Building Air Pressure Testing and MVHR Fitting

Building Air Pressure Testing

Building Air Pressure Testing.
New houses need building air pressure testing to comply with part L of the Building Regulations to provide a measure of the permeability of the building. There is a standard to be met and all testers have to be registered with ATTMA  or BSRIA and all results logged with them. At present there is no legal requirement to test old houses that are being renovated. If, however, you are renovating an old house it will be in your interest to achieve a certain level of air tightness and so testing may be a sensible idea.
Air leakage is caused by either wind pressure i.e. wind blowing on the building forcing air into the building and sucking it out on the down-wind side or the “stack effect” which is when the heat in a building causes hot air to rise it is forced out at the top of the building whilst fresh air is drawn into the building below to replace it. (A big aside here: the stack effect is why tall buildings have revolving doors, the stack effect would either suck normal doors open if they opened inwards or make them hard to open if opening outwards.)
If you are trying for an energy-efficient house, the lower the air leakage the better. Here is another meaningless statistic that everyone quotes, “up to 40% of heat loss is due to poor air tightness”.  It is fairly obvious that draughty houses are colder; it is just that we do not realise quite how leaky houses are. Draughts from leaky doors, windows, loft hatches, letterboxes and cat flaps are only the obvious ones; hidden holes also leak air, for example: where the waste pipe goes through the kitchen wall, where you floor joists are mounted into the walls, where there is a gap under the skirting leaking air from the basement below. Obviously if a house is “too” air tight you will have other problems; the house will be stuffy, you will get condensation and mould growing, gas cookers will have insufficient oxygen to burn properly etc. So permanent mechanical ventilation is needed and required by building regulations, hence MVHR systems.
We offer a “check before your test” service; we will come and de-pressurise your house and help you find leaks so you can seal them. We recommend that you do this before plasterboarding or covering up your “air tightness line”. It is very difficult to find leaks once plasterboard is in place and even more difficult to seal. Common leakage points we pick up during building air pressure testing are: drain penetrations, leaking loft hatches, leaking windows and door frame mountings, joist penetrations, around the eaves, around suspended ground floor skirting. The modern practice of “dot and dab” plaster-boarding makes finding these leaks very difficult.
Finding and sealing leaks retrospectively is difficult if not darn right impossible so “Build tight – ventilate right” is the mantra. The devil is in the detail; so really think about the corners, where your penetrations (drains, services, wires etc.) of the air tightness line are going to be early in the design so you can plan and build accordingly.
When chasing lower and lower air tightness it gets harder and harder.
 An air-tightness fan in place during building air pressure testing

Airtightness fan in place during building air pressure testing

Though often referred to as the “Air-tightness factor or ratio”, it is not really a factor or a ratio – call me a pedant but a factor or ratio has no units.
The measure used for building air pressure testing is the rate air is pumped out of the building to maintain a pressure differential of 50 Pascals divided by the surface area of the house. So units are: cubic metres per hour per square metre at 50 pascals differential! Not surprisingly the units aren’t often quoted and we call (incorrectly) the result a ratio or factor!
The 50 Pa differential is not that large – atmospheric pressure is around 1000 Pa.

How to understand building regulations air pressure testing requirements.

  1. Surface area of the heated building envelope is used for UK Building Regulations.
  2. The current UK Building Regulations Part L require the air tightness to be no greater than 10 m3/hr per m2
  3. If the factor is above 7 m3/hr per mthen other energy-saving measures must be taken to compensate.
  4. If the factor is below 7 m3/hr per mthen the house passes UK building regulations subject to insulation levels etc. being up to required building regulations.
  5. If the factor falls below 5 m3/hr per m then permanent mechanical ventilation will be required. (See MVHR systems.)
To confuse things there are two measures of building permeability of a building, luckily both require the building pressure to have a 50 Pa differential with the outside.
The volume of air pumped (in or out of the building ) per hour to maintain this differential is then compared to the surface area of the building. This is the measure for UK Building Regulations.
The Passivhaus standard works by comparing the volume of air pumped to the “volume of the lived in space” – not the surface area as used by Building Control. “Lived in space”  means the volume of the house inside the insulation layer but minus the volumes of all the walls and floors. (I’m not sure why these volumes are excluded, but there must be a reason! I have asked the Passivhaus trust but have not had a satisfactory answer).
In effect we are measuring the number of Air Changes per Hour at a 50 pascal differential. So the unit will be ACH @ 50Pa. or m3/hr per m3 @50Pa. (To achieve the Passivhaus standard the house must achieve 0.6 ACH or less – a very hard target). Passivhaus standard argues that the rate the air changes in a house will be a better measure for the energy saving that will be achieved. The ACH is also what decides how efficient a MVHR unit will be.
I am an ATTMA certified Passivhaus tester.