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Understanding Moisture

Moisture Problems in Houses

Winter condensation is probably the most common moisture-related issue that affects houses. In its mildest form, it appears as surface condensation on windows. In severe cases it causes decay that might affect the structure itself. In between these extremes, it can manifest itself as mold growth on interior walls, ceiling stains, paint peeling and growth on shower curtains. (In Germany it’s seen as a very serious issue and can be taken through the legal system to rectify).  Moisture is added to room air in a variety of ways. It is also removed in a variety of ways. The balance created between the rate of moisture generation and the rate of moisture removal establishes the equilibrium humidity level (or relative humidity) in a house, and consequently the potential for future problems.

Sources of Moisture

Moisture can be added unintentionally through normal living activities. It is also added from Clothes drying (unvented), Clothes washing, Cooking (none extracted) Baths/Showering (none extracted), Dishwashing, House Plants and Human Contribution. It is also added from ground moisture in the soil beneath our houses and migrates up through the bearers, joist and flooring boards. Exposed earth floors under floors can produce as much as 30 litres of water a day if the ground is wet. Even exposed rock can release substantial quantities of moisture.

Wood absorbs increasing amounts of moisture from the air as the relative humidity increases (usually manifest as sticking doors). It could absorb an additional 4 to 5 percent of its weight in moisture. The floor assembly and partition framing of a typical house will contribute additional litres of water during the heating season. This is only a portion of the total water contributed by all materials.

*Sources Of Moisture  Litres
Cooking (non extracted)  3.0 / day
Clothes Washing  0.5 / day
Baths / Showers  1.5/day (per person)
Dishwashing  1.0 / day
Clothes drying (unvented)  5.0 / load
Portable gas heater  1.0 / up to per hour
Breathing (per hour) (average)  0.2 / per person
Breathing asleep (per hour) (average)  0.02 / per person
Perspiration per hour 0.03 / per person
Pot Plants what goes in comes out

*sourced from Consumer NZ 

"The average four person New Zealand home can contribute up to 20 litres per day"

Understanding Relative Humidity

Air contains varying amounts of moisture; the actual amount of moisture contained in air is referred to as its absolute humidity. More precisely, the absolute humidity is the ratio of the mass of water vapour to the mass of dry air in a given sample of air.The warmer air is the more moisture it can hold. Relative humidity is the ratio of the amount of moisture in the air to the maximum amount of moisture the air can hold at a given temperature. Air is said to be saturated (at 100% relative humidity) when it contains the maximum amount of moisture possible at a specific temperature. Air holding half the maximum amount of moisture at a given temperature has a relative humidity of 50%. High relative humidity near cold surfaces is the single most important factor influencing moisture problems in homes and buildings.

Understanding Condensation

When relative humidity reaches 100%, moisture can condense. The temperature at which air reaches 100% relative humidity is called the dew point temperature. Moisture will condense on a surface whose temperature is below the dew point temperature of the air next to it. So the coldest surface in a room is the place where condensation will probably occur first (called the first condensing surface). Condensation can provide an environment for the growth of mold, mildew, and other biological pathogens. It can also cause deterioration in building materials. The same strategies used to control mold and mildew growth also control condensation on surfaces:

"Increase surface temperatures and reduce moisture levels near surfaces"

Moisture problems related to low surface temperatures may not be eliminated by increasing ventilation, and those related to high ventilation may not be eliminated by increasing surface temperatures. Understanding which factor dominates - low surface temperature or high moisture content will help in choosing the best strategy. For example, consider an old, leaky, poorly insulated home in winter time, which is suffering from condensation, mold and mildew. Since the house is leaky, it has a very high natural air change that dilutes interior airborne moisture levels and therefore maintains a low interior moisture level. Providing mechanical ventilation in this house by installing a fan with out heat will probably not control interior mold and mildew, since the interior moisture levels are already low. A better strategy would be to increase surface temperatures by insulating the house, thereby reducing relative humidity on surfaces. 

House temperatures are dropped when occupants are sleeping and raised to normal levels when awake. However, when temperatures are reduced at night, relative humidity increases, which can cause mold and mildew to grow on cool surfaces. Increasing interior temperatures can often control mold and mildew. Unfortunately, this means increasing energy consumption. An appropriate balance must be achieved between reducing energy consumption and avoiding surface moisture problems.

"Insulating homes lessens the need to maintain high indoor temperatures".

