Cat on a Hot (& Dark!) Tin Roof
Why ALL new roofs should lower solar absorptivity, and how to detail it
Dark roof or light roof? Steel or something else? Our material choices in building design - whether new build or renovation - have implications to the retention of heat and thermal comfort of our homes. Many of our clients are aware of these factors at play, however in the wider community, we still see a strong trend of dark-coloured roofing. A dark-coloured roof can have a surface temperature 35 degrees above a light-coloured roof! (1)
We can loosely describe roofs as ‘warm’ or ‘cool’ by how much heat they are designed to collect and store. We’ve recently seen a debate around the preference of roofs that attract or repel heat in cool-temperate Victoria, as a way to control moisture by ensuring the vapour escapes. This article explains why we have a difference in opinion.
What is solar absorptivity?
Solar absorptance (SA) is a measure of how much heat from the sun a material absorbs. It is expressed as a figure between 0 and 1, where 1 indicates the material has absorbed 100% of the sun’s radiation. A material or colour with a lower SA stays cooler by reflecting heat away. In the context of a hot summer, this figure can be used to judge a material’s performance in protecting the home from excess heat retention.
When we look at the performance of a building, under the lens of either NatHERS or Passive House, the classification of these SA values is taken into account, as it will impact the heat loading on the house. In cool to mild climates, like Victoria, NatHERS and Passive House favour a dark-coloured roof, as it brings a small amount of heat gain into the home across the course of a year. With the higher proportion of energy in Victoria being spent on heating than cooling, this is how the trade-off is justified in our energy modelling. However higher-performance housing with increased roof insulation will reduce this benefit. We believe in an emphasis on optimising and controlling solar gains - through optimised window placement, thermal mass inside the home, and the use of external shading - is a much more intelligent approach to how we use our solar energy.
Our design locations typically fall within climate 6-7; classed as mild/cool-temperate. Additionally, solar intensity has a greater impact in summer than winter. Summertime sun is high in the sky on generally clear, long days. Winter sun is low-angled, often obscured by clouds and in the sky for a shorter period. If you already have solar panels on your roof, you’ll easily understand the solar intensity of summer versus winter based on your power generation. Therefore, the contribution of solar energy on the roof of the home to heat the house in winter is minimal compared to its detrimental effect in summer. We’ve seen in the news recently a suggestion for Australia to start naming its heatwaves in the same way it names cyclones. Extreme heat events kill more people in Australia than any other natural hazard (2); we know this will only be exacerbated as our climate changes.
Comparison of materials and finishes:
Colour, surface and material have an impact on the absorptance of solar energy.
We can look at the difference between these SA values between different colours and finishes in the table below. To minimise excess heat retention, we prefer a low SA, and a high Solar Reflectance Index (SRI):
Steel Roofing Values
Source: Bluescope - STB-2: Urban Heat Islands and their mitigation, published 2023;
Interestingly, official data for the uncoated steel products (Zincalume and Gal) was not available due to their property changing over time. We had expected they would perform very well, however Bluescope steel advised under the NCC classification system, they are classified as M due to the dulling of the surface. As a strong conductor, in the same way steel heats up quickly, it also cools down quickly. It has low thermal mass and will cool down quickly when the temperature drops.
Terracotta & Concrete Roofing Values
Source: Monier Roofing - Energy efficient roof;
Terracotta and concrete roofs have naturally much higher values of solar absorptance, and lower SRI. The values of the Terracotta tiles shown here are a range of red-coloured tiles, while the concrete tiles shown are a light and dark-toned concrete. These materials also have a higher thermal mass, meaning they will delay and prolong their release of energy. So you get the double whammy of higher heat retention over a longer period of time!
Debunking the warm roof theory:
The theory: a darker roof will heat up quicker, and therefore vaporise any condensation under the roofing more easily, hoping that moisture will escape the building.
This approach is completely theoretical and is not tested. It is plausible that for an old tiled roof there would be enough gaps in the roof for vapour to escape. But what about the millions of homes that have been built since the introduction of foil based vapour barriers - that do not allow moisture to escape?
So, what should you do?
Protect your thermal envelope
We’ve discussed in other posts here- the importance of allowing vapour to escape the thermal envelope. By improving building efficiency we increase airtightness and increase insulation and introduce a higher risk of moisture condensing within our thermal envelope, which in turn can cause major building and occupant health issues. Modern building design seeks to ensure this moisture can escape in the form of vapour by ensuring vapour diffusion through the building envelope. This is why Altereco will always detail and specify high-quality Class 4 vapour permeable membranes.
The updated NCC 2022 has now incorporated requirements to support the extensive research undertaken over the past decade, such as the ProClima Australia study 2022 : ventilation under roof sheeting is really important!
Modern building methodology for roof design is to create a ventilated drainage cavity, immediately under the primary roofing material.
Source: ProClima construction details, published 2020;
In reality, we do not need a warmer roofing material when the roofing assembly is detailed appropriately to manage bulk rain water, or condensation and vapour under the roof sheet.
A dark roof in summer is going to significantly increase the temperature of the roof surface, stress your insulation layer, and ultimately influence indoor comfort and the need for additional cooling.
The effect of this on our urban environments
We also want to touch on the broader topic of the Urban Heat Island (UHI) effect as it relates to roofing as well. UHI describes the accumulation of heat within urban areas results in increased local temperatures, making those hot conditions and particularly heatwave conditions more uncomfortable and dangerous (3). This phenomenon has been described since the late 1800’s. The factors which contribute to this largely are the removal of vegetation and development of the built environment. Use of high-solar absorptivity materials, and high thermal mass in the form of asphalt roads, concrete paving, roofing both domestic and commercial, car parking. 2018 Data in the map below shows the Urban Heat Island effect across Melbourne suburbs, where yellow indicates a 5-10°C increase above the non-urban baselines temperature, and orange indicates a 10-15°C increase.
Conclusion
In conclusion, dark roofing materials are best avoided where possible. The heat retention benefits identified by NatHERS for a Cool Temperate climate are outweighed by the wider detrimental effects of excessive heat loads and UHI. To take advantage of the sun’s heat in our cool-temperate environment, we are much better off designing our buildings intelligently through passive solar principles than relying on uncontrolled solar absorptance which has other impacts to our environments, particularly in urban areas.
Altereco therefore recommends:
Choose a roof with a SA (solar absorptance) under 0.45
Always insist upon a ventilated roof cavity that; drains away water and ventilates vapour & excessive heat
Utilise a durable and warrantied Class 4 roofing membrane
Always install membrane with suitable tapes, to ensure all joints are sealed and this secondary line of defence is reliable
References:
(1) Bluescope Steel, Technical Bulletin 39: Thermal Performance of Roofing Materials, published May 2023.
(2) Coates, L, Haynes, K, O’Brien, J, McAneney, Dimer de Oliveira, F, Exploring 167 years of vulnerability: An examination of extreme heat events in Australia 1844-2010, published 2014, Environmental Science & Policy journal
(3) Gregory, J, Azarijafari, H, Urban Heat Islands, published 2021, MIT Climate portal
(4) ABC News, Is the sea of dark roofs raising the heat in Australia's new suburbs?, published Oct 2023.
(5) News.com.au, A state government has acted on growing calls to ban popular dark-coloured roof home design trend, published Jan 2024
ABC News In-Depth, Why Australian suburbs are so hot, published to YouTube, Jan 2022