Cold Roof vs Warm Roof: What's the Difference?

The terms 'cold roof' and 'warm roof' describe where the insulation sits in a flat roof build-up. Getting this right is critical for long-term performance and meeting building regulations.

What is a Cold Roof?

In a cold roof construction, insulation is placed between the joists, below the structural deck. The space above the deck is ventilated to outside air. This was the standard construction method for many years.

What is a Warm Roof?

In a warm roof construction, insulation is placed above the structural deck, keeping the entire structure within the warm zone of the building. This is now the preferred and recommended construction for flat roofs.

Why Warm Roofs Are Now Preferred

• Eliminates the risk of condensation within the roof structure
• Easier to achieve required U-values (Building Regulations)
• More thermally efficient — no cold bridges at joist positions
• No ventilation detailing required
• Insulation layer protects the structural deck from temperature extremes

Inverted Warm Roofs

An inverted (or upside-down) warm roof places the insulation above the waterproofing membrane, protecting the membrane from UV and temperature extremes. This is common on larger commercial roofs and roof terraces.

What Should You Specify?

For any new flat roof or replacement, specify a warm roof construction with PIR insulation above the deck. Your contractor should size the insulation correctly to achieve the required U-value. Cold roof ventilation details are harder to get right and present a higher condensation risk.