You may have heard of thermal performance as an important aspect of modern building design. With net-zero carbon emission being a global target, the focus is on the construction industry to become more energy efficient.
Since the Building Code of Australia (BCA) has set energy efficiency requirements for new homes, thermal performance has become a buzzword that architects and designers keep throwing around.
In this article, we will discuss the importance of thermal performance and how you can improve it in your home.
What is Thermal Performance in a Building?
If you’re only here for the tips, feel free to scroll down and skip this section. But if you’re also interested in the nuts and bolts of heat efficiency, let’s dive in!
Take it from CSIRO, our national science agency: thermal performance is the amount of heating or cooling that makes the interiors of a home comfortable. In other words, it is a measurement of how efficiently your home can retain heat. A building with good thermal performance will need less energy to keep the insides warm and comfortable.
For Brisbane homes, it means keeping our houses cool during hot and humid summers and, at the same time, having a warm temperature in winter – particularly, early in the morning or late at night when temperatures go down to single digits.
The thermal performance of any building is mainly determined by how much heat it gains and loses. Heat can flow freely inside a building through the process of conduction. And you know the good old rule of nature: there’s nothing you can gain that you won’t eventually lose. Invariably your precious heat will escape through any defect in the design or build of the building leading to energy loss.
And it’s not just cracks and crevices that tend to let out heat. It’s also big and bulky, structural elements like walls and roofs that it can seep out through. The rate of ventilation and internal heat loads are also important factors. Besides, the human body exchanges heat with the environment continuously.
The sum total of these losses and gains determines the energy required to maintain a comfortable temperature for the occupants. Actually, it is a complex mechanism with multiple energy systems interacting with each other. This keeps changing depending on the weather conditions and comfort requirements of occupants.
Other than that, here are a few other factors that influence the thermal performance of buildings.
- Design variables like geometrical dimensions of the walls and roofs, and shading devices.
- The physical properties of the building materials like density, specific heat, thermal conductivity, conductance, U-Factor, etc.
- The ambient temperature, solar radiation, humidity, rainfall, and wind speed.
- Internal heat gains due to occupants and lighting and equipment
- The capacity of the air-conditioning (HVAC) systems.
Let’s dig a little deeper.
All these factors determine the Heat Transfer Coefficient (HTC) of a building. This is the rate of heat flow divided by the temperature difference between the inside and outside of the building. It is expressed in units Watts/Kelvin (or Celsius).
HTC determines the Space Heat Demand of a building, which is the amount of heat needed to maintain the interiors at a specific temperature. Higher HTC indicates poor thermal performance and increases the Space Heat demand for a building.
Here’s Why You Should Improve Your Home’s Thermal Performance
Ask any design and construction company about thermal performance, and they’ll tell you it’s key to how sustainable your building is. In the words of Michael Carrocci, the Group Director of South Australian construction farm Buildtec, “There’s no question that building for thermal performance makes cents – and sense – for everyone.”
And just think about it – improving the thermal performance of your home can help you save a lot of money.
In fact, it’s quickly becoming the gold standard of home building. As per the energy efficiency rating system of the Nationwide House Energy Rating Scheme (NatHERS), all new houses and apartments will require a minimum rating of 7 stars out of 10. The goal is to create energy-efficient dwellings that cost less without compromising comfort.
A simple upgrade from a 6-star to a 7-star home can lead to a reduction of 18-28% in heating and cooling energy consumption. This can allow you to save as much as $900 in a year. No wonder, engineers are using sustainable energy design features to reduce the energy requirement of any building,
Beyond that, an upgrade will decrease greenhouse gas emissions and will also reduce pressure on the grid at peak hours. The buildings we live in are one of the largest energy consumers in the world. Reducing your heating and cooling needs will not only ease the cost of living but also reduce the carbon footprint of your home.
Jess Fitzgerald of Carbon Positive points out, “About 20 per cent of Australia’s carbon footprint comes from household emissions. If every Australian could cut that in half, that’s reducing Australia’s footprint by a significant amount.”
Use of U-Value as a Way to Assess Thermal Performance
While exploring the subject of thermal performance, chances are you will come across the term U-value. The U-value of the building elements is important for the thermal performance assessment of any home or office. The U-value or thermal transmittance is the amount of heat that passes through a structure over a given period. The lower the U-value, the better the thermal performance of a system.
In other words, the lower the U-value the more resistance the material offers to the flow of heat through it. So, designers prefer using building materials with low U-values to ensure that the building has good insulating properties.
The choice of U-value will depend on the part of the building you are insulating. For example, the wall and the floor materials will not have the same U-values. Likewise, U-values can differ based on the building regulations of the country that you live in. For example, the Australian Standard AS/NZS 4859.1:2018 lists the methods for the U-value calculation of a window or door in Australia.
The point is, U-value calculations are needed for implementing energy-efficient construction strategies. But there’s one problem.
Calculating the U-value of a system made from multiple materials is a complex process. It depends on other material properties like Thermal Conductance (C-value), Thermal Resistance (R-Value), and Thermal Conductivity (k-Value).
Quite often, the U-value is confused with the R-value. Mathematically, these two values are closely related and are the inverse of the other. That is, U = 1/R + the convection and radiation heat losses.
In reality, there will be more thermal resistance due to the layer of air around the inner and outer walls of a building. Also, air gaps between multiple layers of a wall should be considered.
Unless you are familiar with the engineering standards, it is best to take the help of a professional to calculate the U-value. Or else, you can try using the available online tools.
One of the best ways to evaluate the thermal performance of your home is by conducting thermal performance testing through a reliable company. This will give you the thermal performance score of your home and can help you to identify the areas where an upgrade is needed.
