What is Air Permeability?

Air permeability is the control of air leakage through the fabric of the building. This will include both infiltration and ventilation movement of the building.


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As air permeability engineers we work strictly to the set building regulation. These regulations include both ATTMA TS1 and ATTMA TSL1, of which part L of the building regulations sets all energy efficiency standards for all new homes and specificies that a selection of new homes on all building developments must be tested for Air Leakage.

Air tightness, air leakage or air permeability all refer to the infiltration of cold/hot air into the building and/or the loss of heated/cooled air from inside through gaps, cracks, holes, etc in the building fabric. The loss or cooling/heating of this ‘conditioned air’ through ‘uncontrolled ventilation’ affects the energy consumption of the building, as additional energy will be required to re-heat or re-cool the air. It also impacts on the comfort levels of the building occupiers.

Air tightness plays a significant role in the energy efficiency of buildings. Because: a link has been established between carbon emissions and global warming the production of energy emits carbon the built environment contributes about 50% of carbon emissions heated/conditioned air leaking from buildings requires the use of additional energy to maintain temperatures By limiting the leakage of heated/conditioned air from buildings, it is possible to reduce energy consumption and costs. The government has made commitments to reduce carbon emissions through the European Energy Performance of Buildings Directive (EPBD) and the Kyoto Agreement. Part L is their method for addressing the conservation of fuel and power in buildings based on their overall ‘Emission Rate’ and as such places performance requirements on air tightness. Performance is proven through site testing of the completed building.

Air permeability is an important factor in assessing the overall carbon emission of a building via the appropriate calculation methodology: Standard Assessment Procedure (SAP) for dwellings under 450m² floor area – accredited software has been developed to make calculation easier. Simplified Building Energy Model (SBEM) for buildings other than dwellings – software still under development Before construction: SAP or SBEM is used to calculate a Target Emission Rate (TER) in advance of starting building work. This is based on a range of factors including orientation, glazing, insulation, heating system/fuel, etc.. The details of the above are entered into the software along with a ‘Design Air Permeability’ (what you hope the building will achieve under testing) which must be under 10m³/(h.m²) @ 50 Pa or 15m³/(h.m²) @ 50Pa if a poorer assumed value can be used. The TER is effectively the performance the building would have achieved under 2002 regulations, with a 20% improvement factor applied, in line with wider government targets for reducing carbon emissions. To achieve the TER, you may need to change the specification of products, orientation of buildings, heating system or commit to achieve air permeability lower than the basic regulatory requirement. After construction: The building is then constructed and the specification and configuration of the building may change over the course of the process. The actual details and tested air permeability figure are entered into the software again to give a Dwelling or Building Emission Rate (DER or BER). The actual DER or BER must be lower or equal to TER. This is a regulatory requirement under Approved Document L.

Yes, buildings need to demonstrate a minimum level of air permeability (10m³/(h.m²)) and in the majority of cases this is done through an on-site, pre-completion test. This is not an onerous requirement in itself but the design air permeability for a particular building may need to be lower to achieve the overall carbon emission rate for the building and/or client specification.

All new dwellings (based on a sampling rate) All new buildings other than dwellings ‘Large’ Extensions to buildings other than dwellings There are some exceptions to the above where buildings may be able to assume a poorer air permeability rather than undertake a test. The exceptions are explained below: Dwellings Approved Document L1A – Conservation of Fuel & Power (for England & Wales) requires air tightness testing of new houses and flats to achieve building control approval. Refurbishments of and extensions to existing residential buildings do not require testing. Small developments (1 or 2 houses) may avoid the need to test by accepting an assumed poor value for air permeability of 15 m³/(h.m²) @ 50Pa but this may add costs to other aspects of the building specification in order that the building meets overall targets for emissions. Alternatively it may be possible to re-use existing test evidence where building designs have been repeated within 12 months of conducting the original test. Buildings other than dwellings Approved Document L2A – Conservation of Fuel & Power (for England & Wales) requires air tightness testing of new buildings other than dwellings to achieve building control approval. This includes all buildings using fuel or power for heating ie industrial units, warehouses, schools, hospitals, residential care homes, hotels, offices, retail units, etc. ‘Small’ commercial buildings (with a floor area less than 500m2) may avoid the need to test by accepting an assumed poor value for air permeability (15m³/(h.m²) @ 50Pa) but this may add costs to other aspects of the building specification in order that the building meets overall targets for emissions. ‘Large’ extensions to buildings other than dwellings Approved Document L2B – Conservation of Fuel & Power (for England & Wales) requires air tightness testing of ‘large’ extensions to buildings other than dwellings to achieve building control approval. ‘Large’ is defined as an extension greater than 100m2 AND greater than 25% of the useful floor area of the existing building.

We provide a simple checklist for building preparation, which includes the following: The building should be ‘completed’ All external doors and windows closed All internal doors wedged open All fire dampers, ventilation louvres and trickle vents closed but not sealed Mechanical ventilation turned off with inlet/outlet grilles sealed All combustion appliances switched off Drainage traps must contain water This needs to go to site. The preparation would ideally be undertaken by the contractor in advance of the test body arriving on site to maximise the testing time in a day. BM TRADA will undertake the preparation where this has been arranged in advance. For dwellings it may also be necessary to agree the test programme with the building inspector before arriving on site. Where possible, it is helpful to accurately calculate the envelope area and confirm the fan installation arrangements based on architectural drawings before coming to site.

The objective is to measure the volume of conditioned air escaping through the building envelope via uncontrolled ventilation at an induced pressure difference of 50Pa. The following basic steps are typical: Check site preparation / Prepare site – including temporary sealing Calculate the envelope area if not done previously Explain the process to relevant staff and sub-contractors working on or near the building Take environmental condition measurements – wind speed, temperatures, barometric pressures Install template(s) into suitable aperture(s) Install fan(s) into template(s) Connect monitoring equipment Check the static pressure Take multiple pressure difference readings and record fan flow rate(s) – allowing sufficient time for the pressure readings to stabilise Check the static pressure Process the readings through appropriate software – check that readings fulfil the requirements of the standard If the building fails, attempt to identify/quantify air leakage/infiltration paths Disconnect measurement equipment Remove the fan(s) Remove the template(s)

Depending on: The size/complexity of the building which affects the pre-test checks, equipment installation and test time The amount of preparation that needs to take place The access to the site which affects set up times Assuming the second two factors are within reasonable limits the first factor tends to mean a test time of between 1 and 4 hours: dwellings typically take 30 to 90 minutes for each unit industrial units typically take between 90 minutes and 3 hours hospitals, schools, large office blocks, etc typically taking up to 4 hours.