Frequently asked questions

1. What is air tightness?

Imagine a lovely winter evening. The temperature is at freezing point outside and the heating is running at 75% to keep you snug at 22 degrees. This warm air is known as 'conditioned' air.

Now imagine somebody opens a window right next to you. The air tightness of the house has been significantly reduced, meaning you feel a cold breeze. You would then need to put the heating on full blast in order to try to keep the house warm but all of the heat you are putting into the house will just go out the window. This comes at a cost. Turning the thermostat up costs money!

The same happens in a house that is not air tight. Modern heating systems are designed to be smaller and therefore do not always have the power to heat a building that allows the air to simply escape. Making the house air tight means that your warm air does not escape but continues heating the building.

Heat loss from buildings is also one of the biggest contributors to CO2 emissions in the country.

So what is air tightness testing?

Air tightness testing is simply the way we make sure that the building is not allowing too much air to escape. Air tightness testing is often known as air permeability or air leakage testing.

2. Buildings are meant to breathe

It is important not to confuse air tightness testing with ventilation. Air tightness is about keeping the heat inside; ventilation is about the quality of the air in the building.

An ideal home should be air tight, with a ventilation strategy to match. You should avoid having lots of ventilation and a high air permeability score, and should also avoid having a very good air tightness score and not very much ventilation. There should be a balance between them.

3. So how leaky should my building be?

The maximum leakage allowed under the Building Regulations Approved Document L is 10.00 m3.h-1.m-2@50Pa (the lower the score, the better the performance).

The lower the score the more air tight it is. This could imply particular attention is required when sealing the gaps as they may make all the difference in achieving your target.

Some buildings with low targets are simply too leaky to achieve the set Design Air Permeability Target (DAP) and you should discuss this further with your SAP assessors if you are concerned this is the case. Others may just need some remedial works carried out and can then be retested and achieve their DAP.

To give you a guide, the following failure rates are published by ATTMA:-

DAP* <1.0 Failure Rate 31%
DAP* <1.01 to 2.0 Failure Rate 19%
DAP* <2.01 to 3.0 Failure Rate 27%
DAP <3.01 to 4.0 Failure Rate 20%
DAP <4.01 to 5.0 Failure Rate 14%
DAP <5.01 to 6.0 Failure Rate 9%
DAP <7.01 to 8.0 Failure Rate 5%
DAP <8.01 to 9.0 Failure Rate 2%
DAP <9.01 to 10.0 Failure Rate 3%

* When results are lower than 3.00 m3.h-1.m-2@50Pa, forced mechanical ventilation should be installed to allow for appropriate ventilation, as building sickness can occur.

So what is this m3.h-1.m-2@50Pa?

m3.h-1.m-2@50Pa, pronounced metres cubed, per hour, per metre squared at 50 Pa (Pascals), is the measure of air tightness. The rate of leakage is the number of cubic metres of air, every hour, escaping from every metre squared of the building when the building is subject to a 50Pa pressure difference.

OK, sorry I asked. So how do you conduct an air tightness test?

In simple terms, we place a fan in the door, close all of the windows and turn the fan on. We know how much air is coming through our fan because of our calibrated equipment, so we know how much air is escaping. The actual process is a little more complicated, as shown below:

  1. The building is inspected prior to the test being carried out.
  2. All external windows and doors are closed, trickle vents closed or taped (as appropriate).
  3. All internal doors are opened.
  4. Areas that are allowed to be temporarily sealed are sealed with a low tack tape.
  5. Tubes are run around the property. These are outlets used for measuring the pressure.
  6. A fan is positioned in a doorway.
  7. Environmental measurements are taken - temperature, barometric pressure and static (background) pressures.
  8. The fan is run to create a positive or negative pressure, usually up to around +/-75Pa.
  9. 10 measurements are taken at varying pressure levels.
  10. Environmental measurements are re-taken - temperature, barometric pressure and static (background) pressures.
  11. The data is checked.
  12. A result is calculated and provided.

