Design and Detailing for Airtightness

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Key Principles

1. The Contractor or Project Manager must be made responsible for achieving the airtightness levels set. In particular, this will involve co-ordinating between trades.

2. Inspection remains an integral part of achieving airtightness.

3. Ideally at least 2 pressurisation tests will be undertaken; the first when the building is weathertight, and the second a couple of weeks or so before handover.

4. Experience suggest that making one person (or team) responsible for airtightness is the most effective way to tackle the issue.

5. Remedial airtightness works to existing properties can reap substantial benefits without undue disruption.

It is not yet generally possible within the UK to specify that a building shall be airtight and leave it to the Architect or Contractor to sort out. There is not yet a culture of airtight construction, except perhaps, amongst those who construct superstores.

The responsibility of the Designer cannot be overestimated, for if airtight buildings are to become mainstream, as they are elsewhere in the world, the techniques must be above all simple and buildable, with most if not all of the ‘tricky’ areas designed out from the start. In this way, such techniques can become ‘second nature’ to Contractors and there is less reliance on potentially adversorial inspection and testing.

Ideally too, the Designer will understand the issues sufficient to prepare a sound performance specification – giving achievable targets for airtightness as well as a clear description of responsibilities and procedures, and a clear and practical set of overall and detail drawings, along with a detailed specification.

In the meantime, and even with good documents, there is likely to be a need for effort and vigilance by both the Design Team and the Main Contractor or Project Manager on site. This chapter briefly describes this effort, while the next describes in more detail the actual test procedures and auditing techniques used.

4.1 Plan of Work

The RIBA Plan of Work provides a framework for the entire design and construction process. The table on the next page allocates specific tasks relating to airtightness to each Work Stage to enable a schedule of tasks and responsibilities for the Design team to be prepared according to each project.

4.2 Roles and Responsibilities on Site

Designer / Design Team

The responsibilities of the Design Team are detailed on the following page, showing all stages including site works and beyond.

It is critical that the purpose of pursuing airtightness is explained so that all concerned understand why they are being asked to attend to these issues. The initial briefing of key personnel at mobilisation stage – whether or not this involves the airtightness specialist – is also critical in determining the approach to conducting the works, inspection, testing and auditing etc. which will need to be dovetailed into the many other concerns on site.

On large projects it may be useful for one member of the Design Team to take special responsibility for airtightness issues.

Contractor

The Main Contractor’s principal responsibility is to deliver the airtightness performance overall and the most likely task on any but the smallest jobs will be that of co-ordination between the subcontractors. The Main Contractor must be clear that he carries responsibility for the overall airtightness and in turn must ensure that all subcontractors are clear about the extent of their responsibilities. This is important since there may be some deviation to conventional practice in order for airtightness to be achieved.

As with the Design Team, experience suggests that the best performance has been achieved by Contractors who employ a dedicated individual (or team) to carry responsibility for airtightness, to inspect the works and instruct as required.

For Contractors, the issues of airtightness are intimately linked to issues of good or bad workmanship in general and this can make the issue both more sensitive, but also more difficult to control. Even simple buildings are immensely complex and so the most important aspect of all is the creation of an overall culture of careful, tidy, accurate and airtight construction, something which cannot be simply forced through with a performance specification.

It is easier to specify and draw an airtight detail than to build it, and so the emphasis on inspection and Contractor responsibility has not developed from a prejudice against Contractors, but from a realistic appreciation that this issue cannot be entirely resolved ‘on paper.’ It is genuinely about a culture shift (at least for many in the industry) and this is where the real challenge lies.

4.3 Inspection

The pie chart, below, indicates the disposition of air leakage found in dwellings according to studies undertaken by BRE (9).The studies offer a range of conclusions, the most significant of which is that the greatest volume of air leakage is occurring in areas outwith the ‘normal’ consideration of ventilation, through the myriad of cracks and openings all over the building which is described as ‘background air leakage.’

Pie chart indicating the disposition of air leakage found in dwellings by BRE.

Of the background air leakage subsequently investigated, the principal leakage routes were noted as being:

• Plasterboard dry lining on dabs or battens, often linked to routes behind skirtings etc.
  
• Cracks and joints in the main structure; open perpends, shrinkage & settlement cracks
  
• Joists penetrating external walls, esp. inner leaf of cavity walls
  
• Timber floors, under skirtings and between boards
  
• Internal stud walls, at junctions with timber floors and ceilings
  
• Electrical components, sockets, switches and light fittings
  
• Service entries and ducts
  
• Areas of unplastered masonry walls; intermediate floors, behind baths, inside service ducts

It is perhaps worth mentioning that the BRE results were based on buildings using dry lining on masonry walls and timber floors. Had the masonry walls been plastered, if concrete floors had been used, and if basic airtightness measures were taken, it is likely that the principal problems would occur around service penetrations, and, to a lesser extent, around windows, doors and rooflights. This is the experience of countries where envelope airtightness generally is more developed.

The following table lists many of the most common infiltration problem areas. On larger projects, common problems include:

• Incomplete bulkheads at eaves;
• Gaps where blockwork abuts to steel columns or beams (right);
• Uncapped cavity walls, at eaves (right) and mid-points where cavity walls change to composite panels;
• Gaps along the underside of corrugated roof linings - even if profile fillers are used poor workmanship is common (right);
• Perforated (acoustic) roofs, where the unsealed mineral fibre acoustic layer bridges the eaves of the building, consitituting a major leakage point (right) ;
• Gaps where plasterboard or wall linings are incomplete, commonly above suspended ceilings and to the underside of beams (eg. p13);
• Incomplete door and window reveals (right)
• Services Penetrations into the building, and between zones inside the building (lower right).

