What Is the Difference Between a Warm Deck and a Cold Deck?

Warm Deck Roof

Insulation Location
In a warm deck roof, the insulation is installed above the roof deck and directly beneath the waterproofing membrane. This design ensures that the roof deck and supporting structure are kept on the ‘warm’ side of the insulation.

Thermal Performance
A warm deck roof provides superior thermal performance. By maintaining the roof deck at a temperature closer to the building’s interior, the risk of condensation within the roof structure is significantly reduced. This makes it a more energy-efficient option, ensuring heat loss is minimized, even around roof features such as roof lanterns.

Ventilation
Warm deck roofs typically do not require additional ventilation because the insulation layer prevents warm, moist air from reaching the roof deck. This simplifies the construction process and reduces the chance of issues arising from inadequate ventilation.

Common Usage
Warm deck roofs are the preferred choice for modern construction and retrofits, particularly when incorporating roof lanterns. Roof lanterns in warm deck roofs benefit from improved thermal efficiency and reduced condensation risks, ensuring a comfortable internal environment.

Building Regulations Reference
The design and construction of warm deck roofs in the UK must comply with Approved Document L of the Building Regulations, which governs energy efficiency and the conservation of fuel and power. Compliance includes ensuring the thermal performance of the roof achieves the required U-value (typically ≤ 0.18 W/m²K for pitched or flat roofs with roof lanterns).

Cold Deck Roof

Insulation Location
In a cold deck roof, the insulation is installed below the roof deck, leaving the deck and supporting structure on the ‘cold’ side of the insulation. The ceiling of the top floor serves as the boundary between the warm interior and the cold roof space.

Thermal Performance
Because the roof deck is exposed to external temperatures, it is more prone to condensation, particularly in colder climates. This can lead to issues such as dampness, rot, or structural degradation if not properly addressed.

Ventilation Needs
Cold deck roofs require effective ventilation to manage moisture and prevent condensation. Typically, this involves incorporating a ventilation gap between the insulation and the underside of the roof deck, often with vents in the fascia or soffits to promote airflow. Without adequate ventilation, condensation can accumulate, particularly around roof lanterns, where thermal bridging may exacerbate moisture problems.

Usage Considerations
Due to the challenges of managing condensation and the complex ventilation requirements, cold deck roofs are less common in new builds. However, they may still be found in older buildings or specific retrofit situations. Roof lanterns in cold deck roofs require particular attention to thermal bridging and ventilation design to meet modern performance standards.

Building Regulations Reference
Cold deck roofs must also comply with Approved Document L regarding thermal performance and Approved Document C, which addresses resistance to moisture and condensation. For cold deck roofs, compliance typically involves ensuring proper ventilation is installed as outlined in BS 5250: Code of Practice for Control of Condensation in Buildings.

Relevance to Roof Lanterns

The choice between a warm deck and a cold deck roof has significant implications when installing roof lanterns. Roof lanterns are a key architectural feature, often used to introduce natural light into interiors. However, improper design can lead to issues with thermal performance and condensation.

• Warm Deck Advantage: In a warm deck roof, the roof lantern benefits from being part of the insulated structure. This ensures minimal heat loss and reduces the risk of condensation forming on the glazing or frame.
• Cold Deck Challenges: In a cold deck roof, roof lanterns are more exposed to external temperatures. This increases the likelihood of thermal bridging and condensation unless specific measures, such as thermal breaks and enhanced ventilation, are incorporated.

To meet UK Building Regulations, roof lanterns in both warm and cold deck roofs must achieve appropriate U-values and adhere to moisture control requirements. Careful attention must also be given to junctions between the lantern and the roof to ensure an airtight and thermally efficient installation.