What are the most energy efficient windows?
Monday, 18 October 2021
Energy efficiency is an ever more important topic. For anyone considering an update to their home, considerations of thermal performance will naturally come up. Clearly, it makes sense to choose products and materials that are durable and have a long lifespan. Even more important, though, is the impact that your choice has on the ongoing performance of your home. The average home in the UK loses around 25%-30% of its heat through windows. In this way, choosing energy efficient windows can help to both lower your energy bills and decrease your environmental impact.
Energy Efficiency Scales
Clearly, the most energy efficient windows retain the maximum amount of heat in your home. This results in less energy wasted and a more comfortable home environment. But how is this measured? The BFRC (British Fenestration Rating Council) was founded in 1997 to establish the window energy rating system for the UK. The organisation aims to raise standards and drive performance in the industry. The thermal efficiency of window and door products is rated from A++ to E. Windows are assessed to balance the solar gain, thermal loss and air loss to arrive at an average kWh per m2 per year. A rated windows begin at a score of zero on this scale.
Glazing
As the part of your window with the greatest surface area, glazing is a key part of what makes an energy efficient window. Building Regulations now require any window you install to have a U value no higher than 1.6. A U-value is the rate of transfer of heat through a structure. This means that all newly installed windows these days will be at least double glazed. So does it then follow that the most energy efficient windows are those with the most panes of glass?
Single Glazing
The U-value of single glazing varies since single glazed products may have been installed over a wide range of time. However, the U-value tends to sit somewhere between 5 and 5.6. Clearly, this is a considerable difference to current requirements for new window installation.
Double Glazing
On average, double glazing used to perform at a U-value of around 2.8, so this is probably where existing older windows sit. However, current products can now achieve up to 1.4 in U-value. This is all down to improved designs, materials, and composition of the gas that sits between panes. Multiple glazed windows contain an inert gas such as argon, which is heavier than the air around us. This is incredibly effective at muting noise and retaining heat.
Triple Glazing
Perhaps obviously, the U-value of triple glazed windows is still lower. With the third pane of glass comes an additional air pocket buffer. If the panes of glass were all the same thickness, triple glazed windows would only offer a small increase in insulation. However, one pane of glass is constructed thicker than the others which provides better thermal protection. Triple glazed windows typically achieve a U-value of 0.8 and some as little as 0.5. These numbers suggest that this is the most energy efficient window glazing.
Low Emissivity Windows
No matter how many panes of glass you choose in your energy efficient windows, the glass construction itself has a role to play. Low emissivity, or low-E windows, can be as much as 40% more efficient than conventional double-glazed windows. The glass is covered in a microscopically thin, transparent coating that reflects long-wave infrared energy, or heat. Low-E coatings have been developed to minimize the amount of UV and infrared light that can pass through glass without compromising the amount of visible light that is transmitted.
Window Frames
It’s not just glass that loses heat, though. Your choice of window frame material plays a part too.
Wood
Wood is a natural and sustainable material, and a popular choice for green building projects. Not only can it be easily and conscientiously recycled or disposed of at the end of its life, but it has a naturally high thermal performance. Indeed, of all frame materials, wood is the most insulating material. The crux comes with the maintenance that it requires, since damaged or rotting wood clearly drops in energy efficiency.
PVC-U
This material is widely used in new buildings and re-glazing projects, and for good reason. PVC-U is high performing in all of the areas required by building regulations - frames are energy efficient, have great soundproofing properties and are extremely secure. Alongside all this, there is no maintenance required.
Aluminium
Aluminium framed windows perform extremely well in energy efficiency. The composition of the window is engineered to insulate, enabled by thermal breaks within the aluminium frame. This not only means that your home loses less heat from the inside out, but also that your space does not overheat in hot summer months. ‘Green’ projects favour this material as it’s easily recycled, so the energy required to manufacture it is more sustainable.
Composite
The energy efficiency of this window frame material is exceptional, with a U-value of just 0.8, so thermal insulation is excellent. Window frame ranges that use this timber replacement material tend to be incredibly durable and low maintenance. This is great for how well the window investment ‘pays for itself’ through the longevity of the windows.
Window Structure
The way that your frames and glazing are constructed is another factor to consider in terms of how energy efficient they are. There may be differences in performance due to the window style, or due to how they are put together within the frames.
Casement
Casement windows normally open outwards or inwards like a door. Hinges work on the horizontal plane as well. Casement windows employ thermal breaks within the frame material to prevent heat loss from the seal of the glazing and the frame. The quality of this thermal break can be crucial in terms of how energy efficient the final window product is.
Sash
Sash windows normally open by sliding vertically, although it’s possible to slide horizontally in some instances. Sash windows traditionally have a higher air leakage rate than casement windows. However, improvements in material engineering and design mean that timber replacement options such as our Ultimate Rose range can offer A rated energy efficiency.
Balancing cost
Fitting into a budget is a consideration which comes up frequently, and it is easy to go for the most essential of windows to fit into that budget. However, especially in the case of energy efficient windows, it’s important to think long term. Energy efficient windows will repay an initial investment with reduced household bills, and more comfortable and sustainable living. To find out which options will work for you, just get in touch.