Understanding Humidity in the Home

Understanding Humidity in the Home

Balanced humidity is a key ingredient for creating a healthy and comfortable environment within the walls of your home. In general, humidity is defined as the amount of water vapor in the air. It is most often referred to as a measurement of ‘relative humidity’.

It is important to understand the concept of relative humidity as it can directly correlate with allergy problems, house and building damage, the presence of pests and your overall comfort level.

Indoor humidity that is either too high or too low can cause problems for your health and your home. When the indoor relative humidity is below 30% the air is extremely dry, this increases the likelihood of the spread of cold and flu viruses.

Air that is too dry can also irritate and inflame nose and respiratory passages, dry out skin, and wreak havoc on the foundation of your home. It can cause walls and ceilings to crack, wood floors to separate, and door and window frames to shrink.

When the relative humidity is above 50% the air becomes damp and moist. Airborne allergens such as mould spores, dust mites, and bacteia can thrive and multiply quickly, aggrivating the symptoms of allergy-sufferers.

Excessively humid environments are also desirable breeding grounds for cockroaches, termites, and other annoying pests. Excessively high humidity can spawn musty odours, accelerating wood rot and decay, stain ceilings and walls, and cause paint to flake and peel.

In mathematical terms, relative humidity is the gram-per-cubic-meter measure of the water vapor in the air divided by the gram-per-cubic-meter measure of the maximum amount of moisture the air can hold at the current temperature (also known as saturation, or 100% relative humidity).

A relative humidity of 100% means that the air is completely saturated. This is known as dew point.

The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface.

Digital humidity gauges called hygrometers can quickly give you an accurate reading of the current humidity level in your home.

To avoid the air quality and comfort problems inherent with extremely low or high relative humidity, ideally the relative humidity in a home should fall between 40 and 50%. This means that the surrounding air is holding only half of the maximum amount of moisture it can retain.

At this level, dust mites cannot survive, mould spores cannot reproduce, and household pests, which prefer to live in highly humid environments, are driven away. The air is moist enough so as not to irritate your skin, throat and breathing passages… as well as weaken the physical foundation of your home. This moisture level also lessens the risk of foundation problems related to high humidity.

Both dehumidifiers and humidifiers can help you monitor and maintain an optimal level of indoor humidity year-round. They allow you to easily maintain the right level of relative humidity in your home. Models equipped with built-in humidistats that can be controlled digitally and/or manually help you quickly adjust the relative humidity level to suit your needs.

Dehumidifiers absorb excess moisture in overly humid air.

Humidifiers inject moisture into very dry air.

Maintaining the right relative humidity can make all the difference in curbing symptoms in an allergy-prone household and in keeping critters at bay. This simple practice will also help to ensure the quality of your home’s foundation while making your family feel healthy and comfortable indoors.

Visible mould in the home is a problem, however most of the toxic mould floating around our air is invisible to the naked eye. Thermography inspection can be used to identify problems in the home. 

Thermal image courtesy of Intec Thermographics:

Intec Thermographics provide home inspection surveys and reports for domestic and commercial properties.

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What Causes Mould in the home?

What Causes Mould in the home?

Damp house have been proved beyond any doubt to cause respiratory infections, allergies and asthma.

Visible mould in the home is a problem, however most of the toxic mould floating around our air is invisible to the naked eye.

When we understand what causes it to occur, we can work out how to prevent it.

To begin, lets understand humidity and water vapour in relation to the home.

Water vapour is simply water that is ‘diffused’ or ‘suspended’ in the air. Humidity is ‘the amount’ of water vapour in the air.

Mould LOVES humidity.

Older houses may have changed A LOT since construction. They may not be as well ventilated as they used to be.

For example. the original fireplace may be bricked up now, or an electric fire may be installed in it’s place. Double glazed and Upvc windows and doors may have replaced old drafty wooden ones. Coal fires were used to heat water and keep the house warm. The fire would burn all the old breathed air, along with any water vapour and suck it up the chimney. Fresh air would replace it, being sucked in through the original poorly fitted and drafty windows and doors.

Below: Condensation on a window means the house is humid, there is a high level of water vapour in the air.


Lets also consider the social change we have seen in recent decades.

