Sun Tunnel Installation

The top unit is the collector. It carries the glazing, a dome or a flat pane, sitting on a base flashing that weathers it into the roof covering. Its job is to gather light from the whole sky, not from the sun in one position, which is why a dome collects more at eight in the morning in December than a flat pane of the same diameter does.

The tube is the product. It is a cylinder with a highly reflective inner surface, and every centimetre of it either preserves the light or eats it. Rigid tube comes in fixed lengths with jointing bands, flexible tube arrives as a concertina of aluminium laminate that can be pulled to length and bent by hand.

The diffuser is what the room sees. A translucent panel in a trim ring, set flush with the plaster, its job is to take a hard column of light arriving down a metal pipe and turn it into something that reads like daylight rather than a spotlight.

The two apertures are the part people do not expect. One is cut through the roof structure, between rafters or trusses spaced at 400, 450 or 600mm. The other is cut through the ceiling structure, between joists at their own spacing, which is very often different. Both have to be big enough for the tube, and neither of them is negotiable once the other is fixed.

The Victorian and Edwardian terraces through Tredworth, Barton, Kingsholm and Linden share a plan: two bedrooms front and back, a landing between them with no window, and stairs coming up into that landing. It is the darkest space in the house and it is the classic sun tunnel job in this city.

It is also where the cold water cistern lives. Whoever plumbed the house in 1935 put the tank over the landing because that is the highest point with a joist run to sit on, and the daylight wants the same spot. There are three ways out of that, and the survey picks between them rather than guessing.

The first is to move the diffuser to one side, off the tank footprint, and accept the light landing at the head of the stairs instead of the middle of the landing. This is usually the best outcome and it is often better light, because the stairwell gives it further to fall. The second is a rigid tube with one elbow, taking the run out from under the tank and up to a clear part of the rear slope. The third, where the house is on a modern combination boiler and the cistern is disused, is to take the tank out, which is a plumber’s half day and frees the whole void.

The rear slope on these houses is usually the right one to use. It is often clay tile or Welsh slate at 35 to 40 degrees, it faces away from the street so the top unit raises no questions in a conservation area, and the two storey rear addition below it gives a shorter run than the main roof does. Where the addition’s own roof is nearly flat, a top unit on a kerb is the better answer, and the kerb detail is on the flat roof rooflight installation page.

A sun tunnel is a metal cylinder passing through a cold loft with a warm room at one end. If warm humid air from the room gets into it, that air meets a cold aluminium wall, gives up its moisture, and the water runs down the inside of the tube and appears as drips at the diffuser. It looks exactly like a roof letting water in and it is nothing of the sort.

The fix is at the bottom, not the top. The ceiling ring is sealed to the plasterboard and, more importantly, to the ceiling’s vapour control layer where there is one, so the tube is outside the warm envelope rather than open to it. The diffuser sits on a continuous gasket in its ring rather than resting on the plaster. Where a second diffuser is fitted at loft level, which is an option on most rigid systems, the still air gap between the two panels does most of the work: it raises the temperature of the lower panel, cuts the heat loss up the tube, and means the surface the room sees stays above dew point on a cold morning.

Bathrooms and kitchens are where this matters most, because they generate the moisture. A sun tunnel into an internal bathroom is a good installation and it is a poor one if the ring is caulked with decorator’s filler and the vapour layer is left cut. The same principle governs the reveal detail described on the home page, and it fails in the same way and at the same time of year.

Heat leaves a house up a sun tunnel in two ways: through the glazing at the top, and through the tube wall into the loft. The glazing figure is published and the tube figure usually is not. A double diffuser arrangement with a sealed tube commonly comes out somewhere around 1.4 to 2.0 W/m2K over the whole assembly, which sits inside the limiting standard the regulations quote for rooflights, and an uninsulated single diffuser tunnel through a deep cold loft does noticeably worse than the datasheet implies.

Two details close the gap. The first is an insulating sleeve over the tube where it passes through the coldest part of the void, which reduces the temperature difference across the tube wall and lifts the surface temperature inside it. The second is how the tube passes through the insulation layer itself. Loft insulation is now commonly 300 to 400mm deep on the trussed estates, and a tube pushed through it leaves a gap all round unless the insulation is dressed up to the tube and held there. That gap is a chimney for warm air, and it is the same defect as a badly boxed loft hatch.

