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Air Permeability Testing – Get Your Building Airtight

3 Mar 2018 at 12:13 PM

Once a dwelling is complete, an essential part of our work is testing for airtightness.

Air permeability testing is a crucial phase of any build as the result will significantly affect the energy performance of the building. Here we are going to give you our top tips for ensuring your  building is as air tight as possible. But first some background for the uninitiated…Air Permeability Testing

What is Air Permeability Testing?

As a general rule, current building regs require that new dwellings achieve an air leakage of around 5m3/hm2  or less. That is the air leakage rate per hour, per square metre of envelope area. If we are being precise, there are a few caveats around SAP Calculations and sample testing which can affect this, but broadly this is the target.

The m3/hm2 figure is the headline ‘pass or fail’ result produced by the air testing engineer and demonstrates how much air (typically by depressurising) is being sucked into the building through ‘leakage’ when the fan is operating at 50 Pascals.

Uncontrolled Air Leakage

The key, is that this test measures uncontrolled air leakage. It is not concerned with trickle vents, extract fans or ventilation systems. Controlled ventilation sources will simply be taped up or sealed prior to testing. Therefore air permeability testing is looking for gaps and cracks in the fabric of the building.


Some certification schemes look to go further than building regs standards – for example the Passivhaus standard requires all new builds to reach a maximum of just 0.6 air changes per hour. Whilst this is a huge improvement against a ‘5’, it should be noted that there are slight variations in the testing metrics and procedures.

Whilst testing may take place at any time in the build process, it is common for air testing to take place in the final throes of a project prior to issuing of final SAP Calculations and an EPC.

In a residential build, the test result will be passed to the SAP assessor who will then update the calculations, establish that a pass has been achieved, and issue final reports and an EPC.

Effects in SAP

Importantly, the SAP Calculations will be significantly affected by the air leakage rate, as high levels of uncontrolled air leakage will reduce the energy performance of the building. A SAP Assessor will generally set a design air permeability target of between 5-10m3/hm2.

This is a reasonably achievable performance although in some cases this may need to be set lower. Why? Well the dwelling may be struggling to meet its emissions targets and a low air leakage may compensate for other areas, either arising from poor design, or factors beyond the developer’s control.

SAP ModellingWhilst it would be great for our heating bills and indeed energy calculations if we produced super airtight houses every time, this obviously can cause issues when a dwelling is not designed for very low levels of ventilation.

Ventilation specialists will generally quote a ‘maximum’ air tightness of 4-5m3/hm2 as a healthy rate for a naturally ventilated house. That is, ventilated only with extract fans, trickle vents and windows. Anything tighter and some form of forced ventilation will be required, i.e a mechanical ventilation system.

Some mechanical vent systems have a heat recovery function (MVHR) which draws heat from wet rooms and recycles it for use in the rest of the house.

Top Tips

During the course of testing hundreds of different buildings, we’ve learnt a few things. So we asked our Technical Manager Lee for his top 10 tips for ensuring the best result:

1. Apply an air tightness strategy from the start

“It is critical to set airtightness goals from the very earliest drawings. There is huge value in getting the right advice early on – we can interrogate a set of design stage drawings, review them with design teams and set the project off on the right footing. This is a relatively inexpensive exercise compared to the problems that can be encountered later. Carry out toolbox talks before you build to ensure all of your trades know that this is going to be an air tight build.

It helps to define the line of your airtightness barrier very early – draw a red line through all the elements that separate heated and unheated elements. Appoint an ‘Air Tightness Champion’ to coordinate between all consultants and trades”.

2. Inspect throughout the build

“Put in place an inspection regime during construction – ensure that the air barrier is uncompromised by shoddy workmanship. Failures found during testing can lead to expensive uncovering and remedial work. Again it can pay dividends to have a consultant visit site during the build to review progress.

Leakage commonly occurs due to a cavity wall being breached during construction – very often around floor joists so use appropriate hangers and fixings to avoid this”.

3. Careful with drylining

“Plasterboard installed with dot and dab can lead to problems, as any missed leakage in the brick or blockwork has a potential path from behind the board to the floor slab.Sealing Plasterboard

Either consider parge coating the wall first, or use solid lines of adhesive around all sides of the board and any cut outs for sockets. It’s not always possible to seal around skirting boards so this can really help”.

4. Seal waste and supply pipework

“Ensure all waste and supply pipework is sealed where it penetrates walls and floors. with compatible gunned sealants, or for a larger gap use pre-compressed flexible expanding foam strips. Do not use expanding foam – it shrinks and breaks the seal very quickly. Use only flexible foams which are tested for air tight applications”.Air gaps around pipework

5. Windows and doors

“It’s amazing how many window and door frames we see which haven’t been sealed or fitted properly. Use suitable seals, sealants and draught strips to ensure there are no gaps or cracks around all jambs, sills and lintels”

6. Rad pipes and manifolds

“Manifolds from underfloor systems and radiator pipes are often not sealed into the floor screed – this is then covered up with flooring or carpet. Also many radiator pipes penetrate the wall behind a rad – which again is very hard to spot unless you are looking for it”

7. Don’t forget eaves cupboards and loft hatches in homes

“With many homes designed with rooms in the roof, we see a lot of air leakage through eaves cupboard doors and loft hatches. They are usually an afterthought though – they do separate cold unheated space from the living space so make sure they are sealed just as well as your external doors and windows would be”.

8. Take care around light fittings

“Seal holes around light fittings and pull cords in the ceiling, choose airtight fittings if possible, otherwise install air tight boxes over the fitting in the ceiling void”.

9. Seal around services

“Water, drainage, gas pipes, boiler flues and electricity cables are typically leaky – it’s not unusual to find leakage in meter boxes and around consumer units”.Air Permeability Testing

10. Test twice

“It is wise to test twice – once before the covering up of the membrane or air tight barrier when remedial work can easily be carried out, and again at completion”.

Design it out

Lee added, “Many of these are somewhat superficial fixes – there is no substitute for good air tightness design right from the architect’s drawing table.

Passivhaus builds and the like are different beasts and will need a lot more thought around air tightness barriers and membranes in the construction itself. The most important point on any build is to get all the trades onside and get them thinking about air tightness as they go.

If everyone buys into the air tight theme, with good practices from the start, you can avoid the problems later on.”

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