Airtightness is Next to Godliness...

...so why do we try to achieve it on-site with a mastic gun or expensive tape, rather than designing it in properly?

Spelled out like that, it’s obvious we need to find a better way to ensure airtightness performance. Martin Goss, MD of Offsite specialists Mtech Consult, explains how modern factory production methods can deliver very high levels of airtightness... for FREE!

With new Part L of the Building Regulations almost upon us, requiring a further reduction in the energy consumption of new buildings, there is increasing focus on the insulation performance of the building envelope.

However, as wall, floor and roof elements start approaching U values of 0.1 W/m2K or better, the benefits of adding yet further insulation diminish rapidly. At these levels of insulation performance, fabric heat loss is no longer the key factor in energy consumption and CO2 emissions. Issues such as domestic hot water, lighting and air infiltration take over as the main source of energy demand. On a building designed to comply with the minimum requirements of new Part L, the energy required to equalise the temperature of the air infiltration can be as high as 20% of the total building energy demand. Specifying ever higher U values is not the solution here; instead we need to focus on how we can effectively seal the building envelope to control the air movement across it.

While new Part L does not seek to reduce the minimum value of air infiltration from the current 10m3/m2/hr @ 50 pascals, the demand for a 25% improvement in energy consumption is nevertheless likely to force building designers to reduce air infiltration rates to well below this minimum standard. And 25% is just the start... housing is targeted to become carbon neutral by 2016; other construction sectors will follow.

As constructors who have experienced the challenge of delivering airtightness of 3m3/m2/hr or less on-site with traditional forms of construction will know, this can be an unequal struggle. It inevitably means chasing the leakage paths with a mastic gun in the hope that, at least temporarily, the building envelope will deliver the required performance result; ratings achieved are rarely retained through the service life of the building and often deteriorate rapidly.

Offsite manufacturing delivers airtight construction

This is where Offsite manufacturing of the building elements can really make the difference, routinely achieve levels of tolerance and fit that are not possible on-site.

In a factory production process, product and system design, component interfaces (doors, windows, structural junctions, etc.), assembly techniques and the materials used are all developed, refined, planned and controlled to maximise performance, delivering products that are intrinsically airtight... so it’s ‘for free’!

Whether it’s a wall element, floor cassette or a roof panel; for volumetric modular or closed panel construction, assembling as much of the complete fabric as possible in the factory ensures air leakages are minimised:

• Where pipework and electrical services penetrate the building element, it is relatively easy to seal reliably when the wall, roof or floor panel is at waist height with good access and lighting, and the right equipment and sealant materials to hand. It also avoids on-site contamination from dust, dirt and water, all of which decrease seal performance.
• The same compliance approach is taken where fixings for the internal lining will penetrate the vapour control layer (VCL) or cladding fixings penetrate the breather membrane. Proprietary sealing tapes can be fitted in the factory to ensure potential air leakage paths are minimised when the internal lining is fixed on-site.
• The jointing of breather membranes and VCLs can be maintained to very high standards; specialised adhesive techniques can be used to ensure high levels of jointing integrity. Some high-performance adhesives, requiring specific and stable temperatures, can only be used in a controlled factory environment; thus on-site processes will never deliver the benefits offered.
• Window-to-wall interfaces are well known for air leakage. On-site this can be an awkward detail, but in the factory environment, where windows are pre-installed into the closed wall panel construction, VCLs and breather membranes are fully visible and accessible to the operator, so a fully airtight seal can be achieved & inspected every time.
• Factory process can handle large format materials not permitted on-site... 6m lengths of plasterboard or OSB, large sheets of floor decking, even VCLs & breather membranes in 3m widths... creating elements with fewer joints, thus fewer opportunities for air leakage.
• We can introduce inspection hold points and airtightness testing to ensure panel performance before the application of claddings or lining boards; whilst this is theoretically possible on-site, in practice the work flow rarely makes this a feasible proposition.

And the repetition of factory manufacture, in a controlled, standardised environment, delivers reproduceable reliability with the opportunity to try, test & improve systems even further.

