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Building Tight

Chris Knights

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What is air leakage?

Air leakage is the uncontrolled flow of air through gaps and cracks in the fabric of a building (sometimes referred to as infiltration or draughts).

This is not to be confused with ventilation, which is the controlled flow of air into and out of the building through purpose built ventilators that are required for the comfort and safety of the occupants.

Air leakage commonly occurs at interfaces between construction elements.Typical locations include:

• Wall to roof and wall to floor junctions

• Window and door to wall joints

• Service penetrations

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What is air leakage?

Air leakage paths are quite often tortuous – air may travel some distance through the building fabric between the points of entry and exit.

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What is the air line?

The air line is normally defined as the element(s) that are used to create the airtightness line.

• The air line should be decided at design stage

• It must be continuous around the building

• It must be possible to slice through the building on any plane and see a continuous air line

• The air line should be contiguous with the thermal line

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General Principles of Design & Construction

The basic principles of airtightness are:

• Airtightness is not rocket science, but it doesn’t happen by chance

• The airtightness strategy should be simple, robust and achievable

• The building air line should be identified pre-construction and preferably at design stage

• Responsibility should be given to a nominated individual or team

• Communication of its importance and relevance is key

• Responsibility should be given to a nominated individual or team

• All parties in the process should be involved

• Common details should be generated

• It must be clear who is responsible for sealing each element

• Protocols for checking detailing must be in place

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Elements - Walls

These for the vertical elements of the envelope, some are structural, some not, but all are required to perform an effective barrier between the internal and external space.

Best Practice

• Blockwork used as the air line

• Soft joint to all other components

• All penetrations, such as windows, sealed to the airline element

• Generally, walls should be sealed to the floor and ceiling

Traditional Block and Brick Cavity

It is the most common wall type, but can also deliver the most variation in airtightness performance. This is mainly as a result of workmanship and the airtightness performance of the block work.

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Elements - Walls

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Elements - WallsCommon Pitfalls

• Blockwork sealed to steel beams/lintels, but beams not sealed

• Blockwork hidden behind internal finishes before penetrations are cut, making access to seal the blockwork difficult

• Expanding foam used for filling gaps, but this is often performed ineffectively. If used it should be used appropriately and sparingly

• Mortar seal between wall and beam joint – incorrectly assumes appropriate, mastic seal should be present.

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Elements – Ceilings/RoofsPlasterboard

Plasterboard will commonly form the airline. This may be horizontal, or contain multiple elements for warm roof constructions

The more complex the design the more consideration must be given to the continuity on the air line

Buildings that a full storey on the top floor are potentially the simplest to design an air seal line for. However they can have significantly more breaks in the line that you might first think.

Best Practice

• The ceiling should be continuous

• It should be mechanically sealed to the vertical air line

• All penetrations, such as loft hatches, sealed to the airline element

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Elements – Ceilings/RoofsCommon Pitfalls

• Significant number of breaks in the airline that are not visible and therefore often not sealed

• Service penetrations inside internal walls

• Service penetrations behind boxings

• Ceiling line often broken at internal walls

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Elements – Ceilings/Roofs

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Elements – Ceilings/Roofs

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Elements – Ceilings/Roofs

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Elements – Ceilings/Roofs

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Elements – Ceilings/Roofs