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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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    • CommentAuthorgoodevans
    • CommentTimeOct 16th 2018
    Dave (DJH) - at last we have had a few days of manky, moist and mild weather outside - quite the opposite of March earlier this year. At the moment our leaky house (air) is running at 48% and 20.9 degrees quite close to the overnight absolute humidity of the atmosphere outside.

    It would be nice to get some feedback as to what your house is doing now - and what happened over the summer. and if If you are experiencing/experienced buffering in the other direction - i.e. your house has/had a significantly lower humidity than what the atmosphere outside would indicate.
    • CommentTimeOct 16th 2018
    The past couple of days have been pretty grey and damp (no PV to speak of, so the last set of showers were warm not hot) but today is nice and sunny agin. Temperature inside is currently 24°C, RH 46%, outside about 16°C, RH 79%.

    Summer was pretty warm; I still haven't built my brise soleil. I guess minimum temperature of 25°C or so, max was perhaps 30°C but cooler than outside. RH has been between 45%-50%. I haven't done any calculations recently.
    Wouldn't a house soak up moisture through the summer, then release it through the winter?

    In winter the incoming air is low AH and then gets heated when it enters the house, so gets to a very low RH. This will drive the timber/concrete/plaster/limemortar/straw/ etc to desorb water (dry out) thus buffering the internal humidity. This doesnt happen in summer when the heating is off so the fabric gets damper again.

    So by the mid autumn the house fabric should be at its most humid point of the annual cycle, and be humidifying the air inside it.

    We speculated that an unusual house with porous but insulating material that bridges the construction and no VCL (such as DJH's straw bales) would continuously absorb moisture outside (high RH) and release it inside (lower RH). So you'd have a continuous inflow of moisture which the ventilation would continuously have to dry out, the AH of the extract air would always be greater than the AH of incoming air, even in summer. This could be the source of the 2 gallons of water DJH seemed to be extracting every day. Still interested to see any evidence on this either way?
    24degC 46%RH = 10g/m3 AH
    16degC 79%RH = 11g/m3 AH

    So today the outside air is being dehumidified as it passes through the house, by a small amount, IE the fabric has capacity to absorb water today. Thanks for posting data!
    • CommentTimeOct 16th 2018
    Posted By: WillInAberdeenSo today the outside air is being dehumidified as it passes through the house, by a small amount, IE the fabric has capacity to absorb water today.

    I suspect the data aren't accurate enough to draw any conclusions; it might be closer to say that the internal and external AH are approximately equal and essentially no buffering/exchange is taking place at the moment. :devil:

    The numbers are taken from single meters and the meters are not calibrated, although they seem reasonably consistent. So there may well be metering errors (I'm sure they have a technical name) plus the actual values will differ slightly at other places in the house. In short, there's a lot of noise.
    • CommentAuthortony
    • CommentTimeOct 16th 2018
    I think that we should be using partial vapour pressure as this willtell us which way moisture will be moving. For homes this is generally outwards.
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