In the old days when water had to be heated with coal fires, it was a chore. People had less baths and often shared the water, The amount of water vapour being produced via bathing was low. By the same token laundry day was a similar trial. Most people dried their clothes outside because there were no tumble dryers or radiators to dry them indoors. The amount of water vapour produced from drying clothes was massive, but it was not produced in the home. Even when it was, it went up the chimney. Nowadays people shower every day and never wear the same clothes twice without them being washed. Hot water is available instantly; the kettle, the shower, the steamer, fryer etc…

These two revolutionary changes in housing and living standards combine to result in dwellings with very high humidity. This creates a tendency towards condensation and mould growth. There are of course other issues but those mentioned are the most important.

No ventilation + much more water vapour = Mould City.

Below: Visible mould around a window frame, additional thermal image (via Intec Analysis)

What can we do?

Understand that excessive humidity is the cause of mould growth. Then identify how we humidity can be lowered.

Kitchen, ensure the fan/cooker hood is working correctly and use it frequently. Fans extract air (and the water in it). When working in the kitchen make sure the door is closed and the fan/cooker hood is on to remove the water vapour and prevent it travelling through the home.

Bathroom, when showering or taking a bath, keep the door closed and use the extractor fan. If there is not one installed, open the window to get that water vapour out of the house. A hot shower or bath will produce trillions of water molecules. These are warm (energetic) so they skip from liquid state into air and form water vapour. Extractor fans in bathrooms need to stay on longer to shift huge amounts of water vapour. Consider installing an automated one.

Windows, check the trickle vents, a small horizontal vent at the top of each frame. These should always be open as they provide a route in for ‘make up’ air. When the extractor fans are working around the house these small vents are handy for pulling in fresh, dry air to replace the old air being pulled out by the fans. A lack of vents or having them closed all the time will encourage drafts at lower levels (where you will feel it). This will reduce the efficiency of the extraction fan. Open them all.

Radiators, do not use all the radiators around the house to dry clothes. Wet clothes feel heavy because they are wet. When they are on a radiator the water evaporates into the air then travels to the coldest spot in a room and condensate back to water causing unnecessary problems. If you must dry clothes on radiators use the kitchen or bathroom and use the extractor fans.

Below: Invsible mould around a window frame, additional thermal image (via Intec Analysis)

A cold house is a damp house!

Mould is almost exclusively a problem of the internal environment. Heating, or more precisely lack of heating raises humidity because the ability of air to comfortably hold water vapour is entirely dependent on heat. Warm air can hold a lot more water vapour than cold air. In effect if we cool the air, then we raise its humidity relative to its temperature. Mould will thrive in a house with a high relative humidity.

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Residential Heating Systems

Residential Heating Systems

Thermal Imaging cameras provide us with an image of temperature distribution on a surface or object. This is hugely useful when installing or maintaining heating systems, from a commercial to a domestic level.

You can pinpoint any irregularities in temperature distribution very quickly. Underfloor pipes or pipes behind walls can be located and leakages identified. Heating system performance can be assessed and radiator inefficiencies can be identified, among other things. 

Amazingly, quite a lot of information can be gathered from the thermal condition of a heating system.

Below are some examples (Images from Intec Analysis Ltd. FLIR T450sc)


Radiator with trapped air.

Because air bubbles in the gas central heating system rise, you’ll find air collects at the top of radiators and steadily gathers, making your home colder as the problem worsens. So if a radiator isn’t heating up, or is cool at the top and hot at the bottom, you’ll need to learn how to bleed it.






Radiator with sludge build up.

Sludge is essentially dirty water. Particles of dirt from the water in the system combine with rust (iron oxide) from the inside of radiators, pipes and soldered joints. The older the system is, the more likely it is to have some heating sludge build up. Dirt and rust combine to make a gooey liquid. As this passes through the central heating components it deposits some of the particles. It can block pipes and radiators to a point where the heat is patchy on radiators, radiators don’t work properly or get to temperature, or they don’t heat up at all. It can physically block important components such as the boilers heat exchanger which is incredibly expensive to fix.





Thermal Imaging inspections are an important diagnostic tool to prevent leaks from heating systems becoming serious enough to cause significant damage to a property.

Below: A leak identified from a residential home (Images from Intec Analysis Ltd. FLIR T450sc). 


This was identified in a residential living room ceiling. The yellow square from the thermal image  represents the temperature profile from a water heater located in the cupboard upstairs. The dark area on the thermal image was wet. This was caused by condensation dripping from adjacent pipework on the water heater. Over time it had soaked part of the upstairs cupboard floor and had made its way through the living room ceiling.

Identified with a thermal camera and confirmed with a moisture meter. Note that no visible damage can be seen from the digital image. This was caught just in time and costly repair work to the roof was avoided.

For more information on home thermography inspection including related blog posts please click here.

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