Do I have to do air tightness testing on my development?

If you have built anything new, then chances are you will need to carry out an air test. We are seeing an increasing amount of customers who need to do this on change of use sites too.

Air testing is a factor that is included in your SAP (Standard Assessment Procedure) for dwellings, or part of your SBEM (Simplified Building Energy Model). There are two types of calculations that take into account all factors in the building that will impact on how much gas, electricity and oil (if applicable) will be needed to heat the property per year (known as BER - Building Emissions Rate. They also look at the CO2 emissions (known as the TER - Target Emissions Rate). Factors included are:

  • U Values of external walls
  • U Values of floors
  • U Values of ceilings
  • U values of (internal?) walls
  • U value of windows
  • Efficiency of the boiler
  • The size of the property
  • The direction the house is facing (for sunlight)
  • Ventilation
  • And many more!

One of these is the (Assumed) Air Tightness Testing target. The reason it is assumed is that the SAP or SBEM calculations are done prior to building, so the air testing figure is one of the last to be calculated.

This often has two impacts:

  1. Lots of inefficient products are used so the SAP almost runs out of CO2 allowance. This means the air testing target becomes increasingly lower which is increasingly harder to meet.
  2. The air tightness test target score is not set in stone which is why we never say 'Pass' or 'Fail'. Often in the building industry, materials change, dimensions change and techniques change. These can all have an impact on the SAP or SBEM calculation post-test, which will impact the air tightness target

The SAP and SBEM requirements are required under Part L1A (for dwellings) and Part L2A (for non-dwellings). Occasionally, testing is required under Part L2B (for extensions / refurbishments).

How long does an air test take?

The actual air tightness test itself takes around 5-10 minutes. Most of our time on-site is preparation (temporary sealing, preparing the building etc). This can vary from 10-20 minutes in a flat to 3 hours in a large commercial building.

Do you need to test every plot on my development?

For non-dwellings, covered under Part L2A or L2B of Approved Document E, all buildings must be tested except those with less than 150m2 of useful floor area and where an Assumed Air Test Target of 15 m3.h-1.m-2@50Pa can be used in the calculation. With the government allowing less and less CO2 to be emitted from buildings every time Approved Document L is updated, using 15 m3.h-1.m-2@50Pa is becoming increasingly difficult without having to use extreme amounts of insulation and additional building fabrics.

For dwellings, sample air testing can be carried out, although it does come with penalties.

Sample air testing can be carried out on plots that are identical and if your target is more than 5.00 m3.h-1.m-2@50Pa. 1, 2 or 3 of the plots is tested (we will come back to that) and the result of that test is assumed for the other plots. However, you can only re-use data when the plot you are testing achieves 2.00 better than the plot you intend to use the data for.

So how do I know how many I should test?

Site Sound will assess your development and offer two options.

The first is to test all properties on the development. This may cost more but means you won't need to go the extra mile in making them very air tight which can often save money on building materials per plot, such as mastic and expanding foam. The cost will not rise above the agreed fee unless there are multiple failures or additional visits.

The second cost will be to test a sample. This assumes the minimum testing possible in order to pass the air tightness test requirement. Any instances, as shown in the bottom example above, may result in increased costs.

Once an order is placed, we can provide a plot specific schedule so you and we know which plots have been tested, still to test and which ones have not met the 2.00 m3.h-1.m-2@50Pa rule. We divide the schedule into types so not a single extra plot can be tested by mistake.

What do you define as a different 'type'?

Approved Document L requires you to test at least one of each type (of structure? of construction? of property?). If you have 4 of a specific type, you will need to test 2 of these and if you have 5 or more, you will need to test 3.

