White solar reflective roof coated with The GOAT White liquid roofing membrane, providing waterproofing, UV protection and reduced roof surface temperatures.

White Roofs Work: The Only Catch Is Keeping Them White

The idea that a white roof performs better in the sun isn't marketing. It's physics, and researchers have been demonstrating the benefits of reflective roofs for decades.

But there has always been a catch.

A white roof only delivers its best reflective performance while it stays white.

Dust, pollution, soot, algae and other contaminants gradually build up on a roof surface. As that bright white finish becomes dirtier, it absorbs more solar energy and reflects less of it.

So the real question isn't simply how reflective a white roof is when it's first installed.

The more important question is:

How well can that reflectance be maintained over time?

White is right, and the research proves it!

Researchers at Lawrence Berkeley National Laboratory have spent decades studying reflective roofing and its effect on surface temperatures.

The difference can be significant.

On a typical summer afternoon, a clean white roof reflecting around 80% of sunlight can stay approximately 31°C (55°F) cooler than a grey roof reflecting only 20% of sunlight.

Berkeley Lab research has also shown that increasing the solar reflectance of a roof from around 20% for a dark grey surface to around 55% for a weathered white surface can reduce cooling energy use.

That's a substantial difference from changing the reflective properties of the roof surface.

Why does a white roof stay cooler?

A cool roof works in two important ways.

First, it reflects a high proportion of incoming solar radiation rather than absorbing it into the roof.

Second, it emits absorbed heat, helping prevent excessive heat from building up within the roofing system.

The result is a cooler roof surface, which can help reduce heat transfer into the building below and reduce cooling demand during hot weather.

Reflective roofs can also contribute towards reducing the urban heat island effect, where built-up areas become warmer than surrounding rural areas because buildings, roads and other surfaces absorb and retain solar heat.

So if white roofs work so well, why aren't they everywhere?

The catch: white roofs get dirty

A newly installed reflective roof may look brilliantly white, but roofs live in the real world.

They're exposed to dust, soot, atmospheric pollution, biological growth and general environmental contamination.

Over time, contaminants accumulate on the surface, and changes to that surface can affect its reflective performance.

This is why cool roof performance shouldn't be considered only when a product is brand new.

The Cool Roof Rating Council (CRRC) measures roofing products initially and again following three years of outdoor weathering. The aged results provide a much more useful indication of how the product's radiative properties perform after real world exposure.

Keeping a white roof white

One of the challenges with reflective roofing is maintaining that clean, highly reflective surface over time.

As contaminants accumulate, some of the original reflective performance can be affected.

Cleaning can help maintain the surface, but frequent or difficult roof maintenance isn't particularly attractive to building owners.

That's why the cleanability of a reflective roof coating matters.

Rather than looking only at the headline reflectance or Solar Reflectance Index (SRI) when the product is new, it's worth considering what happens afterwards.

How easily can the surface be cleaned?

How well does it resist contamination?

Can its reflective performance be maintained over the longer term?

Those questions can be just as important as the initial SRI figure.

A modern approach to reflective roofing

Modern reflective roofing systems can tackle the problem differently.

A washable, high cleanability finish coat is designed to make accumulated contamination easier to remove, helping maintain the bright reflective surface responsible for the cooling effect in the first place.

Because there's little point achieving an impressive reflectance figure on installation if that performance cannot be effectively maintained.

A cool roof needs to do more than start white.

It needs to be designed to stay reflective.

Where The GOAT Solar Retrofit fits

This is exactly the thinking behind The GOAT Solar Retrofit.

The system combines a high SRI reflective base coat, with an SRI of 112, with a washable finish designed to help maintain that reflective surface over time.

It's waterborne, has 0 g/L VOC, and can be applied directly over an existing bituminous roof, providing a retrofit option without automatically requiring the existing roofing system to be ripped off and replaced.

The principle is simple.

White roofs work because they reflect solar energy. Keeping the surface clean helps maintain that reflectance. Making the surface easier to maintain tackles one of the practical challenges traditionally associated with reflective roofing.

Related reading

New or newly-laid roof? Choose The GOAT in solar reflective white, see Solar Reflective Roofing. Existing bituminous roof? The GOAT Solar Retrofit goes straight over the top, or ask our technical team.

Back to blog