Skylights and Light Tubes: Bringing Daylight Into Windowless Bathrooms and Stairwells

Every apartment and independent house we work on in New Town seems to have at least one room that was simply never given a window, most commonly an inner bathroom tucked behind a bedroom, a stairwell in a duplex or triplex unit that runs up through the middle of the plan, or a store room that got the leftover corner once every other space had claimed its share of the external wall, and for years the only answer anyone offered these rooms was a tube light left on permanently and an exhaust fan doing double duty as the sole connection to the outside world. That's a genuinely fixable problem now, and we've been specifying skylights and tubular daylighting devices, commonly sold as solar tubes or light tubes, on a regular basis for exactly these rooms over the past several years of practice, enough that we have real cost and performance data from our own installed projects rather than manufacturer brochure numbers, and this piece is about the specific products, the specific installation details, and specifically how we monsoon-proof these openings so a client isn't trading a dark bathroom for a leaking one.

The Two Product Categories, and Which Room Gets Which

It's worth being clear upfront that skylights and light tubes are genuinely different products solving overlapping but distinct problems, and choosing the wrong one for a given room wastes money without fixing the actual issue. A conventional skylight is a fixed or vented glazed unit set directly into the roof or a flat terrace slab above the room it's lighting, and it needs that room to sit more or less directly under an accessible roof plane, which makes it the natural choice for top-floor bathrooms, stairwells that run up to a terrace level, or a store room on the uppermost floor of an independent house. A tubular light tube, by contrast, uses a small roof-mounted dome, typically 25 to 55 centimetres in diameter depending on the model, to capture daylight and channel it down through a highly reflective aluminium or silvered polymer tube, often routed through a false ceiling void or a service duct, to a diffuser panel that looks much like a regular ceiling light fitting in the room below. Because the tube itself can bend and run several metres horizontally as well as vertically before terminating, a light tube can serve a room that isn't directly under the roof at all, which makes it our go-to solution for an inner bathroom on a middle floor of a multi-storey building, or a windowless store room two or three rooms removed from the nearest exterior wall, situations where a direct skylight simply isn't physically possible.

Skylights: Where We Use Them and What They Cost

For top-floor and terrace-adjacent rooms, we specify skylights most often for stairwells in duplex and triplex configurations, which are common in independent houses and larger builder-floor units we design across New Town and greater Kolkata, because a stairwell running from a mid floor up through a top floor to a terrace often has no external wall at any level and would otherwise be lit entirely by artificial light around the clock. A fixed toughened glass skylight unit, typically sized between 60x60 centimetres and 90x90 centimetres for a residential stairwell, using a laminated safety glass with a low-E coating to control heat gain, runs us somewhere in the range of ₹18,000 to ₹35,000 installed for the unit and glazing alone on a typical project, with the structural opening, flashing and finishing work adding a further ₹10,000 to ₹20,000 depending on the roof construction and whether it's a flat RCC slab or a sloped tile roof. For bathrooms specifically, we generally prefer a vented skylight over a fixed one wherever the budget allows, because the same opening that brings in daylight can be fitted with a remote-operated or manual vent that doubles as passive exhaust ventilation, reducing the load on the bathroom's mechanical extractor fan and helping with moisture control, which matters a great deal in Kolkata's humidity. On a recent independent house project in Rajarhat, we fitted a vented skylight above the top-floor bathroom that let the family cut their exhaust fan runtime by roughly half during the drier months, purely because the stack effect of warm moist air rising and venting through the skylight opening was doing part of that job on its own. It's worth noting that a skylight is solving the opposite problem to the one we tackle on an exposed balcony, where the brief is usually to cut down harsh direct sun rather than invite it in, and we've written separately about the louvres and awnings we use for that in our piece on balcony shading devices.

Light Tubes: Solving the Problem a Skylight Physically Cannot

Light tubes earn their place in our toolkit specifically for the rooms a skylight can't reach, and the installation logic is worth understanding because it changes what's achievable in a given flat. The roof-mounted collector dome needs a relatively direct run down to the room being lit, and while the tube can turn corners using 90-degree elbow sections, each bend introduces some light loss, so a light tube serving a bathroom two floors below the roof, or offset horizontally by several metres from the collection point, will deliver noticeably less daylight than one running a short, mostly straight path. For a typical inner bathroom in a New Town apartment on a floor close to the building's roof, a 35 centimetre diameter tube delivers daylight roughly comparable to a 60 to 100 watt bulb on an overcast day and considerably brighter under direct sun, enough to make the room usable without switching on the light fixture through most of the daylight hours, and installed cost for the unit, tube run and ceiling diffuser typically falls between ₹22,000 and ₹40,000 depending on tube length and the number of bends required. Where a bathroom sits on a lower floor with no viable roof access at all, which does happen in multi-storey apartment towers, a light tube simply isn't an option, and in those cases we fall back on the internal glazing and light well strategies we cover in more depth in our piece on daylighting deep-plan apartments, which is worth reading alongside this one since the two problems, and the two toolkits, overlap at the edges.

