If an area looks suspect, check it before covering it up.

If the existing insulation is dry and fit to keep, the design can incorporate it. An extra layer is not automatically an improvement, however. Moving a layer with high resistance to water vapour can make it harder for the roof to dry out. Assess the whole roof build-up rather than just comparing the thermal conductivity of two insulation materials.

Timber conducts heat differently from the insulation between the rafters. A continuous layer on the room side reduces the effect of these repeating thermal bridges. PIR boards can provide the required thermal resistance in relatively little space. That matters where every extra centimetre lowers the sloping ceiling over the usable part of the room.

The U-value of the finished roof depends on all its layers, fixings and junctions. The nominal thickness of the new board alone cannot tell you what it will be. Check junctions at the wall plate, gable walls and roof windows. Even with good insulation across most of the roof, you may still feel the effect of a local gap.

The facing may strongly resist water vapour passing through the face of the board. The finished lining, however, has joints, edges, screws and service penetrations. If these leak air, moisture can reach colder layers. Plan how to connect the new insulation to the rest of the building’s airtight layer.

Do not add another membrane wherever it is easiest to fit. The design must account for the materials already in place and how the roof can dry out. The rooms also need ventilation, independently of the air space beneath the roof covering. A new lining does not remove the moisture produced by everyday life in the house.

The composite board needs suitable support and fixings for the substrate. Follow the system specification, taking into account the board’s weight and the roof pitch. An adhesive method suitable for an upright wall may not suit an overhead lining. Avoid driving screws too deep, as this can weaken the fixing. Support spacing and joint positions should follow the system documentation for sloping or overhead installation.

Plan joints and cuts before installation, particularly around windows. If a supporting framework is needed, include it in the overall lining depth. Interior work can be scheduled separately from reroofing, but adhesives and jointing compounds still have application limits for temperature and humidity. A cold, damp loft is not necessarily fit for installation.

Good insulation reduces heat flow through the roof, but summer comfort also depends on windows, shading and ventilation. Large areas of unshaded glazing can heat a room considerably even when the slopes are well insulated. Review these features as part of the refurbishment. A thin layer of PIR alone cannot guarantee a cool loft during a heatwave.

Noise needs a separate assessment. PIR does not absorb sound in the same way as mineral wool. Sound insulation also depends on the mass of the lining, the framework and any gaps. If rain or road noise is a problem, the whole assembly may need to be assessed. Do not count on a specific improvement without test data for a comparable build-up.

An insulated plasterboard combines two steps: fitting the insulation and lining the room. The joints, corners and faces still need finishing. Plan lighting, sockets and cable routes in advance. Cutting openings into a finished slope later can break the airtight layer or require additional fire protection.

At handover, check the junctions around walls and windows as well as the surface finish. Keep a record of the layers installed. PIR insulated plasterboard is worth considering when the loft benefits from its shallow depth and combined functions. A lasting result depends on understanding the existing roof and fitting a continuous, securely fixed lining.