White roof - reducing solar gain

[Adam2]

We have a few excess solar gain mitigation measures planned and I was wondering about the potential benefits of painting the single ply flat(ish) roof membrane with a white reflective paint. Anyone else done this or looed into it and care to share thoughts on pros/cons. Plenty of products popup when you google this. I have pinged the question over to the membrane supplier to see if they have an opinion  

[SteamyTea]

While it will help, it may not be as good as you think.

Surface texture is the important thing, so big difference between matt and shiny, not do great between colours.

So may be worth looking at it in more detail.

 

https://www.engineeringtoolbox.com/solar-radiation-absorbed-materials-d_1568.html

 

[IanR]

The solar gain you don't want in Summer, can help you in the Winter and shoulder months. Do you know you have a solar gain issue? (is it built yet).

 

If still in the planning stage, could you swap to an insulation with longer decrement delay?

[SteamyTea]

Adding PV, or a home made ST system will take some of the energy away.

[Adam2]

3 hours ago, IanR said:

Do you know you have a solar gain issue? (is it built yet).

Still building but looking into any useful extra options. Hadn't thought much about the roof before. I did model the glazing and have made changes there and will have a brisse soleil planned. In cooler months not sure will ever really benefit from more heat in the top floor bedrooms so no real loss I think.

[SteamyTea]

Infrared-reflecting paint can cool buildings even when it is black

 

TECHNOLOGY 24 April 2020

By Layal Liverpool

 

A new paint reflects infrared with a layer of polymer, which helps keep objects cool

Jyotirmoy Mandal

 

A two-layered paint that reflects infrared light while maintaining its colour could help keep buildings and vehicles cool under hot sun. This could help reduce energy used in cooling, such as by air conditioners.

This coating was developed by Yuan Yang at Columbia University in New York and his colleagues. It consists of a top layer of commercial paint, which provides the colour, and a bottom layer made of a polymer similar to Teflon, which reflects infrared light.

Sunlight contains both visible and infrared light but the infrared accounts for the majority of the solar energy, says Yang.

 

When the sun shines on an object coated with this paint, the top layer absorbs certain wavelengths of light, depending on the paint’s colour, while the bottom layer reflects infrared light, preventing the object from heating up.

A similar cooling effect can be achieved using white paint or metal mirrors, but Yang says the advantage of this new paint is that it can be any colour desired.

Read more: Earthquake-resistant paint could help brick buildings survive tremors

Usually black paint absorbs heat, but painting an object with a black version of this new coating kept it about 16°C cooler than when an object painted with commercial black paint was exposed to the same amount of sunlight.

 

In another test, the new paint coating was found to be able to maintain its colour despite being placed in an oven at 60°C for 30 days.

Yang says this paint could help save electricity and reduce carbon dioxide emissions.

“Solar reflective and thermally emissive surfaces offer a sustainable way to cool objects under sunlight,” says Mingqing Wang at University College London, who was not involved with the work. This could be useful in tropical locations to help keep buildings cool and reduce electricity consumption from air conditioning, as well as to prevent cars, buses and trains from getting too hot, she says.

An intriguing next step would be to try and add more functionality to the coating, for instance to enable the energy from the reflected infrared light to be harvested to generate electricity, says Wang.

 

Journal reference: Science Advances, DOI: 10.1126/sciadv.aaz5413

 

Whitest paint ever reflects 98 per cent of light and could cool homes

 

ENVIRONMENT 15 April 2021

By Matthew Sparkes

 

An infrared camera shows how a sample of the whitest paint (the dark purple square in the middle on the right) cools the board below ambient temperature

Purdue University/Joseph Peoples

 

An extremely white paint that reflects 98.1 per cent of sunlight can cool itself by radiating heat into deep space. It could help keep buildings cool, potentially replacing energy-intensive air conditioners.

Xiulin Ruan at Purdue University in Indiana and his colleagues previously developed an ultra-reflective paint using calcium carbonate particles that reflected 95.5 per cent of sunlight. They have now improved on that by using barium sulphate particles in a paint that reflects 98.1 per cent of sunlight.

This new ultra-white paint absorbs less than half the amount of energy from the sun as the previous paint. Standard commercial white paint absorbs between 10 and 20 per cent of sunlight energy.

 

The amount of sunlight absorbed by the new paint is lower than the amount of energy it radiates through our atmosphere and into deep space, so the material actually becomes cooler than its surroundings. The team plans to carry out experiments with painted tubes carrying water and hopes to create an electricity-free refrigeration effect.

The team hopes that the paint can lower global carbon emissions as houses coated in the paint would need less air conditioning. If the paint is used on a 930 square metre roof, the cooling effect could be as high as 10 kilowatts, which the team says is more powerful than a standard air conditioner.

Read more: Infrared-reflecting paint can cool buildings even when it is black

Ruan says there is a double-pronged positive effect because the paint sends energy away from our planet. “We send the heat to space, we’re not leaving the heat on Earth,” he says. “Traditional air conditioners leave the heat on Earth’s surface, it’s just moved from the inside of your house to the outside.”

 

The team calculated that if 0.5 per cent to 1 per cent of Earth’s surface was covered in this paint, for instance by coating roofs with it, the total effect would reverse global heating to date.

The painted surfaces will need to be kept clean of dust and dirt to retain their reflective properties but the team is working on ways to make it shed particulates.

Ruan is now working on an even more reflective material but says that there may be diminishing returns. “Pushing to 100 per cent is hard. You will get 19 watts per square metre more cooling benefit, so practically it may not be that attractive given the cost,” he says.