Defining types is complicated. Below shows what a different type could consist of:

  • Relative Position
    • Detached, semi-detached or terrace.
    • Lowest floor flat (this could be the floor at ground level or the floor above a commercial premises. It is the lowest floor of a residential development).
    • Intermediate floor flat.
    • Top floor flat.
    • If the separating walls or floor partially adjoin another property, these two properties would be different types (e.g. in high rise flats when the building is staggered).
  • Construction type
    • Any change in construction type, for example if one property had a warm roof construction (no loft) and another had a cold roof, they would be different types.
    • Timber, reinforced concrete, traditional block work - any changes will be a different type.
  • Size
    • A greater than 10% variation in the total envelope area (measured in m2).
  • Significant penetrations
    • If the number of external doors, windows, soil stacks or service entrances increases or decreases by 1, this could be seen as a different type.

Who selects which buildings to test?

Building Control will have the ultimate say in which plots require testing, though Approved Document L1 suggests that the Testing Body should propose the number of plots to test.

When should I book in my air tightness test?

One of the biggest causes of failure is the testing being conducted at too early a stage in the build process. Air tightness testing should be carried out at the end of the process. The air test should be one of the last trades to enter the building, aside from cleaners.

Mastic and caulk are usually required for an air test to pass. If we are called in too early, the mastic and caulk work will not have been completed which will let air escape.

Click below to download our building completion checklist:

Site Managers Air Tightness Checklist - Dwelling

Additional Temporary Sealing Deviations

How quickly can you send me the report?

The air tightness result will be provided instantly on-site for each plot. At the end of the visit the report is sent to our head office (by email) to be checked by either the quality manager or the technical manager. Depending on the payment status, the report can then be released. This typically happens the next working day but can be guaranteed on the same day with a bit of notice.

What do I do with my air tightness test report?

The report should be sent to your Building Control Officer within 7 days of the test being carried out. Failing to do this could result in your test being rejected (although very unlikely).

The report is also sent to your energy assessor (usually the person carrying out the SAP calculations) to create the EPC - Energy Performance Certificate. This is a final confirmation of the SAP calculation once the air testing is confirmed and the builder / contractor confirms that the building was constructed as per the original calculation.

The EPC is required to be sent to the Building Control Officer before a building completion certificate can be issued.

So what happens if the building fails the air test?

The first piece of advice is not to panic. Failing a test is not the end of the world. It just means that some attention needs to be paid to certain areas. We have lots of techniques to find the leaks but, put simply, we just turn the fan around and feel where the air is coming in. We can then look at which option is best to permanently fix the problem.

Often, this can be done during the same visit. Sometimes it will require us to return to site once the work is carried out.

What are the most common air leakage/infiltration paths?

The most common areas of leakage are typically the areas of the building with penetrations through the walls - bathrooms and kitchens. Below is a list of areas that typically fail air tightness testing.

  • Kitchens
    • Behind the kitchen units the plasterboard will not always meet the floor and will not be covered with skirting as per other areas. The air will travel up the dot and dab plasterboard and find a leakage path to outside.
    • Holes around penetrations - Cooker hoods and waste pipes typically break through the external wall. These areas should be made good once the pipe is through using a flexible mastic. This is often not carried out as these areas are out of sight.
  • Living Rooms
    • It is important, particularly with dot and dab plaster construction, that a continuous mastic bead is present around the perimeter of the floor / skirting. If there is a 2mm gap around 80 metres of perimeter, plus the same upstairs, that adds up to 0.32m2 of leakage - the equivalent of a small window being open.
    • Although we are allowed to temporarily seal fire places, gaps that lead into the fire places, particularly gas pipes and around the mantle, will allow air to leak directly into the fireplace and escape
  • Boiler Cupboards & Plant Rooms
    • Boiler cupboards and plant rooms are full of cables and pipes that penetrate in multiple directions. If the pipes are not sealed around, they will allow extensive leakage into walls and floor voids, which will find its way out.
    • Services from outside will often come up through the foundations or concrete base through a pipe. This pipe needs to be backfilled as it will be completely open to outside.
  • Bathrooms
    • Shower trays and boxing behind toilets will often lead to voids. These holes can sometimes be very large and need either to have a mastic seal where the pipes penetrate, or be backfilled using expanding foam.

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