Closed-Off Rooms

Many people close off unused bedrooms or other rooms during heating periods to lower heating bills. Since air and room temperatures are reduced, these rooms can have high levels of relative humidity, leading to mold and mildew growth. Again, the benefits of energy conservation should be weighed against the possibilities of damage from mold and mildew. If rooms are closed off, controlling of interior moisture levels may be necessary.

Exterior Corners

Exterior corners are common locations for mold growth or condensation, for several reasons. Obstructions such as furniture can prevent heat from reaching corner surfaces. Sometimes rearranging furniture to remove airflow obstructions from a corner is all that is needed. Wind typically moves faster at corners, increasing heat loss at corner surfaces. When wind enters corner assemblies and blows through or short-circuits the thermal insulation (wind washing), the interior surface can be cooled significantly. Corners have a greater exterior surface area per unit of interior surface area than other wall surfaces; therefore corner framing construction often results in more wood than insulation.

"Homes with a mechanical ventilation system have less mold and mildew growth than homes with low levels of passive air movement (draughts)".

Thermal Bridges

Thermal bridges are regions of relatively high conducted heat flow in a building envelope. An example of a thermal bridge is the wood stud in an exterior frame wall or ceiling joist. Where insulation is installed between studs or joist, the stud or joist has a greater conductivity to heat flow than the insulation and therefore provides an easy path for heat to escape. The result is a cold spot at the interior face of the wall lining or ceiling lining where it is in contact with the wood. Another example of a thermal bridge is a gap that occurs in insulation due to poor installation practices. Therefore thermal insulation in wall and ceiling cavities increases interior surface temperatures, reducing the likelihood of interior surface mold, mildew, and condensation.

"Typically a 1 cm hole in 1 Sqm of insulation can reduce the R-value by as much as 25%"


Windows are typically the coldest surfaces in a room, and are where moisture is most likely to condense. Condensation may occur either because the interior airborne moisture level is rising, or because the exterior air temperature (and the temperature of the interior surface of the glass) is dropping, so that the relative humidity adjacent to the window rises to 100%.

The more moisture generated in or entering a space, the more moisture will be deposited on the condensing surface. In effect, the temperature of this surface controls the behavior of moisture in the room. To control condensation, window surface temperatures need to be raised by the use of double or triple glazing, and the use of selective-surface (low-e) and inert gas-filled windows. The colder the climate the greater the required thermal resistance of window surfaces. In a sense, the advent of higher-performance windows has led to a greater incidence of moisture problems in the houses. This means higher interior moisture levels without visible surface condensation on the windows. In older houses, the thermally poor window systems limited interior moisture levels by condensing moisture on the windows called the first condensing surface. When condensation occurs, the window is acting as a dehumidifier for the room. The visible condensation alerts the occupant to the need for ventilation to flush out interior moisture. In effect, the windows act as an early warning system to identify excessive moisture.

Surface Condensation

If moisture is generated faster than it can be removed, the relative humidity will rise until condensation occurs. Condensation will occur on a surface when its temperature falls below the saturation temperature (or dew point) of the air adjacent to it. The inside surface temperature of an exterior wall, for example, will depend on the indoor and outdoor air temperatures and on the amount of thermal resistance between the surface and the exterior. The air's ability to hold moisture decreases as its temperature is lowered. Air adjacent to a colder surface loses its capacity to store moisture as it cools, and eventually condensation occurs. Lower humidity levels are usually required in colder weather to reduce this risk.

Mold and Mildew

Mold and mildew (two words for the same thing) are simple plants, of the group known as fungi that grow on the surfaces of objects when the relative humidity is high. Mold discolors surfaces, causes odor problems, and causes deterioration of building materials. Mold can also produce allergic reactions, hypersensitivity, and infectious diseases. Certain fungi found in indoor air produce mycotoxins, which can be carcinogenic (induces cancer), teratogenic (induces birth defects), immunosuppressive (reduces immune system performance), or oxygenic (poisons tissues). Most fungi have microscopic wind-borne spores. These spores are buoyant and can enter homes and buildings as part of natural (wind-and temperature-driven) or controlled (fan-forced) airflow. Although their concentration varies seasonally, mold spores are almost always present in the outside air.

Fungi generally grow when the temperature is between 10deg.C and 38deg.C, with optimum growth occurring between 23C and 35C. However, some types of fungi can grow at temperatures as low as 2C and as high as 49C. Many building materials (wood products, cotton fabrics, wool fabrics, hemp fabrics, organic dust and lint, soaps, oils, paints, adhesives, certain plastics, and vinyl’s) provide nutrients for fungi. Mold needs moisture to produce enzymes and to perform metabolic activities to digest carbohydrates, fats, and proteins. The optimum relative humidity for fungal growth is 70%. Since relative humidity is dependent on both temperature and vapour pressure, control strategies usually focus on either or both of these factors. In heating seasons, mold grows on interior surfaces during the winter. Typically, the interior surfaces of exterior walls are cool (due to heat loss), while moisture levels within the space are high.

