PAS2035 introduces a set of requirements concerning the assessment, design and post works monitoring and evaluation of domestic retrofit projects. It dovetails with PAS203 which details the installation, testing, commissioning and handover requirements for individual measures.

The PAS2035 document actively seeks to encourage a ‘whole house’ approach, requiring assessors to determine a whole house medium term improvement plan. This is to break away from a siloed single measure-by-measure mentality that has long existing in the industry, shifting to longer term holistic thinking about the interplay between insulation, ventilation, heating and hot water system design and renewables. It’s important to consider the interplay between all these facets and to make sure that actions taken today don’t become compromised later down the line or worse still need to be reversed e.g. wall insulation, window reveals and glazing.

That said, with a fabric first approach, it’s entirely feasible that EWI will be specified as a sole or main measure and the reason we’ve been asked to clarify the requirements for air tightness testing and subsequent ventilation strategy.

PAS 2035 and EWI: The Concept

PAS2035 places particular emphasis on the mantra ‘Build Tight, Ventilate Right’. That is to say, we should recognise that when applying measures such as EWI we’re very likely to reduce infiltration and air leakage through the gaps, cracks and service penetrations across the structure. This improved air tightness is fantastic in reducing heat loss and improving resident comfort but it in turn triggers the need to review the ventilation strategy for the building.

Evaluating Ventilation

There are a number of factors to consider when evaluating ventilation options for a building. First and foremost is to recognise the level of uncontrolled background air leakage (infiltration via gaps, cracks and holes in the building fabric), reduce this as far as practicable and then offset this with purposed provided, controllable ventilation. This can take many forms but can include IEV, MEV, PIV, MVHR etc. Ignoring an assessment of the background ventilation (or indeed simply making an assumption about how much of it there is) can lead to properties either being over ventilated (excess air changes and thus heat loss) or mechanical ventilation systems not working efficiently because the leakiness of the building does not allow the systems to balance and effectively draw and expel air from the property itself.

The first port of call when considering the ventilation strategy for a building therefore is to conduct an air tightness test. This is important not only as a simple means of assessing fabric efficiency i.e. less leaky = good but also because it provides that all important assessment of the level of uncontrolled background air leakage and thus what the optimal ventilation strategy is likely to be. Of course, the funding isn’t always there to go to added lengths to reduce the air leakage or specify advanced ventilation systems and therefore again, this is why it’s important to use the air tightness testing as a means of assessing risk and at the very least ensuring that the installed ventilation solution is at least proportionate to the level of air tightness.

When Is An Air Tightness Test Required

Even if EWI is a sole measure, under Section 8.5: Assessment of the PAS 2035 retrofit standard, an airtightness test may be required to test the air permeability of the building envelope, using an approved method, including identification of key leakage locations.

The responsibility for organising the testing lies with the Retrofit Coordinator but PAS2035 introduces air tightness testing in three guises

i) under pathway C clauses 8.5.1 and 9.3.3 on the grounds principally of assessing the pre-works air tightness in the interests of subsequently setting a design standard to reduce the level of air leakage and then validating that this target has been met post works.

ii) For pathway A and B projects, Annex C makes it mandatory to assess the adequacy of the existing ventilation pre and post works. This can be done by following the guidance set out in Annex C or more recently the Insulation Assurance Authority (IAA) has introduced a TrustMark backed competent persons scheme that allows for the use of either low pressure pulse or blower door fan technology to be used as a means of measuring background ventilation and subsequently using these measurements as a basis for ventilation strategy decision making. Following the prescriptive Annex C route tends to mandate the installation of wet room IEV, trickle vents and 600mm2 undercuts beneath all internal doors as a minimum in all cases whereas the IAA background ventilation assessment scheme introduces a means of using measurement as the basis for assessing homes on a case-by-case basis and then making ventilation interventions on a proportional basis. Here wet room IEV remains a core minimum requirement but further interventions are only needed where the existing presence of background ventilation is deemed inadequate.

iii) under section 14 and the post completion monitoring and evaluation requirements. Here it is at the discretion of the Retrofit Coordinator to instruct a level of monitoring and evaluation on projects to ensure that intended outcomes have been realised. Within this framework both intermediate and advanced levels of monitoring and evaluation stipulate post installation air-tightness testing. Of course if this has already been done for reasons i or ii above, these test results simply feedback into the evaluation process.

From the above you will note that where an air tightness test is required there will always be a requirement for pre-retrofit testing whereas post retrofit testing is discretionary in certain cases.

If there is a requirement for an airtightness test under PAS2035 or if the Retrofit Coordinator has used discretion to request one it will always be required pre-works.

Air Tightness Testing Methods.

There are two PAS approved methods of testing the air tightness of a building:

1. The traditional Blower Door Test: A blower door kit consists of a variable speed fan attached to a frame and a flexible panel designed to fit across an external doorway. This can then pressurise by blowing air in or depressurise by sucking air out of the building in order to measure the Q50 (air leakage rate)

2. Pulse Air Permeability Measurement System – a portable compressor used to pressurise a building from the inside in order to measure the Q50

Useful Link: PAS 2035 – Different Types of Air Tightness Tests​ (buildtestsolutions.com)

Who Can Test

Under PAS2035 there’s obviously a requirement for the testing itself to be carried out by trained, accredited and competent persons. As previously stated, the responsibility for the organisation of the testing lies with the Retrofit Coordinator and an appropriately competent Retrofit Designer may be tasked with any design of the resulting ventilation plan.

The choices for any EWI contractor tasked with organising such a test therefore are as follows:

1: Refer back to the Retrofit Coordinator whose responsibility it is to organise the appropriate air tightness testing.

2: Train and upskill: Whilst there are many training organisations there are just two approved accreditation bodies who will be able to advise on localised PAS accreditation training options:

1. Elmhurst Airtightness Scheme (formally iATS but recently acquired by Elmhurst Energy) – Click Here >
2. ATTMA Air Tightness Testing & Measurement Association – Click Here > 

There is also an Insulation Assurance Association background ventilation tester scheme which is open to all: https://www.theiaa.co.uk/faq-myth-buster/  . The requirements of this particular scheme are for individuals to be trained (IAA and Elmhurst are the two current providers, each operating independently of one another) and then for participants to register themselves with the IAA scheme as well as for them to register and lodge their tests via either the Elmhurst Airtightness Scheme or ATTMA. There is no need to have any other form or prior connection to the IAA etc.

All of the above organisations will also be able to advise on cost of equipment and where to purchase.

3: Subcontract: There are many trained and accredited air tightness testers available to carry out any testing requirements under PAS 2035.

Recommended links as follows:

Find an Air Tightness Tester – Build Test Solutions

Ventilation & Air Tightness Testing Service | The IAA CS

Ventilation Strategy

The airtightness test will dictate the ventilation strategy.

Of particular relevance and interest however; you will no doubt be familiar that following the prescriptive Annex C route tends to mandate the installation of wet room IEV, trickle vents and 600mm2 undercuts beneath all internal doors as a minimum in all cases to allow the air to move freely. This has been the ongoing subject of much controversy and debate.

You will therefore be interested to know that the Insulation Assurance Authority (IAA) has introduced a new and fully TrustMark backed competent persons scheme that still allows for the use of either low pressure pulse or traditional blower door fan technology to be used as a means of measuring background ventilation and subsequently using these measurements as a basis for ventilation strategy decision making. However, the IAA background ventilation assessment scheme introduces a means of using the resulting measurement as the basis for assessing homes on a case-by-case basis and then making ventilation interventions on a proportional basis. In this case whilst wet room IEV remains a core minimum requirement, further interventions (trickle vents, undercuts etc) are only needed where the existing presence of background ventilation is deemed inadequate.

Air Tightness Testing Is Required For A Reason