Does a Capped Chimney Need Ventilation?

Why a sealed flue goes damp

A chimney is a tall masonry tube, and masonry soaks up water. Rain on the stack, moisture in the air and condensation inside the flue all find their way into the bricks. In a working chimney, warm air passing up the flue dries the walls, and the draught carries the damp away. When the fire is retired and the top of the pot is sealed, that air movement stops. The flue becomes a still, cold space, and any moisture entering it has nowhere to go. Salts that have collected over years of use get carried into the brickwork, and they show up as brown stains, flaking plaster or wallpaper lifting on the chimney breast below.

The cure is not complicated. It means leaving a path for air to enter and leave the flue, while keeping rain out. That is what a ventilated cap does. Our overview of capping a disused chimney explains the full picture, and this page deals with the ventilation question on its own.

How a flue is ventilated when capped

There are two usual arrangements, and the right one depends on how the chimney is built and what is below it.

A vented cap on the pot or stack

The most common fix is a cap with openings in its sides or a louvred top. Rain cannot fall straight in, but air passes through, allowing the flue to breathe. Some caps sit on the pot like a hat. Others are fixed over the top of the stack with the pots removed, usually when the stack has been rebuilt. A good cap is made of clay, terracotta or stainless steel, bedded firmly, and free of anything that could snap off in the wind.

A vent at the base of the flue

Where a fireplace has been blocked up, a small air vent is often fitted low in the infill, on the room side, so air can enter the flue from below and rise through it. It can be a discreet brick-sized vent or a grille set into the plaster. In some houses, the vent at the top is enough. In others, the combination of both gives better results. Your builder or building control body can say which suits the property.

What goes wrong if you seal it completely

Sealing a flue tight, with a solid slate or concrete slab bedded over the top, looks neat and keeps the weather out. It also tends to cause the damp problem it was meant to avoid. The stack cannot dry, moisture builds, and the effects can appear inside the house a year or two later. The damp may look like a roof leak and send people looking in the wrong place. Our page on damp on chimney breasts explains how to tell the two apart.

A second risk arises where the flue is shared. A chimney that serves two houses, or two floors, may still have another fire connected to it. Stopping the air movement from one side can affect the draw on the other. Do not cap a flue until you know what else is connected to it. For shared stacks, our guide on shared chimney stack repairs covers the responsibilities.

Regulations and who to ask

The principle of keeping a disused flue ventilated is long established, and standards for chimney design deal with it. We do not quote the detail here, because the requirements depend on the age of the house and the work being done. If the work forms part of a larger change, such as taking out the chimney breast or fitting a stove, it may involve your building control body, which may be your council or a private approver. Check with them before you start, and ask the installer for the ventilation details in writing.

Where the house sits in a conservation area or is a listed building, changes to the appearance of the stack, such as replacing pots with a flat cap, may need consent from the council's conservation officer. A like-for-like cap on an existing pot is often acceptable, but ask first.

What to check on your own chimney

Look at the top of the stack from the garden. Is there a pot with a cap, or a flat slab over it? If it is a slab, does it have any gap or vent? Look at the fireplace below. Is there a vent in the blocked-up opening? If neither is present and you have damp on the breast, a vented cap is a likely fix. If you are not sure, a roofer can examine the stack and check the flashings and flaunching while up there, as these cause similar symptoms. We photograph what we find and write up a fixed price. Published UK guides put chimney repair anywhere between £500 and £2,000; the figure for your stack comes after the survey. Return to chimney repairs in Basingstoke for the full set of services.

It is also worth asking whether the flue needs to be capped at all. If the chimney is in good order, a rain cap over an open pot may serve better, and costs little. And if the stack has become a nuisance, removal is possible, which our page on whether to remove an unused chimney considers.