Choosing membranes for an insulated roof

On the room side, the specified layer controls vapour diffusion and helps maintain airtightness. Its continuity matters at walls, windows and services. Warm, moist air passing through a gap can reach a colder part of the roof and release moisture there as condensation.

Allow for tapes and seals when ordering the membrane. Plan the cable runs and lining supports before installation to reduce the number of later penetrations. On an existing roof, establish how the current layers can dry before adding a layer with high vapour resistance.

Outside the insulation, the underlay can drain water that gets past the covering and reduce wind movement through the insulation. Follow the drainage detail to the eaves. For a low-resistance membrane, check whether the manufacturer permits direct contact with insulation or installation on continuous decking.

Older underlays with higher vapour resistance may need a ventilated gap. Filling that space during insulation work can impede drying. Look for the product markings and installation details. If they are illegible, assess the roof build-up before deciding how much insulation can be added.

The Sd value, given in metres, is the thickness of still air with the same resistance to water vapour diffusion as the material. A lower value allows vapour to diffuse more readily. Some vapour control layers have a fixed value, others vary with humidity. Use the stated value or range and the design conditions for your roof.

Indoor ventilation carries away moisture produced in the home. Vapour diffusion through materials and air movement through a leak are distinct processes. The roof detail must provide continuous airtightness and insulation while allowing the intended drying path.

A PIR board can have a facing with high vapour resistance. Assess the facing, board joints and surrounding layers together. If a supplier proposes a board of the same thickness with a different facing, recheck the moisture behaviour of the assembly.

Over-rafter insulation needs continuous details at the eaves, ridge and openings. Confirm that adhesives and tapes bond to the chosen facing. The documentation for the complete system specifies which functions the aluminium facing can perform and which separate layers the roof requires.

Select jointing products for the substrate and the installation temperature. Dust or surface moisture can weaken a bond even if the tape initially sticks. Masonry and timber may need different preparation from smooth membrane surfaces. Use the adhesive specified for each junction.

Group service routes where practical and detail every pipe, cable and structural penetration. Leave enough movement allowance at joins so movement of the structure does not pull a seal apart. A membrane stretched tight around a pipe can tear at its edge.

List every membrane and mark its position on the roof section. Include material for laps and upstands, joint tapes, sealants, penetration seals and any primer. Compare complete systems that fulfil the same requirements, including the work needed to prepare difficult substrates.

Inspect and photograph the joints before the lining conceals them. Record the exact product names alongside the photographs. If moisture appears later, this record helps identify the layer involved and trace how air, vapour or water reached it.