Compare figures for the actual products you are considering rather than relying on general descriptions such as polystyrene or mineral wool.

For example, a 100 mm layer with a thermal conductivity of 0.022 W/(m·K) has a calculated resistance of about 4.55 m²·K/W. A material rated at 0.031 W/(m·K) needs about 141 mm for a similar resistance. These figures cover the insulation layers alone, excluding finishes and thermal bridges. They do not mean PIR is always half as thick: the difference depends on the material used for comparison.

Add the lining, adhesive, any supporting frame and the allowance for levelling the substrate. Factory-bonded PIR insulated plasterboard can simplify the build-up, but you still need its overall thickness. A frame several centimetres deep may affect the comparison more than the difference between the two insulation cores.

Draw each option in section at the window, floor and door. This shows what happens to the window sill, the width of the reveal and the room for trim. In a narrow passage, check door handles and the swing of the door as well. The insulation has to fit the room as it is used, not just the dimensions on a materials list.

For a wall 4 m long, reducing the finished lining depth by 4 cm saves approximately 0.16 m² of floor area, assuming all other dimensions stay the same. That is a geometric calculation only. Both options must first meet the same thermal requirement: a thinner lining should not gain space simply by providing less insulation.

When several walls are involved, include the corners. In a loft, the depth of the sloping lining also affects headroom and where furniture can stand. Multiplying the perimeter by the thickness difference does not capture the whole effect. Draw a section through the bed, desk and walkway: the practical gain can be useful even if the extra floor area seems small.

If a wall separates a heated room from a cooler space, establish where the thermal envelope runs. Insulating on the unheated side can sometimes preserve space in the living area. The choice also depends on access, fire protection, how each room is used and whether the junctions can be detailed and built properly.

Partition walls between rooms at similar temperatures do not all need thermal insulation. Noise control or the strength of the lining may matter more. Thin PIR is not a substitute for an acoustic system with the right mass and layer arrangement. Identify the problem before choosing the material.

Insulating an external wall from the inside changes its temperature and its ability to dry. Assess the existing layers and the conditions in the room. Do not simply cover a damp wall with a new lining. Find and eliminate the source of water first, then check the substrate, particularly at floor level and around the reveals.

An airtight board face works only as part of a continuous system. Edges, joints and cable penetrations all need suitable details. The room still needs ventilation. A slimmer lining may save space, but a leaky junction at a window can still cause a local cold spot and moisture problems.

Compare the total cost of the finished work, including fixings, jointing, and alterations to sills or services. Weigh that against the space available and how you will use it. A few centimetres in a narrow corridor can be worth more than the same gain in a large hall. There is no single financial answer for every room.

Keep a sample and the documentation for the chosen system. Agree before installation how fittings will be secured and where cables will run. PIR with a plasterboard facing can combine insulation with a neat finish. A calculated overall thickness and a checked design make a better basis for the decision than a promise of a fixed percentage of extra floor space.