Solar Thermal Reflective Coatings, and having a Cool Roof.
Here at Efficiency Matrix, we are not rocket scientists but some of the claims/assumptions out on the internet really needs to be straightened out and explained in a manner where most people will be able to understand cool roofs.
Let’s start firstly with an explanation of what the building envelope actually is: It includes the external walls, ceiling, and floor that connects you to the outside environment.
Considering that reflective paint works by deflecting radiant visual energy, we decided to model radiant energy transfer comparisons using different building envelope products.
Before we go into the modelling of what white paint and what solar thermal reflective paints can do, let’s talk about the different spectrums of energy that comes from the sun and the spectrum of energy that comes from building materials and materials that the earth is made of.
When considering different building envelope products, it’s important to understand that energy radiates from all objects, the sun radiates energy primarily on the visual spectrum and near-infrared, and Indoor objects in a house here on earth radiate on the middle to far-infrared spectrum.
- Conduction: which is countered using an insulation R-value
- Convection: which is countered with an airtightness layer
- Radiation — which is countered with a radiant barrier, eg, Low-E films, shiny metallic surfaces and white paints, with additives claiming better near-infrared deflection performance. This is called the albedo effect. Insulation with an R-value can also block this form of energy transfer because this type of energy is transferred into the building envelope via conduction.
As a silver bullet argument, A paint/coating, 1 mm in thickness does not provide any meaningful insulation. As an example to justify this claim, the best insulation in the world (aerogel, k=0.02, so R=t/k=0.001/0.02=R 0.05 m².K/W) has an R-value of .05, where most homes in Australia require ceiling insulation of R 4.1.
High solar reflectance (white paint) reduces direct sun energy absorption (albedo), hence a cool roof energy-saving claim is supported. The above chart looks at how much energy makes the earth’s surface in the visual spectrum range. It’s huge! But it only benefits energy efficiency for a few hours of the day. The sun is only actively forcing its energy on our buildings during 3-4 peak hours of the day and yet we expect comfort 24 hours of the day.
Earth receives energy from the sun in the form of solar radiation—radiation with varying wavelengths along the electromagnetic spectrum. The sun emits strongly in the visible light range, but it also produces ultraviolet and near-infrared radiation. This spectrum is highly dependent on the effect of the atmosphere. The earth radiates heat back to space mostly at much longer wavelengths (Middle to far Infrared at a much lower intensity).
Moving onto the myths that are doing the rounds, regarding high reflective/white paint benefits. At the bottom of this article, we modelled different solutions for keeping heat out of a building and keeping heat inside of the building, using different building envelope materials.
Solar reflective paint can be applied to the inside of a building to keep it warm.
White Paint and Solar Reflective Coatings are only effective in reflecting/rejecting sunlight or energy from the sun. Taking away the sunlight provides no building performance benefit.
Dulux went down this path, and they paid a heavy price of $400000 in Australia.
There are other court cases that have arisen from special reflective paint claims in the US too.
More recently there is this one. Year: 2020
Sarking has a way better performance than what any paint could achieve, due to its high reflective capabilities with medium to far Infrared spectrum energy which all homes radiate internally.
It is indeed a myth. To put this into perspective, sarking (reflective foil) does NOT perform nearly as well as bulk insulation in keeping energy inside a building envelope, so how could a visually coloured thin layer of paint compare to sarking? Introducing a thin layer of paint will NOT insulate the building envelope. The only example of where painting the inside of your house with thermal reflective paint might be of any use, is if you plan on launching rockets into earth orbit from inside your living room. Which is what NASA originally used the thermal reflective paint for.
It could explain why they demonstrated the use of a blow torch in this video to test how “insulative” their product is on TV. In other videos online, testing of these paint products with infrared heat lamps can be found, but unfortunately, such high-intensity near-infrared heat does not radiate from everywhere inside the building envelope other than the actual heater itself.
