Roof insulation: trace the heated boundary before choosing boards

An unheated loft can remain outside the insulated envelope when the boundary follows the ceiling. Living accommodation usually needs insulated slopes and adjoining walls. Compare both arrangements with ventilation and future conversion plans.

Investigate damp timber before enclosing it between new layers. Insulation cannot stop a roof leak. Drying and repairs must address the identified cause rather than follow the insulation delivery date alone.

Rigid PIR can suit limited build-up height and a continuous layer above rafters. Mineral wool fits spaces between structural members. Compare the actual products, including fire properties, rather than material names alone.

Use EPS and XPS in assemblies intended for them. Greater water or compression resistance is not needed everywhere. Check conductivity, loads, water absorption, facings and the board’s intended application.

For a homogeneous layer, R = d/λ with thickness in metres. Assuming d = 0.12 m and λ = 0.022 W/(m·K) gives about 5.45 m²K/W. This illustrates the insulation layer, not the completed roof.

The complete assembly calculation includes other layers, surface resistances and structural effects. Rafters and fixings can alter the result. Confirm thickness by calculating the actual build-up, especially when combining materials.

Plan joints and fit edges according to the system. Gaps around rigid boards need an approved filling method. Arbitrary foam is not automatically compatible with material, airtightness and fire requirements.

Trace junctions at the wall plate, chimney, ridge and roof windows. Insulation must meet adjoining elements. Extra thickness across an easy central area does not correct a thermal bridge around the perimeter.

Join the internal airtight layer continuously at perimeters, laps and service penetrations. Small leaks can carry moisture with airflow. Choose sealing products for the actual surfaces they will bond to.

A PIR foil facing performs its specified function only with correctly detailed joints. Do not assume it replaces every vapour-control layer. External layers must also provide protection and a planned drying route.

Above-rafter fixings transfer loads through insulation to the structure. Selection considers snow, wind, pitch and substrate, not just board thickness. Retain the required protective finish on the room side.

Reaction to fire of a material differs from fire resistance of an assembly. Framing, lining and joints also matter. A service cavity can reduce later penetrations through the continuous airtight layer.

Inspect board edges, perimeter seals and services, then photograph them. Check required ventilation remains open. Correct defects before lining, while junctions are still visible and accessible.

After upgrading, monitor indoor humidity and signs of water. Insulation reduces heat loss but does not replace ventilation of occupied rooms. Recurring damp calls for investigation of the whole assembly.