The Importance of Ventilating Floor Voids by Daniel Lewis
Historical Purpose of Suspended Timber Floor Voids
Historically, suspended timber floor voids were designed to facilitate the evaporation and dissipation of ground moisture. These voids also accommodated uneven ground and ground heave, with fresh air from below aiding combustion in fireplaces within living spaces. Additionally, the unobstructed airflow below ground level (or below the rudimentary Damp Proof Course) helped with keeping the external walls dry and reduced the risk of spalling brickwork.
Suspended timber floors were the prevalent construction method until the mid-20th century. These floors typically required a ventilation gap under the joists and adequate brick vents for continuous cross-ventilation. When Building Regulations were introduced in the 1960s, several legal requirements were established, such as a 150mm continuous ventilation gap and the concrete oversite to reduce moisture ingress from the ground.
Moisture Ingress and Humidity Issues
Significant moisture ingress can occur from floor voids, evaporating from the ground below and leading to elevated levels of relative humidity. High humidity raises the moisture content of timber joists located within the void, potentially leading to damp, mould, and rot, resulting in structural failure if not properly ventilated. Timber exposed to air with 85% relative humidity will reach a moisture content above 20%, making it more susceptible to mould, rot, and infestation.
Air infiltration from the floor void can create health hazards indoors by transferring humid air, fungal spores, microbes, and bacteria into living spaces. During winter, the stack effect within homes (warmer air escaping at the top and drawing make-up air in through the void) can intensify air ingress, increasing humidity in the house and allowing contaminants from the floor void into the living space.
Ventilation and Building Regulations
Appropriate ventilation of the floor void, meeting Building Regulations air change requirements, can minimise some of these risks. However, it will not eliminate all risks as living spaces will still be exposed to the void. Many suspended timber floors suffer from high humidity even with adequate ventilation. Rising water tables in the UK and global warming, which increases rainfall, contribute to water ingress into floor voids, turning them into breeding grounds for dampness, bacteria, mould, and rot.
Recommended Humidity Levels for Floor Voids
To avoid such issues, the median relative humidity (RH) in a floor void should be maintained below 70%, and the wood moisture content (WMC) should be kept below 20%. This is not achievable without correct ventilation.
Why Does Floor Void Ventilation Matter?
Humidity Control: Without ventilation, humidity will rise until the void becomes saturated. Some unventilated floor voids reach relative humidity levels of 90%-100%, which, over time, will result in timber floor rot.
Optimal RH Levels: Adequately ventilated and well-maintained floor voids should remain below 70% RH, minimising the risk of damp, mould, and rot.
Variable Conditions: Conditions beneath houses vary widely depending on water table height, ground condition, exposure, season, external temperature, humidity, and other factors.
Ventilation Solutions: High moisture content problems can be alleviated by adding more vent openings to increase the ventilation rate.
Regulations and Compliance
Building Regulations
Approved Document Part C (5):
- Ventilation openings are required on two opposing external walls.
- Openings should be at least 1,500 mm² per metre run of external wall or 500 mm² per square metre of floor area, whichever is greater.
- Ventilation openings should not resist airflow at any point.
BS 6891 (6): Ventilation requirements for pipework in ducts or voids typically require a minimum Effective Open Area (EOA) ventilation of 0.05m².
BS 5250:2021 (7): Subfloor ventilation should be provided by vents not less than 1,500 mm² per metre run of external wall or 500 mm² per square metre of floor area, whichever is greater.
Enhancing Floor Void Ventilation for a Healthier Home
A comprehensive floor void survey can establish the presence of standing water, timber rot, dampness, leaks, and the moisture content of subfloor timbers, as well as relative humidity (RH) levels. If the floor ventilation is deemed insufficient and a contributing factor to the above issues, you can use an approved air brick requirement calculator to assess and upgrade air bricks effectively, thus improving the ventilation of your floor void.
A standard 215mm x 65mm clay air brick typically allows 1,290mm² of free airflow. Upgrading to an improved plastic air brick can increase this to 7,750mm², providing six times the original ventilation capacity. This significant enhancement can improve the condition of your floor void and create a healthier overall home environment.
Relevant Research and Industry Standards
Pelsmakers (1):
Sealed airbricks over three years resulted in floor void conditions averaging 93% RH and 15ºC in one property, and 90% RH and 15ºC in another, indicating serious concerns.
BRE, EST, Rickaby (2,3,4):
Obstructing or reducing floor void ventilation leads to moisture buildup, increasing the risk of mould growth and timber decay.
References:
- S. Pelsmakers, B. Croxford & C.A. Elwell. Suspended timber ground floors: measured heat loss compared with models.
- Penasco, C., Anadon, L. D. Assessing the effectiveness of energy efficiency measures in the residential sector gas consumption through dynamic treatment effects: Evidence from England and Wales.
- BRE 2000. Good Practice Guide 294, Refurbishment guidance for solid-walled houses - ground floors.
- EST 2006. CE184 - practical refurbishment of solid-walled houses. London: EST.
- Approved Document C: site preparation and resistance to contaminants and moisture.
- BS 6891:2015+A1:2019 Specification for the installation and maintenance of low-pressure gas installation pipework of up to 35 mm (R114) on premises.
- BS 5250:2021, Management of moisture in buildings. Code of practice.