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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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  1.  
    I’d like the forum’s help with a thought experiment (involving some maths).

    For context, we’ve identified a thermal bridge at the corners of a proposed office building. The envelope subcontractor is proposing to change the psi-value test criteria to -2° rather than -5° to scrape a pass, on the grounds that it doesn’t get that cold very often.

    I’d like to demonstrate the amount of water vapour that is likely to be produced by the occupants, as a ball park figure, to illustrate why we’d be better off eliminating the thermal bridge or using a VCL than relying on balmy winters.

    I scraped the following figures from the web:

    • normal respiratory rate for an adult at rest is ~15 breaths per minute.
    • 0.5 litres per breath, so 7.5 litres/minute or 450 litres exhaled/hour
    • when you exhale, you release up to 17.5 milliliters of water/hour (assuming exhaled air is 37°C @100% RH)

    The highest occupancy room is around 120 persons in a room that is 350m² and 3.5m high. So the volume is 1225m³ or 1,225,000 litres.

    Average ventilation rate from AHUs is 12 l/s/person.

    Internal temperature 20°, RH assumed at 50%, Dewpoint is 9.3°

    So each hour, the 120 people are generating 2.1 litres of extra water vapour (120 * 17.5ml). But that is going into a space that receives (12*120*3600) 5.2M litres (?!) of fresh air in the same time period.

    Does that mean I’m worrying about nothing? Or will there be other factors that I haven’t considered? Bear in mind this is just a ‘back-of-the-envelope’ assessment.
    • CommentAuthortony
    • CommentTimeDec 14th 2023
     
    I don’t think the number of people or how much water they produce is that relevant

    RH is, presumably the building will be ventilated

    I always use a vb , but it won’t prevent Condensation if the vb itself goes below dew point condensation will form on it.

    My rule of thumb is that anything below 12C inside the heated envelope is ar risk of collecting condensation

    I would suggest that it would be a good move to mitigate the thermal bridges.
  2.  
    Posted By: tonyRH is, presumably the building will be ventilated

    I always use a vb , but it won’t prevent Condensation if the vb itself goes below dew point condensation will form on it.


    Thanks Tony,

    Yes the room is mechanically ventilated, 12 litres per second per person.

    In this case the facade is curtain wall spandrel (metal tray filled with rockwool) with a plasterboard lining in front. So the envelope itself is well sealed, the debate is just around whether to include a vapour barrier within the plasterboard zone.

    According to the analysis, the metal tray part is fine, the cold spot is occurring on the curtain wall profile itself. One option is to try and open up the plasterboard zone so that it can reach the same temperature (and ventilation) as the room - apparently if the lining is ignored i.e. 'thermally invisible', then the problem goes away.

    I'd still like to look at filling the profile with rockwool or lining it on the inside with slim insulation batts, but this will all require re-running the thermal calcs. I wanted to try and figure out the magnitude of the issue first.
    • CommentAuthortony
    • CommentTimeDec 18th 2023
     
    I do a uni lecture on ventilation, a bit outside the box though. I reviewed the requirements and needs, guidance is a mess! In

    In my view an occupied building ought to be ventilated be pending on the number of people using in it and the types of activities going on.

    Controlling ventilation using CO2 and/or humidity would be better than relying on l/s rates

    An unoccupied building still needs ventilation but at a much lower rate,

    I like thought experiments ?
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