What is ‘Condensation’?


Condensation happens when warm moist air meets a non-absorbent cooler surface. The warm air cools and is unable to ‘hold’ the moisture. As a result, water is deposited on the surface.

Nearly everyone is familiar with condensation. Just about all of us will have experienced the winter morning’s misted or pouring wet bedroom window. Older people still impress a younger generation with recollections of ice on the glass at a time before central heating. 

Even though ice has been largely relegated from windows, water vapour is still around in our homes waiting to be condensed on the walls and windows of bathrooms after bathing or kitchens where cooking pans have been boiling or washing machines churning. 

But as building standards have improved with more ventilation and warmer insulated surfaces, moist air is either immediately removed by extractors or maintained longer and naturally removed by ventilation without having the opportunity to condense on much cooler surfaces.


The Physics


Air in the UK will usually include moisture in the form of water vapour. It is never completely ‘dry’ as in the desert. The amount of water the air is able to ‘hold’ is normally related to the temperature of the air. The warmer the air, the more moisture it can contain. When the air can hold no more moisture it is described as ‘saturated’. 

The ‘relative humidity’ in a room is the ratio of how much water vapour there actually is (its ‘density’), compared with how much the air can ‘hold’ before it becomes saturated. So a room with a fixed temperature containing completely ‘dry air’ has a relative humidity of 0%, whereas that same air at the same temperature at maximum saturation is 100% relative humidity. 


Relative Humidity (RH) = (actual water vapour density / saturation water vapour density) x 100.

Expressed as a percentage. 


Outside of the home, take a look at a weather app on your phone. The forecast will include relative humidity - a measure of how ‘dry’ or ‘moist’ the air is. 

100% relative humidity is known as the ‘dew point’ or ‘saturation point’. If saturated air starts to cool, moisture will condense from the air causing water to be deposited on non-absorbent surfaces.



So, for example, the air in a shower room (hot showers are an efficient way of producing water vapour) will continue to be absorbed by the warm air until it can contain no more. At that, the dew point, the walls of the room and the window panes start to become moist. After the shower if the moisture laden air is not removed/extracted, the air temperature will drop resulting in wet surfaces as moist air continues to produce condensation. 

Understanding when and where any ‘dew point’ is likely to occur on or in the building fabric is an essential part of energy-efficient design.


Types of condensation


Surface condensation

As described above, condensation on surfaces is the more well-known form of condensation. It occurs on non-absorbent surfaces such as glass, tiles and wall plaster.

Interstitial condensation

Equally of concern and a bit more complicated, ‘interstitial condensation’ happens when warm moist air penetrates the building fabric and meets with the surfaces or volumes of a material that are significantly cool enough for the warm air to cool rapidly and release its moisture. The often unseen build-up of moisture within the wall can be extremely destructive over time, particularly in timber frame structures. 

Where a Vapour Control Layer (VPL) isn’t included, an architect will look to produce a steady temperature gradient through a wall or roof from the warm interior to the, generally, cooler exterior. The temperature gradient should plan to be at all times greater than an equivalent saturation point gradient. If at any time the two gradients meet, condensation will occur. Avoiding the occurrence of a dew point will of course reduce the chance of moist air condensing within the building fabric where it might cause damage.



Damage caused by condensation

Where condensation is chronic within a building, damp will occur with all its associated problems:




• Mould (mildew) growth - a fungus that can release toxins into the atmosphere that can have a detrimental effect on health such as asthma, rhinitis, itchy eyes, respiratory symptoms, respiratory infection and eczema.

• Staining of fabric such as wall paper and plaster.

• Decay of the building fabric both from surface and interstitial condensation. 

• Poor performance of insulation if subject to damp.


Avoiding condensation


• Produce less moisture 

Reducing the incidence of water vapour. Though water vapour will always be in the air, adding to it through household activities can put pressure on the building fabric. For example, moisture can be avoided in cooking by simply putting lids on pans; drying laundry to can generate moisture, so try and avoid drying clothes on an airer or radiator without first opening the window and closing the door to the room. Best of all get a washing line rigged up outside.

• Provide adequate ventilation 

Removing moisture from a room is usually the first choice. Traditionally the householder would throw open the windows to remove the water vapour. However with increasing rigour of energy conservation and building air-tightness, mechanical ventilation is preferred by energy standards such as Passivhaus. Mechanical ventilation can provide not only for the removal of stale air, it can also exchange the heat from the extracted air to warm the incoming fresh air - so reducing the necessity of reheating the space - MVHR (Mechanical Ventilation with Heat Recovery). 

• Heating and insulation

Maintaining a warm space ‘stores’ any water as vapour. By contrast, cooling a space (e.g. a kitchen, bathroom or laundry) that has high relative humidity will encourage condensation. The best way to maintain a constantly warm temperature within a house is through high levels of insulation.


Avoiding interstitial condensation

• Even professionals can find themselves tied up in knots thinking about interstitial condensation. 

• Mainly a problem in timber framed building.

• Materials or membranes that limit or prevent water vapour are called vapour control layers (VCL). 

• VCLs usually double as air-tightness membranes. On completion, buildings need to be air-pressure tested.

• For any VCL to work properly, it needs to be installed correctly - without penetrations caused by pipes, wiring or damage. Proprietary tape needs to be used to secure the membranes securely against their background. Foil-backed plasterboard is not a good solution in this respect. 


A traditional vapour barrier preventing any moisture from entering the construction


• A VCL that prevents any water vapour penetrating the structure from the inside of the wall is known as a vapour barrier. A vapour barrier can only act on moisture heading from the inside to the outside and not the other way round. This can cause problems when in the summer vapour can travel from the outside towards the inside. In these instances vapour can condense against the wall side of the barrier and cause damage.

An intelligent membrane. Sophisticated but expensive


• A VCL that controls the amount of vapour entering structure either from the inside or the outside of the wall or roof is called an intelligent membrane. The membrane changes its permeability according to relative humidity, pressure and temperature conditions, so it can vary between being a vapour barrier and a breathable membrane. It is generally taped into place over the insulation, on the warm side.

• Oriented Strand Board (OSB) can also act as a VCL whilst simultaneously acting as structural racking to a timber frame. It can absorb water vapour at low levels of relative humidity (RH) but increasing the RH, the material absorbs very little more water vapour compared with other types of timber boarding such as ply. If used behind and internal finish, it is wise to ventilate the surface of the OSB by including a gap (often a service zone).


Avoid interstitial condensation using the ‘Breathing Wall’ technique

• Another solution is the ‘breathing wall’ technique. A modern ‘breathing wall’ is one which refers to the diffusion of water vapour, not air, through the wall or roof fabric. The flow of water vapour is controlled from the high humidity interior of the building to the lower humidity exterior such that water vapour can escape through the outer layers of the wall more quickly can enter through the inner layers.

• The rule of thumb that makes this technique work says that the inner layers on the warm side of the insulation should have a vapour resistance at least five times greater than the layers on the cold side of the insulation.

• In selecting the successive layers of materials that the wall is built up from, the right materials have to be selected so that the vapour permeability gradually increases from the inner skin to the outer skin.

• It’s worth noting with a word of caution here in that by using a breathing wall, high standards of air-tightness are difficult to achieve.


Whichever system you choose to reduce the risk of interstitial condensation, it’s best to involve the expertise of your architect in making the final decision.