On a personnel level, it is much easier to establish a culture and understanding around the need for airtightness across a relatively small and constant factory workforce with a standard way of working. The disparate groups involved on a traditional construction-site make this discipline and culture much more difficult to embed and retain.

Mtech Consult’s airtight design experience

At Mtech Consult, we are experienced in designing Offsite construction products to achieve excellent values of airtightness, then developing production to ensure the Offsite technology delivers this exemplar performance.

From routine prototype test work it is clear that even in the factory environment, multiple barriers to air flow across the structure work best. It is rarely practical to make a single layer perfectly air tight in the accepted sense. Successive layers, which in a wall construction could include the breather membrane, sheathing boards, VCL and dry lining, can all combine to restrict the overall air flow. Assembly teams need to understand the importance of each of the elements that make up the total air flow barrier; in the factory environment all these layers can be applied by just one assembly team, making compliance significantly easier.

This test work has also highlighted the fact that intermediate floors are a particularly weak area as far as air leakage is concerned; a big concern for apartment buildings. In a traditional built-up floor there are many relatively small components, brought together to complete the overall floor structure. In the factory environment we are now encouraging manufacturers to use large format boards wherever possible; increasing labour efficiency and reducing the number of joints, hence reducing areas of potential leakage.

Building Regulations recognises the benefits of Offsite construction

Factory forces a high level of design discipline: all interface details and tricky areas where breather membranes or VCLs come together have to be fully resolved and detailed so production can proceed in accordance with agreed procedures... there are no ‘work-arounds’ or situations where individual operators make their own decisions on what is satisfactory or how a particular detail is to be achieved.

The airtightness of the factory finished panel element or volumetric module can be reliably assumed from the extensive prototyping and type testing undertaken beforehand to verify the standard details and factory processes. This forms a standard part of the assessment methodology involved in product accreditations such as BBA, BRE or BM TRADA. Indeed, even Building Regulations now accept that factory manufacture of building elements can be relied on to achieve high levels of airtightness. Both the existing and new Part L consider factory produced modules can be type tested, relying on proven factory quality control procedures to ensure on-going production modules achieve the required airtightness; completely eliminating the need for on-site airtightness testing.

This is a very important step forward for the Offsite sector and is the first time industry regulators have understood and accepted the potential benefits of factory based construction. Mtech Consult is continuing to work with the relevant trace associations and BRAC (the Building Regulations Advisory Committee) to identify other areas where factory construction techniques can reduce the need for on-site compliance testing and verification.

Embedding ‘Build It Tight’ on-site culture

The route to airtightness doesn’t end once factory finished elements are dispatched to site. It is essential there’s no loss of airtightness when Offsite elements are joined on-site to form the complete building façade or superstructure. Site jointing systems need to adhere to factory tolerances to ensure the individual elements come together accurately, as intended by the design, and the purpose-designed element-to-element air seals achieve the required integrity.

The high degree of repeat accuracy achieved in-factory – purpose made assembly jigs can deliver maximum tolerances of ± 2.0mm on a dimension of 15m for even very large elements of the structure – allows us to develop and introduce simple to achieve, yet highly reliable, air seals on-site.

So, accurate assembly with good detail design is clearly imperative; however, in our experience it is vital to combine this with site erection operatives who fully understand the need to achieve airtightness. Over the past few years Mtech Consult has put considerable effort into developing and delivering training programmes for specialised site erection teams operating across the UK, installing systems for the major Offsite manufacturers.

This is perhaps the final piece in the jigsaw; ensuring installation teams deliver and handover the intended design on-site. It is clearly much easier to embed the philosophy of airtight construction into a small and highly focussed team of Offsite installers than the disparate and generally fragmented subcontractor teams that make up the bulk of the UK’s construction workforce.

If we are to achieve the level of airtight construction demanded by future revisions of our building codes, then we will either need to rely far more heavily on Offsite construction techniques or embed this “build it right; build it tight” mentality throughout the UK’s construction workforce. Most likely we will need to do both.