Double Glazing – The Benefits Of Secondary Glazing

We recently came across a very interestin feature in Planning Building and Construction Today, PBC Today, which got us thinking about the advantages of secondary glazing as opposed to double glazing.

It MAY represent a cost effective alternative to double glazing – especially with higher interest rates and the lack of specific double glazing grants in teh UK right now.

Secondary Glazing

Secondary glazing is an extra window that you simply mount on an existing window. It’s both an easy and cheap way to, in effect, insulate single-glazed windows and will result in a significant saving on energy bills, added protection and improved soundproofing.

Everest describe secondary glazing simply and well as:

“A glazed panel in a frame that is installed to the inside of your existing window. There are different types of secondary glazing, ranging from a DIY plastic-glazed frame that can be clipped to the inside of a window. Or, a glazed unit that is professionally installed. The main reason homeowners install secondary glazing is when they can’t change their windows from the original architectural style of window, usually single glazed. This is either for listed buildings, properties in conservation areas, or to maintain the architectural integrity of the original design.

The gap created between a window and the secondary glazing creates a thermal barrier that has two main benefits. The gap helps the window to reduce heat loss – this is a huge benefit for period properties that struggle to increase their energy efficiency. The gap is also effective at reducing external noise and can be more effective than specialist noise reduction glass.”

Why Add Secondary Glazing

It’s estimated that some 7% of UK buildings have single glazing. Whilst it may be desirable to upgrade these to double glazing it may not always be possible, or affordable to do so. Specifically, if you live in a listed building you may not be permitted to install double glazing and, if your windows contain stained glass or other architectural features you may not want to change them. Alternatively you may have one or two existing single glazed windows that you simply can’t afford to upgrade or can’t justify the cost of doing so.

In these cases (subject to planning regulations) adding secondary glazing could be a perfect solution. As you’re leaving existing wondows untouched and just adding an additional frame behind them it’s classed as a reversible adaptation and therefore accepted in most cases.

Energy Saving

If sealed and correctly installed a secondary glazed window frame will reduce air leakage and reduce heat loss. Depending on the thermal capabilities of the actual glass being used, the improvement in U-value can make a difference of up to a massive 75% and also improve EPC and BREEAM ratings.

What The Experts Say

PBC Today quote Dr Robyn Pender, senior building conservation adviser at Historic England:

“Despite what is commonly thought, secondary glazing is a much more efficient system than double glazing. Double glazing places the two panes of glass close together into a single window frame and stops heat transferring through the narrow gap between the panes either by evacuating the air from the space and creating a vacuum or replacing the air with a low-heat-transfer gas.

“The combination of vacuums and energy-expensive gases, short lifespans and problems with reuse and recycling mean that ‘insulated glazing units’ (double and triple glazing) can often fail to deliver true energy or carbon benefits: factoring in the lifecycle costs means they come with a very serious risk of maladaptation (costing more energy and/or carbon than they save). Early glass was even more energy-intensive than modern glass but even so, a PVC double-glazed window will have cost more energy in one replacement cycle (20 years) than the original window in 200 years.”

Types Of Secondary Glazing

Typically the secondary glaxing frame will be made of either wood – great for cost and heritage appeal, or metal (typically aluminium) – strong, long-lasting and high-performing.

Whilst wooden frames will often be chosen from an aesthetics point of view the benefits of aluminium are significant. Apart from being strong, long-lasting and high performing, they are also lighter, slimline, discrete, easily removable and comes in a variety of different finishes and colours which can blend in with existing designs.

The glass itself comes in different options too – from standard glass to low-emissivity glass (or low-e glass as it is commonly referred to) which is a type of energy-efficient glass designed to prevent heat escaping. As an alternative to glass you can look at acrylic and polycarbonate options too. These can prove more affordible, easier to fit or add increased impact resistance if that’s likely to be an issue.

More energy efficient still is a secondary glazing sealed unit.

Secondary Glazing Costs

Obviously prices change but, at the time of writing in mid 2023, it is estimated that secondary glazing will cost anywhere between £35 and £60 per square meter.