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UK air changes per hour - airtightness requirement?

I have heard that in the UK/England the Building Regs state there can be "no more than 10 air changes per hour". Is this actually true? Or is in just a standard/advisory figure?
I can't see anything documenting this. I am guessing there is no legal requirement buy it would be very useful for me to get some documentation that confirms this as part of my research.



  • Approved Doc LIA (2010) states:

    ''Compliance with the requirements would be demonstrated if:

    The measured air permeability is not worse than the limit value of 10m3/(hr.m2) @ 50 Pascals''.

    I cannot off-hand remember how to convert m3/m2/hr to AC/hr, but I have it somewhere.

    You say ''I can't see anything documenting this''. Have I misunderstood what you mean? I went straight to Planning Portal and the relevant Approved Doc (L1A) and looked in the index.
  • Jason,

    You might be confusing two measurements?

    Part L Building Regs, AD L1A England/Wales limits air permeability to 10m3/m2h tested at 50 Pascals.
    ACH is a PH standard (0.6 or less).

    The two are connected, but unless you are building to PH-certification, the m3/m2h is the metric to use.
  • thanks Nick

    I was googling around but not searching for the right terms to find it......

    does the 10m3 broadly translate to 10 air changes per hour?

  • thanks DarlyP

    yes I am confusing them a little bit - I watched this very interesting documentary on airtightness and it seems to bring those terms together:

    The chap doing the air testing states that new build can be up to 10 ACH an hour to be compliant......I guess he is technically wrong....but is he broadly correct?

    thanks alot
  • There is no direct correlation between ACH and m3/m2h, it depends on the volume of the dwelling and the envelope area.

    I think he has his terminology mixed up...:smile:
  • It perhaps depends on your definition of "broadly"

    a house sitting on a 1 square metre foundation and 1 metre high has a cubic volume of 1m3. The walls have a surface area of 6 square metres (6 x 1 square metre). at 10 cubic metres per hour per m2 surface area, this box is allowed to lose 60 cubic metres of air per hour. As it is only 1 cubic metre in size it can change its air 60 times per hour

    Do the maths again for a 10m x 10m x 10m box: volume = 1000 m3, total surface area of walls/floor/ceiling = 600 m2, loss allowed = 6000 m3; it's 1000m3 in size and can lose 6000m3/hour so it's 6 ACH

    Let's do a terraced house, 5x10x(eaves 5/ridge 8). Surface area = 288 m2, Volume = 325 m3, Loss = 2880 m3/h, 8.8 ACH
    6x6 bungalow with hipped roof 2.5 -> 5m: 139 m2, 113 m3, Loss = 1390 m3/h, 12.3 ACH

    You might say that "on average the regs as applied to an average house might average out at about 10 ACH" but when you're dealing with compliance, and it's this easy to work out, why guess/approximate/average and potentially fail?
  • thanks cjard

    I think that covers it nicely - well as far as, from an outsiders point of view (ie someone who doesn't work in housing directly but is interested in it, particularly energy efficiency) it is somewhat complex and it seems that the buildings regs in terms of compliance and stringency, from an environmental perspective is woefully lame. Those calculations are somewhat over my head and the ACH seems to make more straight forward sense to me! So from what you have said, if I have this right, is that a housem (using your last example) could be compliant but in in fact end up being 12.3 ACH?

    thanks again (this is all very useful for my research)
  • yes woefully lame and the real situation is even worse! As far as I know you are still allowed to fail and many new homes are never tested.
  • Well....for compliance with Part L 2012/2014, most new homes will need to be tested unless they use PV or other renewables to negate the need for an ATT.

    It will be a steep learning curve for a lot of contractors/developers I reckon....:bigsmile:
  • edited February 2015
    Posted By: Jason MatthewsThose calculations are somewhat over my head and the ACH seems to make more straight forward sense to me!
    It's easy enough when you come to appreciate that:

    ACH is "the volume of air the house may lose per hour, expressed as a percentage of the volume of air the house holds"
    m3/m2/h is "the volume of air the house may lose per hour, expressed as a multiple of the total surface areas of the walls/floors/roofs"

    To work out the surface area of the walls, we add them up. To work out the volume is more involved; mostly, houses are made up of cuboids and prisms, but for these examples I 'cheated' and drew a model house in google sketchup, a 3d modelling program, then asked it the volume of that i'd drawn

    The maths is simple, once you have the volume and the surface areas:

    In a passivhaus, take the volume of the house (in whatever units, let's use m3) and multiply it by 0.6. That's the most volume of air (in whatever units, let's use m3) the house may lose in an hour before it fails

    In a regs house, take the surface area total and multiply it by 10. That's the most volume of air (in m3) the house may lose in an hour before it fails

    If a house is 300 m2 and 300 m3, then the regs will permit it to lose 3000 m3 of air an hour (3 million litres). Passivhaus permits it to lose 180m3 of air an hour (180,000 litres)

    So from what you have said, if I have this right, is that a housem (using your last example) could be compliant but in in fact end up being 12.3 ACH?
    Yes. Remember, the tiny box I mentioned (imagine living in one of those white cube shaped plastic water tanks - they hold 1000 lites == theyre 1 cubic metre). According to regs, that cube may change its air 60 times per hour and still be OK!
  • thanks gents - all great of use this.

    You guys have probably seen this, but if not this is a great read:

    I got the impression that most big developments only a few houses were pressure tested and I assume having a look at the latest part L that is still the case? In other words the developers can still so a rough job on air tightness on all the homes, except the ones they get tested?

    From the latest Part L it says:

    On each development, an air pressure test should be carried out on three units of each dwelling
    type or 50 per cent of all instances of that dwelling type, whichever is the less. For the purposes
    of this approved document, a block of flats should be treated as a separate development,
    irrespective of the number of blocks on the site. The dwelling(s) to be tested should be taken
    from the first completed batch of units of each dwelling type. NOTE: Most larger developments
    include many dwelling types. Multiple units of each type should be tested to confirm the robustness of the
    designs and the construction procedures.

    Does that mean only 3 houses could/would be tested most large estates?
  • ... Yes, if the SAP calcs listed at ATT target at 10m3/m2h, and BC were happy to leave it at that.

    However, if the DS SAP calcs indicate a specific ATT value, then they they will have to be tested, or have a 'penalty' of +2m3/m2h added on to the original dwelling's ATT test result.
    And that penalty may well preclude the dwellings from meeting Part L Building Regs.
    So increasingly, all dwellings will be ATT'd, and no bad thing, if it drives up site/construction stds!:devil:
  • I was getting quotes from some builders today, at least one of them now understands air tightness after having work on some projects that fail the park L test.

    He also said about having to put breed of glue round ALL the sides of insulation back plaster board when doing “dot and dab”. But thought that would be good enough without first plastering the inside of block work, provided the blocks were laid by a skilled person.

    So it is beginning to sink in…..
  • edited February 2015
    Posted By: ringithought that would be good enough without first plastering the inside of block work
    No - even if that's effective as air barrier (is plasterboard surface (not its joints) non-air permeable?), if the principal air barrier (against principal air pressure differences like buoyancy) is inboard you still need an outboard 'wind barrier' (e.g. joint-sealed breather felt) to prevent wind buffetting from driving air currents right through the fabric to the inside barrier. Best anyway to put the principal air barrier fairly well outboard.
  • Its a lot better then what builders expected to do 5 years ago! Still why below what should be done.
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