5 Key Questions to Ask Your Window Supplier

Where does the window sit in the wall?

Your window performs best when it sits within the insulation layer of your wall – the warmest position, and the one that minimises thermal bridging between window frame and wall.

In a traditional cavity wall, it's typically in the middle of the insulation. In a SIPs build, the window usually overlaps substantially with the insulated panel and may extend slightly outward into the gap between the panel and the external cladding or brick face. In ICF construction, most of the window's weight needs to bear over the concrete core, with some overlap into the exterior insulation.

The detail varies enough between construction methods that it's worth discussing with your supplier early. It’s one of the simplest ways to protect the thermal performance in which you're investing.

What tolerances will you work with?

Every opening is different, and the clearance your installer needs depends on how square and consistent your walls are. In a new build with precisely engineered openings – common in SIPs or ICF construction – around 10 mm each side is enough space to fix and seal the window without compromising performance.

In a renovation, things are rarely that neat. The wall behind an existing window can be uneven, out of square, or different dimensions on the inside compared to the outside. That often means slightly larger gaps on one side to accommodate what's already there. This is normal.

A good supplier will walk you through what to expect for your specific build, with properly supported and sealed windows as a priority. If you're renovating, it's worth going in knowing that tolerances may be wider than on a new build. Acknowledgement of this is a sign your installer is working with your building, not against it.

How will the windows be sealed?

Sealing methods vary depending on exposure, the position of the window in the wall, and whether external insulation is involved. There isn't a single right answer, your supplier should be able to explain what they'll use and why.

Some projects suit compressed foam tapes for a clean, consistent finish; others need a different approach because of larger or uneven gaps in the window reveal or high exposure. In all cases, the goal is the same: an airtight, weathertight seal that looks great and performs well for the life of the window.

What are the size limits – and what determines them?

Maximum window and door sizes are more often limited by hardware than by the glass itself.

Inward-opening tilt-and-turn windows can often carry larger, heavier sashes because the hardware is engineered for the load. Sliding doors vary significantly by system – Norrsken's S319A inline sliding door reaches approximately 5.9 m wide, while the S315A lift-and-slide door reaches approximately 8.5 m, because the hardware and panel configurations are different. Fixed panes often allow the largest individual panels, and often accommodate custom shapes more easily than opening sashes.

Ask about sizes early, and be open to changing the product type if your original specification can't achieve the dimensions you need at the performance level you want.

Who's responsible for making good?
When existing windows are removed and replaced, there will always be some degree of disturbance to internal plaster, reveals, and decorative finishes. This is best discussed with your supplier before work starts.
As a glazing supplier and installer, making good internal finishes isn't within our scope. But we'd rather help you have that conversation clearly at the quoting stage than leave it to surface as a surprise after installation. We're happy to talk through what the typical extent of disturbance looks like on different project types so that you can arrange follow-up trades. A clear scope, agreed by all parties before work begins, protects your home and your happiness.

Who's responsible for making good?

When existing windows are removed and replaced, there will always be some degree of disturbance to internal plaster, reveals, and decorative finishes. This is best discussed with your supplier before work starts.

As a glazing supplier and installer, making good internal finishes isn't within our scope. But we'd rather help you have that conversation clearly at the quoting stage than leave it to surface as a surprise after installation. We're happy to talk through what the typical extent of disturbance looks like on different project types so that you can arrange follow-up trades. A clear scope, agreed by all parties before work begins, protects your home and your happiness.