DrillVent – Design Study on timber frame cavity ventilation

Regulation – Guidance notes – Good practice

Northern Ireland – Regulation
The cavity shall be ventilated by means of open perpend joints at the top and bottom of the wall. There shall be at least one open perpend joint for every 1.2 m length of wall.

England and Wales – Regulation
Any framed external wall will meet the requirement if the cladding is separated from the insulation or sheathing by a vented and drained cavity with a membrane that is vapour open, but resists the passage of liquid water, on the inside of the cavity.

Scotland – Regulation
Ventilation of external wall cavities is necessary to prevent the build-up of excessive moisture that could damage the fabric of a building. Ventilation holes can also be used to drain excess water from the cavity that has entered through the outer leaf. Timber frame Interstitial condensation is one of the major problems that need to be addressed in timber framed buildings. Cavities should be vented to the outside air by installing ventilators with at least 300² mm free opening area at 1.2 m maximum centres. A ventilated cavity should be provided for extra protection in severely exposed areas.

Ireland – Regulation
A clear, drained and ventilated cavity of not less than 50mm is a requirement under the Irish building regulations TGD part C 1997 : Site Preparation and Resistance to Moisture. R.3.2. The ventilated and drained cavity is an essential feature for timber frame construction in Ireland – Drainage and ventilation channels (ventilated cavity) are required to all areas that are compartmentalised by fire stops.

NHBC – National House-Building Council
Cavity ventilation is required to control the moisture content of the timber structure. Cavities should be ventilated as described in NHBC Standards clause 6.2 – D4(c):

BR262 – Building Research Establishment – Thermal Insulation Avoiding Risks
3.20(f) Ensure that a clear cavity is maintained behind the cladding and that it is drained and ventilated.

TRADA – The Timber Research and Development Association
Timber Framed Construction 4 – Walls, Identifies the cavity as ventilated Open perpends to ventilate and drain cavity, spaced at max 1.5 m centres. All timber cladding should be designed as a ‘rain-screen’, with a ventilated, drained cavity behind.

Homebond – Provider of structural defect cover – Ireland
External Walls – Cavity ventilation. The cavity in timber frame construction must be ventilated to dissipate any moisture vapor that may collect. Ventilation is generally provided by proprietary ventilators, Fitted in the prepends (vertical joints) in the masonry outer leaf, at 1500mm centres.

Straub,J.F and Burnett,E.F.P. – Research Cavity Ventilation
“Both theory and testing have confirmed the importance of sufficient venting. The four vent inserts tested in our program all restricted airflow to a very high degree; in fact they negated most of the benefits of venting.”

Building Standards – Current Practice and Procedures – Ireland
3.10.5.4 Our research has determined that countries (i.e. Canada, New Zealand and USA) with far more technical and cultural appreciation of timber frame than exists currently in Ireland have suffered wide spread problems in timber frame buildings and that a major contributing factor to these failures was the lack of a drained and ventilated cavity and cladding failures

Terms – matters of specific interest:
  • Equivalent area is a better measure of the airflow performance of a ventilator. Equivalent area is defined as ‘the area of a sharp-edged orifice through which air would pass at the same volume flow rate, under an identical applied pressure difference, as the product or device being tested’.
  • Free area is simply the physical size of the aperture of the ventilator, but may not accurately reflect the airflow performance which the ventilator will achieve. The more complicated the airflow passages in a ventilator, the less air will flow through it. So two different ventilators with the same free area will not necessarily have the same airflow performance. The free area of a vent is typically 25% greater than the equivalent area.
  • Approved Document F, England and Wales 2006 uses equivalent area instead of free area for the use of ventilators. Must conform to EN13141-1:2004.
  • EN13141-1:2004 Ventilation for buildings – Performance testing of components/products for residential ventilation – Part 1: Externally and internally mounted air transfer devices. (The European Standard EN 13141-1:2004 has the status of a British Standard.)
  • Externally mounted air transfer device; device designed to allow the passage of air through the building envelope with the minimum ingress of rain, snow, foreign bodies, etc.
  • Ventilation “is primarily driven by a combination of wind pressure and thermal buoyancy. The provision of vent openings at the top and bottom of the wall will generally allow the most ventilation by these mechanisms.” Wind pressure at the base of a wall is expected to be in the order of 1Pa. Wind pressure will generally be greater at higher levels and on a standard 2 story house, the wind pressure would be expected to be in the order of 4Pa. It is noted that wind pressure is effected by elements such as wind direction and velocity.
  • Ventilation rates: It is conventional to express ventilation rates as Air changes per hour (Ach) and research has shown that the value needed to make a difference in a timber framed cavity wall would be expected to be in the order of 10 Ach. (similar to a ventilated loft space).
  • Natural ventilation is mainly driven by two mechanisms; the stack effect, and wind pressure. The stack effect arises from the decrease in the density of air as its temperature increases. If a boundary separates these two masses of air, a pressure difference will exist across the boundary. At either ends of the boundary, or at openings in the boundary, air will flow across the boundary from high to low pressure regions. Normally, this provides a flow of air into the lower part of the cavity and out of the higher part of the cavity. The ventilation rate achieved through the stack effect increases with the height of the boundary, or stack of air, as the overall pressure difference will be greater.
    Wind pressure influences the ventilation of a building by creating variations in pressure around the outside of the building. Variations in pressure are highly dependent on the building form and the wind speed and direction.
  • Open perpend or open weep hole is considered to be an unobstructed opening with a free area of 650mm². (10mm x 65mm x 90) approximate air flow – 2.13m³/hr – applied pressure difference 1 Pascal Ref. – Straub,J.F and Burnett,E.F.P.
  • Vented air space is defined in BS5250:2002: cavity or void that has openings to the outside air placed so as to allow some limited, but not necessarily through, movement of air.
  • Ventilated air space; cavity or void that has openings to the outside air placed so as to promote through movement of air.

(NB. The information herein contained is intended as a guide only. Regulations/industry approve standards change from time to time and differ from area to area. Please check with your local building control or guarantor for compliance.)