Fire and insulation detailing around a tunnel

Fire separation

Some ceilings provide fire resistance: between a garage and a house, between flats, and on the ceiling of a protected escape route such as a hallway in a loft converted house.

Where that is the case, any penetration has to maintain the rating. Fire rated collars and appropriate detailing exist for exactly this, and it is not a detail to improvise.

It comes up most often in a loft conversion, where the stair enclosure is a protected route with a fire resisting ceiling. A tunnel dropping into that hallway is a penetration of it.

Where we are in any doubt we say so and it is confirmed with Building Control rather than assumed.

Loft insulation

The tunnel passes through it, and the insulation must be maintained around it rather than pushed aside and left.

A gap in insulation around a tunnel is a thermal bridge and a route for warm air into the loft. It is also easy to leave, because it is out of sight the moment the loft hatch closes.

Equally, insulation must not be heaped against the tube in a way that blocks the eaves ventilation path in a cold roof.

Vapour control

The ceiling collar is sealed to the tunnel so warm moist air cannot enter the roof space around it. This is the single most important detail for preventing condensation inside the tube.

Downlights nearby

Worth mentioning. Where a ceiling has fire rated downlight covers, the tunnel penetration is treated with the same care. A ceiling full of correctly rated fittings and one unrated hole is not a fire resisting ceiling.

A tunnel is a hole through the insulation line

That is the thing to hold on to. Everything in this guide follows from the fact that a sun tunnel passes through the layer that separates the warm inside of a building from the cold outside, and wherever that layer is interrupted it has to be made continuous again.

Two layers, actually, and they do different jobs: the insulation, which keeps heat in, and the vapour control layer, which keeps moist air out of the construction. Both are cut for the tunnel and both have to be restored around it.

The cold loft case

The commonest situation. Insulation lies between and over the ceiling joists, the loft above is ventilated and cold, and the tunnel crosses it.

Insulate the tube. A metal tube through cold space is a cold surface. Warm moist air from the room below condensing on it sends water down to the diffuser, where it looks exactly like a roof leak and produces a great many unnecessary roofing investigations.

Insulating the tube along its full run through the cold space keeps the inner surface closer to room temperature so there is nothing cold enough to condense on. It is a cheap material and it is frequently omitted, particularly on kits that do not include it.

Seal the ceiling ring airtight. The more fundamental measure, because it stops room air getting into the tube at all rather than managing it once it is there.

Sealant or tape at the junction between the ring and the plasterboard, done properly rather than approximately. It also stops warm air leaking from the room into the loft, which is a genuine heat loss and a route for moisture into the roof space.

Restore the ceiling insulation around the opening. Loft insulation cut for the tunnel leaves a gap, and a gap in a ceiling insulation layer is a thermal bridge and a convection path. It is dressed back up to the tube rather than left short.

Do not block the ventilation. A cold loft is ventilated deliberately, at the eaves and frequently at the ridge, to keep it dry. A tunnel running across the space should not obstruct that airflow, and insulation pushed into an eaves gap while working around the tube is a common and avoidable error.

The warm roof case

Where the insulation sits at rafter level rather than ceiling level, as in a converted loft or a room in the roof, the picture changes.

The tunnel now passes through insulated construction rather than cold space, which reduces the condensation risk on the tube considerably. What it introduces instead is a more demanding detail where the tube crosses the insulation and the vapour control layer.

Both have to be maintained and sealed to the tube. Insulation packed loosely around a tube is not the same as a continuous layer, and a vapour control layer cut and left is an air path into a warm roof construction.

Getting that wrong produces moisture inside the build up, which condenses out of sight and shows up eventually as damp insulation or decayed timber. That is a considerably worse outcome than the same error in a cold loft, where at least the moisture ends up somewhere ventilated.

Why the vapour control layer matters as much as the insulation

Air leakage carries far more moisture into a construction than diffusion through an intact membrane does.

A taped, continuous seal at the ceiling ring and, on a warm roof, where the tube crosses the vapour layer, behaves completely differently from a loose one. The difference is not marginal.

It is also the reason the two details are specified together rather than treated as separate items: insulation without airtightness lets moist air past it to the cold side, and airtightness without insulation leaves a cold surface for that air to find.

