Weighing up double verses triple glazing
Updated: May 18, 2021
Choosing from the vast array of window and door types and materials can be a daunting task. But at a basic level, perhaps it’s best to start with the relatively simple choice of double verses triple glazing. Even this choice can be complex and without an understanding of the benefits of one verses the other you risk making a poor decision which could result in overpaying. Is triple glazing worth the additional cost? This blog post aims to clarify some of the misconceptions around double and triple glazing in the hope of de-mystifying the choice.
We would always suggest starting with a honest appraisal of what you want to achieve with your project. Whether it’s a simple replacement window project, full refurbishment of an existing property or new build will start to define what you can realistically expect to achieve from your windows. Categorising a list into ‘needs’ and ‘wants’ will mean you can quickly narrow down the choices from the overwhelming number of options on the market.
Super high performance triple, or even quad, glazing can have headline leading thermal performance, but if you are installing the windows into a poorly performing existing wall construction, then many of the benefits will not be realised.
So, let’s dig into how to compare different glazing systems and some of the positives and negatives.
The thermal performance of many building products is described as a U value, and measured in W/m2K (watts per metre squared Kelvin). Basically this measurement tells you how many watts of energy are lost through the product. So, a lower U value signifies higher performance and better thermal insulation. Or to think of it in another way, less heat loss through the product to the outside.
When we think about windows and doors it is important to understand the relationship between the glass U value (often called the mid pane U value) and the whole window or whole door U value.
Glass suppliers will calculate their mid pane U value and pass this information on to the window supplier. This is fairly straightforward as their calculation can just take into account the mid pane performance of the glass (i.e. not including the frame, the edge of the glass etc). The window supplier can then use this to calculate their specific whole window or whole door U value.
Since the performance of the glass and window or door frame can vary massively it is important to compare the whole window or door U value of the products you are considering to get a fair comparison. A supplier could use a super high performance glass with a very low mid pane U value, but if this is combined with a low performance frame then the whole window or door will be compromised.
It should be straight forward for a supplier to confirm to you the whole window or door U value for product they are offering. This would typically be based on a standard sized window or door in accordance with EU legislation. Any reputable supplier will also be able to confirm the actual project specific whole window U value of your quote, which will take into account the actual sizes and types of products you have on your project, rather than standard sizes and types.
Solar gain and overheating
Solar gain, expressed in terms of G value, is another factor you might want to consider, particularly on projects with large areas of glazing which are quite well insulated. Solar gain is the amount of free energy from the sun that passes through the glass into your home. The G value is a percentage value; 100% meaning that all the suns energy passes through to the inside, and zero percent meaning than none passes through. This can be a massive benefit, or a massive pain. Which, really depends on the whole building performance. A well insulated new build property may suffer from excessive solar gain, meaning it will tend to overheat. In the worse case scenarios this can be very uncomfortable for the occupants and require the significant costs of installing, and running, air conditioning.
Other ways to reduce overheating from solar gain are to introduce a special solar control coating to the glass. These typically offer G values down to around 0.25 (or 25%) but add around 10-15% cost to the windows. Standard double glazing typically has a G value of between 0.6 and 0.7 (or 60% to 70%), whilst triple glazing typically achieves between 0.45 and 0.55 (or 45% to 55%). So you can already see that triple glazing allows less solar gain, so can reduce overheating from the sun. Giving you the double benefit of solar control and improved thermal efficiency.
The acoustic performance of windows and doors is often overlooked, but arguably can have a bigger impact on your day to day life than the thermal performance. And, similarly to thermal performance, it is important to compare the acoustic performance of the window or door system you are buying in terms of the glass and frame combined, not just the glass.
Acoustic performance of building products is typically expressed in units of dB Rw, decibels reduction weighting. To simplify this a little, it is basically the measured sound reduction the product gives in a laboratory test. During the test, the window or door is installed in a prepared opening with a noise source (a multi directional speaker) on the inside and a measuring device (microphone) on the outside. The noise source operates at various frequencies to simulate a range of real life noise types in one test. Because dB Rw is a measurement of the reduction the window provides, the higher the dB Rw figure, the better it’s acoustic performance.
