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The problem of solar heat gain from skylights

14 July 2017
A skylight can be a welcome addition to nearly any home. A skylight can provide spaces with plentiful natural light and they can help to generate extra warmth. This additional heat is desirable during winter time, but the problem of solar heat gain can arise when summer arrives.

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This is due to the fact that windows – including skylights – have less regulations around thermal properties than ceilings, floors and walls, for example. Yet too much warmth through your skylight can lead to overheating, not just of the physical space but the people occupying it as well! It’s important, therefore, to understand the process known as ‘solar heat gain’ and prevent the worst wherever possible.

What is solar heat gain?

Solar heat gain refers to how much the temperature of an object, space or structure increases when exposed to solar radiation. Some materials can be more absorbent or more resistant to this radiation when struck by sunlight.

Glass, in particular, is more sensitive to short-wave radiation which it absorbs and radiates at longer, heat-creating infrared wavelengths. This increase in temperature – as a direct result of the sun shining through the glazing – describes solar heat gain. The concept is well understood nowadays thanks to the concept of the ‘greenhouse effect’, the process of solar heat gain seen behind greenhouse glass which is often used to refer to global warming.

Skylight positioning and solar heat gain

If you want to maximise or minimise the potential for daylighting and passive solar heat gain from your skylight installation, you must consider the skylight’s position. Because skylights are located on the roof, they can add a lot more heat to a house than traditional windows. Consequently, they can result in unwanted heat gain and heat loss if not carefully placed.

The direction that the skylight’s roof faces makes a significant difference to thermal performance. North-facing roofs receive constantly cool illumination, for instance, and the least direct sunshine. East-facing roofs are ideal for maximising light and solar heat gain during the morning, while west-facing skylights provide more during afternoons. The most potential for winter solar heat gain is seen in south-facing skylights, with the trade-off of unwanted solar heat gain in the summer

Installing skylights according to this advice can help prevent unwanted heat gain, but for best effect combine the above with careful selection of skylight type and materials.

Preventing unwanted solar heat gain

There are multiple ways of preventing unwanted heat gain. Options include installing the skylight in an area shaded by trees, or the adding of a movable window covering to the skylight’s interior or exterior for instance.

Skylight glazing

Generally, skylights are glazed with plastic or glass. However, some skylight units can be installed with special glazing more able to manage solar heat gain.

Some examples of these advanced technologies include low-emissivity (low-e) coatings, heat-absorbing tints and insulated glazing. Some manufacturers utilise multi-layer glazing with a transparent insulation material housed between the panes.

You can usually judge a product’s thermal properties at a glance by referring to the SHGC, or “solar heat gain coefficient”, an independent rating of a product’s thermal performance. The lower the number provided, the less solar heat that will be allowed to enter.

Depending on the performance you expect, multiple skylights can be installed with different glazing in specific locations in the house.

Skylight shape

Skylights can come in multiple shapes and sizes. Recent developments in the industry have resulted in designs that utilise lens-like, mirrored or sun-tracking elements to provide extra daylighting without causing daytime heat gain or night-time heat loss.

Most tubular skylights feature a mirrored “light pipe” and diffusing lens in order to extend daylighting potential without increasing the product’s size. Since they use this solar collector, and they are smaller than other skylights, they minimise winter heat loss and summer heat gain far more effectively than alternative models.

Unfortunately, the trade off with tubular versions is that they do not provide two significant benefits of skylights, namely: rooftop views and extra ventilation.

Slope of the skylight

Solar heat gain is affected by the tilt or slope of the skylight. The principle is quite simple: a low-sloped skylight admits more heat in the summer and less in the winter. This is the exact opposite of what is needed!

A general rule of thumb in the industry is to attempt to achieve a slope that equals your geographical latitude plus five to fifteen degrees. If the latitude of your location is 40o, then the optimal slope will have an angle for 45o to 55.o

As you can see, there are multiple considerations to think about if you want the perfectly-performing skylight. However, we guarantee that with a properly installed skylights the benefits more than pay for themselves!

All of Sunsquare’s skylight products feature outstanding thermal performance. Find the rooflight that’s right for your property!

Written to help architects, surveyors and home improvers alike understand every UK building regulations.

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