As performance requirements tighten and compliance documentation becomes increasingly scrutinised, IGU manufacturers are being urged to take a closer look at the implications of substituting core components.

When presented with a lower-cost component alternative, the change may seem minor at first – particularly if the substitute product looks similar. However, according to Thermoseal Group, the performance data behind the product is what ultimately matters.

“We occasionally hear that if a spacer looks the same and saves money, it can be tempting to make that swap,” says Mark Hickox, sales director at Thermoseal Group.

“But thermal performance isn’t visual. It’s calculated, certified, and embedded in the overall window rating.”

Window U-values and Window Energy Ratings (WERs) are typically determined through simulation, using verified thermal conductivity values for each individual component within the IGU.

If a spacer bar is replaced with a product that has a different conductivity value, the overall thermal performance of the window can change.

“If the new spacer performs worse than the one originally specified, the window may no longer meet the rating it was designed to achieve,” Mark explains. “If that recalculation doesn’t take place, manufacturers could unknowingly be supplying a product that doesn’t align with its declared performance.”

As the industry transitions from SAP to the Home Energy Model (HEM), the way window performance is calculated is becoming significantly more technical.

Under SAP, compliance modelling often relied on standardised assumptions and area-weighted averages. HEM, by contrast, uses detailed, element-by-element modelling with individual windows assessed according to their specific size, orientation, frame factor, U-value and solar gain (g-value).

As a result, performance declarations are expected to carry greater technical scrutiny.

Manufacturers will need to demonstrate precisely how published U-values have been derived, including the exact specification of the glass, frame, spacer and sealant system used.

Most importantly, substitutions without recalculation are unlikely to align with the explicit modelling approach under HEM.

So, what should manufacturers look for when assessing equivalence?

Across Europe, thermal conductivity for warm edge spacers is standardised through the IFT Rosenheim test method. These reports provide indicative conductivity values within the defined frame types and incorporate both the spacer value and the sealant value used within the IGU.

Together, these form the recognised ‘two-box method’ value for fair comparison.

To validate a substitution, IGU manufacturers should request the original IFT conductivity report and confirm that the sealant system used performs no worse than the one referenced in the simulation.

Typical conductivity values are 0.24 W/mK for hot melt and 0.40 W/mK for polysulphide, polyurethane or silicone systems. The BFRC will validate IFT reports before allowing performance figures to be used within registered documentation.

Dimensional consistency is also critical.

Many high-performing rigid spacer tubes measure 6.5mm in height by airspace width, consistent with traditional aluminium systems. Alternative hybrid or plastic tubes are often 7mm in height. Without recalculating the simulation, this dimensional difference alone can invalidate an existing U-value or WER.

Flexible foam systems require similar scrutiny. The most common UK size is 4.8mm thick by airspace width. Thermoflex, for example, has a conductivity value of 0.125 W/(mK) and is the best performing in the world.

While it can be substituted into simulations for other foam systems, the reverse is not automatically true. A lower-performing alternative may reduce the rating of the window if recalculation is not undertaken.

“In some configurations, particularly where laminated glass is involved, even small differences can shift a window from an A rating to a B,” says Mark. “That’s why substitution should always be based on certified equivalence – not assumptions.”

The principle is straightforward: component changes are possible, but they must be validated by data.

Comparing total spacer and sealant dimensions, confirming conductivity values using the two-box method, and ensuring documentation aligns with declared performance are all essential steps.

As compliance expectations across the sector increase, the margin for error decreases.

For IGU manufacturers, ensuring that performance data truly stacks up is not simply good practice – it is fundamental to protecting credibility, certification, and long-term trust within the supply chain.