AIRTIGHTNESS

 

What is Airtightness

 

What is airtightness anyway? Everyone is familiar with the feeling of an unwanted cold draught coming through a poorly-sealed door or window, and the frustration of needing to turn up the heating system to keep warm. When all the air leaks in a building are added up, they can amount to a substantial source of heat loss.

 

But there are other good reasons for airtight construction. For example, draughts can carry moist air, which can condense when it hits any cold surface inside your walls or roof. This can cause dampness, which can lead to mould and rot, as well as carrying unhealthy mould spores into your home.

 

Airtight construction uses building materials to minimise heat loss and eliminate moisture and mould in the wall and roof assemblies. In conjunction with a ventilation system, airtight construction goes a long way towards creating a low energy, high performing healthy home.

 

It is best practice to have the air control layer on the interior side of the ‘thermal envelope’ with the exterior layer being the weathertightness / windtightness layer (building wrap etc). In some types of construction for example SIPs, the air control layer may be part of the wall system, providing appropriate attention is given to all joints and junctions.

 

Building Code Requirements

 

Airtightness is referred to only partially in the New Zealand Building Code, and the code has no set criteria for air permeability or air permeability testing. This is in contrast to most other developed countries where air permeability testing has been commonplace for some time. In Australia, air permeability testing was included into the National Construction Code (NCC) in 2019 as an optional compliance path for P2.6.1(f) building envelope sealing.

 

“Compliance with P2.6.1(f) is verified when a building envelope is sealed at an air permeability of not more than 10m3/h/m2 @ 50Pa reference pressure when tested in accordance with AS/NZS ISO 9972 Method 1”.4

 

In the New Zealand Building Code, Clause E3 – Internal Moisture deals most explicitly with moisture originating from within the building. https://www.building.govt.nz/building-code-compliance/e-moisture/e3-internal-moisture/

 

While the objectives, functional requirements and performance criteria stated in E3 are sound, there is little guidance given and no standard of measurable verification required for these provisions. Minimising the air permeability of the building envelope is a reliable method of controlling the moisture that might enter the building structure. Standardised air permeability testing, as outlined in this section of the Healthy Home Guide, therefore provides a solution for the missing verification in E3.

 

As well as Clause E3, Airtightness is referred to in Clause H1 – Energy Efficiency. H1.3.3. states “Account must be taken of physical conditions likely to affect energy performance of buildings, including … (c) the airtightness of the building envelope…”

 

Similarly to E3, Clause H1 provides reference to a desired performance outcome, but is silent on how this is to be specified, constructed or verified. By adopting internationally accepted standards for air permeability testing, the energy efficiency objectives of a building can be accurately measured and verified.

 

Residential Tenancies (Healthy Homes Standards) Regulations 2019

 

This document has a Draught Stopping Standard. Landlords must make sure the property doesn’t have unreasonable gaps or holes in walls, ceilings, windows, skylights, floors and doors which cause noticeable draughts. All unused open fireplaces must be closed off or their chimneys must be blocked, to prevent draughts. https://www.tenancy.govt.nz/healthy-homes/draught/

 

Air Permeability

 

Airtightness refers to the permeability (or ‘leakiness’) of the building envelope. A non-airtight building is susceptible to infiltration, which is the uncontrolled flow of air through the building elements. Wherever air is allowed to flow, heat and moisture will also flow. Hence infiltration dramatically impacts the energy efficiency of the building envelope as well as effecting health, comfort and durability. Airtightness prevents uncontrolled air movement through the building envelope, thus providing for a building that can perform as designed.

 

4https://ncc.abcb.gov.au/ncc-online/NCC

Fresh Air Supply

 

A common misconception is that airtight buildings cannot have an adequate supply of fresh air. On the contrary, airtight buildings facilitate internal conditions that can provide a much healthier environment than one in which air is allowed to move through the building envelope unchecked. The supply of fresh air to the living spaces of a building is a matter for ventilation and is accordingly dealt with separately to airtightness. Infiltration, whether through unintentional gaps, holes and cracks, or through intentional pathways such as trickle vents or passive stack vents, does not reliably supply an adequate quantity or quality of fresh air.

 

Measuring Air Permeability

 

Air permeability is a measure of the degree of airtightness of a building. In the past it has been common to use air changes per hour (ACH) or hr-1 as the unit of measurement for airtightness. ACH refers simply to the number of times that the entire volume of air inside the living space would be fully exchanged at a given pressure difference to the exterior. Typically, ACH is given at the reference pressure of 50 Pa. This relatively high pressure differential is chosen in order to minimise the impact of environmental factors such as wind, ambient temperature and relative humidity which can all influence the amount of air moving through different parts of the building envelope at any particular time.

