Air barrier design

Location of the airtightness barrier should be an early consideration of design development. But determining where exactly in the construction element the barrier should be is not a straight-forward process. Mark Siddall of LEAP reviews the range of strategies available.

Internal airtight cladding

eg plaster, parge or plasterboard

Uses common sheet properties

Can be checked relatively easily and rectified where necessary

The sheet is unprotected

Risk of puncturing during construction and building life

The joints must be sealed carefully even against floors and roofs e.g. sensitive to movement and subsequent crack formation

Internal sealing layer

eg foil

Vapour barrier can naturally be used for air sealing as well

Large size foil sheets can be used, with few joints as a result

Certain difficult construction problems

Accuracy required at joints

Risk of puncturing during construction and building life

Services installation penetrations cause problems

Drawn-under sealing layer

eg paper or foil

The air sealing layer is protected against damage

Electrical installations, DIY projects, and future rewiring etc (during the operational life) is possible without the sealing strip being damaged

Good prospects of achieving high level of airtightness

Reduced risk of puncturing during construction and building life

Moisture damage risks not known

The effects of supplementary insulation and carpentry and furnishing, e.g. on moisture conditions in the sealing strip, in particular, are unknown

Requires double wooden frame

External air sealing and wind protection

The wind protection system's air sealing properties can be used

It can form a part of a pressurised rainscreen

Allows early installation, inspection, testing and remediation without disruption to programme and before substantial completion

Allows for the conventional scheduling of sub-trades

Barrier not penetrated by floors, partitions, or electrical services

Avoids the requirement for cable seals/ polypan or strapping to accommodate services

It can be repaired externally before being covered

Allows ease of site access and has fewer complicated junctions

Electrical installations, DIY projects, and future rewiring etc (during the operational life) is possible without the sealing strip being damaged

Allows single expenditure for multiple benefits

Significant risk that airtightness is so good that moisture can condense inside the construction (ensure suitably vapour permeable wind barrier)

The layer is affected by the external climate consequently materials and joints are exposed to more extreme moisture and temperature conditions in order to ensure long term integrity suitable methods for sealing the envelope should be considered.

Stringent requirements on internal vapour barrier

Provided that a reliable and effective vapour barrier system is employed on the warm side of the building fabric moisture diffusion can be attenuated.

WARNING:
A combined external wind/ air barrier could permit windwashing. This would occur as a result of the air/ wind barrier acting like a diaphragm whereby warm internal air is drawn into the depth of the wall element assisting heat loss and thus increasing the U-value. This action could also draw moisture into the structure.

There is also the risk that the convective currents that are present within a building (induced by opening external doors, MVHR etc.) could assist the vapour transport through any unsealed joints in the vapour barrier. This action could also draw moisture into the structure.

Combination of internal and external air sealing

Double safety for air sealing

Use of double sealing layers is uneconomic

An airtight wind protection can cause moisture damage if inappropriately specified

Homogenous construction

eg cellular concrete

Simple design

Electrical cables can be included without jeopardising airtightness

Limited choice of materials

Connection details to other materials have to be solved separately

All building sections should be able to be carried out applying the same system which limits the method and choice of material

About the author:

Mark Siddall, principle at low energy architectural practice LEAP, is an architect and energy consultant specialising in low energy and PassivHaus design. He was project architect for the Racecourse Passivhaus scheme and has a keen interest building performance. In addition to architectural services his practice provides project enabling and education for clients, design teams and constructors.

LEAP website: www.leap4.it