Technical Insulation Solutions

For new dwellings, flat roofs must meet limiting U-values to limit heat gains and losses through the building fabric. The maximum (limiting) U-value for flat roofs is 0.16 W/m²K (area-weighted average).

• Best practice: Design to the notional building specification, which uses a U-value of 0.11 W/m²K for flat roofs, to help meet overall targets for primary energy rate, emission rate, and fabric energy efficiency. Ensure insulation is continuous, tightly fitted without air gaps, and extends to meet wall insulation. Assess load-bearing capacity before adding insulation.

When extending an existing dwelling or replacing a flat roof element (e.g., during waterproofing renewal), new fabric elements must meet limiting U-values as new thermal elements. The maximum U-value for flat roofs is 0.15 W/m²K (area-weighted average). This applies to elements in extensions and new or replacement elements in existing dwellings.

For refurbishment of an existing flat roof (renovating a thermal element, e.g., replacing waterproofing or renovating more than 50% of the element's surface, or during a major renovation affecting over 25% of the building envelope), check the existing U-value. If it is worse than the threshold of 0.35 W/m²K, the roof must be upgraded where technically, functionally, and economically feasible (e.g., simple payback of 15 years or less).

For extensions or replacements in non-domestic buildings, new thermal elements (including flat roofs) must meet limiting U-values of 0.18 W/m²K (area-weighted average). This is particularly relevant for buildings over 1,000 m² where consequential improvements may be triggered.

For refurbishment (renovating existing flat roofs, e.g., if more than 50% of the surface is renovated or during a major renovation), if the existing U-value exceeds the threshold of 0.35 W/m²K, upgrading is required where feasible.

• Improved U-value target for renovated flat roofs: 0.18 W/m²K.
• If the target is not feasible: Aim for the best U-value with a 15-year payback, not exceeding 0.70 W/m²K.
• If the existing U-value is 0.35 W/m²K or better: Upgrading is not required, allowing for waterproofing overlay without additional insulation, subject to condensation checks.

In all cases, if there is little or no existing insulation, upgrading to the relevant improved U-value is recommended where feasible to enhance energy efficiency and avoid condensation. Let our technical team advise you on the most suitable insulation and waterproofing solutions tailored to your project.

XPS - Extruded Polystyrene

•Manufactured using extrusion: a continuous process which results in a closed-cell structure with a smooth skin on the top and bottom of the board

•XPS’ closed-cell structure prevents water penetrating the structure of the insulation board and provides long term strength and durability

Mineral Wool

•Mineral wool is a furnace product of molten rock at a temperature of about 1600 °C through which a stream of air or steam is blown

•The final product is a mass of fine, intertwined fibres with a typical diameter of 2 to 6 micrometres.

VIP - Vacuum Insulated Panel

•Form of thermal insulation consisting of a gas-tight enclosure surrounding a rigid core, from which the air has been evacuated

•VIPs have about 1/5 the thermal conductivity of conventional insulation, and therefore about five times the thermal resistance

The chart provides a comparison between various insulation types, illustrating the thickness required for each to achieve a U-value of 0.16, as an example. Vacuum Insulated Panels (VIP) may be the only viable option in scenarios where there are strict limitations on the overall thickness of the roof membrane. While Polyisocyanurate (PIR) generally offers strong thermal performance, Mineral Wool may be a more suitable choice in situations where non-combustible insulation is necessary, such as over compartment walls, though it will need to be considerably thicker to reach the same thermal standard. In addition to thermal efficiency and fire performance, other factors also influence the choice of insulation. These include material properties such as density and the range of board sizes available, all of which should be considered to ensure the most appropriate solution for the project.

Insulation Effectiveness: The U-Value calculation helps to determine the effectiveness of insulation in a building envelope. Lower U-Values signify greater insulation effectiveness.

Effect of Thermal Bridging: Thermal bridging occurs when there's a path of least resistance for heat flow, typically through structural elements. Thermal bridges can significantly increase the U-Value and reduce the overall thermal performance.

Standard Conditions: U-Values are usually calculated under standard conditions (usually 0°C outside, 20°C inside), and assume steady state conditions, with no effect from solar gain or wind speed.

Regulations: Building regulations often specify minimum U-Value requirements for different parts of the building envelope (walls, roof, floor, windows, etc.), to ensure a minimum level of energy efficiency.

Glaser Method: This is a widely used method for condensation risk analysis. It's a steady-state calculation that considers the diffusion of water vapour and the condensation risk within the structure, but it doesn't account for the storage and redistribution of moisture, limiting its accuracy.

Dynamic Simulation: More complex, dynamic simulations can provide a more accurate condensation risk analysis, accounting for factors like moisture buffering and variable internal and external conditions.

Prevention and Mitigation: Proper design, adequate insulation, controlled ventilation, and appropriate use of vapour barriers can help prevent or mitigate condensation risk.

Assessment Tools: Various software tools can be used for condensation risk analysis, such as WUFI, which provides both standard (Glaser method) and more advanced hygrothermal (moisture and heat transfer) analysis.

Regulations: Certain building regulations require a condensation risk analysis to ensure adequate moisture management in the building's design.