How Do I Choose and Install Energy-efficient Windows?
Well-fitted, quality windows can greatly improve the energy efficiency of your home. Here’s how to choose wisely
Professional advice from: Kit Knowles of Ecospheric; Rich Tyers of Rich Tyers Studios; Alan Budden of Eco Design Consultants
For windows to be truly energy efficient, they need to fit snugly into walls.
“Energy-efficient glazing is similar to conventional glazing, but with extra attention to detail to ensure it’s airtight,” Alan Budden says. Special air-tightness tapes can be used, internally or externally, to help implement this, he says.
Rich Tyers explains that thermal bridging is the most important thing to think about. “Thermal bridging is the gaps in junctions between elements such as a wall and window,” he explains, and says that installers need to “ensure the window unit is aligned with the insulation layer in the wall.
“A common problem,” he continues, “is when a window is fixed to the external leaf [wall] of masonry, creating a cold bridge around the perimeter of the window that leads to mould in winter.”
“What we’re doing nowadays is taking the frame and swallowing that into the fabric of the building, so you see more pure glass,” Kit Knowles says. “This gives windows a much better psi value [a unit of heat loss], which is the thermal performance associated with the installation of that unit.
“Good installation is key,” he stresses. “Done badly, it can massively undermine the value of a unit. People often install glazing into miss-sized apertures – building frames that may be hollow or thermally transparent. I’ve often a seen a brand-new, triple-glazed, high-performance unit essentially sitting on a 100mm gap.”
Rich and Kit recommend both timber and aluminium for high-performance windows. “My favourite is an aluminium/timber hybrid frame,” Rich says
“Timber tends to be cheaper and less carbon-intensive to produce, and can last indefinitely as long as it’s maintained,” Kit says. However, if you want to minimise the efforts of maintaining timber, Kit agrees with Rich that a good compromise is a timber unit clad in aluminium. “You still get clean, modern looks, but also increased durability,” he says. “You’ll probably get 60 years out of an ‘alu-clad’. They tend to be around 30% more expensive than timber alone.”
“When looking at windows, ensure the unit is ‘thermally broken’,” Rich says. “This means the window frame has an insulating material sandwiched between the inside and outside layers of material (timber, aluminium, uPVC, and so on). This blocks a certain amount of heat from escaping through the window frame and reduces the likelihood of condensation and mould internally.”
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When shopping for windows, U-values are a key factor to be aware of. These figures reflect the amount of heat lost through a window, and represent watts per square metre per degree (W/m²K).
“People often think higher is better,” Kit says. “But with U-values, lower is better. Double-glazed values are typically 1.2 to 1.4; triple glazed are 0.6 to 0.8.”
Be aware that different U-values might be quoted by different companies. The Uf is for the frame; the Ug is the glass, and the Uw is for the overall unit. “The only one that really matters for comparison is the Uw, which is for the whole unit,” Kit says.
“We always specify to reduce the amount of frame – the subdivisions – as these lose more heat and allow less light in to heat the space,” Alan says. “A 1m sq opening window can be 32% more efficient than one of the same size with an opening window on one side and a fixed one on the other, and is probably 30% to 40% cheaper.”
“I recommend taking up every opportunity to find the right product for you,” Rich says. “Local trade shows provide opportunities to meet manufacturers and try out their products. Manufacturers often offer the chance to look around their workshops or factories and see how products are made. There’s a permanent exhibition of products at the National Self Build & Renovation Centre (NSBRC) in Swindon.”
As well as understanding U-values, Rich recommends reading up on G-values, a metric that reflects how well glazing transmits solar heat from the sun (the lower the figure, the less solar heat will be transferred indoors). Typically, double-glazed glass has a higher G-value than triple-glazed.
After you’ve made your initial search on Houzz, there are a few organisations to bear in mind if you want to double-check for accreditation.
Rich recommends FENSA (Fenestration Self-Assessment Scheme) or, for low-energy construction, the AECB (Association for Environment Conscious Building).
Kit advises checking the Passivhaus Trust and the International Passivehaus Association’s Certified Passive House database.
Our experts are in agreement here. “If the choice is between double and triple glazing, the answer is almost always go triple,” Kit says. Triple glazing tends to only be around 15% more expensive than double-glazing, he adds.
“We always recommend triple glazing with a centre-pane U-value of 0.55W/m2K or less,” Alan says. “This is not only to make them efficient, but also more comfortable for occupants. Even if the temperature outside drops to below -5°C, the inside surface temperature of the glass will be above 17°C, so the temperature difference between this and a room temperature of 21°C is no more than 4 degrees (the point at which the difference is detectable by the human body as a draught).”
A layer of inert gas, such as argon, which commonly sits between the glass in double- or triple-glazed units, will help to insulate (and soundproof) your home and fight condensation.
You might think a triple-glazed window would be more at risk of overheating in summer, but that’s not the case. “Triple glazing reduces solar gain, which in turn can reduce over-heating in summer,” Alan says.
While homeowners won’t want excess energy to escape in winter, ventilating windows is important to avoid rot and mould.
“We nearly always recommend mechanical ventilation with heat recovery (MVHR), as this can keep most of your heat in,” Alan says. “It’s up to 90% efficient, compared to just allowing cold air in via trickle vents in the windows.”
“Ensure you seek advice on planning requirements from your local council or local consultants [such as architects or planning consultants] when seeking to replace windows in a building that’s in a conservation area, is on a world heritage site, is listed, or has a similar constraint,” Rich says. “You might need Planning Permission. The windows might be protected and, therefore, may need to be restored rather than replaced.”
If you have a period property in a non-conservation area, it’s down to personal preference. However, with a greater choice of window options today, there’s no reason why you can’t have an energy-efficient window that remains sensitive to the period aesthetic of your home.
“We would suggest that doors have an overall U-value of 0.8 W/m2K or better,” Alan says. “A fully-glazed door is often more efficient than one with fixed and glazed portions, and spy holes are thermally inefficient.
“We wouldn’t normally recommend a conservatory, as they overheat in summer and waste energy to heat in winter,” he continues. “If you do want a conservatory, do the traditional thing and make it with single glazing. Studies by UCL have shown these to be more energy-efficient, as you won’t be tempted to heat it in winter.”
“It’s important to remember that windows don’t work in isolation, but as part of the energy-efficiency strategy for the whole building,” Rich says. “When thinking about glazing upgrades and other passive upgrades to the home, keep in mind the following principles:
- Quality is better than quantity. A well-installed double-glazed window will achieve a better outcome than a poorly installed triple-glazed window.
- Minimise thermal bridging. With windows and window openings, ensure the window is aligned with and connects to the line of insulation within the wall. If the window is installed in such a way that there’s a gap, you’ll get a cold patch and increased risk of mould.
- Think about solar gain. Large areas of south-facing glazing will overheat the building. East- and west-facing glazing also creates an overheating risk. External shading such as a brise-soleil, sails or landscaping features [such as a tree or pergola] will help to minimise this.”
Have you installed energy-efficient windows in your home? Have they made a difference? Share your tips in the Comments.