Thermal performance requirements for roof and wall insulation in Poland

Under the previous technical conditions for building partitions in rooms with a temperature of at least 16°C, the maximum Uc was taken as 0.20 W/(m²·K) for external walls and 0.15 W/(m²·K) for roofs, roof slabs and ceilings above an unheated attic. These are not values introduced only in 2026. Before applying them to a specific project, the correct legal basis must be established.

As at 1 November 2026, transitional provisions must be taken into account. Art. 102a of the Building Law provides for the possibility of preparing the specified designs under the earlier provisions for 18 months from 20 September 2026, subject to meeting the statutory conditions, including submission of the investor’s declaration. Separate rules apply to some other works. The designer should establish which regime applies to the given case; the U-value table alone does not determine this choice.

Other conditions may apply to rooms with a lower design temperature. Therefore do not automatically transfer the requirements for a house to a cold warehouse or an unheated garage. First determine the use and the design temperatures. Only then select the insulation and the parameters of the insulated wall or roof sandwich panel.

Lambda describes heat conduction through a material. The thermal resistance R of a single homogeneous layer can be calculated by dividing its thickness in metres by lambda. The U-value describes heat transfer through a building component. It is therefore neither another name for lambda nor a parameter resulting solely from the thickness of one board.

Calculation example: 10 cm of material with a lambda of 0.022 W/(m·K) gives an R of approximately 4.55 m²·K/W. This does not yet mean a specific U-value for the finished roof. The calculation of the building component takes into account the other layers, the surface heat transfer resistances and the required corrections. Joints and thermal bridges in the details require separate attention.

Selected PIR boards with aluminium facings have a declared lambda of 0.022 W/(m·K). This allows a given thermal resistance to be achieved with a smaller thickness than for a material with a higher lambda. This matters with a low parapet, a terrace threshold or a limited attic height. However, the parameter must be checked for the specific product.

Not all PIR boards have identical facings, edges and applications. The variant for under a roof membrane may differ from a product for another assembly. Also select the fixing and the method of sealing the joints. An insulation board does not automatically replace waterproofing, and its facing does not guarantee the tightness of the entire surface without careful installation.

Heat also flows through rafters, fasteners and gaps in the insulation. A layer laid only between structural elements behaves differently from continuous insulation above them. The junctions of the roof with the wall, the wall plate and windows also matter. A large thickness in the middle of the slope does not automatically fix a weak detail at the eaves.

Before purchase, ask for solutions for these locations on a drawing. You need to know how to connect the wall insulation to the roof and maintain the tightness of the layers. For sandwich panels, check the conditions of the declared U-value and how the joints are made. A catalogue parameter does not justify leaving gaps or sealing the junction incorrectly.

This conclusion cannot be drawn from the parameters table alone. The scope of application of the requirements depends on the type of project and the regulations for the given building. New construction, alteration and ongoing maintenance are different situations. For planned works, it must be established which elements are subject to assessment under the current conditions.

The regulations also apply to building components covered by alteration. This does not automatically mean that all other parts of the house must be altered with every roof repair. The scope should be established in the documentation. A technical assessment may also show that wider works are justified on energy grounds, even if they do not arise from the legal obligation alone.

Safe behaviour of the building component with respect to moisture and fire is also required. An assembly with a low U-value may be defective if it traps moisture in the timber or has a leaky layer on the interior side. Check the vapour barrier run, ventilation and the ability to dry out. Fire conditions must be assessed for the solution used.

In a new building, the primary non-renewable energy demand indicator is also assessed. Very thick insulation alone does not determine the overall energy performance. Installations, ventilation and the energy source also have an influence. The choice of boards should therefore follow from the whole design, not solely from a comparison of advertised lambda values.

Commission a calculation for the actual layer assembly and the planned temperature. State the available height, the substrate, the loads and the method of finishing. Ask for precise designation of the board, facing and edge. When comparing PIR, EPS, XPS and mineral wool, compare equivalent applications and the same required thermal effect.

In the quote, include tapes, fixings, cut-offs and edge details. Then check that the delivered product matches the calculation assumptions. A change to a product with a different lambda or a different purpose requires reassessment. This allows the planned result to be achieved without accidentally increasing the insulation thickness.