Having said that, not all daylighting solutions are equal. Leaks, haze, intense glare, thermal inefficiency, and impractical installation are common problems that can arise from poor design or inadequate materials. In this article, we’ll explore what a skylight can bring to a project and how to avoid daylighting disasters.
Why Overhead Glazing Outperforms Vertical Windows
The science behind this is simple and it’s important to consider it during the planning phase of a building. A skylight provides approximately three times more daylight compared to a vertical window with the same area. This is because the sky above is completely bright and a vertical window can only capture a portion of that light. On top of that, overhead glazing spreads the light further inside the building, reducing the difference between a well-lit perimeter and a darker core.
Daylight Factor (DF) calculations confirm this fact by measuring what percentage of a room gets a minimum level of natural light. A DF of 2% or higher indicates that the room gets enough light to be used without artificial lighting during the day. Skylights can help achieve this over a much larger surface of the room compared to vertical windows alone. When considering commercial construction that aims for BREEAM or LEED certification, for instance, the roof design will determine the overall daylight autonomy of the building, not just the placement of walls and windows.
Regarding passive solar design, this has a big impact as well. South-facing (or north-facing in the southern hemisphere) skylights will bring in the most sunlight during the winter when the sun is low. With the right calculations and maybe an overhang, these skylights can also provide shade in the summer when the sun is high, avoiding overheating the building. But again, this is something that needs to be carefully planned and calculated during the design phase, not added as an afterthought.
Thermal Performance: The Tension Between Light and Heat Loss
Every window or roof opening weakens the thermal isolation of a building. What is essential, then, is to manage the weakness rather than turning a blind eye to it.
The key parameter here is the U-value that expresses how much heat is lost through the assembly. This is important because, in the end, U-values are typically higher than you think. Triple glazing provides U-values of 0.5-0.6 W/m²K for the glass itself. Frame materials can adversely affect the performance of the glass itself while also adding significant material usage to the design. Aluminium, steel, or other energy-intensive materials serve as high thermal conductors and will cool the interior air next to the frame quickly, leading to condensation forming there because air cannot hold as much moisture at colder temperatures. An insulated frame and well-thought-out construction make a big difference here and can significantly save in materials and heat loss over the life of the skylight.
Albeit fixed windows have lower U-values compared to operable ones, an often underestimated feature in skylights is thermal bridging through the frame. The continuous profile that holds the glass unit can provide a direct path to the outside for warmth trapped within the building. Warm-spacer-bars at the glass edge help here.
Another technology that doesn’t quite get the attention it deserves in specification discussions is low-E coatings. As long as they are applied to the inner face of a window (low-E hard coat), these coatings suppress longwave radiation from the warm interior escaping over the cold glass and therefore perform better in winter without losing almost any light entrance. The best units make VLT values of 70% without problems.
Managing Solar Gain Before It Becomes a Problem
Thermal performance in winter and solar gain in summer are two separate problems that pull in opposite directions. A glazing spec that solves one can easily worsen the other. The Solar Heat Gain Coefficient – referred to as the g-value in European standards – measures the fraction of incident solar radiation that passes through the glazing into the space. For a roof that sees high solar exposure, a high g-value that’s welcome in January becomes the source of overheating by July. A floor-to-ceiling office under an unshaded rooflight with a g-value of 0.6 will be unusable on a summer afternoon.
Dynamic solar control coatings applied to the outer glass surface selectively reflect infrared radiation while transmitting visible light. This allows the g-value to be tuned without simply blocking light with tinted glass – a common but crude solution that darkens the space unnecessarily. For projects where overheating is a genuine risk, integrated shading – either internal blinds within the sealed unit cavity or external automated louvers – provides the flexibility that a fixed coating can’t.
The calculation isn’t complex, but it does need to happen. Sum the skylight area, apply the g-value, model against the thermal mass of the space and the ventilation strategy, and identify whether passive control is sufficient or whether active shading is needed. Doing this at RIBA Stage 2 is straightforward. Doing it after construction is expensive.
Structural Safety and Overhead Glass Specification
Glass installed overhead needs to meet safety standards above and beyond that of vertical glazing. If a vertical window breaks, you have a hole in the wall. If an overhead light breaks, you have falling glass.
Laminated safety glass with a PVB (polyvinyl butyral) interlayer is the standard solution to this. When laminated glass breaks, the interlayer holds the fragments together, maintaining the integrity of the pane rather than allowing shards to fall. For overhead applications, the inner lite of an insulating glass unit should always be laminated – this is the last line of defence between the glass and the occupant below.
