F.A.Q. and Resources

All new buildings of all types need to be tested to evidence energy efficiency.  It is possible to avoid testing individual houses and small commercial buildings however, it is not advisable. This is because the air permeability value in energy performance calculations will be estimated and likely poor. This makes it difficult and/or expensive to achieve the overall Energy Performance of the building.  

On larger residential developments, it may be possible to test a sample of units rather than every single one but, this only applies to developments started under older versions of Part L. In practice, a large proportion of the units will still need to be tested as relatively small variations in size, position and openings will be considered a different type. In addition, to use tested results on untested plots, you will need to achieve ‘2’ better on the tested units, which is difficult to achieve. 

ATTMA is the Air Tightness Testing Measurement Association. It operates the largest and longest-established approved competent persons scheme for air testers. Air testers must be a member of such a scheme and must hold the appropriate level of registration for the project they are undertaking. Level 1 is only for dwellings and small, single-fan buildings other than dwellings. Level 2 is required for the majority of buildings other than dwellings. 

The maximum air permeability allowed on any tested unit is ‘8’ (‘10’ for older versions of Part L), however, because of the trade-off of factors to achieve the overall calculated energy performance of the building/unit, it is more typical to need to achieve ‘4-6’.  

‘10’ is the equivalent of a 20p piece-sized hole in every square metre of the envelope area. The envelope area is the ‘external’ surface of the unit/building. This covers the total area of the floors, wall and ceiling which border the outside, or a neighbouring unit for flats and attached houses. This is calculated by the air testing company.    

Air permeability is expressed in m3/(h.m2)@50Pa, this represents the volume of air being lost, every hour, through unplanned ventilation (gaps, cracks and holes). It covers every square metre of the envelope area when there is a pressure difference of 50Pa between the inside of the building/unit and the outside. 

Targets for air permeability tend to be significantly lower than the maximum allowed by Building Regulations and they will only get lower and less flexible as efforts to achieve Net Zero ramp up.

The following provides an indication of what the target numbers mean in terms of areas of concern, construction methods, etc:

The following numbers refer to air permeability in m3/h/m2@50Pa

 8-10

• Building Regulations requirements
• Should be a formality if the house is fully finished – including flooring being laid, bathrooms and kitchen sealed, doors/windows commissioned, etc
• But watch out for large openings for services behind bath panels or in utility rooms/cupboards and for eaves storage that is not boarded/sealed

5-8

• Should be readily achievable on all house types but property needs to be fully finished – including flooring being laid, bathrooms and kitchen sealed, doors/windows commissioned
• On masonry construction with dot-and-dab plasterboard lining, it is also important to be confident that air cannot escape between the bottom of the plasterboard and the floor – this would ideally be achieved by sealing the top and bottom of the plasterboard with a continuous ribbon of plaster but, if this is not possible/has not been done, by filling the gap with expanding foam before skirtings go on or, worst case, sealing under the skirting boards using flexible sealant
• Make sure this is done in areas that are not visible i.e. behind bath panels, behind kitchen/utility room cabinets, etc

3-5

• ‘5’ is the most commonly required target for houses to ensure a compliant Energy Performance Certificate can be produced
• On masonry construction, in most cases this can be achieved with the same considerations as ‘5-8’, plus:
  
  • Extra attention should be paid to openings around services, especially SVPs e.g. open-ended boxing behind toilets that feed into the ventilated loft space
  • Sealing around doors and windows
  • Electrical fittings – fully fitted, no gaps/holes around socket plates/downlighters, etc
• If the target is lower (tighter) for masonry construction houses this could be difficult to achieve, especially on smaller properties that may have a similar number of penetrations in the envelope area but the air leakage is divided over a smaller area
• A ‘3’ is probably the limit of what could reasonably achieved on most houses of masonry construction but this would be difficult to achieve and would require a great deal of attention to air tightness from design through to completion by the main contractor and all trades
• It is possibly for masonry construction apartments to achieve tighter results, aided by fire stopping between floors but be aware that we often find that the top floor units are considerably worse as the air is able to escape through a ventilated roof space
• On timber frame/SIPs/CLT, etc where a vapour control layer/membrane (VCL) can be utilised to ‘bag’ the property, it is possible to achieve better results and test at an earlier stage but using these methods does not guarantee better performance – design, communication and inspection will still be key to ensure that the air tightness barrier is maintained throughout. If the VCL is not robustly and tightly sealed together and to any penetrations through it, performance can still be very poor
• On steel frame apartment blocks, figures of around a ‘3’ should be achievable assuming the air barrier has been well designed and installed, and maintained after windows/doors and services have been fitted and sealed/fire-stopped. But we have seen issues on this type of construction where air is able to bypass the air sealing details via a ventilated ceiling void/plenum – any air that could get into the ceiling void was able to escape around poorly sealed external louvres/ducting
• On reinforced concrete frame construction apartments with a SFS facade, these values should be readily achievable assuming that the façade system is well designed and installed, doors and windows are well-fitted and all services have been sealed/fire-stopped. We have seen issues on this type of construction where air is able to bypass the air sealing details via a ventilated ceiling void/plenum – any air that could get into the ceiling void was able to escape around poorly sealed external louvres/ducting, we have also seen scenarios where air is able to escape via hollow concrete floor beams (through openings in the beams to release water)
• For target of under a ‘5’ it is more important that appropriate ventilation is provided to ensure moisture control, air quality and comfort