The other related Myth, which is also spoken about,
Painting your roof with a solar reflective paint can assist the performance of a building in winter and cold nights
Solar reflective paint does not keep your house warmer in winter because it is solar VISIBLE reflective, not infrared reflective. Thus a house warmed by its heating system in winter loses some heat through the roof by infrared radiation to the night sky and it is not blocked by solar reflective paint.
Urban Heat Island effect is not considered in the modelling simulations, which is critical for showing cool roof benefits, when mechanical plants are air cooled and roof mounted.
The heat island effect is only significant when there is no wind (i.e. local high energy density in cities), but almost always (in Melbourne, Australia specifically), at peak loads there are hot northerly winds, and this disrupts the heat island effect, thus heat island effect on rooftop equipment performance is nullified. Other cities may be less windy and Urban Heat Island may come into play, but not likely.
Sarking Effectiveness is no Myth
Sarking can reflect and keep a home warm, but can also keep a building cool in warm climates.
It most definitely helps, but…
Dust and mould buildup on its surface over time can degrade its performance. Foil building wraps need to remain shiny and clean for them to perform optimally. This happens naturally with vertical foil in walls and foil facing down which remain dust-free for 50+ years, hence providing reliable reflective insulation. However, foil facing upwards does degrade from dust deposition and the Australian Standard 4859 describes this degradation in its performance.
There are some products coming to the market now where there are layers of foil with an appropriate gap in between. These types of products can produce some great results.
The internal cavities of walls/ceilings and subfloor
The internal cavities of the building envelope whether they be walls, subfloor or the roof area can be problematic areas for condensation which leads to unhealthy mould and material damage. It is very important the insulation and vapour barriers are designed to avoid condensation.
There are many potential solutions for stopping heat transfer from extremely hot days and cold days, and it is best to seek an expert for the optimum cost-effective solution for your building. For example, solar reflective paints have great benefits in hot sunny locations and if the roof has poor insulation, but is less cost-effective if the roof is already well insulated. Often the most cost-effective retrofit is sealing a roof with spray foam insulation, as this can also reduce thermal bridging at frames that are significant thermal bridges.
What the modelling says
We modelled the following insulation/energy deflection building products, systems.
- Solar reflective coating paint
- White Paint-[White 16] (Performance Data Adopted from Alucobond)
- Black Paint (Performance Data Adopted from Alucobond)
- Brushed aluminium Roofing (Performance Data Adopted from Alucobond)
- Sarking/Foil Paper
- R-4 Bulk InsulationThis modelling is only looking at radiant temperature/energy coming from the suns full solar spectrum onto objects that the building envelope is made of.
Real solutions for improving the performance of a roof.
If a building does NOT have insulation, using a standard off the shelf white paint (White 16) or a solar reflective paint CAN improve comfort within a building during a summer hot day but not in cold winter climates (Solar Reflective Coatings may slightly outperform standard white paint). If there is no insulation in your building, consider using spray foam insulation instead.
If there is insulation, consider the below retrofits.
- Repairing sarking/foil that is torn and ripped under a tin roof, with foil tape.
- Fix up gaps in the insulation on your ceiling.
Understand that real energy efficiency performance is realised by improving airtightness and ensuring that insulation is abutted consistently to this air barrier throughout the building envelope. A solar reflective coating does not meaningfully improve energy efficiency over a simple white (white 16) coloured paint if your ceiling is consistently insulated and airtight. And even if the ceiling isn’t consistently insulated, the roof area temperature is not lowered enough using solar reflective paint to make a quantifiable difference.
Some of these Solar Thermal paint products may offer better durability for longevity over standard white paint which may add significant value to an asset.
A video by Melbourne University discusses the benefits of a simple light (White is the best) colour as opposed to a dark colour for roofs. Mainly beneficial for uninsulated roofs during summer ONLY. eg Heritage or Storage Sheds.
by Joseph Cheung & John Konstantakopoulos
For other more important things to focus on, watch this video.