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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorSimonD
    • CommentTimeAug 20th 2021
     
    I'm wondering whether anyone can point me in the direction of some books and resources to get a deeper understanding of relative humidity in houses.

    I'm building a house and I've recently put in some temp and humidity sensors in the house at varying locations. My aim is to install some under the suspended timber floor to monitor humidity there over the long term.

    Now, what I'm finding slightly peculiar is that I've had a sensors installed central in the house on the ground floor. This is currently a shell with no partition walls, and no plumbing or anything else. The sensor, which I originally located under the suspended timber floor was showing me a relative humidity of 100% and maybe reducing to 95% if I was lucky. Temperature fluctuated pretty much around the 16 deg C mark.

    I've taken the sensor up and put it directly on the subfloor and it is consistently showing relative humidity levels between 90-95%, currently 93 at a temp of 19.1 deg C. This is with the ground floor being fully ventilated with open doors and windows (with these closed and no ventilation the figure was 90% @ 18 deg C this morning).

    I've taken moisture readings of all my timbers with floor joists showing 16% and timber wall studs 13%, masonry walls also measuring 13%. All fine at least

    Outside relative humidity is fluctuating but pretty much at around 70-75%.

    Am I merely seeing an increase in RH due to air temperature reduction as air travels into the house or is something else going on here?

    I'd like to understand what's happening here as it's a good 20% above the figures I'm getting on the 1st floor of the house.
  1.  
    First thing would be to put 2 sensors next to each other to see if the readings are the same. (sorry f you have already done this)
    • CommentAuthorrevor
    • CommentTimeAug 20th 2021
     
    I do not know of any books on the subject but I am not surprised at your findings as you are building new. It will take quite some time for the house to equilibrate once it has all been closed in and allowed to warm up.

    Expect your timbers to be down at the 9% to 10 %. This is why so much shrinkage in builds, the timber if you are lucky will come in at 15%.

    Some of the timber for my build came in a stack it had water puddling between the cupped boards. Having built the roof had to come back some time later to pack the purlins supporting the dormer rafters as they had shrunk quite a lot.

    Typically a modern house with heating will have RH about the 55% mark this will give timber an Equilibrium moisture content of around 9%.The upstairs you say is lower which is understandable as it will be warmer and dryer due to heat rising.
    Does not sound like you have had any plasterers in it will get a lot wetter after them. I do not fit any door casings until after plastering and house has dried out, and casings are kept indoors to equilibrate to 9 to10 % before fitting.

    If you google "equilibrium moisture content" that will lead you to some information you will find useful.
    • CommentAuthortony
    • CommentTimeAug 20th 2021
     
    RH is a bit confusing as it is temperature dependant

    When temperature changes so does RH
    •  
      CommentAuthordjh
    • CommentTimeAug 20th 2021
     
    Agree with everything Peter and revor have said.

    As regard books, perhaps start instead at https://ukcmb.org/ (UK Centre for Moisture in Buildings) who have published various things. There are lots of other things on the web once you start searching. I've started a couple of threads about humidity buffering that may have useful links in and there are other threads on here.

    How have you calibrated the sensors? Have you put them all together somewhere and tested that they all show the same, for example? What are their specs? Both as regards accuracy etc and also operating environment. Some have upper limits of 95% RH for example.

    13% for masonry sounds very high. I suspect some error there.

    When you say 'subfloor' what are you talking about? Some part of the suspended floor or a concrete oversite underneath, for example? Is there a DPM underneath somewhere?

    FWIW, I think it's taken about five years for my house to settle its moisture levels after completion. I believe up to two years is usual for more normal constructions.
    • CommentAuthorSimonD
    • CommentTimeAug 20th 2021
     
    Posted By: revorI am not surprised at your findings as you are building new. It will take quite some time for the house to equilibrate once it has all been closed in and allowed to warm up.

    Expect your timbers to be down at the 9% to 10 %. This is why so much shrinkage in builds, the timber if you are lucky will come in at 15%.

    Does not sound like you have had any plasterers in it will get a lot wetter after them. I do not fit any door casings until after plastering and house has dried out, and casings are kept indoors to equilibrate to 9 to10 % before fitting.

    If you google "equilibrium moisture content" that will lead you to some information you will find useful.


    Thanks, I should have been a bit more specific, the house is 'technically' renovation and extension, but as we began to demolish in prep, the old building had to be stripped back much more than anticipated. Very little left of orginal building, however, 3/4 of the old exterior masonry walls remained which are encasing this area so no new masonry work has gone on here, only about 6.3 cubic meters of concrete for new pad foundations and oversite went in wet.

    This whole area has been closed in for 6 months with just floor joists installed.

    I've also been careful to ensure all my timbers were below a certain moisture content at installation. None of which were above 16% and osb subfloor is/was at 8%.

