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


‚Äč

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.


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


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: www.leap4.it

 

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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