Case Studies

Resolving An Overtight Building Air Pressure Test

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The Problem

The property in question was a 4 bedroom, 2 storey detached property in Scotland, of timber frame construction with a warm roof (insulation within the pitch of the roof elements), which had an Air Permeability (AP) target of 5 m3/hr1/m2@50pa.

The building employs Intermittent Mechanical Ventilation (IEV) in all wet rooms and window background ventilators (trickle vents) in all other rooms, referred to as a System 1 Ventilation strategy.

The original air test result was 3.68.

The property significantly exceeded its designed AP target (overtight) for the ventilation strategy employed.

Although it comfortably passed as far as the air test is concerned, the ventilation strategy employed would not satisfy the building regulations and action was required.

Air Testing

Our Solution

The first course of action was for the client to liaise with the project architect to discuss the result and come up with a ventilation strategy that would be satisfactory to the Building Control Officer (BCO).

In most cases, the actions required would be to upgrade the ventilation from IEV to Decentralised Mechanical Extract Ventilators (DMEV). These ventilators run continuously as opposed to intermittently with IEV and are far more effective in extracting excess humidity in the air. These are referred to a as a System 3 ventilation strategy and are generally acceptable for AP results between 3 - 5 and are relatively simple to install in place of IEVs.

In some cases the BCO may require DMEV systems to be commissioned.

The Air Test Engineer cannot do anything to assist in cases where ‘over-tightness’ is an issue. They cannot, for example, allow trickle vents or windows opened to increase the test result figure. Doing so would increase the risk to occupant health and comfort and increase the risk of humidity damage (mould & rot) to the cosmetic finishes and fabric of the property.

In most cases, a further air test is not required after a DMEV upgrade as all intentional ventilation is temporary sealed for an air test. So, upgrading the ventilation strategy would have little to no difference in the first test result. An AP test is to establish uncontrolled air infiltration rates through the building envelope (floor, walls, and roof) and not the ventilation system.


A higher proportion of new builds nowadays due to better insulation requirements and building products and methods etc are encountering ‘overtight’ situations which can be costly and inconvenient to redress.

It may be prudent to discuss with your designer / architect the ventilation strategy and contingency plans prior to building. It may also be prudent to default to DMEV ventilators in the build specifications, as they will allow for a greater margin of comfort in the air test results.

If the property AP result comes in below 3 with either a system 1 or system 3 ventilation strategy, then the BCO may insist on a centralised / whole house mechanical ventilation air handling system being installed or other designed ventilation measures implemented before a habitation / completion certificate can be issued. These can be very costly and impractical to incorporate into a building not designed for these measures from the outset.

Some houses are specifically designed to be extremely airtight and have the appropriate ventilation strategies included. These are usually Mechanical Ventilation with Heat Recovery (MVHR) System 4 ventilation strategies that allow for results between 0 - 3 or over depending on target.

System 2 ventilation is referred to as ‘Passive Stack’ ventilation. It is a rare strategy in domestic new builds. Generally, most new builds employ either system 1, 3 or 4 as their ventilation strategies.

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