Is a metal roof suitable for an energy-efficient house?

The thermal resistance of the insulation and the continuity of its joints are central to roof performance. Thermal bridges and uncontrolled air movement also matter. Addressing them in the design allows a metal-covered roof to perform well. The outer covering alone tells you little about the energy demand of the house.

Thermal conductivity, or lambda, describes a material. The U-value describes heat transfer through an entire building element. To calculate it for a roof, include layer thicknesses, junctions and any elements passing through the insulation.

PIR can provide considerable thermal resistance in a relatively thin layer. For example, a 100 mm board with a declared conductivity of 0.022 W/(m·K) gives about 4.55 m²·K/W. Dividing thickness in metres by conductivity gives the resistance of this layer. Calculating the U-value of the whole roof is a separate step.

This can help where the overall build-up or the eaves detail limits the available depth. Boards come with different facings and edge profiles. Select a product intended for the proposed construction. A PIR insulation board on its own cannot serve as the load-bearing roof or a finished waterproof covering.

Lower conductivity allows a thinner layer to achieve a comparable resistance. Other considerations may favour another material. Mineral wool can offer advantages in fire performance and vapour permeability. EPS may be economical, and certain XPS products suit locations subject to moisture and compressive loads.

Compare products suitable for the same intended use. Check compressive strength, behaviour in contact with water and fire classification. PIR boards cannot all be described as non-combustible, and a roof assembly may have a different classification from its insulation core. The design has to meet the requirements together.

Warm, moist indoor air can flow through gaps at joints and service penetrations. As it cools, water may condense inside the roof. Plan the airtight layer and its connections as a continuous system. Insulation fitted between structural members does not by itself stop air leakage.

Pay particular attention to the wall plate, gable walls, roof windows and services passing through the lining. Choose tapes and seals compatible with the surfaces they bond to. Simply butting faced boards together does not automatically produce an airtight joint.

In a typical ventilated pitched roof, an air space beneath the sheets helps remove moisture and heat. Keep the full designed path clear and size the inlet and outlet to suit the roof. Covering the openings for appearance can impair performance. This airflow serves a different purpose from ventilation of occupied rooms.

Anti-condensation fleece beneath metal sheets can hold droplets temporarily and needs conditions in which to dry again. Its function is limited. A house still requires thermal insulation and a properly designed build-up on the room side. Assess the fleece in that wider context.

A dark surface can become hotter in sunlight. Summer temperatures in the loft also depend on insulation, ventilation, glazing and shading. Consider these features together when selecting a roof colour or trying to keep the loft cool.

Plan rooftop equipment before the roof is closed. Its fixings, penetrations and added loads need to be allowed for in the design. Drilling through finished layers creates further points to seal. Set out cable routes and maintenance access while the underlying layers are still accessible.

Check insulation continuity, airtight joints and the details against the drawings. Photographs taken before covering are valuable for this inspection. On some projects, an airtightness test also helps locate leaks. Interpret the measured result against the design target for the whole building.

Tile-effect metal sheets and trapezoidal steel sheet offer a lightweight covering for a suitably designed roof. When setting the budget, allow for detailing and inspecting the insulation and airtight layer as well as purchasing the metal. These details and their execution strongly influence the energy the house will use.