PIR insulation
Internal insulation and moisture: what to establish before choosing PIR
Insulating a wall from the inside requires assessment of moisture behaviour across the complete construction, rather than a comparison of PIR board thicknesses alone. Existing masonry, rain exposure, airtight connections and room ventilation influence the choice. An insulation name does not establish suitability for a particular wall or determine whether an additional vapour control layer is required. Before ordering, arrange an assessment and agree details at windows, floors and services. This guide organises the client’s decisions, from defining renovation constraints and the design scope to accepting the work and fitting furnishings later. It is not a universal installation method: details must follow the building assessment and the selected system’s documentation.
Why insulate on the inside?
State what prevents external insulation: a protected façade, the property boundary or a limited renovation scope. This starts the comparison of options; it does not automatically make every board suitable for the room side.
Measure the space available after allowing for finishes, trims and furnishings. Distinguish PIR core thickness from the total proposed lining depth in your enquiry. Two similarly named quotations may include different layer combinations.
Assess the existing wall first
Do not cover unexplained damp with a new lining. Leaks, rising moisture and substrate condition need investigation before design. The inspection findings should identify necessary repairs or further assessment.
Give the designer information about render, coatings and earlier façade repairs. Leave unconfirmed layers as questions to resolve. An assessment of the actual wall matters more than an example from another building.
Compare complete solutions
Compare the board, its facing, fixing method and finish together. For PIR, consult the documentation for the exact variant. Another board’s properties, or those of the core material alone, do not describe the completed wall.
Ask how moisture risk was assessed and why that construction was selected. Where a hygrothermal assessment is required, it should use the established conditions. A low thermal conductivity declaration does not replace this assessment.
Draw the junction details
Have junctions with reveals, floors and internal walls shown explicitly, rather than leaving them to the installer as a standard detail. These locations need continuity of insulation and the required airtight layers.
The drawing should also show where insulation ends when only one wall is renovated. Mark unresolved details. This makes it easier to price the actual scope instead of discovering omitted junctions as extras.
Include services and ventilation
Position sockets, cables and fixings before the lining is installed. Penetrations through layers requiring airtightness need suitable details. Ask where a cabinet or shelf can safely be fixed after completion.
Agree room ventilation at the same time. An insulation board does not provide air exchange. If windows or the ventilation system are also changing, assess those changes together instead of treating them as independent purchases.
Plan checks before concealment
Arrange inspection of junctions before they are covered. Photographs and material records help locate layers later, but do not replace workmanship assessment. Record deviations and how they were corrected.
At handover, collect final drawings, operating guidance and restrictions on later alterations. Keep them for the next contractor. A well-chosen system also needs use that does not compromise its designed connections.
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