To respond to some of the recent posts (thank you Maria, Paul, Kate & Robert), plus to invite interested parties to come up with further questions for us to consider.
Getting the envelope area and volume right for the air permeability and air change rate calculations is essential, as Paul comments the result will be wrong if either the airflow and pressure measurements are wrong or if the area and volume are wrong. So if you cannot work out a building's volume and envelope area, you can't carry out accurate tests. In commercial building testing, which is normally to ATTMA (Air Tightness Testing & Measurement Association) QA requirements rather than BINDT (British Institute for Non-Destructive Testing), which generally covers testers just undertaking air leakage testing of dwellings, we have long accepted that the calculation of areas and volumes off CAD or similar packages helps us to cope with complex shapes, also to deal with unconditioned areas that are outside the envelope for air leakage purposes, such as plant rooms, some fire escape stairs.
But overall it is the responsibility of the test engineer to be satisfied that the envelope is calculated to a satisfactory degree of accuracy, in line with ATTMA Technical Standard 1 (TS1) requirements, and that this is fully documented i.e. a marked up sketch plan if they have measured up directly on site, or records of drawings used and the calculation undertaken when the envelope area and volume is precalculated off-site (which is normally the case for commercial buildings). If presented with a calculation from an architect or other specialist the test engineer should check that it makes sense. One common error when calculating envelopes for dwellings is to use the areas in the SAP calc and then forget to add in the party wall, which although a non-heat loss element for SAP, needs to be included in the envelope area for air permeability calculations.
My advice is to always calculate the floor, roof and wall areas seperately, and to then add these together to get the total envelope. One simple check is: is the floor the same as the roof? If the building is flat roofed, or has a cold roof construction, it should be; on the other hand, if it is warm roofed, or has dropped eaves or sections of sloping ceilings, the roof area would normally be greater than the floor area. If the floor area is given as being bigger than the roof area you can pretty much guarantee that there is a mistake! Another rule of thumb is that the volume (in m3) will normally be fairly similar to the total envelope area (in m2), say with 15% to 20%, unless the dwelling is particularly off in shape (such as scissor flats, for example). The more complex the shape the less reliable this rule of thumb.
Fundamentally, ensuring that testing is reliably and accurately undertaken is a function of the knowledge and awareness of the client and to some extent the amount they pay for testing. If you get the cheapest tester you can find, you probably won't get the best service, particularly if anything goes wrong - some leak location exercises after failed tests have been known to be very cursory, and in commercial buildings are sometimes only commenced once additional payment has been agreed. Not surprisingly, you get what you pay for!
Whilst I wouldn't deny Paul T's comment about poor practice and even corruption in large testing organisations, I don't necessarily think it is limited to large testing organisations. Indeed, what about the corruption on the other side? I have been blatently asked by builders on at least a dozen occassions - "don't bother about the test, how much for the certificate?" Now, none of those are AECB members, but the pressure is often on to get buildings through. The key things to ensure good practice and reliability are once again, knowledge, awareness and involvement of the client/architect and the choice of testers. Large testing organisations work mostly for large builders and not surprisingly develop relationships, want to keep these clients happy, so may tend to lean a little far towards helping them get satisfactory results.
Of course, since often the mainstream construction industry is often only trying to achieve a Building Regulations pass, or perhaps a slightly more onerous air permeability target of 5, some have the attitude that fudging the numbers a bit to achieve something that is a pathetically poor target anyway makes little difference. And the knwledge that buildings often worsen by up to 20% in the first year, is quietly ignored!
With regard to MVHR, the Canadian Super-E standard for dwellings requires every unit to have MVHR and to achieve an air change rate of 1.5 ACH @ 50 Pa. Moreover they routinely achieve 1.0, including in Super-E in the UK (although there aren't a lot of these yet). So I think an absolute maximum for MVHR should be 1.5 ACH @ 50 Pa, with a normal target of 1.0 ACH @ 50 Pa. Of course, if the fans are the lowest possible consumption, that helps the economics and the carbon balance. Large dwellings and complex duct systems can increase the resistance to flow and hence the energy absorbed in making the MVHR work. In several buildings I have advocated the through-wall MVHR units to avoid lengthy duct runs, although these do have additional leakage risks and maintenance requirements, and lower efficiencies in practice I'm sure.
I'm involved in a current project where it seems the builder would only tender on the basis of an air permeability target of 5 m3/hr/m2@50 Pa, but where the installation of MVHR systems has gone ahead regardless. I think this is very unsatisfactory and the developer should have known better - mind you, if the target was dropped to 1.5 as I and others advocated the builder would have been stuffed, since they are struggling to get the 5 in some units. In this case I think it very likely that the carbon footprint of this so-called green development will be worsened since they'll be using extra electricity to operate the MVHR and not getting a good return because most of the warmed air will be escaping through the holes.
The number of air leakage tests ultimately depends upon the target (harder targets frequently need more tests) and the skills and experience of the builder and often the effectiveness of the site supervision. As a general rule, I would always allow 2 tests and if going for Super-E (ACH of 1.5) or PassivHaus (ACH of 0.6) I would allow three. Site leakage audits, particularly a couple of weeks before testing is due, can often also be very useful, particularly on commercial projects or multiple dwellings.
The PassivHaus target of 0.6 ACH @ 50 Pa is meant to ensure that there are no draughts within the dwelling with a velocity of 0.1 m/s or more, which is the level at which our bodies notice them. If you can prevent stronger draughts, the proposition is that the demand temperature can be dropped by 2 degrees, with consequent huge energy savings. Certainly experience of PassivHaus or other airtight buildings does suggest that comfort conditions are very good, but whether the same savings would be demonstrated in the UK we'll have to wait and find out.
Anyway, this has been a very long post, but hopefully still useful. Please keep coming back with questions, further issues, so we can get the best possible FAQs out the end.