Aluminium sill vs concrete sill on an EWI project: which wins?

If you are pricing an external wall insulation job, the window sill is not a detail you can leave until the end. The wrong call here causes water ingress, failed SEAI inspections, and callbacks that cost more than the sill ever would have.

The problem with concrete on an EWI build

Concrete sills were designed for a different wall. On a standard block build with no insulation board, a concrete sill sits flush and drains cleanly. Put 100mm of EWI on that wall and the whole geometry changes. The sill is now recessed. Water runs back to the wall instead of away from it.

To fix this on site, you have two options. Cut the sill back and relay it, which means breaking work already done. Or build out in front with render and silicone, which fails in two winters.

Neither option is acceptable on a job that has to pass SEAI inspection.

What aluminium does differently

An aluminium sill is made to your exact projection. If your EWI build-up is 120mm, the sill projects 120mm plus the required overhang. It arrives on site ready to fit. No cutting, no shimming, no silicone to fill gaps.

The drip detail is formed into the profile. Water breaks away from the face of the wall exactly where it should. The powder coat finish matches your window frame RAL colour. There is no maintenance requirement and no cracking over time.

Thermal bridging

Concrete conducts heat. On a deep energy retrofit, a concrete sill running full width across the window reveal creates a thermal bridge that undermines the insulation performance you just paid for. Aluminium with a thermal break in the profile eliminates that path.

Weight and lead time

A concrete sill for a standard window opening weighs 15 to 25 kilograms. It needs two people to lift and position. An aluminium sill for the same opening weighs under two kilograms. One person, one visit.

Concrete sills come from a builders merchant in standard widths. If your opening is a non-standard size, you wait or you compromise. Aluminium sills for EWI are made to measure with a three to five day turnaround from Fabricator.ie.

The verdict

On a standard new build with no external insulation, concrete is cheap and it works. On any EWI project, any ICF build, or any retrofit where the wall thickness changes, aluminium wins on every metric that matters: drainage, thermal performance, weight, lead time, and fit accuracy.

The concrete sill was never designed for the wall you are building today.