Main Areas of Heat Loss in a Home
Needless to say, standing on a cold floor on a chilly winter night feels miserable. Not only does heat loss result in higher fuel bills, but it also causes a high amount of energy waste. Besides, it can wear out your HVAC or boiler system at a quick rate.
In reality, heat loss rates are different for different houses. Listed below are the main sources of heat loss in any building.
- Walls: Around 30 to 40% of the heat loss in a building takes place by conduction through the walls. For composite walls, heat transfer can take place through both conduction and convection. The cracks around the joints with windows and doors can be hotspots for heat loss.
- Roofs: Roofs account for around 25% of heat loss in a home. If you live in an apartment with neighbours above you, the roof heat loss will be minimal. But for any building exposed to the atmosphere, cracks, holes, or improperly placed vents in the roof or attic can lead to heat loss.
- Floor and Basement: The foundation of the building below the floor accounts for 15% to 20% of the total heat loss. This is because the thermal performance of concrete or cement slabs in the foundation is poor as they are not good insulators. Then again, most homes do not have insulation covering the floor area which leads to higher heat loss.
- Doors and Windows: You may think that an open door and window cause maximum heat loss, but it is not so. Typically, doors and windows contribute to 10% of the heat loss in a home. A major portion of this is caused by radiation through the window glazing. Air leakage through badly installed doors and windows is another reason. However, windows can also gain some solar heat which can help in balancing the heat loss.
How to Improve the Thermal Performance of a Building?
And now to the most important part.
For new homes, orientation is one of the vital aspects of improving thermal performance. This involves designing the building while keeping in mind the path of the sun and wind patterns in the area. Good orientation based on the climate zone will not only decrease heating and cooling loads but will also improve the comfort factor.
For instance, in Australia, the solar pathway is to the north. So north-facing rooms will receive maximum solar radiation in winter.
Here are some other effective ways of improving your home’s thermal performance.
Step 1. Add Insulation
One of the easiest ways to optimise the thermal performance of any building is by adding high-performance thermal insulation. Since most modern buildings have cavity walls, injecting insulation in them is relatively easy. The same process can be used for Australian homes made from brick veneer. Other than that, insulating the roofs and floors can also help save your energy bills.
The cost of insulation depends on the area and also on the insulation type. Insulations with higher R-Value materials are more expensive. For example, R3.0 insulation can cost $6-$8 per square metre. On the other hand, R6.0 is priced around $14-$16 per square metre.
However, you can avoid using high-value insulation unless you are living in an extremely cold or hot climate. Another option is to use energy-efficient cladding that can prevent heat loss and improve the efficiency of the HVAC system.
In addition, you can also use a green roof that acts as a layer of thermal insulation. This is a roof that is partially or fully covered by plants or vegetation. Quite simply, rooftop gardening has benefits beyond aesthetic appeal.
Step 2. Draught-proof It
Draught-proofing your home is an effective and cheap way to reduce energy loss. Heat loss by air leakage in a building is mostly through cracks, porous materials and other unintentional openings. While ventilation is a controlled airflow in and out of a building, air leakage is uncontrolled.
Draught-proofing involves closing all the gaps that let warm air out and allow cold air in. That said, the areas in a home where there are open fires or a lot of moisture need ventilation. Kitchens, bathrooms, and utility rooms are such areas. Also, don’t block Intentional ventilation like extractor fans, wall vents, and underfloor grills.
Make sure there is no airflow through the gaps around the door and windows. If your home has a chimney, draught-proof it when you don’t use it. Shut all the doors, windows, and vents in unused areas of the house. Internal doors connecting to a non-heated room will also need draught-proofing.
Some other areas that require draught-proofing are cracks around floorboards, loft hatches, and areas around pipework. This will reduce heat gain when an air conditioner is operating.
Step 3. Reduce Heating and Cooling Loads
The fact is, with every degree increase in the heating or cooling; your energy use increases between 5% to 10%.
The first step to prevent that is to replace old electric appliances and choose more efficient options with higher energy ratings. Install energy-efficient lighting, HVAC systems, and office equipment to reduce interior heat gain.
It is important to check the duct work in the forced air systems in your home. People tend to take their ducts for granted! And it’s far from being a “set-and-forget” thing. Leakages and insulation damage in air ducts can affect system efficiency and air distribution. For air handlers or furnaces with a filter, make sure the filter is not clogged. Fully insulating water heaters and hot-water pipes is also an effective way to prevent heat loss.
If you have an old HVAC unit, think of replacing the compressor, and adding condenser fan controls to improve performance. You can also add programmable thermostats that can change the temperature automatically for the best performance. Install ceiling fans in every room to move the cool air around more effectively.
Finally, in areas of high summer heat, use lighter colours on the roof to decrease the amount of solar radiation being absorbed. Using roofing tiles with high “solar reflectance” can also have the same effect and reduce your peak cooling demand.
Step 4. Adequate Glazing
Using double- or triple-glazing on the windows can reduce much of the heat loss through them. This arrangement consists of two or three layers of glass separated by spacers. To further improve the thermal properties, the in-between space is filled with an insulating gas like argon. Check your local building regulations to find out the best glazing option for your location.
Note, a newly defined property of window glass is solar heat gain coefficient (SHGC). This concept can help you to increase or reduce solar heat gain through the windows or skylights. For example, in colder climates, choosing SHGC values between 0.30 and 0.60 will help in gaining solar heat.
A home with superior thermal performance will be more energy-efficient and environmentally friendly. And even if your home is older, there are small steps that can be taken to improve overall energy efficiency. So, before choosing an architect for a new home, make sure that they are experienced in thermal performance assessment for buildings.
Remember, not all measures for improving the thermal performance of a house will suit all situations. It’s best to take the help of an experienced building professional to apply the right selection of methods to improve thermal comfort.