Mold growth can be controlled in a number of ways:

  • By preventing the interior surfaces of exterior walls and other building assemblies from becoming too cold.
  • By limiting interior moisture levels.
  • Adding insulation to a wall, floor or ceiling raises the temperature of the surface.
  • Controlled ventilation and control of moisture sources limit interior moisture levels.

When there is excessive ventilation or excessive air change by infiltration and exfiltration during the heating season, uncomfortably low relative humidity can also occur. When relative humidity drops below 20%, membranes in the human respiratory system begin to dry, and defenses against infection begin to fail. At low relative humidity people wearing contact lenses become uncomfortable, and static electricity discharges can affect equipment and people. Relative humidity should be maintained above 25%.The higher the interior relative humidity, the higher the thermal resistance (R-value) necessary to control relative humidity adjacent to interior surfaces. Carpets located on cold surfaces such as concrete slabs are particularly sensitive to dust mite growth. Like mold, dust mites grow at about 70% relative humidity. Carpets on cold surfaces should be avoided.

Testing for Mold

If you can see it or smell it, you have mold. The human nose is far more accurate than all the testing money can buy.

Moisture Removal

Moisture can be removed in three basic ways:

  • By diffusion through the building envelope.
  • By mechanical dehumidification
  • By the replacement of interior air with exterior air.

Moisture removal by diffusion occurs as the moisture migrates through the wall linings in the direction of lower water vapour pressure towards the exterior. Mechanical dehumidifiers remove moisture from room air by blowing it through a series of cooling coils. Water is condensed on the coils where it is collected and removed. The greatest proportion of moisture is removed from houses by the replacement of inside air with outside air. When cold outside air is introduced into a house and heated, its relative humidity is reduced, and it can absorb additional moisture.

"The constant replacement of inside air with outside air carries away moisture, the higher the air replacement rate, the lower the humidity level"

The air exchange routes may be as direct and obvious as vents, flues, or open doors and windows. They may also be indirect, through the fabric of the building: at the junction between foundation and walls, or through the numerous cracks and openings that exist around windows and doors that enclose the heated space. Moist air that escapes directly, without passing through walls or roofs, does not normally create problems. When the exfiltration route does pass through these spaces, however, condensation can occur within them giving rise to a variety of potential problems.

Outside and inside air are exchanged due to the pressure differences between the two,

  • Created by mechanical equipment (such as exhaust fans or heating equipment),
  • Wind pressures,

The buoyancy of warm air creates a pressure difference between outside and inside air in the same way that hot air creates a draft in a chimney. Vents installed to exhaust air from clothes dryers, kitchens and bathrooms, like chimney flues, tend to depressurize houses and induce air leakage inward rather than outward. Mechanical ventilation systems incorporating heat exchangers that bring in fresh air from outside the home will reduce condensation problems while maintaining a constant fresh air environment. This method alleviates the need to have natural ventilation. The concept with all mechanical ventilation systems is to expel indoor stale air and replace it with fresh out side air; the systems described below will have variations on this.


When you take a cold can of drink outside on a warm day, you might notice that it becomes wet on the outside of the can. As warm air loses heat, it begins to lose its ability to retain moisture; the cans colder surface collects water from the warmer air, creating condensation on the can. Dehumidifier’s work exactly the same, as warmer inside air passes over a cold coil in the dehumidifier it releases its moisture into a container, much the same as condensation on windows. Can work in small closed spaces but once the device is subjected to larger open spaces the efficiency drops and becomes much less effective. Not suitable as a long term solution but can be effective for short durations only. Dehumidifiers only remove moisture; they do not remove indoor air pollutants. Dehumidifiers are also expensive to run – they cost $0.14-$0.42 per litre of water removed, this becomes a hidden heating cost.

Methods for Ventilation  and Condensation Control

  • Exhaust Ventilation
  • Forced Ventilation
  • Balanced ventilation
  • Passive Ventilation

Considerations about mechanical ventilation systems:

  • Heat: wasted or recovered
  • Damage: potential for moisture damage seen or unseen
  • Air Quality: outdoor or roof space air

For the purpose of description a fan has two sides

  • Exhaust: the pulling out of air from a polluted indoor environment and taking to outside.
  • Supply: the pushing in of air from outside into the indoor environment.