The tube must also stay dry on the outside. It is not a flue and it does not want a ventilation gap, but it does want to sit clear of any point where roof ventilation air is being deliberately drawn, so the eaves path is not blocked by insulation pushed up against the sarking while the tube is being fitted. A blocked eaves gap creates a cold spot above the top unit, and cold spots in roofs behave predictably.

Light arriving at the bottom of a tube is a column. Left alone it puts a bright disc on the floor and leaves the rest of the room dim, which is why the diffuser is more than a cover plate.

An opal or frosted flat panel is the standard fitting. It scatters evenly, it looks like a recessed light fitting when the sun is not on it, and it sits flush with the ceiling. It gives the most even spread and it loses a little light doing so.

A prismatic panel has a moulded pattern that refracts light sideways rather than scattering it, pushing more of the output out towards the walls. In a corridor or a long landing this is the better choice, because the light is thrown along the space rather than dropped in the middle of it.

A shallow domed diffuser projecting slightly below the ceiling line gathers light from the tube across a wider angle and radiates it into the room more like a pendant. It is the most efficient of the three and the least discreet.

Colour matters too. A tube with a neutral coating gives light that reads white. Cheaper coatings can push the light slightly blue or slightly warm, which shows up badly next to a window in the same sightline, and shows up not at all in a windowless cloakroom. Where a tunnel lands in a room that also has a window, the neutral option is worth specifying so the two sources match.

A sun tunnel normally serves the top floor, because that is where the loft is. A ground floor hall in a two storey house is a harder problem and it is not an impossible one. The tube continues down through the first floor inside a boxed enclosure, usually taken through the back of an airing cupboard, the corner of a built-in wardrobe, or a void alongside a chimney breast.

The arithmetic has to be faced honestly. That run is likely to be four to six metres with at least one change of direction, so only a top specification rigid tube is worth fitting, and the light arriving is a fraction of what the same tube would deliver over two metres. In a hall with no daylight at all, a fraction is still a change of character. In a room that already has borrowed light through a glazed door, it may not be.

The construction side is a proper piece of work rather than a variation. The tube passes through a floor, so the joists at that level are structural and the opening between them is trimmed. It passes through a fire separating floor in a three storey house, which brings the same considerations as the protected stairwell above. And the boxing has to be built, plastered and decorated in a finished part of the house, which is where the time goes.

It gets surveyed as a two aperture job with a third opening in the middle, priced accordingly, and described in those terms before anybody agrees to it.

Central Cheltenham brings a problem that exists at the ceiling rather than the roof. Regency and early Victorian houses through Lansdown, Montpellier, Pittville and the Bath Road terraces have lath and plaster ceilings: riven timber laths nailed across the joists with a lime plaster coat pushed through them, and a hundred and eighty years of nail sickness behind that.

Cutting a 350mm circle in one is not the same operation as cutting plasterboard. The plaster is held by keys of lime squeezed between the laths, the laths are held by nails that have often rusted to nothing, and vibration travels. A saw cut that would take ninety seconds in plasterboard can crack a ceiling two metres from the hole and lift keys across a whole bay. The method is different: the line scored through the plaster coat first, the laths cut individually rather than swept through, the surrounding area supported from above where the keys are already weak, and the ring bedded rather than screwed hard against old plaster.

Then the decoration. A cornice, a ceiling rose or a plaster centre limits where a diffuser can go before any other consideration, and a tunnel placed to miss those features is placed correctly even if it is 400mm off the theoretical best position.

On a listed building, the ceiling is part of what is listed. Altering it needs consent whether or not anybody outside can see it, and that includes the internal opening as well as the top unit. The sensible order is photographs and a pre-application conversation with the conservation officer before anything is ordered, exactly as it is for a rooflight. The shallow upper slopes behind these parapets suit a kerb mounted top unit rather than a pitched base flashing, because the pitch is below what a pitched kit is drawn for.

The interwar and post-war semis across Longlevens, Hucclecote, Tuffley, Warden Hill and Charlton Kings have hipped concrete tile roofs at 30 to 35 degrees, cut roof construction with a purlin and struts, and a very consistent first floor plan. Bathroom over the hall, small window on the side elevation or none at all after an extension went up next door, and a landing with borrowed light.

A hip is bad news for a roof window because the slope narrows towards the corner and the usable rectangle is much smaller than the roof looks from the garden. It is close to irrelevant for a sun tunnel, because a 250 or 350mm aperture fits in the narrowing part of a hip perfectly happily. That is the single best reason to consider a tunnel on this kind of house.

