Limiting air infiltration and energy loss

Over the past decade, Dubai’s energy demand has increased in sync with the rapid urban development and population growth of the UAE. In particular, the residential villa stock has grown by more than 300%; from 20,000 villas in 2000 to over 60,000 villas in 2009. In order to limit energy consumption, local authorities introduced building legislation (2001 and 2003) that prescribes minimum insulation levels for external walls and roofs.

The resulting constructive solutions focus on the use of a mid-plane insulated prefabricated block to attain the prescribed maximum wall U value (0.57 W/m2 K), however the reinforced concrete frame typically remains non-insulated, and thus introduces significant thermal bridges in the building envelope.

Simulation results show that with appropriate external wall insulation strategies alone, energy savings of up to 30% are realised.

The complexity of any modern building envelope requires that consideration is given to achieving insulation continuity and airtightness early in design. This two stage process should be done at both schematic and detailed design stages.

Consideration at the schematic design stage involves the primary construction and insulation method (masonry or steel frame insulation, externally insulated RC frame, etc.) and selecting the primary air barrier elements (plaster finishes, sheathing boards, etc.). The choices made dictate the philosophy for the remainder of the design and construction process.

At the detail level it is important that the design builds upon the above strategy showing the builder how to maintain insulation continuity and airtightness.

Airtightness

The airtightness of a building, or its air permeability, is expressed in terms of air leakage in cubic metes per hour per square metre of the building envelope area when the building is subjected to a differential pressure of 50 Pascals (m3/(h.m2)@50Pa).

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The building envelope area is defined in this context as the total area of all floors, walls and ceilings bordering the building, including elements adjoining other air conditioned spaces.

Air leakage is defined as the flow of air through gaps and cracks in the building fabric.

Uncontrolled air leakage increases the amount of heat loss as warm air is displaced through the envelope by colder air from outside. Air leakage of warm damp air through the building structure can also lead to condensation within the fabric (interstitial condensation), which reduces insulation performance and causes fabric deterioration.

The Air Barrier Line

The air barrier is a layer within the building envelope which will adequately restrict the passage of air between the internal and external environments. The barrier should closely follow the line of the inside face of the insulation in the exposed elements of the fabric of the building.

Consideration should be given at an early stage as to which layer of each exposed element of the fabric will form the primary air barrier, and to the junctions between them.

Air barrier materials are materials that are used anywhere in a building assembly to stop the movement of air into or out of the conditioned space (water vapor can also be transported by air). Air barriers can be mechanically fastened building wraps, self-adhered membranes, fluid-applied materials, insulating boards, non-insulating boards, spray polyurethane foam, poured concrete, metal, glass, and a host of other materials.

Thermal Bridging & Energy Loss

Traditional design and construction practice has concentrated on insulating exposed walls, floors and roofs of buildings, to reduce thermal transmittances (U-values).

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Until recently there has been limited focus on the energy losses that occur at the junctions between construction elements and around openings, or on those losses that occur because of uncontrolled air leakage. As standards of insulation have improved, the proportion of the total energy loss that may be attributed to these causes has increased.

In designing and building for low heat loss, both good insulation and control of air infiltration are needed. Good attention to detailing is necessary during installation for insulation to work effectively and to ensure unwanted air infiltration is eliminated as far as practicable. This translates into “thermal continuity of the insulation” and “air tightness of the building”, which are now key requirements of the Dubai Green Building Regulations.

Design Considerations

– Keep it simple! Simple designs are more likely to get built right.

– Decide which layer of the construction provides the air barrier. Stick with this.

– Minimise the number of different types of construction within the thermal envelope – wherever one form of construction meets another, problems are likely to occur.

– Pay careful attention to the design of junctions between elements to ensure continuity of the air barrier. Think the construction sequence of each detail through, to ensure that it can be built. Change details if it becomes apparent they do not work, or if site staff identify better ways of doing them.

– Favour simplicity of form – complex forms increase the number of junctions within the thermal envelope, each of which increases the likelihood of discontinuities.

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– Minimise penetrations of the thermal envelope, whether by services or structure or construction. A services space inside the air barrier line can help reduce service penetrations.

– Where penetrations are unavoidable (soil stacks, ventilation exhausts and intakes, water supply, electricity and gas supplies), develop appropriate details for their proper execution, for making good damage to insulation, and for re-sealing pipes and ducts to the surrounding air barrier.

Air Tightness Strategy at Design Stage

  1. Simplify the built form where possible.
  2. Define the line of the air barrier as early as possible. Mark up large scale sections with a bold coloured line.
  3. Consider and rationalise construction sequencing.
  4. Redefine the air barrier route and insulation strategy in critical areas to simplify details and avoid problems.
  5. Decide and specify which materials will form the air barrier; Consider:

– Material air permeability

– Buildability

– Position within the construction

– Long term durability

  1. Consider junction details between air barrier materials:

– Practicality of forming the seals on site

– Durability of the seals, especially where not accessible for future remedial work.

  1. Minimise the number of service penetrations through the external wall.
  2. Consider how service penetrations will be sealed.
  3. Rationalise service routes and penetrations.
  4. Highlight air barrier critical elements and junctions on construction drawings.
  5. Apportion responsibility for sealing critical junctions to specific trades.

Further Reading:

Air Barrier Association of America:
http://www.airbarrier.org/

Whole Building Design Guide:
http://www.wbdg.org/resources/airbarriers.php

Air Barrier Brochure:
http://pdf.archiexpo.com/pdf/grace-construction-products/perm-a-barrier-air-barrier-systems-brochure/2545-240591.html

Peel & Stick System v’s Liquid Applied Air Barrier Video: https://www.youtube.com/watch?v=9xe2k8LNjj0

Detail showing 'Tyvek' air barrier on masonry / rainscreen construction

Detail showing ‘Tyvek’ air barrier on masonry/rainscreen construction

Detail showing 'Tyvek' air barrier on steel framed metal cladding construction

Detail showing ‘Tyvek’ air barrier on steel framed metal cladding construction

Example detail showing 'Tyvek' sealing tape around air barrier at structural opening

Example detail showing ‘Tyvek’ sealing tape around air barrier at structural opening