What is a twin wall flue?

If you would love a wood burner but your home has no chimney, a twin wall flue is usually the answer. It is a factory-made, insulated chimney system that can be built through the inside of your house or up the outside wall, giving a stove a safe route for smoke and gases to leave the building.

In short

• A twin wall flue is two layers of stainless steel with insulation packed between them.
• It acts as a complete new chimney, so you can fit a stove in a home without one.
• It can run internally through ceilings and the roof, or externally up a wall.
• It must be installed to Building Regulations (Approved Document J) and the manufacturer's instructions, including set distances from anything combustible.
• Planning permission may be needed in some locations, especially for an external flue.

How a twin wall flue is made

Each section has an inner stainless steel tube that carries the flue gases, an outer stainless steel skin, and a layer of insulation, usually mineral wool, sandwiched between the two. Sections lock together to form a continuous insulated chimney.

The insulation does two important jobs. First, it keeps the outer surface much cooler than the gases inside, which allows the flue to pass safely through floors, ceilings and roofs when the correct clearances and fittings are used. Second, it keeps the flue gases warm as they rise. Warm gases draw better, so the stove pulls air in properly and less tar and moisture condense inside the flue.

How it differs from single wall flue pipe

You will often see a short length of plain, single skin flue pipe coming off the top of a stove. Single wall pipe is uninsulated and gets very hot, so it is only used to connect the stove to the chimney or twin wall system within the same room, and it needs a much larger distance from combustible materials. Once the flue leaves the room or passes through a ceiling, it must change to an insulated system.

Where can a twin wall flue run?

Internal route

The flue rises from the stove, passes through the ceiling and any upstairs room, through the loft and out through the roof. Fire-stop plates are fitted where it passes through each floor, and upstairs it is often boxed in. Concealed sections need access for inspection, and the roof penetration is sealed with a flashing and storm collar. Because more of the flue is inside the warm house, an internal route usually draws very well.

External route

The flue leaves the stove, goes straight through the external wall at the back of the stove using a wall sleeve, then rises up the outside of the house, supported by wall brackets, to finish above the roofline. This avoids work through upstairs rooms and is popular for extensions, garden rooms and bungalows. It is more visible, which is why planning rules can come into play.

Key safety rules

A twin wall flue is part of your home's fire safety, so it has to be designed and fitted correctly. The main points an installer will check include:

• Distance to combustibles. Every system has a stated minimum distance from combustible materials such as timber joists. For many systems it is around 50mm, but the manufacturer's figure for that specific flue is what counts.
• Flue height and outlet position. Approved Document J gives a general minimum flue height of 4.5m for solid fuel appliances and sets out where the outlet must finish in relation to the roof ridge, windows and nearby structures. The stove manufacturer may specify more.
• Bends. Building Regulations guidance limits the number and angle of bends, as too many reduce the draw and make sweeping harder.
• Support. The system needs proper supports and wall bands at intervals set by the manufacturer.
• Correct rating. The flue must be suitable for solid fuel and rated for the temperatures your stove will produce.
• Notice plate and certificate. A notice plate recording the flue details should be fixed in an accessible place, and the installation must be signed off. You can read more in our guide to stove installation regulations in England.

Do you need planning permission?

For most houses in England, fitting a flue is normally permitted development if it rises no more than one metre above the highest part of the roof. In designated areas, which include conservation areas and National Parks, a flue should not be fitted on the principal elevation or a side elevation facing a highway. Listed buildings will usually need listed building consent, and flats do not have the same permitted development rights. If any of these apply to you, check with your local planning authority before work starts.

Twin wall flue or chimney liner?

If your home already has a chimney, the usual approach is to line it with a flexible stainless steel liner rather than build a new flue alongside it. Twin wall is the choice when there is no chimney, when the existing one cannot be used, or when the stove is going in a room far from the chimney breast. Our guide to fitting a wood burner without a chimney covers the options in more detail, and you can compare with chimney liner installation.

How much does a twin wall flue cost?

The cost depends on the length of flue, the number of floors and fittings it passes through, whether it runs internally or externally, the roof type and access. A short external run on a single-storey extension is a very different job to an internal flue through two floors and a loft. For a broader picture of what affects the price, see our stove installation cost guide. Any figures are only a guide, as every home is different, so ask for a written quote.

Next steps

The best route for a twin wall flue depends on your room layout, roof and what lies above the stove position. A site survey will confirm whether an internal or external route makes sense, the clearances needed and whether any permissions apply. If you are considering twin wall flue installation, get in touch to arrange a survey and a written quote.