    You are indeed spot on re plasterng, that's going to happen soonish.

    I'll have a look at equilibrium moisture content.


    Posted By: Peter_in_HungaryFirst thing would be to put 2 sensors next to each other to see if the readings are the same. (sorry f you have already done this)


    Posted By: djh

    How have you calibrated the sensors? Have you put them all together somewhere and tested that they all show the same, for example? What are their specs? Both as regards accuracy etc and also operating environment. Some have upper limits of 95% RH for example.



    Indeed, did that when originally setting the units up. There was minor variation, like 1-2% maybe, and interestingly with a slight difference between battery or usb powered, but I also noticed it took a few hours for the units to first equalise.

    I'm using Shelly H & T sensors fed straight to their app (they're okay but I've found a few annoying features to how they work). However, for long term monitoring I'm probably going to use Raspberry Pi with hardwired sensors (DHT22) as they can be hardwired up to 20m. The wifi in the Shelly sensor isn't something I'll trust.

    Posted By: djhAgree with everything Peter and revor have said.

    As regard books, perhaps start instead at https://ukcmb.org/ (UK Centre for Moisture in Buildings) who have published various things. There are lots of other things on the web once you start searching. I've started a couple of threads about humidity buffering that may have useful links in and there are other threads on here.

    13% for masonry sounds very high. I suspect some error there.

    When you say 'subfloor' what are you talking about? Some part of the suspended floor or a concrete oversite underneath, for example? Is there a DPM underneath somewhere?

    FWIW, I think it's taken about five years for my house to settle its moisture levels after completion. I believe up to two years is usual for more normal constructions.


    I'll have a look at ukcmb thanks. The manual with the moisture meter I have says that 13% reading is a 'medium' moisture level and 20+ is high. With these being clay bricks, it's okay I think for now. But yes, there may indeed be some unreliability as the meter is mostly made for timber measurement with a masonry measurement setting.

    The sub floor is the osb sitting on top of my joists. It's all suspended timber floor with sufficient underfloor ventilation and concrete oversite. No dpm.

    Posted By: tonyRH is a bit confusing as it is temperature dependant

    When temperature changes so does RH


    That is part of the problem. I actually don't know how these sensors are set up to work or how well the temperature compensation works. On a couple of occasions the sensors have moved in the direction opposite to what I would expect.
    • CommentAuthorborpin
    • CommentTimeAug 21st 2021
     
    In a house under construction, high humidity is not unusual.

    I'd get a large dehumidifier and plumb it into a drain to control the humidity. Right now, those dry timbers you installed are anything but dry now (after 6 months closed up).

    I decided it was cheaper to buy than hire one. Something like this https://www.ebay.co.uk/itm/203561928494?hash=item2f653c872e:g:3RYAAOSwX89hDY0o

    You could rig it to a smart switch so you can switch on/off depending on humidity level.
    • CommentAuthorEd Davies
    • CommentTimeAug 21st 2021 edited
     
    Posted By: SimonDI actually don't know how these sensors are set up to work or how well the temperature compensation works.
    AIUI they directly measure relative humidity - i.e., the raw electrical signal out of the physical sensor is, at least to a first order of approximation, proportional to the relative humidity of the material in the sensor which in turn follows the air RH with a time lag for absorption/desorption. This is the same sort of reasoning where the humidity in materials like wood or brick is proportional to the RH of the air, not the absolute humidity of the air.

    I.e., any temperature compensation is for second-order effects rather than because they measure absolute or specific humidity or anything like that, then convert that to RH by looking at the temperature. Support for this notion: if they were measuring AH or anything it would be nutty not to at least give the option of outputting that, which the DH22, for example, doesn't.
    • CommentAuthorbhommels
    • CommentTimeAug 21st 2021 edited
     
    Posted By: SimonD
    I'm using Shelly H & T sensors fed straight to their app (they're okay but I've found a few annoying features to how they work). However, for long term monitoring I'm probably going to use Raspberry Pi with hardwired sensors (DHT22) as they can be hardwired up to 20m. The wifi in the Shelly sensor isn't something I'll trust.

    If you are going the RasPi route anyway, can I recommend the SHT range from Sensirion? Much better than the rather outdated DHT types, and still very affordable. I used SHT-35 breakout boards for my home monitoring system.
    At work we looked into affordable RH/T sensors and we reckoned the Sensirion SHT-3x series offers the best price/performance. FYI the IST HYT-939 is the most accurate - but much less affordable at £ 60+ a pop.
    • CommentAuthortony
    • CommentTimeAug 21st 2021
     
    Opening windows would be my advice
    • CommentAuthorrevor
    • CommentTimeAug 21st 2021
     
    Posted By: tonyOpening windows would be my advice


    Agree particularly if you have condensation on them.

