiant temperature, relative humidity, activity in each space, clothing level, air movement around the home, the level of light each area receives and if there is a potential of glare in your home. On top of this, we can open different windows at different times to allow for your preferences, schedule and needs. This then becomes a video showing you how the temperature around your home changes throughout the day during the warmer summer days. The below image shows an example of a daylight analysis for a home. The closer an area is to the colour red, the more light it receives. An office is usually designed to about 75% daylight. The biggest surprise in our experience is often how bright the bedrooms are and how dark the home office is.

The image shows how static models usually view a home: without internal walls, where the heat gain from cooking or from where people spend time is the same for each square meter of the home. Heat coming into one room will here be assumed to be instantly distributed around the entire home as no walls are holding the heat contained in the room it came in.

Summary comparison of different types of thermal model creation​

It is important to pick the right way of analysing your building. If it is just a quick check of approximate annual heating costs, the static model can deliver quite good and fast results. If you are after a more in-depth analysis to find out exactly where the points of opportunities in your building are to make it more efficient or comfortable, the dynamic approach is the one to take.

Get in touch for your thermal modelling needs.