 

ACH is a relatively simplistic measure and has the disadvantage of favouring larger buildings. The larger the building volume, the easier it is to achieve a low ACH. To measure ACH the volume of the building must be calculated along with the flow rate through a calibrated fan at a range of pressures.

Effect of Airtightness on heating demand.

 

Airtightness and Health

 

When warm, humid air from the inside of a building is able to freely move into the structure of the building, moisture contained within that air will likely condense on a cold surface within the building element. Even if condensation does not lead to liquid water, the increasing relative humidity that results from a decrease in temperature across the thermal envelope, will provide conditions for mould growth. Mould growth within walls combined with sub-optimal conditions within the living space creates a detrimental living environment for the occupants of the home. Airtight construction prevents this air movement from occurring thus minimising the likelihood of mould growth within the structure of the building. Combined with making the envelope easier to maintain constant temperature and humidity, airtightness drastically improves the healthiness of the building.

 

Airtightness and Durability

 

Mould growth can lead to decay directly but is also symptomatic of other forms of decay. Increasing moisture loads within the structure of a building will encourage degradation of timber, steel and other building materials. A building structure that optimises drying capacity is one that will remain durable. Airtightness is a key component of controlling moisture flow into the structure of the building. For optimal drying potential, airtightness systems that are vapour open should be used. This allows building elements to continue to dry out throughout the life of the building, instead of trapping moisture in.

 

A range of products and systems are now readily available in New Zealand for the construction of airtight and vapour open building elements. If a designer is concerned about a particular system in a given location, then a hygrothermal analysis using modelling software tools such as WUFI® should be carried out to verify that the assembly will dry out over time instead of accumulating moisture5.

 

Airtightness and Comfort

 

Draughts are not only expensive due to heat loss and unhealthy due to coldness and dampness, they also cause discomfort which significantly impacts the occupants’ experience of a home. Airtight construction eliminates the occurrence of draughts and unplanned air movement. When combined with an effective ventilation system and heating protocol, an airtight building is also much easier to maintain at a consistent temperature throughout the entire living space. Reducing the variability of temperature and humidity within the building is another way of drastically increasing comfort.

 

It is important to understand that "infiltrated air" is not "fresh air". It is air leaking through cracks and construction flaws in the home. Cracks are where mice, insects & dust mites live and defecate. These cracks and holes are places where water condenses at various times of the year, producing mould, rot & mildew that affects your health and degrades your home's structural integrity.

 

Designing for Airtightness

 

It starts with design. The first thing to state about delivering airtightness is that it is not just something for either the design or build team to worry about – it’s the responsibility of both, working together, and it requires a degree of collaboration that has not been always been the norm in our house construction. It means everyone working towards a common goal, rather than individual sub-contractors trying to get a job done as quickly as possible.

 

Achieving airtightness starts at the design stage. This is because simple and clear design makes airtightness much easier to achieve on site. Airtight design ideally starts with making your building as simple as possible. Air most typically leaks at junctions where different building elements meet (roofs and walls, walls and floors, window openings, service penetrations, and balcony connections, to give just a few examples). Designing a simple building form that limits the number of these junctions will make airtightness easier for the build team to achieve.

 

5A directory of WUFI® trained professional is available at https://www.wufi.co.nz/nz/wufi-professionals/

Vertical section of a typical dwelling showing key junctions where air leakage id likely to occur. Image: CEREMA CC-BY-SA 3.0

 

 

 

Of course, this should not preclude interesting design and architectural expression! But the more complex a building is, the more challenging it will be to achieve high levels of airtightness. Simple junctions with easy to understand details are also critical.

 

Set a clear target

 

It’s also important to know what your airtightness target is from the outset. Are you simply trying to meet a good standard, or aiming achieve the best standard? A clear goal will keep the design and build teams focused. Set a target for your initial test that is significantly better than whatever minimum you must achieve. This is in case the airtightness layer subsequently becomes damaged after initial test, for example during the final fix.

 

Where is your air barrier?

 

Designers should set out a clear air barrier strategy for the entire building. This should identify what material forms the air barrier for all the key building elements – walls, roof, floor etc. – and where exactly it is in the construction build-up. It should also specify how these different elements will be connected in an airtight manner at key junctions.

 

This air barrier strategy should be communicated to the entire design and build team, and all sub-contractors. This can be done via a single clear drawing of the building that shows the air barrier and what materials comprise it, as well as how key junctions and penetrations are to be sealed.

