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Design and Detailing for Airtightness home | introduction | context | designing | implementing | testing | details |
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| 3 Designing for Airtightness | |||||||||
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The Performance specification may be the only document needed by the Architect / Designer / Client if the building is to be procured through Design and Build or similar route. However, it is more likely to be part of a suite of documents including detailed drawings. The performance specification allows appropriate targets to be set for the project, along with a description of how the process is to be conducted, in terms of scheduling, audits and testing, and potentially remedial works. Given the increasing use of specialist subcontractors, particularly in larger projects, it is also critical that the performance specification sets out both the responsibility for, and constructive guidance regarding the co-ordination of trades with respect to the final air permeability of the completed envelope. A sample specification clause is shown below, which could be adapted for specific use. Once appropriate targets have been set for the project, the next task
is to identify zones which require greater or lesser airtightness levels.
Ideally, these zones need to be identified on a drawing which also identifies
the specific air barriers in red. |
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Showing conditioned (heated or cooled) areas as distinct from unconditioned, with overall airtight separation highlighted in red dashed lines. The example highlights the value of simplicity at an early stage; allowing unheated spaces to project into heated ones like this will complicate the process of constructing effective air barrier layers later. Heated zones need to be kept separate from unheated zones such as roof voids, delivery bays etc. whilst service shafts may require particular attention. Boiler rooms with large flues and intake vents may need to be separated. Entrances are often significant sources of draughts. Lobbies with doors set apart by around 4m, so that one door closes before the second is opened, can be effective, whereas in highly trafficked areas revolving doors are likely to be preferable. Tall buildings, with atria, stairways and service shafts all of which rise through the building can be prone to ‘stack effect’ air movement whereby warm air rises, dragging in cooler air from outside at the lower levels creating more acute air leakage problems. A number of tactics may be employed to reduce the effects, but in any event issues of airtightness are likely to be highlighted in these cases. |
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With the zones and air barriers located, it is necessary to design the air barriers themselves. To be effective, the air barrier must:
The last of these is important since there is evidence that the airtightness of some constructions will tend to decrease over time, and in particular the first period after completion. There are a number of strategic measures which can be employed to simplify the business of designing an airtight building. Since service penetrations in and out of a building provide a major source of air leaks, one strategy is to collect all such penetrations into one accessible area, see right. In construction types such as steel and timber frame, it is usually wise to employ a specific membrane or layer as the air barrier, rather than rely on sealant between, for example, the sheathing boards. Such a membrane can usually double up as the vapour barrier if used internally and gives the Designer the opportunity to consider and address airtightness explicitly, rather than as a function of other elements. Bear in mind that most membranes are flimsy and will need support in all areas. Another strategy is to employ service voids. Creating a service void internally allows for alteration and maintenance of services and finishes without recourse to penetrations through the air barrier. This allows for long term good performance in contrast to membranes which are liable to penetration at all service points, necessitating careful sealing of each and every penetration, not only initially, but over the years of alterations and maintenance to come. Generally, it is better to conceive of the joints in airtight layers as ‘positively’ connected, anticipating differential movement and decay of adhesive or chemical bonds. For example, where different components of a curtain walling system are liable to differential movement, it is clear that a joint whereby the two components are held together with a positive mechanical connection across a compressed gasket is likely to remain airtight longer that a simple butt joint with a mastic sealant between. Finally it is clear that complex solutions to airtightness are likely to be more prone to poor execution and potentially to greater vulnerability to differential movement, failure of sealants, dislocation of components and so on. It is important therefore to aim for the simplest solutions to providing an airtight layer, using the fewest separate materials, junctions and penetrations, and the easiest installation and maintenance. It is worth making a point of considering each and every specified component with regard not only to its own intrinsic airtightness characteristics, but with regard to the connections between it and adjacent components. It is important to provide explicit details and guidance at specific, and particularly tricky detail areas. On design and build contracts it may be necessary to allow for some form of review of proposed solutions and procedures. The following provide a few examples whereby airtightness can be simplified at the earliest design stages. However good the workmanship, blockwork on its own can never be considered airtight. Once plastered, on the other hand, it may be considered extremely airtight, with concern only for those edges and corners where cracking or gaps can appear. This may be contrasted with the more common practice of drylining block walls with plasterboard on battens or dabs. In addition to the intrinsically non-airtight block wall behind, this form of construction typically gives rise to a wide range of air leakage paths behind the boards and into floor, partition wall and ceiling cavities. From the perspective of airtightness, drylining should be avoided unless great care is taken. See right. Similarly, timber floors are difficult to seal well without a good deal of care. On the continent - and to an increasing extent in the UK at large - concrete floor systems are being used for both ground and first floors (often for other reasons such as acoustics, fire and the desire for underfloor heating) and these are easier to make adequately airtight. Hollow planks however can leak into cavities and require to be sealed at their ends. One important and often quoted example is the timber first floor connection with a block wall inner leaf. Who is responsible for ensuring absolute airtightness when the timber joists rest on the wall and are infilled between with block and mortar? Presumably the bricklayer, but is it then his fault if the timber is installed at the wrong moisture level and subsequently twists and warps, leaving cracks around every joint? Is it really feasible to attempt to tape or mastic seal around them all, and what if the underside of the ceiling is to be exposed? (See right) Far better perhaps, to do away with the joist-onto-wall detail altogether and replace with joist hangers(8). Increasingly, the designer should be seeking solutions which are intrinsically airtight because of the design, rather than continuing as before while accepting an increased use of duck tape and mastic on site! Whilst these may get you through the initial airtightness tests, they are are sort term solutions and not likely to lead to the anticipated energy savings for the Client in the long term. A good review of the various materials and components which allow the Designer to create an air barrier may be found in the BRE Report BR448: Airtightness in Commercial and Public Buildings. 3.4 Detailed Specification Beyond the performance specification illustrated earlier, it is important that the issue of airtightness becomes embedded within the standard specification vocabulary. Where an equal or approved alternative may be allowed, it is critical that an airtightness performance specification is part and parcel of that equality of performance. For example, it may no longer remain satisfactory merely to specify a membrane, but in addition to specify the fairly precise nature of the sealing, overlapping and potentially the subsequent layers as well. Simply offering a performance specification and ensuring the responsibility resides with the Contractor is all very well, but it is important too to offer solutions that will enable a satisfactory outcome to be achieved. |
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Footnotes: 7. It is likely that TM 23 is going to be revised and in the meantime UKAS approved testers (which must include ATTMA members) are testing instead to BS EN 13829:2001(1) Thermal Performance of Buildings: Determination of Air Permeability of Buildings - Fan Pressurisation Method. 8. Manthorpe Building products (01773 514 200) and www.manthorpe.co.uk produce a ‘joist seal’ or boot which allows joists to be built into block walls without the attendant disadvantages noted above. back to top | contents | next chapter
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