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Understanding Maximum Humidity Volume Under the New Indoor Air Quality Standard

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Understanding Maximum Humidity Volume Under the New Indoor Air Quality Standard

By Ruth MacEachern

Product Manager

Jul 12, 2023

In recent articles we have looked at the importance of indoor air quality in domestic dwellings, UK legislation on air quality in residential buildings, and air flow requirements in British homes. In so doing, we have addressed the targets outlined in the government’s Future Homes and Building Standard, and perhaps most pertinently, amendments to Approved Document F. In this next edition we will turn our attention to relative air humidity in residential properties. In doing so, we will ask: what is relative air humidity?, why is it important?, and to what extent is it regulated under new laws and directives? Finally, we will take a brief look at potential means by which to ensure compliance with humidity requirements.

What is indoor relative air humidity and why is it important?

In simple terms, air humidity is a measure of the quantity of water vapour in the air. More specifically, relative humidity (RH) is the amount of water vapour present in air expressed as a percentage (%RH) of the amount needed to achieve saturation at the same temperature. Relative humidity is therefore closely linked to room temperature and is highly sensitive to changes in room temperature. Consequently, with ventilation and air flow responsible for replacing stale air with a fresh supply of clean air, relative humidity is also inextricably linked to air quality. 

Furthermore, many of the allergens and pollutants  that we seek to eradicate through effective ventilation enjoy hot and humid atmospheres – mould being the example that most readily springs to mind. As such, the ability to measure and control relative air humidity in a home is crucial to ensuring the comfort and well-being of residents. Government guidance for inspectors investigating “Sick Building Syndrome” also list low humidity among factors that may be responsible for the phenomenon. Symptoms of Sick Building Syndrome include eye, nose, and throat irritation, headaches, rashes and itching, and respiratory conditions.

If this is not reason enough to concern yourself with humidity levels in your new build properties, then this final reason should provide the necessary motivation: it is a legal obligation.

What are the regulations concerning relative air humidity in dwellings?

 Approved Document F makes the following statements with regard to moisture in residential dwellings:

The performance criterion for moisture is that there should be no visible mould on the inner surfaces of the external walls of a properly heated dwelling with typical moisture generation

Mould can grow whether the building is occupied or unoccupied, so the performance criterion for humidity should be met at all times, regardless of whether there are occupants

It is also stated that achieving a good indoor air quality includes reducing the release of water vapour and air pollutants, although guidance is not provided on such strategies. What the document does provide is a table providing percentage relative indoor air humidity values for various moving average periods, above which it is unlikely that moisture criteria will be met:

Table B3 Indoor air relative humidity

Moving average period

Indoor air relative humidity%

1 month


1 week


1 day


Possible methods by which to ensure compliance with humidity requirements

There are a number of ways to ensure compliance with humidity requirements. These naturally include the installation of suitable trickle vents in high-moisture rooms such as kitchens and bathrooms and the installation of an extractor hood above a hob. However, perhaps the most effective solution is the installation of a whole house mechanical ventilation system. A Mechanical Ventilation with Heat Recovery system removes stale air and replaces it with a supply of fresh air. What’s more, the systems are particularly energy efficient, as heat is removed from the extracted air and returned to the fresh air supply, thereby reducing heating costs and allowing occupants to remain in close control of the air temperature and the relative air humidity within their homes. 

If you require advice or further information on how to measure relative air humidity in your residential projects or are unsure as to which ventilation systems would be best suited to your specific property, the experts at Envirovent would love to hear from you and offer their assistance. 

Alternatively, if you have left it too late and are already experiencing mould problems in your building, our article on mould removal may prove a valuable source of advice