Can a sun tunnel go through a loft conversion?

The consequence

The tube has to be boxed in as it passes through the converted space. That means a boxed column or a section of bulkhead in a habitable room, typically 300mm to 450mm across depending on the tunnel size.

Some people are entirely happy with that, particularly where it can be run in the corner of a room or inside a wardrobe or a cupboard. Others are not, and it is worth seeing the actual dimension marked out before committing.

Routing

The best routes are through cupboards, wardrobes, en suites or the corner of a room where a boxing reads as part of the architecture rather than an addition.

A boxing in the middle of a wall is much harder to make look deliberate.

Fire separation

Important here. A loft conversion has a protected escape route, and the ceilings enclosing it are fire resisting.

A tunnel passing through that construction is a penetration of it and has to maintain the rating, with fire rated collars and appropriate detailing. Where we are in any doubt we confirm it with Building Control.

Output

A two storey run is long. Expect meaningfully less light than a single storey installation, and specify the largest diameter and rigid tube to compensate.

Under four metres and reasonably straight is workable. Beyond that we would be honest about diminishing returns.

The alternative

Often better: a rooflight in the loft room itself, and a glass floor panel or borrowed light bringing some of it to the floor below. More expensive, and it delivers considerably more.

Yes, and it needs boxing in

Where a loft has been converted, there is no open roof space for a tube to run through. The tunnel has to pass through the converted room, and that means it is visible unless it is enclosed.

That is entirely doable and it changes the job from a straightforward tunnel installation into a tunnel plus a joinery and decorating exercise.

The routes

Through a void that already exists. Behind a knee wall, in a cupboard, through a bulkhead over a stair, or in the space between a sloping ceiling and the rafters. If a route like that lines up with where you want the light, this is much the best answer.

Boxed in through the room. A cased duct in a corner, treated as a piece of joinery and decorated to match. Honest and neat if it is designed rather than improvised.

Within a partition. Where a stud wall runs in the right place, the tube can sometimes go inside it.

What to check first

Whether the roof position and the ceiling position below can be lined up, because every bend costs light and a tunnel that has to dogleg around a converted room loses a great deal.

Moving the ceiling position slightly is frequently the better answer, and it is worth being flexible about where the light lands.

The structure

A loft conversion has a designed structure: new floor joists, frequently steels, and a roof that may have been strengthened. Nothing in that is cut for a tunnel without checking, and the route has to work around it rather than through it.

A tube fitting between rafters is usually straightforward; one that would cross a structural member is not.

Insulation and sealing

In a converted loft the insulation is at rafter level, so the tube passes through insulated construction rather than a cold void. Where it crosses that insulation layer the detail has to maintain both the insulation and the vapour control layer, sealed to the tube.

Get that wrong and you have a cold bridge and an air leak in a warm roof, which is a worse outcome than in a cold loft.

The honest alternative

If the room needing light is the loft room itself, fit a rooflight rather than a tunnel. A tunnel earns its place where the light has to travel past a room to reach one below.

What changes once the loft is a room

Before conversion, a sun tunnel runs through open loft space and the route is a matter of dodging a few timbers. After conversion, that space is occupied, insulated and plastered, and the tunnel has to pass through a finished room or around it.

Three routes exist, in order of preference.

Through a void that survived the conversion. Most loft conversions leave eaves storage and a void behind the knee walls. A tunnel can frequently run in there and come down through a ceiling below without entering the finished room at all.

Boxed in through the room. A visible boxing in a corner, plastered and decorated. Honest, effective, and acceptable in a landing or a bathroom where it can be tucked against a wall.

Not at all. Where the conversion has used the full width and depth and there is no route, the answer is a roof window into the loft room itself rather than a tunnel through it.

What it must not do

Puncture the vapour control layer of the conversion without resealing it, or pass through the insulation line uninsulated. Both create a cold bridge into a warm room and produce condensation that gets reported as a leak.

The tube through any cold zone needs insulating, and every penetration of the vapour layer needs taping back to a sealed condition.