Glass suppliers will often have test data to hand for their glass, but how it is installed in the window or door and the frame performance can have a massive impact on the overall performance. So, like U value it is important to ask for data including the window and frame together.
Whilst often sold as being great for sound control, in reality changing from double to triple glazing doesn’t necessarily improve the sound reduction. Sound is a complex thing, but generally what stops it are dense materials of varying thicknesses. Using double glazed units with specifically specified glass thicknesses, and sometimes acoustic interlayers in laminated glass, can often achieve higher acoustic reductions than triple glazing.
The additional weight of triple glazed units can be an important consideration. Typically adding an additional 10kg/m2 just for the glass, you need to be sure the window or door system is designed to operate flawlessly with this additional load.
Often, UPVC and aluminium windows require reinforcement to handle this additional weight. However, because most timber and aluminium clad timber systems originate from colder, typically Scandinavian climates, they are usually designed from the start for triple glazing.
Considering any required reinforcement is important not only in terms of the weight, and cost, but it can also affect the performance. An example of this would be adding steel reinforcement to a UPVC or aluminium window, which would significantly reduce the thermal performance of the frame, steel being a very effective heat conductor. Again, a reason to always think in terms of whole window U value, not just the glass U value.
More importantly then is to consider the implications for installation. Hauling that additional weight around site is not easy and requires a team of installers who are used to working with heavy units and therefore have the health and safety implications covered off. These are also not the types of products you can just sling in the back of a van. We use dedicated experienced hauliers with mechanical offloading to get your delivery as close as possible to the property. Using a combination of Hiab (a crane on the delivery lorry) and Moffett (a small forklift on the delivery lorry) offload we overcome these issues on a daily basis.
The cost of upgrading from double to triple glazing can vary massively from window system to window system. As mentioned above, some systems will require additional reinforcement to handle the weight of triple glazing, so the cost may not always be proportionate to the increase in performance.
Our Rationel and Velfac window systems are fully factory glazed. To lift the glass into the windows the factory uses overhead cranes with glass suckers. This means there is no difference to them between fitting a double or triple glazed unit. The cost uplift is therefore just down to the supply cost of the glass itself, and therefore, depending on the project can be as little as a 5%-10% increase.
The improvement in thermal performance between double and triple glazing is significant, but how does this actually translate into cash savings on your energy bills? We’ve seen some suppliers claiming up to 50% saving on energy bills! However, the reality is that whilst the window U value can be around 50% lower, this doesn’t mean a 50% saving in your pocket unfortunately.
To really understand the savings it would be necessary to take into account the whole building performance, including walls, floors and roof as well as the windows and doors. This really requires the services of a specialist professional. For a refurbishment project this would be a Retrofit Coordinator, who would be certified under the PAS2035 standard. For a new build this would be the Accredited Energy Assessor, sometimes called the SAP assessor. This professional would take into account all elements of the project including any free energy from solar gain through the windows.
What really makes a practical difference to energy savings is the air-tightness of the building. Even small gaps of around 1mm lead to significant heated air loss through the building fabric. Several small gaps can quickly mount up to start convection air currents drawing cold air from outside into the building. This not only costs you money, but significantly reduces the comfort levels within the building. If your ankles are always cold, but your head is warm, no matter how much you whack up the heating it’s never going to be comfortable inside on a freezing cold winter day. Airtightness and how we can help to achieve improvements is a complex subject we will devote a whole blog post to in the future.
You can now hopefully better understand some of the many considerations to make when selecting and comparing window and door quotes. Essentially, to recap, it is important to compare the whole window or whole door U values of the offered products, not just the glass U value. We are on hand to help you understand this and are always willing to chat though any questions you might have and explain in practical terms and hope to help you make an informed choice.
One of our aims at okoHaus is to combat climate change by helping our customers make informed decisions. Choosing triple glazing over double glazing on the face of it is good for the environment, being higher performance and therefore allowing less energy to be lost during heating of your home. But considering the additional carbon (embodied energy) used in producing the extra pane of glass, will the heating energy savings ever offset the additional environmental cost of manufacture? Probably not if the whole building is not up to a similar level of performance. A discussion of embodied energy will be the subject of a future blog post.