 

Journal reference: ACS Applied Materials & Interfaces, DOI: 10.1021/acsami.1c02368

 

White roofs can cool cities

 

EARTH 3 February 2010

CITIES can battle the “urban heat island” with paint. Highly reflective white roofs could cool cities by an average of 0.6 °C, according to a global simulation.

Dark city surfaces like roofs and roads absorb and radiate heat, leaving cities up to 3 °C hotter than surrounding areas. A team at the US National Center for Atmospheric Research in Boulder, Colorado, combined climate models with a simulation of how temperatures are modified by city landscapes.

They found that in a hypothetical world in which cities sported highly reflective white roofs, urban temperatures were on average 0.6 °C cooler than in cities with existing, mostly black roofing materials. In the real world, says lead author Keith Oleson, the benefits might be slightly less as rooftops get covered in dust (Geophysical Research Letters, in press).

[MikeSharp01]

You could engineer a solution using a series of rotating mirrors arranged on poles with black rear panels so you can either have mirror up or black surface up and get the best of both worlds. If you actually need the sun to hit the under roof you can set the mirrors to normal to the roof. (That may need some care in aligning the rows so the sun can get in.) 

[SteamyTea]

How to keep your house cool in the heatwave

Should you open your windows or close them? Can plants help, and should you get air conditioning? This is your guide to staying cool at home during a heatwave

 

ENVIRONMENT 25 July 2019

By Sam Wong

 

Most UK homes haven’t been designed with heatwaves in mind

Rudi Gobbo/getty

 

Scorching hot summers were once rare in the UK, but thanks to global warming, they are now more likely to occur. However, most buildings in the country haven’t been designed with sizzling heat in mind, and many people struggle to keep their homes and offices cool when temperatures get above the mid-20s.

So what can you do to improve the situation? Some sources, including the National Health Service website, advise people to keep windows closed. That’s a big mistake, says Sue Roaf at Heriot-Watt University, UK.

The main mechanism our bodies have for cooling down is sweating. This works much more effectively if air is moving over our skin. “You need ventilation to actually cool the body down,” says Roaf. That is why a hot room feels much more comfortable with a fan.

 

The NHS website says: “Shut windows and pull down the shades when it’s hotter outside. You can open the windows for ventilation when it’s cooler.”

The NHS said its advice is based on the Heatwave Plan for England. But the advice in that document is more nuanced. It says: “Keep windows that are exposed to the sun closed during the day, and open windows at night when the temperature has dropped.” Elsewhere, it says: “Windows and other ventilation openings should not be closed, but their openings reduced when the outdoor air becomes warmer than the air indoors – this should help keep rooms cool whilst allowing adequate ventilation.”

In 1995, 739 people died in five days in a heatwave in Chicago. Many of them couldn’t afford air conditioning and didn’t open their windows because they were worried about crime.

 

“This heat is very dangerous,” says Roaf. “If you’ve got an overheating flat or house, go somewhere else.” Go to an air-conditioned shopping centre or cinema, for example, or stay with a friend or relative with a cooler house if possible.

If it stays very hot in your bedroom at night, move your mattress to the coolest room in the house.

Blocking out light on sun-facing sides of the house can also make a big difference. “Keeping the sunlight out will prevent some 800 watts per square metre coming through the windows,” says Roland Ennos at the University of Hull, UK.

The best way is to have shutters fitted on the outside of windows, blocking the light before it gets into the house. If that isn’t possible, closing curtains will help.

Read more: Our buildings make this heatwave worse – here’s how to cool them down

There are other things you can do to help cool your house in the long run. Growing plants outside your house can help to shade the walls from direct sunlight, once again blocking something like 800 watts per square metre, says Ennos. “Wisteria is great up the south side, as are climbing roses, and reasonably manageable, while ivy on the other sides can help keep the house cool in summer.”

In a city like London, green roofs on buildings cool the surrounding area, lowering the peak temperature by around 4°C.

Alternatively, you could paint your house white so that the walls reflect more heat, although it will need regular repainting.

If a heatwave lasts for many days, even a well-shaded house with thick walls will heat up. At this point, you might be tempted to get air conditioning. From a societal perspective, that’s a terrible idea. Air conditioners shunt hot air outside, making the surrounding neighbourhood warmer by up to 2°C, according to one study. What’s more, they use huge amounts of energy, contributing to the burning of fossil fuels and causing more global warming.

[joe90]

I was told my south facing conservatory would badly overheat and the house with bifolds into it. I know down here in the south west it’s generally cooler than the south east etc but with all its doors and windows open for the short period it’s sunny here it’s not an issue. Temps can get a little high but hey we (used) to fly to hot countries fir those temps. I can shut the bifolds to keep the heat out of the house, open house windows at night to cool the house and closed during the day to keep the heat out. I did notice yesterday that around midday the 500mm roof overhang kept direct sunlight from the upstairs windows so the bedrooms are not overheating. I can re engineer the UFH to cool the floor if I wish but not found it necessary so far. The main thing I love about my conservatory is the rest of the year it’s a great place to be, heating the house in the shoulder months, nice place to sit even in pouring rain, crisp sunny days in mid winter etc. I would not be without it.

[Adam2]

Some interesting reading there and good to know there may be some benefits to roof coatings if needed.  

[Conor]

If you're talking flat or low pitch roofs, a green sedum roof is an excellent way to reduce heat gain. Also adds a little insulation for the winter.

[SteamyTea]

1 minute ago, Conor said:

If you're talking flat or low pitch roofs, a green sedum roof is an excellent way to reduce heat gain. Also adds a little insulation for the winter.

I did a small scale experiment on this a decade or so ago.

Basically made no difference.

An inch of insulation is better.

I also did some stuff about painting things different colours.

Still up over at the 'other place'.