    I think you are overthinking the issue when you have results you understand what will you do?

    Wait until all the wet trades are finished. Not much point in wasting energy with dehumidifiers just let circulation of air do its job.

    If you have control over it do not fit timber goods doors frames skirtings etc before the house has dried out and provide stop beads for plasterers to plaster to around openings, The gaps between frame and opening will be covered by the architrave. Alternatively what I did was fit a boards to the end of the wall protruding 12 mm for plasterers to work up to then remove them when plaster dried off.
    •  
      CommentAuthordjh
    • CommentTimeAug 21st 2021
     
    Posted By: revorI think you are overthinking the issue when you have results you understand what will you do?

    I don't think there's anything wrong with trying to understand what's going on with the readings from a sensor, even if the results are not useful in themselves.

    Thanks for the mention of Shelly. They look like interesting devices.
    • CommentAuthorSimonD
    • CommentTimeAug 23rd 2021
     
    Posted By: bhommels
    If you are going the RasPi route anyway, can I recommend the SHT range from Sensirion? Much better than the rather outdated DHT types, and still very affordable.


    Thanks for the heads up on these. I've ordered a handful of them today. The ones I've bought are SHT30s and SHT 31s (difference due to stock issues), caged and pre-wired so I can bung them under the floor and connect them up at a later date.

    As for thinking all this through, I know my current results do not correlate with typical new house build high humidity. I've been building this house myself, so first there's the extended timescale between phases which allows for a significant drying out time, second, I've used very little wets, and third, the whole space gets fully ventilated every day.

    Hence why I'm following up with the humidity query and trying to gain a full understanding. I think I may be closer already after weekend investigations.
    • CommentAuthorbxman
    • CommentTimeAug 27th 2021
     
    Hi
    FYI

    I believe It is very easy to check any relative humidity meter by placing it in a sealed container along with a source of common salt that is submerged in water this will I understand show as 75% in all circumstances when things have stabilized .
    • CommentAuthorEd Davies
    • CommentTimeAug 27th 2021
     
    Posted By: bxman: “I believe It is very easy to check any relative humidity meter by placing it in a sealed container along with a source of common salt that is submerged in water this will I understand show as 75% in all circumstances when things have stabilized .”

    That's interesting and useful to know. Just for clarity, it needs to be a saturated solution of the salt. A web search for “salt 75% relative humidity” is illuminating. Apparently different salts will give you different calibration points:

    https://www.engineeringtoolbox.com/salt-humidity-d_1887.html
    • CommentAuthorrevor
    • CommentTimeAug 27th 2021
     
    Posted By: Ed DaviesPosted By: bxman: “I believe It is very easy to check any relative humidity meter by placing it in a sealed container along with a source of common salt that is submerged in water this will I understand show as 75% in all circumstances when things have stabilized .”

    That's interesting and useful to know. Just for clarity, it needs to be a saturated solution of the salt. A web search for “salt 75% relative humidity” is illuminating. Apparently different salts will give you different calibration points:

    https://www.engineeringtoolbox.com/salt-humidity-d_1887.html" rel="nofollow" >https://www.engineeringtoolbox.com/salt-humidity-d_1887.html


    That's it interesting but not surprised. At work we were very interested in humidity control and I believe the instruments were sent off to a calibration lab for checking. When I worked in a chemistry lab we often had to keep moisture sensitive substances in a vacuum jar that had calcium chloride in it as desiccant. A weather station cabinet will be equipped with wet and dry bulb thermometers and Dew point and RH calculated from tables or charts.
    • CommentAuthorrhamdu
    • CommentTimeSep 3rd 2021 edited
     
    Posted By: tony
    RH is a bit confusing as it is temperature dependant

    When temperature changes so does RH


    I agree. Much better if humidity sensors gave dew point or, failing that, absolute humidity. Unlke RH, they are actual measures of how wet/dry the air is. If you know the dew point of the room air is 11C and your infrared thermometer says the skirting board is at 10C, you will get condensation at that spot.

    Simon it sounds like your sensors can measure RH and temperature, which is fine because you or your software can calculate AH or DP from those. You've got the basic point that RH falls as the air warms up and it seems to me your problem isn't so much with the concepts as with a bunch of surprisingly high readings.
    •  
      CommentAuthordjh
    • CommentTimeSep 3rd 2021
     
    Posted By: rhamduI agree. Much better if humidity sensors gave dew point or, failing that, absolute humidity. Unlke RH, they are actual measures of how wet/dry the air is. If you know the dew point of the room air is 11C and your infrared thermometer says the skirting board is at 10C, you will get condensation at that spot.

    RH is what is relevant for biological processes though, both human comfort and mould growth. If you put an RH meter at the place where you are concerned about mould, you can directly read how likely it is. And temperature of course, there's a range where mould is active.
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