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What Causes Condensation and Damp in Houses?

Eugene is a trained engineer and self-taught home improvement enthusiast with almost 40 years of professional and DIY experience.

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

What are the Causes of Damp and Mold?

  • Poor ventilation
  • Insufficient heating in rooms
  • Non-existent insulation of walls
  • Water sources such as damp clothes, plants, cooking and bathrooms
  • Rain penetration through cracked walls, leaking roofs and windows
  • Water soaking up through walls and floors (rising damp)

Damp Can Be Caused by Internal Moisture Sources

Damp is often the result of internal moisture sources. Water evaporates from these in a warm room and turns into invisible vapor in the surrounding air. When warm, moist air hits cold internal surfaces such as ceilings, uninsulated walls, and windows, its temperature falls and the capacity for holding moisture is reduced, causing condensation, typically at the tops of walls, corners of rooms, and the bottom of window panes. Over time, this can cause unsightly mold growth and black patches on walls.

Moisture-laden air comes from several sources:

  • Cooking and boiling of kettles.
  • Showers, baths and towels.
  • Transpiration in plants produces water vapour as moisture is absorbed through roots and evaporates from leaves. Damp compost in pots is also a moisture source as it dries out.
  • Damp laundry on radiators or clothes horses.
  • Humans breathing.
  • Kerosene and gas (butane/propane) heaters.
  • If you live in a country with a damp, humid climate, the air itself will be highly moisture-laden at times.
Drying laundry indoors creates moisture in the air

Drying laundry indoors creates moisture in the air

Leafy plants produce moisture from their leaves. Compost in flower pots is also a moisture source

Leafy plants produce moisture from their leaves. Compost in flower pots is also a moisture source

Damp Can Also Be Caused by External Moisture Sources

Moisture can also make its way into a home from outside through walls, roofs and floors.

How Does Moisture Get Into a House?

  • A bituminous felt underlay or polymer membrane acts as a secondary barrier on roofs and prevents rain that 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, lead or polymer 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 mortar 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 (downspouts in the US) can allow water to run against walls. Cast iron gutters especially can corrode and crack if not maintained and painted. This often leads to water overflowing at the backs of gutters or leaking out of joints and then hitting the fascia board which then ends up rotting. If there are cracks in render or timber cladding, this water can soak inside.
  • Cracked or loose roof and ridged tiles can allow water penetration into the roof space.
  • 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 travelling 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.
  • Compromised damp proof membranes. 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.
  • No cavity between inner and outer walls. 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.

How to Cure Damp

Repairs to Roofs and Exterior Walls

  • Sort out problems with cracked and broken tiles on roofs
  • 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.

Rectify rainfall drainage problems

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  • Make sure your gutters and downpipes (downspouts) are free of leaves and other debris and none of the joints between sections are leaking or have separated.
  • Fit drains (gullies) that will carry water away from the foundation of your home Constant wetness around the base of a wall can wick up through through masonry, leading to damp problems inside.

Improve Ventilation

  • Ensure there is adequate ventilation while cooking or showering by opening windows or installing extractor fans or hoods over ranges.
  • Install vents and rooms.

Insulate and Provide Sufficient Heating

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 a composite sandwich product consisting of a layer of plasterboard (drywall), an expanded polyurethane foam backing and impermeable aluminium foil membrane which can be bonded to the wall. Another method is to nail timber laths to the wall, fill between them with expanded polyurethane 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.
Install gullies (drains) to carry water away from the building's foundations.

Install gullies (drains) to carry water away from the building's foundations.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: I have an insect problem in my home. There are beatles for sure, plus some kind of biting mite. My family room vinyl floor has been eaten up by something, and it is continuing to do so. What started as a quarter sized area turned into plate sized within months. The question I have is my dark ceiling beams have a white powdery film on them. Could it be mold?

Answer: It could be a mildew-type mold, and it may or may not be a health hazard, or at least it will likely rot the beams. This link shows pictures of different types of fungi that can cause the decay of timber:

You could try posting on the DIYNot forum here. In fact, this link shows a question about mold similar to what you describe:

© 2012 Eugene Brennan


Eugene Brennan (author) from Ireland on February 20, 2016:

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.

Micheal from United Kingdom on February 20, 2016:

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.

Eugene Brennan (author) from Ireland on July 02, 2013:

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.

Linda Bryen from United Kingdom on July 02, 2013:

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

Arizona's Restoration Experts, LLC on December 11, 2012:

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

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