Air barrier design

Mark Siddall

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

Pro Uses common sheet properties
Pro Can be checked relatively easily and rectified where necessary
Con The sheet is unprotected
Con Risk of puncturing during construction and building life
Con 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

Pro Vapour barrier can naturally be used for air sealing as well
Pro Large size foil sheets can be used, with few joints as a result
Con Certain difficult construction problems
Con Accuracy required at joints
Con Risk of puncturing during construction and building life
Con Services installation penetrations cause problems


Drawn-under sealing layer

eg paper or foil

Pro The air sealing layer is protected against damage
Pro Electrical installations, DIY projects, and future rewiring etc (during the operational life) is possible without the sealing strip being damaged
Pro Good prospects of achieving high level of airtightness
Pro Reduced risk of puncturing during construction and building life
Con Moisture damage risks not known
Con The effects of supplementary insulation and carpentry and furnishing, e.g. on moisture conditions in the sealing strip, in particular, are unknown
Con Requires double wooden frame


External air sealing and wind protection


Pro The wind protection system's air sealing properties can be used
Pro It can form a part of a pressurised rainscreen
Pro Allows early installation, inspection, testing and remediation without disruption to programme and before substantial completion
Pro Allows for the conventional scheduling of sub-trades
Pro Barrier not penetrated by floors, partitions, or electrical services
Pro Avoids the requirement for cable seals/ polypan or strapping to accommodate services
Pro It can be repaired externally before being covered
Pro Allows ease of site access and has fewer complicated junctions
Pro Electrical installations, DIY projects, and future rewiring etc (during the operational life) is possible without the sealing strip being damaged
Pro Allows single expenditure for multiple benefits
Con Significant risk that airtightness is so good that moisture can condense inside the construction (ensure suitably vapour permeable wind barrier)
Con 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.
Con Stringent requirements on internal vapour barrier
Con Provided that a reliable and effective vapour barrier system is employed on the warm side of the building fabric moisture diffusion can be attenuated.

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

Pro Double safety for air sealing
Con Use of double sealing layers is uneconomic
Con An airtight wind protection can cause moisture damage if inappropriately specified


Homogenous construction

eg cellular concrete

Pro Simple design
Pro Electrical cables can be included without jeopardising airtightness
Con Limited choice of materials
Con Connection details to other materials have to be solved separately
Con 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:


Read more about Building Physics:
  • Heat transfer: Conduction, Convection & Radiation  MORE
  • Insulation materials: thermal properties MORE
  • U-value for dummies MORE
  • Air tightness MORE
  • Thermal bypass MORE
  • Decrement Delay & Thermal buffering MORE
  • Thermal mass MORE



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