PIR or XPS on a flat roof? Start with the layer arrangement

In a conventional assembly, waterproofing protects the insulation. On an inverted roof, thermal insulation sits above the waterproofing and encounters different water and temperature conditions. That distinction is fundamental to material selection.

Inverted roofs use appropriately specified XPS above the waterproofing, secured by the designed ballast. Not every XPS suits every inverted roof. Confirm product properties, separation layers and the complete assembly, including water movement and intended roof use.

PIR boards can be used over a load-bearing deck beneath waterproofing in a suitably designed assembly. Resolve vapour control, fixings and material compatibility. Keep insulation dry during installation. Do not simply seal wet boards beneath the roof covering; assess their condition first.

Do not treat aluminium-faced PIR as an automatic replacement for XPS above waterproofing. Low water absorption alone is insufficient. Facings and documented use matter, together with position in the section, fixing method and covering compatibility.

PIR can be useful with low parapets, restricted thresholds or terrace refurbishment. One illustrative comparison might use lambda 0.022 W/(m·K), while a thinner version could have 0.023. These are examples. Do not apply the lowest conductivity to an entire thickness range without checking the actual product.

Compare XPS using data for the selected model and thickness as well. Include falls, supports and paving in the total height. The insulation core is only part of the section; saving a few centimetres matters when it improves an actual junction, not merely a comparison table.

Roofs may carry ballast, planters, equipment and maintenance traffic. Air-conditioning bases and terrace supports create concentrated loads that need separate assessment. A large kPa figure in a leaflet does not permit arbitrary loading.

A board might list 150 kPa at 10% deformation, with another version at 120 kPa. These illustrative test values do not define permissible pressure over years of use. Permanent loads require assessment of long-term behaviour and load distribution. The same principle applies to XPS and rooftop equipment supports.

PIR comes with different facings, which are not all suitable for the same bonding or welding method. The roof instructions and waterproofing documentation must form a compatible set, including any separation layers.

Mechanical fixings must suit the deck and wind uplift; bonded systems have their own requirements. Do not copy a fixing count from another roof. When refurbishing, assess layers that will remain. Sealing a wet existing build-up beneath new insulation can create further problems.

Plan weather protection during the work too. Material delivered in the morning may meet rain before the covering is installed. Limit each day’s exposed area to what can be closed safely. That organisational cost is absent from a comparison of board specifications alone.

As a purely illustrative calculation, take insulation prices of PLN 80 and PLN 100 net per m². Across 100 m², the difference is PLN 2,000 net before waste and other layers. These are worked-example amounts, not current offers. A real estimate needs two quotations with the same supply and installation scope.

Include vapour control, waterproofing, fixings, flashings, falls and delivery. For inverted roofs, include ballast and structural requirements. Without a current offer for the selected XPS, there is no basis for naming a price winner. Compare the same outcome and scope.

Prepare a section and load schedule. Define drainage and intended use, identifying equipment and maintenance routes. Then choose a product documented for the position designed for it.

Compare PIR where suitable waterproofing protects the insulation. XPS may be advantageous in inverted assemblies designed for it. Compatibility of every layer ultimately decides. Excellent conductivity cannot correct a wrong material position, and high strength cannot replace effective drainage.

Only where the specific product documentation and roof design expressly provide for that arrangement.

No. Compare the complete roof, its loads and current quotations.