 

For New Zealand conditions, it is best practice to have the airtight layer on the interior side of the ‘thermal envelope’ with the exterior layer being the weathertightness / windtightness layer (e.g. building wrap).

 

Service penetrations through the air barrier should also be kept to a minimum. One way to achieve this is by designing a service cavity on the room-side of the air barrier, which can also provide a space to install extra insulation.

 

Types of air barrier

 

It is crucial to specify high quality materials for your air barrier, otherwise it may fail during the lifetime of the building.

 

Some common materials used as primary air barriers are:

 

● A vapour control membrane, such as pro clima INTELLO® or INTELLO® PLUS. Vapour control membranes are commonly used with timber construction but can be used in many wall types. As well as providing airtightness, they are designed to control the movement of moisture to protect building elements.

 

● A plaster layer on masonry/blockwork.

 

● Rigid Air Barrier (RAB). Rab board is now commonly used in New Zealand.

 

● Oriented Strand Board (OSB). However, overseas research suggests the airtightness of regular OSB can vary greatly from product to product. The Passive House Institute says leakage through OSB can account for 20 to 40% of air leakage in a passive house built to 0.6 air changes per hour. Ensure that specialist airtight OSB boards are used.

 

● Min 7mm H3.2 Plywood.

 

 

Upskill for Airtightness

 

It is essential that the builder or project manager and airtightness champions are trained in all relevant aspects of airtightness. This can be arranged by asking an airtightness expert to provide training or by attending training courses. ‘Toolbox talks’ can be organised on site for trades, to provide a practical demonstration of their airtightness responsibilities on site. Pro-clima offers workshops in airtightness for architects and designers as well as for builders. Representatives of high-quality airtightness products are often happy to send representatives to building sites, too, to help ensure their products are installed properly.

 

Communicating detail

 

More detailed drawings showing the air barrier at individual junctions, sections and penetrations can be prepared, with the air barrier clearly identified in a distinct colour. These should be large, clear and easy to understand. Consider providing sequenced illustrations of the key stages involved in installing the airtight barrier at complex junctions. Any careful sequencing needed to install a membrane or create an air-tight seal should be annotated on the consented drawings.

 

Three-dimensional models, drawings and renderings of details may also help to aid installation.

 

Healthy Home Air Permeability Guidelines

 

There are two parts to the Superhome air permeability guidelines for healthy home design.

 

1. The testing standard: setting out the process, protocols and reporting requirements for conducting air permeability tests.

 

2. Air permeability targets: which provide the guideline targets for achieving the desired levels of healthy home performance.

 

Testing Standards

 

For air permeability testing in New Zealand, the Technical Standard of the Air Tightness Testing and Measurement Association (ATTMA)6 should be adopted. This standard in turn is based on AS/NZS ISO 9972:2015 Thermal performance of buildings – Determination of air permeability of buildings – Fan pressurisation method.

 

Testing should be carried out by an ATTMA Registered Air Tightness Tester7 and test results completed in accordance with ATTMA TSL1 should be uploaded to the ATTMA Lodgement Scheme.

 

Air Permeability Targets

 

Without any existing requirement for air permeability testing in New Zealand, it may appear arbitrary to define targets in relation to desired net energy savings. However, targets can be confidently set based on the general trend of a small sample of tests already carried out in New Zealand by BRANZ and others, as well as a large dataset of tests carried out in all types of buildings around the world for a number of decades. The Passive House standard also has a well established requirement of 0.6 ACH (approximately equal to 0.6 m3.h-1.m-2 for a typically dimensioned house). The success of the Passive House standard in achieving health, comfort and energy efficient goals that are in line with the objectives of Superhome, suggest that it is appropriate to adopt 0.6 m3.h-1.m-2 as the best practice target for Superhome.

 

6https://www.bcta.group/product/test-standards/

7A direcotory of ATTMA Registered Air Tightness Testers is available at https://www.bcta.group/attma/members/air-tightness-testers/new-zealand/

 

RECENT BRANZ RESEARCH http://www.buildmagazine.org.nz/articles/show/airtightness-of-apartments

Source: Rupp, S. & McNeil, S., Airtightness trends, BRANZ Build 166, June 2018.

 

At the other end of the scale, we recommend that all houses should have a Blower Door test carried out to measure the permeability of the building envelope, regardless of whether or not a target has been set for the building. As BRANZ research has shown, while there is a general trend towards increasing airtightness in New Zealand homes during the last half century, there is still a broad range of results for newly built homes. It would benefit the industry to know the airtightness of each new home that is built.

 

Recommended Healthy Home guidelines for Permeability are set out below: