The majority of basement transformations are limited on the same front, natural light. No matter how well the space is designed and decorated, the absence of natural light makes it less inviting and compelling.
The good news is that there are some strategic solutions to bring natural light into a basement and make it feel brighter and more spacious.
Excavate and Terrace Your Lightwells
A solid window well bolted to the outside wall is a measure of last resort, not a decision about how to build a basement. If you’re doing a conversion, the external lightwell is as important as any internal layout choice.
Terraced lightwells, stepped excavations intersected with pale stone or light-colored render, operate like miniature amphitheatres, bouncing daylight in multiple directions and ultimately down and into windows. Top tier wider than the bottom, and each retaining tier leaning outwards to actively cast light, not shadow. Sprinkled with short, pale, foliage lightwells are hardly distinguishable from the stylish garden feature you accidentally created.
The windows themselves need to be ordered double or triple glazing with low-emissivity coatings. Below grade, glass is susceptible to thermal bridging and all that well-designed well is for nothing if the room the other side of the window is chilly and coated in condensation.
In this instance, U-value performance is not an optional extra, it’s the difference between a room and a damp room.
Walk-On Glass at Ground Level
If you are lucky enough to have a flat roof terrace or a garden directly above your basement extension you are sitting on the most effective source of natural daylight there is. Light can be channeled from right above the room by incorporating flush-fitted, structural walk-on glazing panels into the ground floor slab. Light then travels in the same direction it naturally travels in and, because it isn’t bouncing off a wall or fence, none is lost.
This approach does not attempt to cheat the geometry of daylight. Horizontal glazing captures the sky directly, rather than at an obtuse angle as with vertical window-sunpipes. There are no moving parts or electronic systems to fail.
If the basement room below is used for sleeping the system will also work for ‘borrowed’ moonlight. For homeowners who wish to have a usable outside terrace but would like to maximize daylight entering the room below, floor lights are available from manufacturers such as Sunsquare Skylights.
Cutting through an existing concrete slab and then inserting a high-performance, high-spec glass panel is not a job that should be attempted without specialist advice. Load paths will need recalculating, edge beams introducing and waterproofing detailing may have been compromised. Budget for a structural engineer working alongside your glazing supplier.
Open-Tread Staircases Beneath Rooflights
A rooflight above a stairwell does more than add aesthetic appeal, it functions as a light distribution system. With a multi-story open-tread staircase, sunlight can pass from the roof right down to the ground floor of the building. When there are no tread risers, there are no horizontal surfaces that can disturb the bar of light.
This solution is most effective when the staircase is designed as a light shaft rather than as a circulation facility. For this, glass or perforated metal balustrades, light-colored treads, and a closely aligned vertical are recommended. This is one of the most cost-effective methods because it doesn’t require the construction of new walls or excavations, instead, the existing verticals of the building are used.
Tubular Daylighting Devices For Windowless Rooms
Basement bathrooms and utility rooms have one common problem: they are often too remote from any external wall or overhead glazing to benefit from normal daylight. Tubular daylighting devices, rigid, extremely reflective tubes, carry daylight from a roof mounted dome right through the building. They can be easily routed through existing service ducts, airing cupboards, and utility chases with minimal structural disruption.
The light output won’t be equivalent to a full window but for a bathroom or storeroom, a 350mm diameter sun tube can provide enough daylight to function for most of the day without using electric light. For bedrooms and home office basements circadian rhythm disruption is a real design consideration, occupants working or sleeping below grade without any exposure to natural daylight experience measurable effects on sleep quality and alertness.
Internal Glass Partitions and Transoms
Solid partition walls block the light as soon as it enters the room. Glass screens, however, allow it to flood through, so, here, a steel-framed internal glass partition enables two spaces to be illuminated in turn from the single source. Glass screens are also an effective way of screening off a stair or preventing some other hazard without blocking light.
Similarly, high transom windows which continue through partitions from one room into the next, allow a single source to illuminate adjacent spaces. This works particularly effectively in conjunction with internal glass partitions. Of course, the glass doesn’t have to be completely transparent: filmed or reeded glass will exclude direct views without absorbing too much of the available light.
The Underlying Principle
All these methods work on the same principle, bring light down from above, bounce it around the room as much as possible, and ensure that it reaches every corner. Basements need not be gloomy, they’re just rooms with a greater need for good lighting design.
