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Fundamentals: Humidity buffering - revisited

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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.
    •  
      CommentAuthordjh
    • 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.
  3.  
    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?
  4.  
    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!
    •  
      CommentAuthordjh
    • 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.
    •  
      CommentAuthordjh
    • CommentTimeAug 16th 2021
     
    I just posted an update. The website gave me a mysql connection error and lost the post. :cry: (too many connections) :devil: I can't be bothered reposting now. Maybe I will later. :shocked:
    • CommentAuthorEd Davies
    • CommentTimeAug 17th 2021
     
    Posted By: WillInAberdeenWouldn't a house soak up moisture through the summer, then release it through the winter?
    I think so, yes. This is unfortunate as the moisture being absorbed in the summer will tend to increase the temperatures inside by releasing latent heat whereas the release of moisture in the winter will tend to cool the house - i.e., you'll have to supply heat to evaporate it. This says to me that you want to keep the amount of buffering to a minimum consistent with keeping the RH reasonable on a day-by-day and week-by-week basis, not seasonally.
  9.  
    I've thought some more about this since 2018 -

    1) ventilation rates need to be much lower in winter than spring and autumn when outdoor air carries more moisture - all those rules about "0.5ach irrespective of season" are baloney

    2) during high summer the outdoors air is more humid (AH) than indoors, so any ventilation makes the house wetter, but

    3) the house inevitably has a large buffer capacity deep inside its many tonnes of timbers and masonry. This is weakly connected to the inside air (takes weeks to equilibrate) so is important for regulating humidity through the summer.

    4) opening windows at strategic times of day/year is a great way to flood-ventilate the humidity out of the fabric and so reset the humidity buffer

    5) the humidity buffer should ideally be 'just enough' to control the humidity through the summer/winter. But allow reasonable daily ups/downs, because resisting these is energy expensive as Ed suggested and also requires lossy ventilation.

    7) the year-round average indoor humidity should be as close as possible to the year-round average outdoor humidity, as the difference must be paid for by heating/ventilation. For me that means indoor humidity around 60%RH, I'm not paying extra to push it down to 40%.

    Edit: I like to submit my post after a few minutes typing and then edit it after submission, less chance of work getting lost.
    •  
      CommentAuthordjh
    • CommentTimeAug 17th 2021
     
    Posted By: WillInAberdeen1) ventilation rates need to be much lower in winter than spring and autumn when outdoor air carries more moisture - all those rules about "0.5ach irrespective of season" are baloney

    I do ventilate less in winter but mainly to minimise heat loss. In winter I run the MVHR at a higher rate for part of the time for IAQ and then run at minimum rate most of the time, which is sufficient. Outside winter I tend to run at the higher rate most of the time, and when the weather is warmer we'll open windows to increase ventilation to reduce internal temperature.

    2) during high summer the outdoors air is more humid (AH) than indoors, so any ventilation makes the house wetter,

    Are you defining 'high summer' as those times when the outside temperature is greater than the indoor temperature? If so, I'll agree, but most of the time even in summer the air is cooler outside. We've fitted a bunch of flyscreens so we can do more night ventilation during summer.

    My post that got lost was about my view now, which is that the humidity buffering in our house took five years to equilibriate. The house behaves more 'normally' now. Humidity is just below 50% in summer.

    7) the year-round average indoor humidity should be as close as possible to the year-round average outdoor humidity, as the difference must be paid for by heating/ventilation. For me that means indoor humidity around 60%RH, I'm not paying extra to push it down to 40%.

    Not sure about this. Given the need to provide some ventilation and the possibility of varying its rate over time it seems to me that the integral over time can be made to vary. But there's no way I would want to live in a house with 60% humidity again anyway. I would pay to avoid that.

    When I'm paranoid I do a CTRL-C before posting :bigsmile:
    • CommentAuthorEd Davies
    • CommentTimeAug 17th 2021
     
    Posted By: WillInAberdeen7) the year-round average indoor humidity should be as close as possible to the year-round average outdoor humidity, as the difference must be paid for by heating/ventilation. For me that means indoor humidity around 60%RH, I'm not paying extra to push it down to 40%.
    I'm not sure that follows as you can choose to ventilate more when the outdoor humidity is lower than average for the time of year.
  12.  
    If you ventilate more, then you have to pay for it, in fan power and ventilation heat losses. Another way to look at it is to ventilate by the normal amount on drier days and less on humid days, and so save energy, at the price of higher average indoor humidity.

    Think of it as a 2nd Law problem: which of these states has the greatest entropy:
    a) your house is allowed to be at similar absolute humidity as its surroundings or
    b) is kept at half the humidity of its surroundings or
    c) is kept at zero humidity (zero molecules of water allowed into the house) or
    d) is kept artificially humidified?

    Hints:
    1 the state of greatest entropy, is the one that requires the least work to be done to maintain it, in the form of heating or fan power
    2 steeper concentration gradients require more work to maintain them.
  13.  
    Didn't that Fraunhofer study show that 90% of humidity buffering occurs in the first 10mm of wall?
    Doesn't CO2 and Humidity activated HRV negate any humidity absorption benefits of natural materials?
    •  
      CommentAuthordjh
    • CommentTimeAug 18th 2021
     
    Posted By: WillInAberdeenIf you ventilate more, then you have to pay for it, in fan power and ventilation heat losses. Another way to look at it is to ventilate by the normal amount on drier days and less on humid days, and so save energy, at the price of higher average indoor humidity.

    Well (a) the cost of ventilation is a lot less than the cost of heating, so you can substitute a lot of the first for a smaller amount of the second, and (b) if you ventilate more at some times and less at others then you can achieve the same average rate but be exposed to a lower (or higher) average external humidity.

    But we're getting into theoretical arguments about very small costs that really aren't worth it.
  15.  
    My interest is because ventilation losses account for about a quarter of my heating load at present, I think this is fairly common, so seems worth putting some effort into reducing unnecessary usage.

    And yes, as mentioned above, I'm pretty strategic about exactly when and how much I ventilate. Others may not be.

    I could target a lower RH% and make the house even drier than its surroundings, but then I'd have to ventilate more, and so use more heating to make up the ventilation losses, so that seems unnecessary and wasteful.

    YMMV!
    • CommentAuthorrevor
    • CommentTimeAug 18th 2021
     
    Posted By: djhBut we're getting into theoretical arguments about very small costs that really aren't worth it.


    Totally agree. Surely one wants conditions that is comfortable. . You don't want it to dry or you end up with static and sparks fly as you kiss your partner! and too moist you get to feel clammy.

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