Another common issue is porous blockwork, particularly when internal walls are drylined rather than plastered or painted. Where this is likely to be unavoidable, it may be worth requiring blockwork to be tested for air permeability, and to have an AP value (by an accredited lab) that is no more than 50% of the target Air Permeability for the overall building.

Common Locations for Inspection
(Applicable to all types of Construction)

Foundation / Ground Floor
Check wall and floor dpcs form an adequate air tightness layer, is a separate layer needed?
Check gaps at perimeter insulation strips
Check potential movement gaps between loadbearing structure such as columns and adjacent non-
loadbearing slab

First and Intermediate Floor Levels

Concrete floors:
Check joint between the floor and plasterboard to walls
Check gaps between concrete planks, or beam & blocks are sealed at the wall
Check voids under floor finishes and service run penetrations

Timber floors:
Check a membrane seal has been incorporated if required
Check any membrane used is supported between joists

Eaves and Verge
Check continuity of airtight layer between wall and roof / ceiling

Ceiling level beneath the roof
Check for separation between deliberate roof ventilation and the conditioned zone
Check for service penetrations and hatches which pass across the airtight layer

Boundaries between different wall envelope systems
Check all systems have a dedicated airtightness layer assigned, and that these can be constructed
to be continuous across dissimilar elements

Windows and Doors
Check that the frame to wall junction is properly sealed and continuous with the wall airtight layer,
particularly at cills
Check the windows and doors have appropriate weather seals between the opening unit and the
frame

Services penetrations
Check for proper seals at service entry points, and at points of entry into conditioned zones. These
may also require fire protection

Main Entrances
Check that the whole entrance area is separated from the conditioned zone by an inner airtight layer

Lift Shafts, Service Cores, Delivery Areas / Car Park
Check these have been separated from conditioned zones with air barriers and draughtproofed access doors

[Based on Notes produced in BRE BR448: Airtightness in Commercial and Public Buildings.]

4.4 Testing and Audit Schedule

In many cases to date, an air leakage test has been carried out a week or so before practical completion. If the result is poor – a high rate of leakage – then a great deal of work suddenly needs to be done, often to areas which have been covered up and the whole business can be both costly and time consuming, just at the point where in many contracts there is already considerable pressure on Contractors.

Far better therefore to schedule the air leakage test at a time where remedial works are relatively simple to perform. On the other hand, it is important that a test is undertaken close to handover so that the Client and Design Team can be sure that the completed building accords with the performance specification.

Ideally therefore, two tests at least should be carried out. The first should be undertaken as soon as a meaningfully air- and weathertight envelope has been installed. Ideally, all air barriers are still accessible and any defects can be readily put right. This test, plus the audit techniques which are likely to accompany it, may be used to ensure an acceptable airtightness performance and give a good indication of where subsequent works may advantageously targetted.

In this way, the second and final test serves simply to confirm the performance of the building, hopefully at a slightly improved level from the first test, without the need for costly and complex operations late in the day.

Such a test schedule is nonetheless costly in itself, but for those who have been involved in such testing schedules, experience suggests that this remains the most cost effective way to deal with the issue. Certainly it is worth avoiding excessive remedial works at the eleventh hour. With a sufficiently good first test performance, it may even be possible to dispense with the final test, if this is deemed acceptable to the Design Team Leader or Client.

It is often the case that the envelope is not sufficiently complete on the due date for testing. This then necessitates a complex process of temporary sealing of the incomplete areas. It is harder then to ascertain the location of the leaks and allowances are made which may prove misleading. Experience suggests that this is not ideal and it would be better to put off the test for a week and carry it out when the envelope is complete and ‘as intended’.

On larger projects, more tests may be needed, or more specific tests of individual areas required. Large projects with multiple units of a similar nature may benefit from either pre-installation component testing, or insitu testing of one installed component to establish acceptable airtightness levels early on. See also Section 5.4.

4.5 Remedial Airtightness Works

With airtightness testing and a general awareness of airtightness issues developing around new build situations, the principal area of concern, as with energy efficiency in general is the existing building stock. In terms of airtightness, the UK building stock is considerably worse than comparable northern latitude countries [10] and there is a good deal of room for improvement.

Either as a stand alone measure, or as part of a package of energy efficiency measures generally, there is scope for remedial works to most of the existing UK building stock. Relatively simple measures may in many cases be sufficient, using a wide range of sealants to control air leakage. However, it is important that such measures are combined with attention to the ventilation requirements of buildings where, to date, insufficient ventilation has been ‘augmented’ by infiltration and exfiltration which, if reduced, could lead to other problems.

As with thermal insulation, there is an extent to which controlling some of the air leakage merely diverts the flow of air, inward or outward, to another defect or gap, but there is such scope for improvement that even fairly basic efforts are likely to reap substantial environmental, financial and comfort benefits for owners and occupiers alike.

There are many examples of remedial works described in the various publications noted in the references. Some of the more successful measures included carefully sealed secondary glazing installed where old windows had to be kept for conservation purposes, draughtproofing of doors and entranceways generally, and installation of lobbies in well trafficked reception areas, attention to draughtproofing of existing windows and targeted use of flexible sealants to ill fitting components and joints between different construction types.

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Footnotes:

10. See, for example, BRE Information Paper 01/00, January 2000 and Limb, MJ. Ventilation and Building Airtightness: an international comparison of standards, codes of practice and regulations. AIVC Technical Note 43, Coventry February 1994.

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