Cool roof

Solutions & Applications · Waterproofing & Flat Roof Energy Efficiency

A dark Greek flat roof reaches surface temperatures in summer that are well above the air temperature. This heat is transferred to the building envelope, increasing the cooling load whilst simultaneously causing the waterproofing to deteriorate. A cool roof reverses this logic: instead of absorbing solar radiation, it reflects it and radiates back the heat it ultimately retains.

What Makes a ‘Cool’ Roof — Two Properties, Not One

A cool roof is not defined by its colour but by two measurable parameters that must co-exist:

Property	What Matters	Why It Matters
Solar reflectance (SR / albedo)	The proportion of incident solar radiation that is reflected	Any radiation that is reflected is not converted into heat on the flat roof
Thermal emittance (ε — emittance)	The ability of a surface to dissipate the heat it has absorbed	Without radiation, heat is trapped even if the surface reflects it
SRI Index	Combines reflectance and emissivity into a single figure (ASTM E1980)	The only unit that allows for a direct comparison of materials

Why ‘White’ Alone Is Not Enough

A glossy metallic surface has high reflectivity but low thermal emissivity — it reflects, but whatever it absorbs is retained and causes it to heat up. In contrast, a reflective elastomeric coating combines high reflectivity with high emissivity, which is why it remains cool.

In the specification, we always require both values — or the SRI index that combines them. A material that is simply described as ‘white’ has not specified anything measurable.

Benefits on Four Levels

Level	Benefits
Building	Lower flat roof and indoor temperatures; reduced air-conditioning load; improved comfort conditions on the top floor
Waterproofing	Smaller temperature fluctuations mean less thermal expansion and contraction — extending the service life of the waterproofing layer
City	When applied to many buildings in the same area, it helps to reduce the local air temperature — mitigating the urban heat island effect
Environment	Reduced energy consumption for cooling, and therefore reduced CO₂ emissions

Liquid-applied Coating or Membrane? Two Approaches to a Cold Roof

Criterion	Liquid-applied Coating	White Bituminous Membrane
Typical use	Upgrading existing waterproofing	New build or complete refurbishment
Application	Coating — flameless, no removal required	Sheet bonding, factory-produced thickness
Waterproofing thickness	Reinforces the existing layer	Self-contained waterproofing layer
Complex geometries	Seamless membrane — ideal for multiple penetrations	Requires careful overlaps
Maintenance	Can be renewed with a new layer	Local repairs

Liquid-applied coating with a European Technical Assessment: ESHA DESMOSEAL

When the cold-applied top coat also serves as the waterproofing for the flat roof, the documentation is just as important as the colour. ESHA DESMOSEAL is a single-component liquid-applied coating based on silane resins, which cures on contact with air and produces a brilliant white film. The film does not yellow and is not affected by airborne particles or dirt — in other words, it specifically counteracts the mechanism that causes reflectivity to diminish over time.

Documentation	ETA-18/0654 (DIBt) in accordance with ETAG 005 · CE in accordance with EN 1504-2
Service life	W3 — 25 years (ETAG 005)
Scope	Substrate gradients S1–S4 · climate zones M & S · mechanical wear P1–P4
Flexibility	Elongation at break > 300 per cent; tensile strength 2.0 N/mm²
Breathability	Water vapour permeability 30 g/m²/h (ISO 9932)
Ageing	4,000 hours of artificial ageing (UV + humidity) whilst maintaining properties and elasticity
Consumption	1.0–1.5 litres/m² → membrane thickness 1.0–1.5 mm

Apply cold, using a roller or spray gun, in 2–3 cross-layered coats of up to 0.5 mm each, allowing at least 3 hours’ drying time. Cracks are first filled with ESHAPOLYSEAL 1K; where reinforcement is required, ESHATERRA 50 reinforcement is inserted between the coats. On concrete, PRIMER PU or PRIMER AQUA 2K is recommended as a primer.

Step-by-step guide

1. Substrate inspection: the condition of the existing waterproofing is recorded — blisters, open joints, standing water. The cold-applied material does not rectify any existing defects.
2. Cleaning: removal of dust, salts, algae and decayed areas. Adhesion is assessed at this stage.
3. Drainage check: ensure there is a sufficient drainage gradient. Standing water on a cold roof leaves salt deposits and reduces reflectivity locally.
4. Priming: use a suitable primer depending on the substrate — concrete, old bituminous membrane or mosaic tiles require different preparation.
5. Reinforcement of critical points: upstands, guttering and penetrations are reinforced locally prior to the application of the main layer.
6. Apply in two coats: in a criss-cross pattern, allowing for the drying time and adhering to the stated consumption per m². Reflectivity depends on complete, uniform coverage.

The Overlooked Factor: Ageing

Reflectivity decreases over time due to pollution, dust and biological growth. This is why international assessment protocols specify values for both new and aged surfaces — and it is the latter that matters in practice.

A simple periodic cleaning of the flat roof restores much of its original performance. Without this, the system silently underperforms. Therefore, when selecting a liquid-applied coating, it is not only important how white it is on the first day, but whether it remains white — materials with a coating that does not yellow or retain dirt, such as ESHA DESMOSEAL, lose their reflectivity more slowly.

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