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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorjms452
    • CommentTimeApr 2nd 2021 edited
    Just found this page (on a site I have an interest in). What's the GBF hive mind thoughts on this one?


    'Building owners are often told to install whole-house mechanical ventilation with heat recovery when it is not required.'

    'As a rule of thumb, installing whole-house MVHR is only worth it if:

    you have eliminated all thermal bridges
    the walls have a U-value of 0.12 to 0.15
    and the air permeability of the thermal envelope (building) is at or below 1.5 air changes per hour (ACH) when tested at 50 Pascal (Pa) (equivalent approximately to 3 m3/m2.h @ 50 Pa for average dwellings) (NB: the Passivhaus target is 0.6 ACH @ 50 Pa).'

    I have a few issues with this:
    Has anyone ever eliminated all thermal bridges? addressed maybe but eliminated...
    It doesn't define what worth it is (cost, energy, comfort, noise ...).
    Fundamentally the energy saving from whole house MVHR (in a house that needs the ventilation) is the same whatever the wall u-values & thermal bridges...
    • CommentTimeApr 2nd 2021 edited
    jms452 asked: "Has anyone ever eliminated all thermal bridges? addressed maybe but eliminated..."

    How do you define 'thermal bridge'?

    I don't like their suggested alternative of single-room HRV in wet rooms. Too great a likelihood of cold draughts IMHO.

    If a house needs the ventilation (i.e. is sensibly airtight) then it's a straight choice between a non-heat recovering system (PIV or MEV) and an MVHR (including also the various passive and distributed options). Not sure how to compare the capital and operational costs, let alone the carbon costs.

    The article is quite old (2013) so perhaps some of its concerns have passed their sell-by date? Maybe it's worth asking them if they still think that?
    • CommentAuthorEd Davies
    • CommentTimeApr 2nd 2021
    I'd think “As a rule of thumb, installing whole-house MVHR is only worth it if … you have eliminated all thermal bridges” is back to front. Having a few difficult-to-deal-with thermal bridges is a reason in favour MHRV because, apart from the increased heat loss, the main problem with thermal bridges is that they're likely to be condensation spots and an important function of MHRV is to reduce indoor humidity and hence reduce the risk of condensation.
    • CommentAuthorMike1
    • CommentTimeApr 2nd 2021 edited
    Superhomes haven't got it right, IMHO.

    Thermal bridges and U-values relate to heat transfer through the fabric of the building, which impacts the heating / cooling systems, rather than the ventilation system. Airtightness is - or was - the factor that relates to ventilation & MVHR, not fabric losses. As you mention, jms452.

    It was thought that MVHR started to be worthwhile when airtightness fell below 3 air changes / hour (for example by R Lowe in 'Ventilation strategy, energy use and CO2 emissions in dwellings'). However that particular paper was published 20 years ago; MVHR systems are now more energy efficient, the carbon intensity of the grid is now much lower, and the assumptions underlying that and other more recent reports have been questioned.

    Last year's Passivhaus Trust paper 'The case for MVHR' suggests that MVHR 'performance outperforms natural ventilation by between 20 and 30% in terms of carbon emissions for all levels of airtightness'. https://www.passivhaustrust.org.uk/guidance_detail.php?gId=46

    On top of the thermal factors, improved air quality & the elimination of trickle vents, etc., are also major benefits.
    • CommentAuthorjms452
    • CommentTimeApr 2nd 2021
    Great reference Mike, I'll mention it to SuperHomes.
    • CommentTimeApr 2nd 2021
    Excellent :bigsmile: The Good Practice Guide it refers to looks quite useful too; albeit slightly out of date now.
    • CommentAuthortony
    • CommentTimeApr 2nd 2021
    The notion that air tightness has to be below a certain level for it to worthwhile

    MVHR is about air quality, economics and quality of life. Air tightness is only relevant when it is windy and the windier it the greater the levels of unwanted and uncontrollable ventilation will be. On a calm day ventilation is still necessary. To rely on air leakage (even TV’s) is not good for quality of life or for heat losses.

    Economically the arguments are only justifiable on windy days

    All buildings should be reasonably air tight and properly ventilated, MVHR offers near perfect air quality, good quality of life, most likely longer life too, these have nothing to do with economics.

    PIV should be illegal and though good for the building and landlord is horrendously expensive for the bill payer though they will get good air quality so long as they don’t live in a polluted city.
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