The short answer
- Choose steel for a listed building or where the conservation officer wants original steel windows matched. Steel gives the slimmest frames and bars.
- Choose steel-look aluminium for extensions, conversions, new homes and many unlisted houses. It is thermally broken, lighter, and usually costs less.
- Either way, the width of the frames and bars, the glazing beads and the colour decide whether the window looks right.
Side by side
| Steel | Steel-look aluminium | |
|---|---|---|
| Frame | Hot-rolled or cold-formed steel sections | Extruded aluminium with a thermal break |
| Sightlines | The slimmest | Slim, a little deeper |
| Insulation | Thermally broken systems exist; check the U-value | Thermally broken as standard |
| Weight | Heavy | Lighter |
| Protection | Galvanised and powder coated | Doesn’t rust; powder coated or anodised |
1. Looks and sightlines
Steel is strong enough to make very slim frames and glazing bars, which is what gives original metal windows their fine, graphic look. Aluminium sections are a little deeper, because they hold a thermal break and a double glazed unit. Good steel-look systems get close with narrow profiles, slim glazing beads and bars that run through the glass or are bonded to both faces of it.
Compare section drawings of both at the same scale, and look at the glazing bars: they are where a cheap imitation shows first.
2. Insulation and condensation
Original single-glazed steel windows are cold, and condensation runs down the frames in winter. A replacement window in an existing home has to meet Approved Document L: a U-value of 1.4 W/m²K or better, or a Window Energy Rating of band B. Thermally broken aluminium meets this comfortably. Thermally broken steel systems exist too, but check the certified figure for your sizes. In historic buildings, building control can accept a different approach where meeting the limit would harm the building’s character.
3. Conservation areas and listed buildings
Where original steel windows survive in a listed building, repair or like-for-like steel is usually expected, and the work needs listed building consent. On an unlisted house in a conservation area, the council may accept slim aluminium when the sightlines and bar pattern match; an Article 4 direction can bring the windows under planning control. Our guide to steel windows in conservation areas covers repair, secondary glazing and consent.
4. Durability and care
Steel must be protected against rust: hot-dip galvanising under a powder coat is the usual specification, and scratches should be touched up. Aluminium doesn’t rust, and its powder coat or anodised finish only needs cleaning. Steel is heavier, which matters for large doors and screens: the hinges, the structure around the opening and the installation all have to allow for it.
Our steel-look windows
Our steel-look windows and screens are thermally broken aluminium, with matching doors and fixed screens.
Questions
Is steel-look aluminium as slim as steel?
Nearly, but not quite. Aluminium frames are a little deeper because they hold a thermal break and a double glazed unit. Good systems keep the frames and glazing bars close to the proportions of steel.
Are steel windows energy efficient?
Original single-glazed steel windows are not. Modern thermally broken steel systems with double glazing can meet Part L, but check the certified U-value for your sizes.
Can I replace steel windows with aluminium in a conservation area?
Often on an unlisted house, if the sightlines and glazing bars match and no Article 4 direction applies. Listed buildings usually need steel, or repair of the originals, and listed building consent.
Which costs more, steel or aluminium windows?
Steel usually costs more than steel-look aluminium for the same window. The size, the glass and the number of glazing bars affect both.
Sources
The official documents this guide is based on. Check the current edition before you order.
- Approved Document L: conservation of fuel and power (GOV.UK)
- Traditional windows: their care, repair and upgrading (Historic England)
- What permission might I need to make changes to my older home? (Historic England)
- Listed building consent (Historic England)
- Permitted development rights for householders: technical guidance (GOV.UK)