What Causes Damp (Mold) in Houses?


Damp, Mould, and Mildew

Damp can be a serious problem in homes, causing damage to plaster and leading to mold on walls, ceilings, curtains, and clothing. It has several causes but can often be remedied or reduced in severity. This article examines how damp is caused and how to deal with it.

Bituminous felt underlay on the underside of a tiled roof
Bituminous felt underlay on the underside of a tiled roof | Source
Flashing around a chimney
Flashing around a chimney | Source

How Does the Water Get In?

  • A bituminous felt underlay or polymer membrane acts as a secondary barrier on roofs and prevents rain which is blown up under roof tiles or slates from making its way down into a building. Sometimes this membrane can become compromised and water can pass through, leaking down onto the tops of walls or ceilings and soaking through plaster. The symptoms are damp spots or mildew at the tops of walls or bubbling on paintwork on ceilings.
  • Copper or lead flashing is used around the perimeter of chimneys where they merge with a roof surface. The upper edge of flashing is covered by brickwork or render on the chimney, the other edge overlaps the roof covering. This prevents rain running down between the chimney and roof. Flashing can crack and deteriorate, but it's more likely that cement covering the flashing on a chimney ends up crackled and crumbles away, allowing water to penetrate and enter the attic space and possibly soak down through walls to rooms below. Flashing is also used where extensions to buildings butt up against existing structures.
  • Damaged or blocked gutters and downpipes can allow water to run against walls. If there are cracks in render or timber cladding, this water can soak inside.
  • Window sills are normally sloped or designed in such a way that water drips from the underside of the sill rather than running back towards the wall. A concrete sill should have a slot on the underside which acts as a barrier and prevents water traveling any further. This slot shouldn't be blocked. Cracks and gaps around sills and windows can allow water to run or soak inside a building.
  • When block and brick buildings are constructed, a damp proof course consisting of a layer of plastic is inserted between the footings (wall foundations) and lowest course of walls. A membrane is also installed under concrete floors before the concrete is poured. This acts as a moisture barrier and prevents moisture soaking upwards through the wall or floor and out through the plaster. If these damp-proof courses are entirely absent or become damaged, water can soak upwards by capillary action through masonry, known as rising damp. The symptoms of this are dampness at the base of walls or under floor coverings, efflorescent salt deposits on walls, crumbling plasterwork, and wallpaper detaching from wall surfaces.
  • Raised flowerbeds shouldn't be built up against outside walls either, as this may cause moisture to bypass the damp-proof course. Also pavements should be constructed so that water runs away from the wall. Timber on the ground floors of houses should be ventilated from beneath. This is to prevent dry rot and also to flush out moist air under the floor which can cause wet rot. Usually there are at least two vents near the base of exterior walls and these shouldn't be obstructed by plants or raised flower beds.
  • Modern masonry walls are built as two separate walls with an intervening cavity which is filled with insulating material. Older concrete, stone, block, or brick walls don't make use of this double wall construction technique, and there is no cavity to act as a moisture barrier. So moisture can soak in through cracks in exterior rendering or damaged or bad pointing on brickwork.

Rising damp causing damage to plaster
Rising damp causing damage to plaster | Source
Vents at the base of walls should be unobstructed by vegetation and raised flower beds
Vents at the base of walls should be unobstructed by vegetation and raised flower beds | Source

Damp Caused by Internal Moisture Sources

Damp is also the result of internal moisture sources which cause condensation when warm, moist air hits cold internal surfaces such as ceilings, walls, and windows. Once warm air cools, its capacity for holding moisture is reduced, and the water collects on cold surfaces, typically at the tops of walls and corners of rooms. Over time, this can cause unsightly mold growth and black patches on walls.

Moisture-laden air comes from several sources:

  • In a kitchen, cooking and boiling generates lots of moisture which ends up condensing on the cold surfaces of the room.
  • In the bathroom, showers and baths are a source of moisture.
  • Plants are a source of moisture, as is the compost in pots.
  • Damp clothes taken from washers produce moisture.
  • Humans breathing out water vapor.
  • Kerosene and gas (butane/propane) heaters generate lots of moisture.
  • If you live in a country with a damp, humid climate, moisture-laden air from outdoors will condense on cold surfaces.

Slot on the underside of a window sill (cill). This prevents water from running back against the wall and should be clear of cement or other debris
Slot on the underside of a windowsill (cill). This prevents water from running back against the wall and should be clear of cement or other debris | Source

Curing Damp: Repairs to Roofs and Exterior Walls, Ventilation, Heating, and Insulation

  • Sort out problems with roofs, downpipes, and gutters and make sure they are cleaned out.
  • Check that there are no cracks in render, pointing on brickwork isn't compromised, and there are no gaps or cracks in cladding on timber framed buildings.
  • In older buildings which lack a damp-proof course or where the DPC has become damaged, a chemical DPC can be installed.
  • Keep the levels of pavements and flower beds below the level of the DPC.
  • Ensure there is adequate ventilation while cooking or showering by opening windows or installing extractor fans or hoods over ranges.
  • Inner walls of buildings can be insulated. This raises their temperature closer to that of the room and makes condensation less of a problem. There are several ways of doing this. One way is to use plasterboard (hardwall) with an expanded polystyrene backing and foil layer which can be bonded to the wall. Another method is to nail laths to the wall, fill between them with expanded polystyrene sheets or rockwool insulation, and nail plasterboard sheets to the laths.
  • Don't remove clothes from washers until you are ready to dry them. Leaving clothes in baskets or drying them on radiators generates lots of moisture.
  • Plants can produce lots of moisture, so either get rid of your plants or improve ventilation!
  • Providing better heating in rooms raises the temperature of the air and all surfaces within a room. Warm air holds more moisture which can be ventilated out and condensation is less severe on warm surfaces.
  • Dehumidifiers are useful for air conditioning and removing moisture. If you live in a humid climate, moist air from outside can be a constant problem, in winter or throughout the year. It condenses on surfaces if the temperature drops. A dehumidifier is helpful in these circumstances.

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Comments 5 comments

azrestoexp profile image

azrestoexp 3 years ago

We deal with mold and mildew on a daily basis, always good to remind people of simple ways to help prevent a dangerous problem.

liesl5858 profile image

liesl5858 3 years ago from United Kingdom

Our home is full of condensation and moulds on the ceiling during winter. Thanks for sharing this great hub about the causes of damp. Helps me understand more about mildews and moulds

eugbug profile image

eugbug 3 years ago from Ireland Author

You're welcome Linda and thanks for dropping in! With the climate we have in Britain and Ireland, condensation can be a problem. Older houses are susceptible to damp because moisture can easily make its way through solid walls. Cold walls also exacerbate the problem.

molometer profile image

molometer 9 months ago

Damp and mildew are a real pain especially in this latitude. It can be quite tricky trying to balance ventilation with keeping warm.

Good informative hub Eugene. Made me rethink about the indoor plants.

eugbug profile image

eugbug 9 months ago from Ireland Author

Thanks Michael. Indoor plants can also get infested with aphids which produce horrible sticky honeydew which ends up on furniture, window sills and panes of glass. Mildew seems to thrive on this.

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