What does solar reflective index (SRI) mean? Why is it important?

The Solar Reflectance Index (SRI) is an indicator of the ability of a roof surface to return solar energy to the atmosphere. Roofing material surfaces with a higher SRI will be cooler than surfaces with a lower SRI under the same solar energy exposure, especially on a sunny day. Pirta paint has an SRI of 116.8.

Cited from https://coolroofs.org/documents/CRRC-SRI-Document_2022-07-12.pdf

What does emissivity mean? Why is it important?

Emissivity is a measure of how effectively a surface emits thermal radiation. The emissivity number is measured against a perfect black surface (black body). Where a black body’s emissivity is 1, a mirror’s is 0. Pirta’s emissivity is 0.997 - 0.998.

Emissivity is relevant because it allows for passive cooling by radiating heat away from the painted surface even when an object is not in direct sunlight.

What is the difference between reflectivity and the solar reflect index (SRI)?

Reflectivity is a measure of the ability of a surface to reflect heat and light away from the surface. SRI is a measurement that incorporates reflectivity and emissivity to give a picture of the overall surface cooling ability of a material.

How does Pirta paint cool?

Pirta cools because it has the ability to reflect and emit more heat than it absorbs.

Are there any limitations to what Pirta paint can be applied to?

Pirta can be applied to any solid/rigid surface providing the surface has been appropriately primed. The surface must be clean and dry and free from dust/loose material and grease.

What makes Pirta different from other paints?

While many paints claim to be reflective, Pirta paint’s reflective abilities surpass all commercially available paints. In addition, Pirta paint is highly emissive, which allows a building to naturally give up the heat gained by being exposed to the sun.

How does Pirta paint cool a surface? Does it just cool the surface or does it reduce the heat inside of what is painted?

Pirta can reduce the surface temperature of a structure. However the cooling effect inside the building depends upon the construction. For example a painted metal bridge. the paint is in contact with the metal and will remove heat from the metal cooling the bridge. On a building the degree of cooling inside the building can be affected by the insulation value of the walls and roof. Heat gain via the windows and any sources of heat inside the building, people, computers, etc.

Where can I get Pirta paint?

If you are interested in participating in our pilot paint projects continuing through 2023, please fill out the contact form on our website (bottom of page).

Who designed/created Pirta paint?

Pirta paint was invented by father-son duo Howard and Rob Atkin, and tested by the University of Leeds. You can read more about their discovery in our press release.

Are there any limitations of what can be painted with Pirta paint?

Wet, oil contaminated, porous, flexible surfaces, textiles or living things cannot be painted with Pirta paint.

When was Pirta paint patented?

Our patent was finalised in April 2023.

How is Pirta paint tested?

Pirta has been extensively laboratory tested and measured for its reflectivity, emissivity and SRI both in the Pirta labs and through the University of Leeds. We have also conducted small scale tests in both the UK and Southern India using test rigs exposed to the sun. Larger scale testing to commence through the summer of 2023.

Can Pirta paint cool in cooler climates or does it only work in hot areas?

Due to its high emissivity, Pirta paint can bring surface temperatures down as long as they are above the ambient temperature. The emissivity also allows Pirta paint to cool a structure in shade or at night, though to a lesser degree.

Many paints include toxic compounds. Is Pirta paint non-toxic?

Pirta is a low VOC water based paint. All ingredients have been classed as non-hazardous.

Is Pirta Paint safe?

Our system is water-based and contains very low VOCs and biocide contents. We have designed the product to be as safe as possible and inline with accepted international norms. Fire safety tests are yet to be carried out, but provisional tests have shown good results.

Will Pirta paint have any effect if I paint inside with it?

The system has been designed as an external product.