An occupant of a building will notice “draughts” far more readily than a general cooling of a room caused by poor thermal performance of the walls, floor and roof. Buildings are not designed to be leaky! So, “build tight, ventilate right.”
It is a fair assumption that a building which meets its Air Tightness Testing (ATT) target will be constructed broadly in line with its envelope design and hence is a good measure of overall quality.
Air Tightness Testing Explained
The ATT of a building is measured by putting the internal space under pressure with all external openings closed, as they would be during normal operation.
The rate in which to air is escaping is then measured and expressed as the volume of this air, m3, per area m2, of the envelope, over time, per hour. Therefore, a typical result expressed would be 5m3 of leakage, per m2 of envelope, per hour.
Building Regulations allow for a maximum leakage rate of 10m3/m2/hour, although the maximum for each building is defined in the Part L assessment specifically for that building.
What is a Good Air Tightness Test Result?
A “good” target result is between 3 and 5m3/m2/hr. A lower result than this will mean that extra ventilation will need to be considered to ensure adequate indoor air quality. A building doesn’t breathe, but its occupants do!
Compliance & Regulations
Part L of the Building Regulations, as part of the overall SAP (dwellings) OR SBEM (non-dwellings) calculation, requires the air tightness testing to be entered at the design stage. This then demonstrates overall compliance of the building as part of the whole assessment.
Why is Air Tightness Testing Crucial In Construction?
Energy Efficiency
A building is designed and must be compliant with all Building Regulations.
Part L demands a certain level of energy efficiency which considers the thermal performance of the envelope, (including AT); and the efficiency of services (to heat, cool and provide hot water), together with renewable energy sources.
Mechanical and electrical plant, which heat and cool the internal space of a building, are designed for a particular leakage rate. Any higher, and energy is literally leaking through the walls, floor and roof, therefore putting a higher demand, wasting energy and increasing running costs.
Indoor Air Quality
As with many things, AT, is a balance between producing an airtight building, and ensuring that the building is sufficiently ventilated to allow stale air to be extracted and replaced with fresh. This historically has been done with vents in external walls and natural air leakage.
If this leakage is being measured, to reduce it, then consideration of this AQ must be made. A natural AT of above 5m3/m2/hr will probably be enough for this to occur naturally, below this (certainly at 3m3 or lower), mechanical ventilation should be considered. This, in turn, has implications on energy consumption to operate.
Extended a Building’s Lifespan
An airtight building is a good indicator of a building’s quality. Meeting its target demonstrates that the design was sound, and the construction was executed accordingly.
It also gives assurance that the building will be comfortable to occupy, and more energy efficient with no draughts.
Air Tightness Testing by Stroma Built Environment
Stroma Built Environment offers a complete air tightness testing service from design review, and site visits to ensure air barriers are robust, through to testing at completion.
With 20 years of experience, we can collaborate with the project team through the lifecycle of the project to aid compliance, in a regulatory environment which is demanding ever lower airtightness results.