Understanding Heat with Cool, Dark, Zinc Steel Roofs and Concrete Tiles – the Facts!

What Are Cool Roofs?

Typically everyone writes their article about Cool Roofs stating “specially designed roofing systems that reflect more sunlight and absorb less heat compared to standard roofs“. What this simplistic statement always ignores is the most important part of a cool roof – Thermal Transmittance or Thermal Conductivity.

• Summer Penalty – Dark Roofs
• Cool Roof Winter Argument
• Zincalume®and Galvanised materials
• Hot Concrete Tiles
• Choosing the Right Roof Heat Protection
• Advanced Thermal Insulation Cool Roof Solutions

Cool roofs are designed to reduce the amount of energy or heat transferred to the building’s interior, but strictly rely on ‘reflectance – light bounce’ and emissivity to improve indoor comfort while lowering energy costs associated with cooling systems. True, however high performance cool roofs achieve this through high solar reflectance, high thermal emissivity and low thermal conductivity. The trifecta for sustainable thermal surfaces. The conversation regarding low thermal conductivity in the Solar Reflectance Index is missing along with some other important thermal facts.

Key Features of Cool Roofs

1. Solar Reflectance: Measures how much solar energy is reflected off the surface. Higher reflectance values indicate better cooling performance.
2. Thermal Emissivity: Indicates how efficiently a material releases absorbed heat. High emissivity materials quickly shed heat, helping keep surface temperatures low.
3. Thermal Conductivity: This is a fundamental property that determines how much heat is transferred through the roofing material. Roofs with low thermal conductivity, such as those treated with advanced coatings, significantly reduce heat transfer into the building.
4. Surface Materials: Cool roofs can be made from reflective coatings, tiles, membranes, or specially treated metal surfaces.
5. Durability: The surface coating needs to work when dirty for other 20 years without maintenance.

Benefits of Cool Roofs

• Energy Efficiency: By reducing the cooling load on air conditioning systems, cool roofs lower electricity bills.
• Improved Comfort: Cooler indoor temperatures, especially in buildings without air conditioning.
• Extended Roof Life: Reduced thermal cycling (heating and cooling) minimizes wear and tear on the roof.
• Urban Heat Island Mitigation: Cool roofs help lower overall city temperatures by reducing heat absorption in urban areas.

The “Summer Penalty” of Dark Roofs

Dark roofs are typically made of materials with low reflectivity and high absorption rates, such as asphalt shingles, dark-coloured tiles, or untreated metals. These roofs absorb most of the solar energy they receive, converting it into heat that radiates to the interior of the building and the surrounding environment.

The Summer Penalty Explained:

• Higher Cooling Costs: Dark roofs can reach temperatures of up to 80°C (176°F) in the summer sun, transferring significant heat into the building below. This increases reliance on air conditioning, driving up energy bills.
• Thermal Discomfort: Heat radiated by dark roofs makes indoor spaces hotter and more uncomfortable, especially in buildings without adequate insulation or cooling systems.
• Environmental Impact: The additional energy consumption associated with cooling contributes to higher greenhouse gas emissions.

Addressing the Winter Argument

Critics often claim that dark roofs provide a “winter heating benefit” by absorbing heat during cold months. However:

• Winter solar intensity is significantly lower than summer, providing minimal heat gain.
• Heat absorbed by the roof often radiates back to the cooler ambient air rather than transferring into the building.
• Proper insulation plays a far more critical role in retaining indoor heat during winter.

The net result is that the cooling penalty of dark roofs in summer vastly outweighs any minor winter benefit.

Zinc and Galvanized Roofs: Misunderstood Poor Performers

Zincalume® and galvanized steel roofs are popular choices in many regions due to their durability, lightweight properties, and resistance to corrosion. They also have a shiny, metallic appearance that suggests high reflectivity. However, their thermal performance is often misunderstood and overrated.

Reflectivity vs. Emissivity:

1. Reflectivity: Unpainted Zincalume® and galvanized roofs can reflect a moderate to high portion of solar energy, with solar reflectance values typically between 0.6 and 0.8. This reduces the light portion of the solar spectrum that’s converted into heat on the surface.
2. Emissivity: These materials have low emissivity, often around 0.1 to 0.3, meaning they retain much of the heat they absorb instead of efficiently releasing it back to the environment. This can result in higher surface temperatures over time.
3. Thermal Conductivity: Zincalume® and galvanized steel have relatively high thermal conductivity, allowing absorbed heat to transfer more easily into the building, which increases indoor heat loads in summer.