The purlin is the obstacle. It crosses each slope at roughly mid height with struts running down to a load bearing wall below, and the tube’s straight line from a central ceiling position frequently arrives exactly at it. Two answers work. Take the top unit higher up the slope, above the purlin, which lengthens the tube but keeps it straight. Or take it lower, below the purlin and nearer the eaves, which shortens the tube but pushes the aperture towards the gutter where the base flashing needs a full course of tiles below it to lap onto. Which one wins depends on the purlin depth and where the ceiling position sits, and it is a measurement rather than a preference.

Ceiling joists in these houses are usually 400mm centres in 50mm stock, so a 250mm tunnel fits between them untouched and a 350mm needs a trimmer. Both get quoted so the choice is the customer’s.

Bungalows and dormer bungalows are the best case for this product in the whole county, and there are a great many of them around Tuffley, Abbeydale, Churchdown, Hucclecote and the vale villages. Every room is on the top floor, the loft is directly above the ceiling of the hall, the bathroom and the bedrooms, and the run from slope to ceiling is often under a metre.

Short runs change what is worth specifying. At 700mm the bounce count is low, so a flexible tube performs respectably and the difference between specifications narrows. Two 350mm tunnels down the length of a central hall will light it properly, and no elbow is needed anywhere because the slope is directly above.

The constraints move to the roof. Bungalow roofs are often shallow trussed at 22 to 30 degrees with a low void at the eaves, so the aperture has to sit far enough up the slope for the tube to stand up in the space available, and far enough from the ridge to clear the ridge board and any ridge ventilation. On a dormer bungalow the first floor rooms already have windows in the dormer cheeks, and the dark spot is the inner landing between them, which is exactly where the remaining loft void sits.

Bungalow lofts are also where the water cistern, the boiler flue and the accumulated storage of forty years all live at once, so the trial fit through the void before cutting matters as much here as anywhere. Coverage across the vale and the villages is set out on the areas pages.

Most sun tunnels in a house go in under permitted development, and the figure that decides it is the same one that governs every other roof alteration: the work must not project more than 150mm beyond the plane of the existing roof slope, and it must not be higher than the highest part of the existing roof.

A flat glazed top unit sitting within 60 to 80mm of the covering is comfortably inside that. A standard dome standing 100 to 150mm proud is inside it in most cases and can be marginal on a thick covering, because the measurement is from the plane of the slope and a 25mm stone slate has already used part of the allowance. A tall dome on a kerb, which is what a flat roof adaptor produces, is a different proposition and gets checked rather than assumed.

Two other conditions come up. In a room formed in a roof, side facing glazing has to be obscured and non-opening below 1.7 metres from the floor, which a sun tunnel does not run into because it does not open and it is in the ceiling. And flats and maisonettes have no permitted development rights for roof alterations at all, so an application is needed regardless of how small the aperture is.

Article 4 directions remove rights street by street, and both Gloucester and Cheltenham have areas covered by them, as do the Cotswold conservation areas. The local planning authority is the only body that can confirm the position for a specific address, and where a sale is anywhere in view, written confirmation before work starts is worth the fee.

There is very little to do to a sun tunnel and the little there is gets ignored, so it is worth setting out. The top unit collects what the roof collects. In a shaded position under trees, moss and leaf litter build up around the base flashing and algae films the dome, and light output falls off gradually enough that nobody notices until it is washed. Once a year with a soft brush and water, at the same time as the gutters, is enough.

The diffuser is wiped from below with a damp cloth. Dust settles on the room side and it is the easiest few per cent of output to get back. Insects appearing inside the tube are a sign rather than a nuisance: they mean air is getting in somewhere, usually at a tube joint band or at the top unit’s clamp ring, and the joint gets taped and made good properly.

Condensation appearing at the diffuser on cold mornings, in a tunnel that was previously dry, usually means the ceiling seal has been disturbed or the loft insulation has been pulled back from around the tube and never replaced, which happens after somebody has been in the loft chasing a cable. Both are quick to put right and neither is a fault in the unit.

The polycarbonate on an older dome eventually yellows and crazes under ultraviolet exposure, and glass does not. Where a tunnel is twenty years old and dim, a new top unit and a new diffuser on the existing tube is often all that is wanted, which is the same logic as the assessment on the replacing a failed or misted rooflight page.