New Windows in Stone Mullions

Homes with stone mullions will encounter a conundrum sooner or later: how do you replace the windows? Old stone mullion builds were built to house an incredibly thin single glazed steel window frame. In most cases this means that nothing other than a new incredibly slim metal frame will be able replace the original.

So…what are you gaining by making that “upgrade”? Upgrading to a double (or triple) glazed unit will do something…but there’s no point in upgrading the glass if you’re going to loose all the heat through a solid steel frame – you’ll be back at square one, except feeling far worse financially.

Limited Options

We can easily imagine that the fully like-for-like steel frame replacement isn’t going to any thermal insulation improvement, so we need to find a better solution.

Normally, we don’t recommend aluminium frames. Regardless of how well they’re made, there’s not a lot that can be done to fix the thermal problem. Having a thermally-conductive metal both inside and outside is still the same setup as a saucepan, and they’re very good at transferring heat.

Occasionally, we see plastic frames squeezed into the stone space. Aside from our normal bias of upvc being an environmental disaster and mostly poor quality, putting a bright plastic frame into an old stone mullion feels and looks like traditional sacrilege…

On the other hand, whilst we love our high-performance timber and aluminium-timber frames, they simply don’t work in this setting. Measuring from inside to outside, the depth of the stone mullion is normally less than 50mm. Our high-performance frames are at least 115mm – over twice as thick! Since stone mullions are structural, we cannot and should not alter them to suit thicker windows. Therefore, timber and alu-timber frames would look almost as ridiculous as the plastic option.

Thermally-Broken Aluminium

As a result of a stone mullion being the cold spot; never being sealed, always needing to be free to breath to prevent damp and mould, we should think about the insulation of the windows differently.

Paying for and installing the world’s most well insulating product isn’t necessarily beneficial here. Our key aim with any stone mullion replacement project is not to give a customer the biggest improvement possible, but to give them the biggest improvement that’s sensible.

So let’s re-think thermally-broken aluminium…

• It can be made incredibly thin…
• …without losing strength and solidity
• A polyamide break in the middle, plus having space for a fully modern triple glazed unit means that insulation will be far better than a solid steel
• Aluminium frames are very low maintenance and resistant to change over time

Our go-to product is therefore the Smart Alitherm Heritage. This frame is specifically designed to be as efficient as possible in a stone mullion opening. Paired with triple glazing, it can achieve an overall U-value of 1.3 overall.

Honestly, the insulation values will never be as good as our thick-framed Danish or German systems, but very few products are. Sometimes, they need to be forgotten in order to engage with a realistic solution. Most of us must consider the costs we’re signing up for; bearing in a mind a single bank of stone mullion units can add up to the same number of frame sections as an entire house, a stone mullion replacement can get quickly out of control if top-end products are used.

At the end of the day…

In the end, the situation requires compromise. Metal windows most would avoid quoting for most homes are now the go-to, as they fill the requirements where our Danish and German windows cannot.

If you’ve got to the end of this blog and you do want an idea of how much upgrading the windows in your home would be, then why not check out our “How to measure for a quick ballpark quote blog”

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