Air Tight for Part L

15 May 2006

Reducing air leakage from buildings is among the most effective ways of improving energy efficiency to meet the Government's reductions targets of 25 per cent. Charlie Greenaway explains how the rules have changed since April - and may tighten further in future

THERE CAN BE FEW CONSTRUCTION PROFESSIONALS still unaware that new revisions to Part L of the Building Regulations came into effect in April. Part L, which has been progressively amended over the past five or six years, is that part of the Regulations governing the conservation of energy in buildings. Previous revisions have concentrated on thermal efficiency - reducing the U-value of various building components to prevent heat loss - but the most recent revisions go further, extending new requirements for reducing air-leakage from buildings.

Designers and builders of commercial and industrial premises might well think that this is nothing new. Since 2002, all new industrial and commercial buildings of more than 1,000sq m floor area have had to comply with new airtightness requirements and must be pressuretested for compliance. But that is no reason for complacency as since then only about 20 per cent of the relevant buildings have actually been air-tested. The past four years were an opportunity to get the calculation methodology for the new Part L right, but it is an opportunity missed.

Air leaking into and out of a building greatly inhibits the structure's energy efficiency. Chilly draughts through doorways, around windows and through walls and floor structures easily undermine the cosiest thermal insulation. Similarly, warm air contained within a building will soon escape through high-level leaks in ceilings, roofs and window openings.

Calculating thermal efficiency in structures such as walls and windows is relatively easy and manufacturers and designers have developed specifications that can be relied upon to achieve a given U-value. However, factoring-in the effects of air leakage adds a whole new dimension to the calculations - and a very variable one at that. Most common causes of air leakage are gaps around service pipe penetrations in walls, ceilings and floors, gaps around windows, doors and service ducts and the intersection between main building elements. Even porous concrete blockwork can contribute significantly to air leakage from buildings.

The Building Regulations' new minimum 'envelope integrity specification' for air-tightness permits a maximum air leakage of 10cu m/h/sq m and is an essential part of the Government's overall strategy to achieve a 25 per cent improvement in the energy efficiency of buildings. Many people in the industrial and commercial property sector might sit back and think they've nothing to fear because they already work to this air leakage specification. But there is a major challenge in the latest Part L revision for them.

As the individual limits have not changed, so some people probably think that nothing which affects the commercial property sector has changed. But the overall standard has been significantly tightened. Using the Government's Standard Assessment Procedure (SAP) calculation method designers have, since 2002, enjoyed a degree of flexibility while still achieving the desired overall building energy efficiency rating. So, for example, if a designer wanted a larger window area, it was possible to sacrifice the thermal performance of the glazing package if you could compensate by making commensurate energy savings in other packages, for example, by using ultra-efficient boilers or adding extra thermal insulation in the walls or the roof.

But in this latest revision of Part L, the Government aims to improve overall energy efficiency by more than 25 per cent. So while individual U-values and air-leakage limits remain the same, there is far less scope for trading off one element of the building against another. Increasing the air-tightness of a building is probably the single most effective way of further improving overall energy efficiency in any type of building. The maximum air leakage permitted under the new Part L is 10 cu m/sq m/h at a pressure of 50 Pa. But it is a relatively simple matter to improve upon this standard in new build and thereby achieve significant energy savings.

Another consideration is that of postcompletion testing. When a building is completed it will receive an asset rating, which determines its energy performance. If the asset rating of the building as built fails, improving airtightness beyond the Part L minimum will be one of only a few options available to bring the building up to the required standard.

Air leakage is one of those areas where you can make a real difference very easily and at little cost. Using the kind of high-performance impermeable and semi-permeable building membranes manufactured by Anderson Monarflex, voids and cracks can be intercepted and the building envelope effectively sealed. It is possible to build a fairly air-tight structure using traditional building methods without the use of membranes, but the standard of construction and the tolerances required are impractical on a day-to-day basis.

A large proportion of energy lost through air leakage escapes through the roof structure. In buildings with pitched roofs, the 'Monarperm Sealed Pitched Roof System' for non ventilated roofs will easily help towards achieving Part L, while protecting against condensation. This system can be optimised by including an air leakage barrier such as A.L.B.200 on the warm side of the insulation and a vapour permeable underlay (for example, Monarperm 700) located on the cold side of the insulation.

On metal roofs, a Monarflex vapour control layer on the warm side of the thermal insulation will prevent warm, moist air reaching the metal surface and condensing. In some cases a breather membrane such as Monarflex SP 400 may be required on the cold side of the insulation. This solution is also suitable for metal wall cladding systems as well as roofs.

Advanced modern membranes such as Monarperm offer a simple and cost-effective solution to air leakage. It does not require ventilation, it is affordable, it controls condensation, it is simple to install and it will reduce air permeability by at least 70 per cent, and further reductions can be made by sealing loose laps with a suitable self-adhesive tape. Although the maximum air leakage permitted under the new Part L is 10 cu m/sq m/h, we are now looking at an overall improvement bringing that figure down to 5 or even 3 cu m/sq m/h.

Compared to the cost of other energy saving measures, improving air-tightness is cheaper not only in up-front costs but also in the long term.Lighting and heating systems are expensive and they're consumable - their efficiency declines over time and they eventually need replacing. But creating an air-tight structure is comparatively cheap and if you get it right, it's right for the lifetime of the building.

Anderson Monarflex will soon be launching a new range of ancillary products to complement its membranes including a neoprene seal to cut air-loss via gaps around pipes and other services and an air-leakage detail tape designed to seal around door and window frames and bond directly with the vapour control barrier.

Many designers will adopt lower air-tightness targets to meet the overall carbon reduction target required by Part L. Currently the regulation will require a 20 per cent improvement in energy efficiency for domestic buildings, but the next round of revisions in 2010 - in just four years time - will probably double this to a 40 per cent improvement. This will almost certainly have higher air-tightness targets.

....Charlie Greenaway is product manager at Anderson Monarflex