Why cutting corners can be costly

Thermoseal Group’s sales director, Mark Hickox, explains why properly sealing your IGU corners is a simple but essential step in maximising unit longevity – and avoiding costly warranty claims down the line.

Corners are critical. In the manufacture of IG sealed units, it’s the areas you don’t see – like improperly sealed spacer joints – that often pose the biggest risk to long-term performance. At Thermoseal Group, we’ve seen how a simple oversight in sealing can lead to premature gas loss, moisture ingress, and costly replacements.

Around 90% of the European IG market now uses dual seal systems, comprising of a primary sealant, typically polyisobutylene (PIB), and a secondary sealant such as hot melt, polysulfide, silicone, or polyurethane. Each plays a vital role.

The PIB primary seal isn’t just a handling aid. It’s the first and most important barrier against gas escape and moisture intrusion – and to work properly, it must be continuous around every edge, including the corners.

That’s why good practice requires the PIB to be extruded slightly longer than each spacer bar length – for example, 1.05 metres for a 1-metre bar – creating what we call a “PIB tail.” This ensures coverage over corner key joints, and a comprehensive seal.

And if you’re not using PIB tails? A simple strip of foil tape over the corner key before applying PIB can still do the job – as long as the result is a fully sealed perimeter. It’s this attention to detail that can mean the difference between a unit that lasts a decade and one that fails in just a couple of years.

Once your primary seal is in place, the secondary seal provides added structural support. But relying solely on the secondary seal for gas retention is risky – especially under real-world conditions where thermal cycling causes expansion, contraction, and eventual fatigue. Any weak point, particularly around corners, will show up under long-term performance testing.

Flexible spacers applied robotically produce just one joint, but even this should be taped at the junction to guarantee sealing integrity.

But why does it matter?

Because, for example, when windows are installed in new-build homes with a specified whole-window U-value of 1.0 W/m²K, any early gas loss can push that up to 1.4 W/m²K – making the unit non-compliant within just a few years of installation. That could mean replacements, disputes, and spiralling costs.

At Thermoseal Group, we supply a comprehensive range of sealants specifically designed for insulated glass construction. Our PIB sealants offer outstanding moisture resistance and minimise inert gas loss – critical for maintaining thermal performance.

Every sealant in our range is independently tested to EN1279 parts 2 and 3 and continually monitored in-house to exceed these standards.

In our laboratory, we test for everything from glass and spacer adhesion to tensile strength, low moisture vapour transmission, and gas permeability. That means our customers can trust they’re working with sealants built for longevity – and with full traceability.

In real world applications, the results speak for themselves. Failing to seal corners correctly risks invalidating guarantees, pushing U-values beyond compliance, and damaging reputations. But with the right sealants and the right technique, you can give yourself an insurance policy against performance, compliance, and your reputation.