Building Air Pressure Testing and MVHR Fitting

Building Air Pressure Testing

Building Air Pressure Testing.
Though we still do Air Permeability Testing we only do as an aid to people building energy efficient houses. We no longer test for Part L of the Building regulations. The annual calibration and ATTMA (The trade body) membership costs are now around £850.00 and we do not do enough testing to justify this cost. For people building to Passivhaus standard I have done the Passivhaus training and can come and do the tests you will want to do as you build your house to find and fix air leaks. I just can’t do the final test when the house is finished.
New houses need to pass a Air Permeability Test to comply with part L of the Building Regulations to provide a measure of the permeability of the building.
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.)
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 and “up to 40%” means 1% of the heat could be.. you get my point. 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. Permanent mechanical ventilation is needed (and required by building regulations) if the house falls below 5.0m3/hr per m2 . If you accidentally fall below  5.0m3/hr per m2  you might consider fitting a Passive Input Ventilation system rather than drilling random holes in you walls!   This sucks air out of your attic (usually) and blows it into your house where it seeps out through the holes in the house.
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 (especially behind bath panels), leaking loft hatches, leaking windows and door frames, joist penetrations, around the eaves, around suspended ground floor skirting.

When you start drawing up your plans and elevations you need a continuous surface line around the whole house marking the ‘air tightness line’. Mark this line on all drawings in RED! Through this line any joist, door, window, pipe, cable, duct, vent, overflow or expansion pipe that has to pass through causes a penetration and has to be sealed. You will need special traps for condensate and expansion pipes for your boiler, hot water tank etc. Your windows and doors will need airtight seals.

Air tighness taping is essential for successful installation of an MVHR

Air tightness is essential for successful installation of an MVHR. Here OSB board is being taped at its joints to form and air-tightness plane

Finding and sealing leaks retrospectively is difficult if not darn right impossible so “Build tight – ventilate right” is the mantra. The modern practice of “dot and dab” plaster-boarding makes finding these leaks very difficult. 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

How to understand building regulations air pressure testing requirements.

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 permeability 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. 
  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. The SAP design will call for a certain standard to have been met – usually the SAP assessor uses 5 as a default setting.
  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 but it discriminates against smaller houses. The larger the volume of the house the larger the divisor so the easier to get a lower result).
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 was an ATTMA certified Passivhaus tester, but the volume of testing I did did not warrant the annual calibration and membership fees (In excess of £850) and so I de-registered. If you need a registered tested for building regulations see ATTMA’s web site.