Exhaust Ventilation

Examples are bathroom fans and kitchen rang hoods. The principal is to take the polluted inside air directly from its source to the outside while the polluted source is being made (spot ventilation) and typically only used when you are showering or cooking. While very good for local pollution control, is not used in New Zealand for whole house ventilation. In some countries where the climate is much warmer exhaust ventilation is used as whole house ventilation to take the inside warm air to the outside. This type of house ventilation should not be used in colder seasons as it would exhaust all your warm air to the outside and replace it with colder outside air.

Forced Ventilation

While advertised as an efficient and capable whole house ventilation system and in some circumstances this has some validation, but for technical purposes is the same as exhaust ventilation but operating in the opposite direction and is continuously on. Some system do slow down or stop the fan when the roof space becomes colder than the house but that is when your mostly likely to experience condensation. The principal is to supply the house with a steady supply of air by forcing it into the house from the roof space. It can control condensation by creating a constant air flow and that is what controls condensation in principal, a steady and controlled air flow. These are factors to consider before deciding on forced air as a whole house ventilation system.

1 Heat Wastage: The claim is they are supplying dry air from the roof space to the house, it then needs at some point to leave the house so that more air can be supplied. It does this by escaping through natural places that are built into the building fabric the same way as draughts enter your home. If you heat any space in your home then the forced air will carry the heated air to the outside and replace it with more roof space air. The claim of using warmer roof space air to ventilate the house with could be valid if it was always warm, which of course its not especially in winter and at night, You will now need to add more heat to make up for what you have wasted. If you can do the mathematics you will probably have to spend considerably more on extra heating than what the ventilation system cost to run, and over time it could cost you more than the initial investment. These types of ventilation systems may be offered with the sale of a 2k/W heating element to boost the temperature delivered into the home when it has very cold supply air. They do say however that it will not provide any additional heating but may take the chill of the cold air. From anecdotal evidence it does not achieve this and cost a fortune to run. 2k/W running at 20 cents per k/w will cost $9.60 per day or $288.00 per month and achieve very little.

2 Potential Damage: As our newer homes are becoming more airtight, forced air systems are struggling to operate because of the lack of natural air leakage passages so the air will try to escape through the building fabric. As warmer, higher moisture holding air passes through your internal wall linings, light switches and power points it comes in contact with a colder wall cavity especially if your walls are uninsulated. The warmer air then cools and releases its moister content into the wall cavity if due point temperature is reached. This has the potential to cause decay and rot to the structure itself. A recent Beacon report IEQ7570/3 2010 ( which tested 10 of these systems (Christchurch and Wellington) highlighted some alarming facts.

(Extract: Using humidity ratio measurements, monitoring of the roof space air and the air delivered into the house shows that in winter during the day there is more moisture in the roof space than in the house. However, the system is turned on by a thermostat based on roof space temperature which will increase with solar heating over the day.  This means that the increased operation of the ventilation system during the day actually increases the moisture in the house during the day. Equally, operating the ventilation system at night takes drier air from the roof space, thereby reducing the level of moisture in the house. However the roof space temperatures are colder at night and will cool down the house as well

This backs up there evidence that the delivery ducts was often at 100% saturation, which means these types of systems actually introduce moisture. Another highlight was the coldest room in the houses often had extremely high moisture contents in the wall lining, above the point where mold can occur. In the USA, forced air systems do not comply with the National Fire Code, as potential roof space fire smoke will be forced into the home.

3 Air quality: New Zealand’s building code NZS 4303:1990 calls for outside air to be used for house ventilation, not roof space air. The code does not distinguish between opening windows, doors or mechanical devices it must come from only outside. The air used to ventilate with forced air systems uses roof space air that has been brought into the roof space from outside. When the air enters the roof space it comes in contact with numerous numbers of contaminants and pollutants. The pollutants can be as small as viruses through to bacteria, pollens, dust and fungi. If a filter is specified with these systems then you need to make sure what range of filtering it is catering for and what is the level of efficiency. Polluted odours and gasses from dead rodents or birds in the roof space can not be filtered against with these systems. A filter of coarse needs to be replaced on a regular basis and that is one of the main problems with these types of systems. If not changed then you are in danger of increasing the amount of pollutants into the home. When any filter begins to clog up, the air pressure around the filter will naturally increase and force even smaller contaminants and pollutants through. Dry air does not necessarily mean fresh air so be careful when this term is used in marketing.