Thickness and configuration are based on span and load. Structural silicone glazing systems, where glass is bonded directly to a structural frame with no mechanical fixings, will require the glazing contractor to submit the adhesive joints and frame design for independent engineering sign-off. Walk-on glass requirements are even more complicated – with typical thicker laminate builds, slip-resistant surface treatments, and engineering to suit point loading and crowd loading.
None of this is insuperable, but it needs proper engineering input, not just a product datasheet.
Flat Rooflights, Pitched Glazing, and Roof Lanterns
The decision may come down to aesthetics and available budget, but each type of rooflight has its inherent performance characteristics. The style ladder is as follows:
Flat Rooflights
Sleek and minimal, these offer clean lines and contemporary looks. For projects across these formats, addlite.co.uk supplies a range of high-performance architectural glazing systems designed for both domestic and commercial applications. This is often the most affordable way to flood an extension with light, but it’s also the most likely to suffer from seal failure, accounting for up to 70% of all glazing leaks if not correctly specified/installed. This is because a completely flat surface can’t shed water effectively. Most long guarantees will specify a minimum pitch to help water drain away naturally. Alternatively, new ultra-hydrophilic self-cleaning coatings spread rainwater and dramatically reduce the amount of standing water required to clean the glass.
Pitched Rooflights
The elegant simplicity of slanting glass means it usually complements any given architectural style. As with lanterns, this configuration helps rainwater to naturally run off the surface, reducing the likelihood of leaks. The steeper angle to the sun also means most designs only require a self-cleaning glass option rather than a mechanised in-roof cleaning system.
Roof Lanterns
The initial impact is greatest thanks to the sheer amount of light these designs can deliver into a room, as well as the richness and interest of light from above and all around. Best teamed with vaulted ceilings.
Ventilation, Acoustics, and Indoor Air Quality
A stationary skylight has one function to fulfill. On the other hand, an operable skylight has two functions.
Thermally activated opening skylights are controlled by temperature and CO₂ sensors, so hot, stale air can escape through the roof, the room’s natural exhaust outlet. This stack effect operates mechanically free: Warm air exits from the high-level opening skylight, and cooler air enters from low level. This system is particularly effective for rooms with high sensible heat gains – commercial kitchens, server rooms, and direct gain south-facing living areas – as it can reduce mechanical cooling demand by natural/assisted cooling.
Acoustics are an often-overlooked performance consideration, especially in our increasingly noisy world. The sound of rain on glass is one of the most common complaints about skylights in homes and hospitality venues. Built with acoustic PVB interlayers of varying thickness during the lamination process, the thicker the interlayer, the more the sound insulation as the difference in thickness breaks up the resonant frequency at which the glass itself amplifies sound. An appropriate acoustic lamination can reduce rain impact noise by 6-10 dB compared to standard float glass, which is the difference between noticing but being able to ignore and being genuinely disturbed.
The Case for Natural Light as a Health Outcome
Mostly, daylighting has been justified on the basis of energy. Less artificial light required, less energy consumed. That case still holds true – in well-designed spaces, natural light can bring artificial lighting energy usage down.
The stronger justification these days is physiological. We now spend 90% of our time indoors, and the lighting environment indoors directly influences the operation of our circadian rhythms – the natural internal clock and master control of sleep, hormone production, and even mood. Circadian rhythms respond to the application of high-intensity, blue-spectrum light in the morning to trigger ‘wake’ hormones. Virtually no electric lighting provides this, but daylight does every time between 9 am and noon. The relationship between poor daylighting, SAD, sleep-cycle health issues, and reduced productivity is well known to building health researchers.
Biophilic design as a conceptual framework brings in a second, higher level of justification, describing positive environmental inclusions like a view to sky, weather, and natural light rhythms as simply good for us as occupants, over and above any effect on our internal ‘wiring.’ Buildings that “live” with the time of day and the quality of light are simply more interesting and desirable to be in. This is becoming a big factor in commercial development leasing, where tenant satisfaction surveys are now starting to deliver lease value premiums or tenant retention savings.
Getting the Specification Right From the Start
The design choices that impact whether a glazing installation will work well and deliver the client’s ambitions for the life of the building are almost all made before anyone knows which brand will supply it, let alone before it’s put out to tender. Skylight location, orientation, and area relative to the floor plate, g-value choice, ventilation, drainage, frame specification – these are design rather than procurement decisions.
Designers that view a skylight as a product from a catalogue – I’ll have one of those, that size, with that finish, please – tend to get disappointing results. Designers that view it as an engineered part of the building envelope, where the same level of scrutiny must be applied to thermal, structural, and acoustic performance as to the roof build-up around it, and the same level of co-ordination in relation to services and structure, will deliver the client a building that simply works better in all dimensions.