Monsoon-Proofing: The Detail That Determines Whether This Was a Good Decision

Kolkata's monsoon is genuinely the make-or-break factor in whether a skylight or light tube installation holds up, and it's also where we see the worst outcomes from installations done by general contractors without proper roof detailing experience, so we're specific about this with every client before work begins. The single most important detail is the upstand, meaning the raised kerb the skylight unit sits on above the roof deck, which needs to be a minimum of 15 to 20 centimetres tall so that ponding water on a flat RCC roof during heavy rain, which is common on Kolkata terraces during July and August, can never reach the seal line even during a clogged-drain event. We always run a continuous waterproofing membrane, typically an APP-modified bitumen or a liquid-applied polyurethane system, up and over the upstand and at least 15 centimetres onto the surrounding roof deck, rather than relying on sealant alone at the joint, because sealant-only details are exactly where we've seen other installations fail within a monsoon or two. For light tubes specifically, the roof-mounted collector dome needs a similarly robust flashing collar, and we specify a dome material rated for UV resistance since Kolkata's roof surfaces see intense direct sun for large parts of the year outside monsoon, which degrades a cheap acrylic dome faster than most clients expect. We also always recommend a rain-sensor-linked closure or, at minimum, a robust static vent screen for any operable skylight over a bathroom, and we schedule a site check with every client roughly one monsoon season after installation as a standard courtesy, specifically to catch any early sealant fatigue before it becomes a ceiling stain.

Getting the Diffuser and Room Finish Right So the Light Actually Gets Used

An underrated part of this work is what happens at the room end, because a beautifully installed skylight or light tube can still underperform if the room around it doesn't help distribute the light it delivers. We generally pair any skylight or light tube installation with a high-reflectance white or very pale ceiling and upper wall finish in the room it's serving, since a light-coloured ceiling around a light tube diffuser bounces a meaningful amount of additional daylight across the room rather than letting it stay concentrated directly under the fixture. For bathrooms, we also pay attention to fixture and tile finish, favouring a glossier tile in pale tones on at least the upper wall band so daylight entering through the skylight or tube carries further into the room rather than being absorbed at the first surface it hits, a detail we've found makes a genuine difference in how bright a windowless bathroom feels by mid-morning.

A Commercial Use Case: Internal Meeting Rooms and Circulation Cores

The same windowless-room problem shows up constantly in the commercial fit-outs we handle across Salt Lake's office corridors, where an internal meeting room or a circulation stairwell in the middle of a floor plate often has no external wall at all, and while a light tube isn't always practical at commercial scale given ceiling void depth and structural constraints, we do specify skylights over internal atriums and stairwell cores in low-rise commercial buildings where roof access allows it, for much the same reasons we use them residentially. If you're planning an office fit-out and the internal meeting room is one of the spaces on your mind, it's worth reading alongside our piece on designing meeting rooms that actually work, which covers the acoustics, AV and table shape decisions that matter once the lighting question is settled.

A Note on NKDA Approvals and Structural Checks Before You Cut Into a Roof

Because both skylights and light tube installations involve cutting an opening into a roof slab, and because our studio is empanelled for NKDA submissions in New Town and regularly works within KMC and Bidhannagar Municipal Corporation jurisdictions as well, we always check whether a proposed opening affects the structural slab reinforcement, the building's approved plan, or a shared terrace in a multi-unit building before proceeding, since an opening that seems purely cosmetic from inside a flat can have real structural and approval implications from the roof side, particularly in older constructions where as-built drawings don't always match what's on paper.

Let's Look at Your Windowless Room and Figure Out What Fits

If you've got a bathroom, stairwell or store room in your New Town or greater Kolkata home that's never seen daylight and you're not sure whether a skylight or a light tube is even physically possible given where that room sits in your building, that's exactly the kind of question worth putting in front of us before you commit to anything, since the right answer depends entirely on your roof access, your floor level and how far the room sits from the nearest usable roof plane. Get in touch through our contact page and we can look at your plan together.