Honesty, integrity and transparency.
Image from superhomes.org.uk
Windows are one of the most important things to consider when you think about improving your home – these have significant effects in terms of how much heat is lost or retained, how comfortable your home can be due to the amount of outside noise coming in, and how much you spend or save on energy on a year-to-year basis (to name a few things to begin with).
When people talk about improving homes’ windows, replacing these with double glazing windows tends to be the main thrust of the conversation. But what if double glazing units aren’t right for your home? Is there an alternative?
Consider secondary double glazing.
Simply put, secondary glazing is a pane of glass and its frame that is fitted on to an existing window unit.
Like Double Glazing Units, secondary glazing improves your home’s energy efficiency by helping minimise heat loss, and it also helps reduce the external noise coming in through your windows. Home owners benefit from the savings they get on energy bills, and their homes are generally more comfortable thanks to the reduced noise coming in from the outside.
Unlike double glazing units, secondary glazing is less expensive, since it doesn’t replace the entire window unit – it’s simply an add-on (and a very useful one at that). It can be fitted on to an existing window’s structure, preserving the home’s aesthetic when you look at it from the outside. Secondary double glazing units generally have a wider air gap compared to double glazing units, so they reduce outside noise coming in better than most DGUs can.
You can also install secondary double glazing yourself if you’d like to, which is something you can’t do with double glazing units. DIY secondary glazing kits are available, though, of course, you also have the option of having secondary glazing companies do the whole job for you as you would with DGUs.
This primarily depends on the type of home that you live in, then on what you want from this particular improvement in terms of benefits. First, if your home is listed or in a conservation area, it’s quite likely that you’ll have to go with secondary glazing in order to preserve your home’s aesthetic, something that isn’t possible with double glazing units as they replace the entire window; this has a significant effect on how a house looks from the outside. (Just to be sure, check with your local council to see about your home and what options are available to you).
Older homes in general may not be able to support double glazing units due to the fact that these are generally quite heavy, and the older window structures in these homes may not be able to properly support these.
If you find yourself in the situation where both are viable options for your home, you have a number of factors to consider – for example, double glazing units are generally superior in terms of the savings you’ll get on your energy bill, while secondary glazing is generally much better at reducing the noise coming in from the outside – make sure you check out the head-to-head comparison section below to see how they stack up against each other.
The general consensus amongst experts, professionals, and consumer advocacy groups is that secondary glazing is not as effective as double glazing.
But why do some people say that secondary glazing can be just as good as – or in some cases, even better than – double glazing?
When people make the argument for secondary glazing being better than double glazing units, they usually cite data from a 2008 study commissioned by Historic Scotland and carried out in Glasgow Caledonian University. The purpose of this study was to consider which improvements to a typical sash window found in most traditional (or in other words, older) homes would be most effective in reducing heat loss, as part of the efforts to minimise homes’ carbon footprints and increase energy efficiency.
So what did that study find, and what does it mean for homeowners?
Here’s what Jonathan Taylor of BuildingConservation.com says:
The findings showed that the chosen secondary glazing system actually outperformed the chosen double glazing system, with a U-value that was more than adequate for current building regulations.
The findings do not mean that secondary glazing is always more effective than double glazing, as many factors affect its performance… the analysis does show that, from a heat performance perspective, the use of properly installed secondary glazing provides perfectly adequate levels of insulation which can often be more effective than double glazing
In that study, it is important to note that the specific type of double glazing unit that was used was a slim profile double glazing unit with low Emissivity glass, filled with argon gas. It was against this particular configuration of double glazing unit that secondary glazing was found to be more effective in minimising heat loss, after draught-proofing and considering and controlling for other factors. 
The study apparently did not consider other types of double-glazing configurations which can be more energy efficient than what was used. For example, a double glazing unit filled with xenon or krypton will be more effective at reducing heat loss than one filled with argon gas, or one that is vacuum-sealed, which would practically eliminate heat loss through convection – something that simply isn’t possible with secondary glazing. This is just one factor among others – type of glass, construction, etc. – that can give a double-glazing unit an advantage in energy efficiency versus secondary glazing.
Long story short – and according to Which? – secondary glazing in general “isn’t as effective as full double glazing”, though it still remains a good option as “it could still save you some money on your heating bills - and it's much cheaper to fit”. Double glazing is certainly more expensive, but it can also provide up to twice the amount you can save on your energy bills versus secondary glazing. 
So all things considered, how does Secondary Double Glazing stack up against Double Glazing Units? Having known what experts say about each and taking into consideration the plethora of factors that go into each setup that can be unique depending on a homeowner’s needs, here’s a comparison of the two that can help give you a good, general set of ideas that can help you out with your decision-making:
|SECONDARY GLAZING||DOUBLE GLAZING UNITS|
|BEST SUITED FOR||
Listed homes, homes in conservation areas, older homes you want to preserve aesthetically
Most modern homes and buildings, as well as older homes with window support structures that can properly hold DGUs weight-wise
|Can you do it yourself?||Yes||No|
Can at times match the energy efficiency of some DGUs
Some configurations of DGUs (particularly the higher-end ones) are unmatched in energy efficiency
Generally better at noise reduction compared to most DGUs in general
Depends on configuration; most DGUs reduce noise well enough, but generally not as well as Secondary double glazing
|Ease of Maintenance||
Requires more time and effort to clean and maintain (particularly if you use the removable type)
Generally easier to clean, since you don’t have to mind the sealed surfaces and areas
Secondary glazing generally lasts a very long time, far longer than DGUs in terms of durability
Good DGUs last for at least 15 to 20 years, possibly more with good care and maintenance
Can at times match the savings that some configurations of DGUs achieve; can depend on other methods of insulation in use at home
Generally considered to bring more long-term savings compared to SDGs; can depend on configuration and type of DGU
At the end of the day, the improvements you can make to your home ultimately depends on what you can do with the type of home you have and where it’s located, then on what you want from these improvements in terms of benefits, and finally, according to the budget that you have. Consider all your options, make as informed a decision as you can, and go ahead and start making your home more energy efficient.