Balanced Ventilation

1 Heat Recovery: Like the previous two system principals that have one or more fans operating in one direction a balanced ventilation system has two fans operating in both directions. What the term balanced means is that air is supplied and exhausted at the same rate. Every room in the house has ideally a supply or exhaust diffuser mostly installed at the ceiling level. All living spaces have one or more supply vents, while all high humidity spaces have one or more exhaust vents. There is not a separate fan in each space but is ducted from two larger fans in the unit itself. The first fan brings in only fresh outside air through ducting that is connected to an outside vent and redistributes it via a ducting network to all living spaces (lounges, dining rooms, bedrooms).The second fan pulls the delivered supply air from the rooms to the exhaust vents (kitchens, bathrooms) and passes the air back to out side. It’s a bit like leaving your front and back door open permanently. Like the other two systems, condensation is controlled with constant air movement and replacement. As with forced air system air is expelled to the outside, but with the balanced system the outgoing air passes through a heat exchanger before leaving.

How this works is a heat exchanger is a device that passes warm air on one side of a material to the colder other side of the material. The heat exchange transfers heat from the outgoing air to the incoming air in the same way that a radiator in your car transfers heat from your engine to the colder air from outside passing through the radiator. The two air streams don’t physically mix together. The major benefit is that most of the outgoing air that contains your valuable heat is passed through to pre heat the incoming colder air from outside. The difference is forced air = wasted heated, balanced air = recovered heat.

The energy efficiency aspect of balanced air compared to forced air is considerably higher, and can have a negative operating cost during the heating season. Unlike forced air systems balanced ventilation systems work at ther highest efficiency in the winter months. The principle of heat exchangers is there efficiency increases when both sides of the heat transferring conducting surfaces are at there greatest temperature difference. 

2 Potential Damage: The likely hood of damage is eliminated because no moisture holding air escapes through the building fabric.

3 Air Quality: Balanced ventilation systems source its air from outside so they comply with NZS 4303 requiring fresh out door air for acceptable ventilation. There is no requirement to filter out door air so no high level of filtering, nor replacement cost. However out door air filtering can be added as an option if your local area is susceptible to various sources of pollution that occupants may be effected by. Examples are pollen and fire smoke which need there own special filters, once the source has been indentified then the appropriate filter can be installed. However most balanced air system don’t require filtering. As our newer homes are becoming more airtight, forced air systems are struggling to operate because of the lack of natural air leakage passages, while balanced air systems are becoming more necessary to maintain indoor air quality.

Passive Ventilation

This is the purposely designed openings (controlled or uncontrolled) into the building at build time. Designed to introduce air flow from outside into rooms or the whole building to stimulate air movement to assist heating or cooling and the replacement of stale air. Although very practical and becoming more understood it requires careful analyses by the designer of air movement, air pressure, placement and quantity. Used a lot out side of New Zealand but is not mentioned in our building code as a standard practice. Could be achieved in an existing home but the lack of quality and purposeful hardware can make it difficult.  

Condensation Control with Heating

The useful property of applying heat to a room or environment is primarily to heat the space and make you feel warm; the now warm environment can be used to control condensation.

This is how it woks:

When you buy a new kitchen sponge it is very dry and hard but when you allow it to soak up water for the fist time it will take some time until it becomes totally saturated. If you can imagine air being much the same it will take on moisture until it is totally saturated, at which point it is released as condensation. So think of air as a sponge which will soak up moisture from the environment depending on the temperature. The warmer the air then the more it can soak up moisture. If you open up a window or two preferable at opposite positions in a room or environment just a little bit that you are heating, then some of the warm air will be drawn to the outside. Now you are probably saying I don’t want to do that because I will loose my valuable heat, you are correct but not as much as you think.

As the air is moving from one opening to the other the warmer air inside the room acts like a sponge and soaks up moisture that is then taken outside, by the laws of physics new fresh outside air is introduced at the same rate. The new introduced air from outside will be drier and I hear you saying I don’t want to spend more money heating cold air.

This is where the science trick comes in, as air from outside is colder its ability to hold moisture is lower so it is more easier to heat,Colder drier air is more efficient to heat than warmer moist air.”So just like mechanical ventilation is a good tool for reducing condensation so is passive ventilation in conjunction with heat. Remember it’s the regular air change that is the main tool for condensation controll.


Do you have problems with condensation, mould or mildew growth?

Or would you like to know how you can ventilate your home with fresh outdoor air.

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