Go to Forum Home Building Design What is the minimum permissible ach/hr?

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    • #30622
      Mark Siddall

      NHBC requirement D15 gives mandatory air changes ranging from 1 per hour (for bedrooms) and up to 2 per hour (for bathrooms, kitchens and toilets). Based upon my earlier calcs this is considerably higher than the rate of 0.3 to 0.4 air changes per hour recommended to meet the Gold standard.

      Has the NHBC been approached by the AECB?

      Building Regs Part F 0.12 states that ’where special measures are taken [to achieve air tightness levels of less than 3-4 m3/hr/m2] additional ventilation provisions may be required’ could this be taken to mean MV? If not has building control been approached at a national level?

      If these matters are not resolved then it could inhibit the adoption of the AECB Gold (and Silver?) standards. Given that 6000 Passive Houses have been built on mainland Europe, and that the Gold standard very closely reflects the PH standard, there is surely a case for the NHBC revising the requirements.

      In this context the following may be relevant to discussions with the appropriate bodies: –

      In Design with Energy by Littler and Thomas (1984) they cite BRE documentation (Ventilation requirements BRE Digest 206 1977). In an example Littler and Thomas note that in “a closed living space of say 50m3 with four people in it the ’intermediate’ situation (referring to a graph), curve 2 of fig 3.4, calls for about 2lts/sec per person or roughly one-half an air change per hour.” Later on they discuss other air contaminants including Radon and site Brundrett as stating “present evidence suggests that to avoid danger the minimum ventilation rate in Britain should be 0.2 ach/hr”(Building Services March 1981).

      Whilst Littler and Thomas also note that Fuller in the US believes that 0.5ach/hr can keep indoor pollutants below critical levels (“What’s in the air for tightly built houses” in Solar Age June 1981).


      BRE Background ventilation of dwellings: A review (Report 162)
      The report suggests “a minimum whole house ventilation rate of 0.5ach/hr with provision of an additional 30 m3 of controllable ventilation in each room. Under typical conditions this allows for up to 3.5ach/hr in a lounge if ventilation is used to the full.” I note that Report 162 also states that the whole house ventilation rate of 0.5sch/hr was dependant upon a RH of 70% and U-values not exceeding 1.0w/m2k for walls and 0.5w/m2k for the roof.

      As the U-values of the Gold standard fall much below this target (i.e. they are <0.15w/m2k) and the RH is generally maintained at a low level (with the exception of cooking), I would question whether this higher air change rate noted in Report 162 is required in order to prevent condensation within the UK climate (i.e. the surface temperatures will be higher thus reducing the likelihood of condensation occurring.)


    • #33588
      Mark Siddall

      Thanks and agreed. The point was really about the AECB seeking wider acceptance/recognition of the standards and the comflicts between existing standards and those set by the Gold standard (which is not to say that current UK standards a better, rather that UK standards could do with revision to include, reflect and accomodate current European best practice).

      The issue of task ventilation is pertinent to the other topic that I have raised on cooking and NO2. https://aecb.net/forum/index.php?topic=761.msg2795#new

      (You picked up on my Freudian slip “air tightness levels of less than 3-4 m3/hr/m2” oops. I know what I meant…all the same glad you clarified.)


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