1-3

• For Low Energy schemes
• Ventilation must be appropriate for these levels of air tightness to provide ensure moisture control, air quality and comfort
• Very difficult to achieve using masonry construction – better performance may be possible using wet plaster combined with the areas mentioned above but would be a risk to target this level of air tightness using traditional construction
• Other constructions, it is possible to achieve sub-‘3’ results on timber frame, steel frame and concrete constructions, especially on larger apartments but it would be a risk to commit to these targets without specific design consideration and plans for how to monitor air tightness on site. It would be advisable to conduct preliminary testing on the first completed apartments to confirm that the target is being achieved and, if not, what areas need to be improved

Less than 1

• For Passivhaus
• Very difficult to achieve – requires specialist design and materials, champion on site, multiple preliminary testing at various stages
• Typically achieved with timber frame system or concrete frame and huge attention to detail by all concerned

The test is a pressure test with fans used to create a pressure difference between the inside and outside of the building/unit. Knowing how much air is being used to maintain this pressure, provides a comparable measure of the volume of air escaping through gaps, cracks and holes. A negative or positive pressure difference can be created. The fan or fans are typically installed into a door using a ‘blower door fan’ template. The test itself only takes a few minutes but set up and preparation means that testing a typical house or flat takes 15-60 minutes. For larger buildings, it may take 2-6 hours depending on size and complexity. The pressure differences generated are small, so no damage will be done to the building or occupants. 

Timing is key, the majority of test failures occur because the building/unit simply isn’t sufficiently complete. However, leaving it to the last minute puts added pressure, especially if there is a problem. We are happy to advise on timings. 

If you have several dwelling units of similar design to test, it is preferable to spend time/effort on the first completions (e.g. show home). This will help to establish what jobs need to be finished to get the desired result. This can inform the timing of future tests and the results that should be achieved. 

For houses and flats of standard construction, it is possible to achieve reasonable air tightness without having to use specialist products and techniques. It requires close control of trades whose work impacts the air barrier: 

Dry liners – to ensure there is a continuous ribbon of plaster at the foot of the boards 

Window/door fitters – external doors and windows are sealed around and create a good seal when closed 

Plumbers – sealing around pipes and ductwork 

Electricians – sealing around cabling 

Bathroom and kitchen fitters – ensuring pipe work is sealed where it penetrates the air barrier and/or there is continuity of sealants. 

It is also important to have good attention to detail in the final finish e.g. mastic sealing, to ensure continuity is achieved, especially in bathrooms and kitchens. This means the unit must be nearly ready for occupation before being tested, which creates an issue if the building/unit fails.    

When targeting a high level of air tightness (4 and under as a guide) then specialist membranes and tapes should be used to create a continuous air barrier within the construction detailing. This means that you are not relying on decorative finishes and mastic seals.    

For unusual constructions or commercial buildings, the air barrier tends to need much more careful consideration and design. This is because of the complexities of achieving continuity between the walls and the roof, as there are more ‘hidden’ areas (suspended ceilings and raised floors) and a greater number/scale of service penetrations (air-con, IT and fire sealing). It is important to have an appreciation of the air barrier and what jobs need to be completed when planning the air test. 

The easiest and most common way to identify air leakage paths is to feel them during or after the test. By depressurising the building/unit (drawing air out using the fan), the air paths should be identifiable as draughts. Where it isn’t easy to feel or pinpoint the draughts, smoke can be used to make the leakage paths visible.    

In many cases, it can be relatively easy to quickly address the issue permanently and repeat the test. Where the failure is more significant, or difficult to access, a further visit may need to be scheduled. If the result is close to achieving the target or design air permeability, it is worth checking with your energy assessor as to whether the performance is good enough to achieve the overall energy targets.