How to compare conductivity, thermal resistance and U-values for PUR, PIR and EPS SIPs

SIP describes a structural insulated panel. The product documents identify the facings, core and thermal conductivity of the actual configuration.

Record conductivity in W/(m·K) and check whether the figure is a declared value or a design value. Measure or obtain the core thickness separately. Including the OSB faces in the foam thickness would overstate the resistance of the insulation layer.

Under the same conditions, a homogeneous layer with lower conductivity transmits less heat at the same thickness. A particular PUR or PIR core may therefore offer more resistance per millimetre than the EPS being compared. Load capacity, fire behaviour and sound insulation require their own data.

EPS and polyurethane foams both come in several grades. Compare two identified products using their respective values. If both walls must achieve the same thermal target, the main difference may be the insulation depth required. The finished section must also satisfy the structural design.

For a homogeneous layer, use R = d/λ with thickness d in metres. For example, a 0.12 m core with an assumed conductivity of 0.022 W/(m·K) gives a resistance of about 5.45 m²·K/W. The result covers the core alone.

For comparison, take EPS at an assumed 0.036 W/(m·K). At 0.12 m its resistance is about 3.33 m²·K/W. To reach approximately 5.45 m²·K/W at that conductivity would require about 0.196 m of EPS. These are worked examples using stated assumptions. Substitute the figures declared for the products under consideration.

In a simplified one-dimensional calculation, the U-value is the reciprocal of the total thermal resistance. Include all layers and the appropriate internal and external surface resistances. OSB, internal linings and any further layers contribute to the result. Taking the reciprocal of core resistance alone omits them.

Establish whether a figure covers the centre of the panel, a panel with repeating joints or a wall area including openings. These scopes can give different results. Compare like areas and calculation assumptions before selecting a wall thickness.

SIP joints can contain timber, splines and fasteners. Corners, wall bases and reveals have different geometries again. Include these paths in the assessment of the envelope and the building’s heat balance. A joint may conduct more heat locally than the uninterrupted polyurethane core.

Ask for the joint details used in the calculation and compare them with the installation drawings. If the spline type or spacing changes, check whether the original calculation still applies. This gives the designer a figure tied to an identifiable construction.

In addition to heat conducted through layers, air can escape through joints and service penetrations. The conductivity of the foam does not quantify that loss. At design and handover, check the continuity of the air seals, window and door connections and the planned ventilation.

Heating bills also depend on envelope areas, climate, indoor temperature and system efficiency. SIPs with a polyurethane core can reduce energy demand, but the amount follows from the building calculation. A percentage difference in conductivity cannot be applied directly to the heating bill.

List the core material, core thickness, conductivity and remaining layers for each option. Record how the calculation treats joints and the area covered by the stated U-value. Add the load-bearing, fire and acoustic requirements alongside the thermal figures.

Where the specified PUR or PIR has lower conductivity, it can achieve high resistance within a shallower section. EPS with higher conductivity needs greater thickness for the same core resistance. Use the calculated thickness difference to assess the available space in the particular SIP design.