Why Low Emissivity Matters:

• Heat retained by low-emissivity surfaces tends to radiate into the surrounding air and the building’s interior, counteracting the benefits of their moderate reflectivity which increases the thermal conductivity
• During hot summer days, Zincalume® roofs can still contribute to significant heat gain within the structure and the urban heat island.

The Winter Argument for Zinc Roofs:

• In winter, low emissivity does little to retain meaningful heat in colder climates because any absorbed heat dissipates rapidly to the cooler ambient air.
• Like dark roofs, Zincalume® roofs provide minimal winter heating benefit compared to the energy they can trap and radiate during summer.

Concrete Tiles: The Hottest Roof Type in Australia

Concrete tiles are a common roofing material in Australia due to their affordability and availability. However, they come with significant drawbacks in terms of thermal performance.

Thermal Mass and Heat Retention:

Concrete tiles have high thermal mass, meaning they absorb and store large amounts of heat during the day. This characteristic causes:

• Heat Storage: During hot summer days, concrete tiles store solar heat and slowly release it into the building, keeping interiors warm even during cooler evenings.
• Thermal Lag: The delayed release of heat creates discomfort and increases the reliance on cooling systems at night.

The Role of Colour:

Dark-coloured concrete tiles exacerbate the problem by absorbing even more solar energy. Studies show that dark concrete tiles can reach surface temperatures exceeding 80°C, making them one of the hottest roofing materials in Australia. This leads to:

• Increased Cooling Costs: High heat retention forces air conditioning systems to work harder.
• Urban Heat Island Contribution: Dark concrete tiles amplify heat in urban areas, contributing to higher ambient temperatures.

Addressing the Issue:

To mitigate these effects, reflective coatings like Super Therm® can be applied to concrete tiles. These coatings:

• Reduce heat absorption by reflecting up to 96% of solar radiation.
• Prevent heat build-up in the thermal mass of the tiles, improving indoor comfort and reducing energy costs.

Choosing the Right Roof Heat Protection

When selecting a roofing material, it’s essential to consider both short- and long-term performance in your specific climate.

For Warm Climates:

• Cool Roofs: Provide the best energy savings and comfort by reflecting most solar heat, efficiently emitting absorbed heat, and having low thermal conductivity.
• Avoid Dark and Concrete Roofs: These significantly increase cooling loads and contribute to urban heat island effects.

For Cold Climates:

• While some argue for dark or metallic roofs for winter heating, the benefits are often negligible when compared to the overall efficiency of a well-insulated home. A light-coloured cool roof with proper insulation still provides significant advantages year-round.

For Mixed Climates:

• High-performance cool roof materials or coatings, like those with ceramic or reflective technology, strike a balance by reflecting summer heat and minimising winter heat loss.

Advanced Thermal Insulation Cool Roof Solutions

Modern cool roofs go beyond simple light colours. Innovative coatings, such as Super Therm®, combine high solar reflectance with advanced ceramic insulation properties. This coating targets all 3 methods of heat from the sun’s solar spectrum and target solar reflectance, high thermal emissivity and low thermal conductivity:

• Reflects and blocks over 96.1% of total solar radiation
• Safe and environmental friendly
• Meet and set thermal insulation coating standards
• Provide additional thermal insulation, reducing heat transfer to the building
• Are effective across a wide range of climates, reducing cooling loads in summer and helping maintain comfortable temperatures year-round
• Lasts over 20 years and works when dirty without maintenance.

Summary

Roof selection is a critical decision for energy efficiency, comfort, and environmental impact. While dark roofs, Zincalume® roofs, and concrete tiles may seem appealing for certain climates, their thermal performance often falls short due to issues like heat retention, low emissivity, and high thermal conductivity.

Cool roofs, particularly those leveraging advanced insulation coatings with Super Therm®, provide a superior solution for most climates, delivering measurable energy savings and a more comfortable living environment. Investing in a cool roof isn’t just about reducing energy bills—it’s a step toward sustainable living, lower carbon footprints, no urban heat load and a cooler planet.