Fire, and where it applies

In a single family house, a sun tunnel through a loft is not usually a fire compartmentation issue, because the loft is not a separate fire compartment from the rooms below.

Where it matters is anywhere a ceiling is providing fire resistance.

A loft conversion. The ceilings below a converted loft frequently have a fire resistance requirement as part of the escape route protection. Cutting an opening in such a ceiling means the penetration has to be dealt with so the resistance is maintained.

Flats and maisonettes. Compartmentation between dwellings is a serious matter, and a tunnel passing through a compartment floor or ceiling needs proper fire stopping. This is a genuine constraint rather than a formality, and it sits alongside the fact that flats have no permitted development rights at all.

An integral or attached garage. The ceiling between a garage and a dwelling above frequently has a fire resistance requirement.

Where a tunnel passes through more than one storey. A run from the roof through a loft and then through a first floor into a ground floor ceiling crosses a floor, and whether that floor has a resistance requirement depends on the building.

Where any of those applies, the detail is a fire rated collar or a proprietary fire stopping arrangement around the penetration, specified rather than improvised. Manufacturers of tunnel systems publish details for the situations their products are approved in.

Downlights, and a common trap

Related and worth mentioning because the two are frequently installed in the same ceiling at the same time.

Recessed downlights are themselves penetrations of the ceiling, insulation and vapour layers, and they have their own requirements around fire rating and insulation contact.

Where a tunnel diffuser and several downlights are competing for space in a small bathroom ceiling, the accumulated penetrations matter more than any one of them. That is an argument for the optional light fitting inside the tunnel diffuser, which puts light and daylight through a single opening rather than several.

Building Regulations

A sun tunnel is a rooflight for regulatory purposes, so a new one engages Part L, and where the opening cuts structure it engages Part A. In practice a tunnel usually fits between rafters and between ceiling joists without trimming, which keeps it simple.

Where a fire resistant element is being penetrated, Part B applies to the detail.

All of it is notifiable, and it is the installer’s responsibility to handle rather than the customer’s to arrange.

The three lines to ask for on a quotation

Insulation to the tube where it crosses unheated space, named rather than assumed.

An airtight seal at the ceiling ring.

And, where the ceiling being penetrated has a fire resistance requirement, how that is being maintained.

The first two are the ones most often dropped from a cheap price, and their absence does not show until the first cold spell, by which time nobody connects the water at the diffuser with a line that was missing from a quotation months earlier.

The penetrations a tunnel makes

A sun tunnel passes through two layers that matter: the ceiling, and the roof. Depending on the building, the ceiling may be doing more than holding plaster.

In a house of two storeys with a loft above, a ceiling into a roof void is generally not a fire separating element and the question is straightforward.

Where the ceiling separates a habitable room from a loft conversion above, or where it forms part of a protected stairway enclosure, it may have a fire resistance requirement. Cutting a hole in it then has consequences that need addressing rather than assuming.

In a flat or maisonette, ceilings between dwellings are separating elements and a tunnel through one is not a simple job.

The protected stairway case

Worth naming because hallways and landings are exactly where sun tunnels go.

In many houses the escape strategy depends on the stairway being protected, which means the enclosure around it, including the ceiling, has a required fire resistance.

A tunnel penetrating that enclosure has to maintain it. That is a building control conversation and it should happen before the work rather than after.

Insulation, which is the everyday issue

The tube passes through unheated roof space. In winter the loft is close to outside temperature, so the tube wall is cold.

Insulate it along its whole length through any unheated space, continuously, without compressing to nothing at the joints. Purpose made insulated sleeves exist and are worth using.

Without insulation the tube is a cold pipe running from the roof to a hole in your ceiling, and any moist air that reaches it will condense on it.

Where the loft insulation meets the tunnel

The ceiling insulation has to be dressed up to and around the tube, not simply cut away to make room.

A gap in the loft insulation around a tunnel is a cold spot in the ceiling of the room below, and it shows in exactly the way a bridged rooflight reveal does.

The seal that does most of the work

The diffuser to the ceiling.

It stops warm damp room air entering the tunnel in the first place, and it is more important than the insulation because it addresses the cause rather than the symptom.

A gasket or continuous silicone onto a clean surface, not a decorative bead run over an existing gap.

And tape the joints between rigid tube sections, so moist loft air cannot enter from outside the tube either.