Overheating & The Solutions
2022 saw the UK’s first “level 4” heatwave and though the UK has avoided sharing the pain, July 2023 has set new global temperature records. Overheating is not a problem for later, and some forward planning is probably necessary before rushing out to buy portable fans which are costly to run and won’t solve the overall problem.
Insulating and double-glazing are as effective against overheating in summer as they are against heat loss in winter, since the same principles of thermal transfer apply—heat moves from the hotter area to the cooler so insulation helps keep heat out if the interior is cooler. But glass is a very effective material when it comes to heat transfer, so properties with ground-floor extensions with large roof-lights and conservatories are at greatest risk of overheating. Elsewhere, west-facing windows in bedrooms are particularly affected as they will heat up late in the day. East-facing windows will heat up in the morning and reach a peak around mid-day.
The best solutions will solve the cause of the problem, reducing cooling demand, rather than trying to correct the effect of the problem with energy-intensive air conditioning. Once we’re used to air conditioning, it’s very difficult to go back to passive cooling methods. Yes, internal blinds and curtains can help, but over time, because they don’t stop the glass from heating up, that heat will still be transferred into the room; moreover, if the whole point of the glazing was to provide a lovely view and connectivity with the garden, internal curtains will defeat it’s purpose. Depending on the situation (whether planning permission allows the measure, architectural issues, etc) the following can be more effective:
External awnings and shutters are hands-down the best at reducing overheating. Horizontal awnings can be attached above windows and glazed doors at the rear of the property on south, south-east or south-west facing facades. Awnings are ideal since they preserve the view outside and are most effective between mid-morning and mid-afternoon when the sun is hottest. Vertical external shutters are effective for windows facing east and west. Ones with removable louvers offer flexibility. For east and west facing windows at ground level, tall plants positioned to each side are a simple way to increase shading.
Window film can be applied to the inside of windows also helping to reduce glare. Film by 3M and other top brands are worth the initial extra cost over budget brands—they last longer and deliver proven results. Film is only recommended where overheating in a room is significant, since it also reduces the effect of solar warming in winter.
look to install systems that can be retracted are incredibly useful and though costly, will be more than worth it if your glazed space is prone to overheating. Special materials which reflect UV rays but let light through are literally a brilliant solution.
So-called “lazy” room fans, which move quite slowly, use much less energy than air conditioning units. These simply move existing air around, helping the body to maximize its natural sweat-wicking properties to cool itself. The effect is pleasant and can be a successful solution, where a “light touch” is preferred, and especially in conjunction with external blinds.
Cross-ventilation can perform the same function—it’s not always possible to improve cross-ventilation but in a home or maisonette it might be feasible to install an opening rooflight above the top landing of the stairs either straight to the roof or to an external wall. This can be used to vent hot air out of the property—these can have mechanical openings or be accessed manually with a rod.
Single mechanical ventilators can perform a similar job, reducing moisture build up at the same time. Modern ones are available that are almost silent. These are useful for example in roof extensions.
There is some buzz around fans that blow moist air (traditionally called “swamp coolers”) but these are most effective in very dry conditions and could increase moisture levels indoors to an unacceptable level and should be used with caution.
Roof extensions often become overheated. Insulating ceiling and walls is required for all recent new-builds, but many extensions built between 1970 and the mid 00’s will be under-insulated. Though costly, it can be worth it to re-insulate, which may involve removing plasterboard and insulation and installing new more effective insulation types. Compressed insulation, or even aerogel (a very thin insulating product) can use even less space than traditional roll-type insulation. Even more importantly, re-insulating can fill any gaps which are common in earlier extensions. Gaps can mean that existing insulation is performing very poorly, if at all.
Not yet popular in the UK, light-coloured roofing materials are beginning to be used in tropical countries which experience heat on a year-round basis. If an extension has a flat roof and is very prone to overheating, look to refinish the roof with a surface that contains reflective mica or similar.
More practically at the moment, light coloured decking reflects heat while dark colours absorb heat, and the same goes for stone and other paving options. The lighter the colour, and the more reflectivity in the material, the cooler it will be throughout the hotter days. If you are lucky enough to have garden space, the greenery, potential shading, combined with light coloured paving, can help protect your home with its cooler micro-climate.
Ventilation
Good background ventilation is key on very hot days and should be retrofit prior to installing any kind of air conditioning. “Stuffiness” arising from poor ventilation will make it harder to keep cool. Trickle vents, door vents and room ventilators can all help to keep air moving and whole house ventilation will help. Trickle vents are “passive”—requiring no energy to work—and they are effective since they work continuously. The small amount of heat loss they create is balanced by the benefits of the fresh air they provide. Modern trickle vents are usually fitted with particulate screening.
Rotating fans do not ventilate, rather they move existing air around. However, they can be effective as this can help evaporate sweat and they don’t use a particularly large amount of electricity.
Air conditioning is not a substitute for ventilation since it provides no fresh air, though it creates some air movement as it throws out chilled air. Air conditioning should be installed only when absolutely necessary, as the energy use is high. Some models of air source heat pump known as “air to air” models used for smaller flats, loft conversions and the like, can be used in heating or cooling modes.
Advice
Be sure to get advice if insulating so that you don’t waste your investment. Insulation must be done right or not at all. Loft insulation is relatively easy to install, but any insulation installed inside walls or internally should be designed and installed by verified, reliable firms. If in any doubt, engage a third party to review the design and the work on site. A Retrofit Coordinator is perfect for the job.
Melissa Merryweather