Humidity in the House....Talk to me!

[dlockers]

Hi folks,

I am trying to wrap my head around humidity in the house (bedrooms, attic), the workshop, and my garden room. I feel I am misunderstanding some crucial bits around how it is calculated and what my sensors are showing me. For example, right now I am seeing an outside temperature of 8 degrees, with a humidity of 91%.

The house.bedrooms is pretty well sealed up - double glazed, insulated subfloor, modern doors. I do have trickle vents on the windows but these are closed (although they aren't fully sealed or insulated). We have started to notice condensation on the OUTSIDE of the windows, which is presumably a good thing.

Bedroom with 1 outside solid wall:
Current Temp: 17.8*
Current Humidity: 74.6%

Bedroom with tiled facade/single wood thickness wall:
Current Temp: 17.9*
Current Humidity: 74.1%

Bedroom with 25mm insulation on external walls:
Current Temp: 17.5*
Current Humidity: 74.6%

Bathroom with 25mm insulation and extractor (3 hours following showers etc):
Current Temp: 18.3*
Current Humidity: 76.7%

The house.attic is definitely draughty but no specific alterations to enable ventilation have been made. I have 50mm PIR down the hipped roof and 270mm insulation. I assume I get draught as a side effect of the eaves not being sealed - I can't see any visible ventilation holes. I have allowed 50mm between the PIR and the roof.
Current Temp: 11.7*
Current Humidity: 85.3%

<outlier as data from same time not available>
The workshop is very poorly insulated - single skin wood cladding, the roof is floating with gaps around all the eaves, there is a sizeable gap around the door.
(sensor has dropped off so using data from 1st Nov)
Temp: 6.6*
Humidity: 82%
[outside was 6* and 90% from what I can tell)
</outlier>

The Garden Room is totally sealed with an AC system.
Current Temp: 17.4*
Current Humidity: 80.9%
-- Interestingly I can't seem to get the humidity much lower, even with the Split-AC system on Dehumidify mode?

So what I can't work out - from a principled point of view is:
* Is the humidity number I am looking at "correct"? I have read something about relative humidity and water content at different temperatures.
* Is there a problem? It seems even I ventilate, because the outside humidity is so high, I can't really achieve anything?

Thanks!

 

[Shoza]

House humidity should be 45-60 max for comfort and health. My old Victorian terrace sits around 55-60 with no special arrangements.

 

[Sinbad2000]

The humidity number you're quoting there is relative humidity.

The warmer the air is, the more physical water it can contain - RH is the percentage "saturation" of the air with water vapour at that particular temperature.

So air that's the same "wetness" will have say a RH of 47% at 25 degrees and a RH of nearly 100% at 12 degrees (ie, if you took some outside air at 12 degrees 98% humidity, brought it inside (through a door/window/vent etc) and heated it up to 25 degrees (without adding or removing any moisture) the RH will be 47%.

With your high RH at relatively high temperatures, I'd say you need a bit more ventilation.

 

[throwaway4372]

Is it always that high, or just temporarily due to the weather?
(high humidity happens even in my new build if it's high outside)

 

[Orifice]

So what I can't work out - from a principled point of view is:
* Is the humidity number I am looking at "correct"? I have read something about relative humidity and water content at different temperatures.
* Is there a problem? It seems even I ventilate, because the outside humidity is so high, I can't really achieve anything?

Thanks!

I have very similar readings for temp/humidity in all of my rooms, similar situation with insulated external walls (75mm).

I also have a PIV in the loft, which brings humidity down marginally (a few % at best), and have read that the "average" humidity should be less. Not sure how that is meant to work as outside its 86% humidity at the moment and without a dehumidifier that is just not going to happen. Open the windows and again it will drop the internal humidity slightly due to the breeze, but still not below 75%.

Seems to be another of those numbers people get hung up on, providing you have enough air movement and surfaces insulated to prevent them dropping below the dew point then it should be fine. Even in the height of summer when it was 28 degrees in one of the rooms, humidity was still 61%.

 

[Sinbad2000]

Not sure how that is meant to work as outside its 86% humidity at the moment and without a dehumidifier that is just not going to happen. Open the windows and again it will drop the internal humidity slightly due to the breeze, but still not below 75%.

See my post above. Cold wet air, when you heat it up, drops its RH considerably. Cold wet air has a LOT less water in it than hot wet air.

 

[alec]

I can't seem to get the humidity much lower, even with the Split-AC system on Dehumidify mode?

I find dehumidifiers are less efficient in cold, heat the air up to 20-21 first. As mentioned above, then air has more water in it to remove.
For me, 18C and 70+% humidity is panic mode. It means there will be condensation around cold corners and windows overnight. Means mould in hidden spots.

Airing with 8C outside should help, that air will be drier than indoors. But heating the air up is the best humidity soak

 

[Trifid]

I also have a PIV in the loft, which brings humidity down marginally (a few % at best), and have read that the "average" humidity should be less. Not sure how that is meant to work as outside its 86% humidity at the moment and without a dehumidifier that is just not going to happen. Open the windows and again it will drop the internal humidity slightly due to the breeze, but still not below 75%.

I added one of these this year, I think it increased the humidity, as when it rains the house gets more humid very quickly. I do like the fresher air, but in autumn when the central heating isn't needed but the outside air is very humid makes it a losing battle. I resorted the dehumidifier for a few days last week and probably should have just accepted running it earlier.

 

[Scuzi]

@Sinbad2000 has it right.

You have two options to reduce the relative humidity (RH) in your house.
1. Increase temperature.
2. Remove humidity.

1 is easy as you just jack up the heat and your RH will drop. It comes at a cost though.
2 can be achieved either by ventilating, or sealing and dehumidifying.

Your workshop will be a good example of where edge cases can’t just be solved with ventilation. Assuming it isn’t heated, you’ll find in the coming weeks and months times where the temperature drops significantly down towards freezing. Your workshop and all the contents will drop to that temperature eventually. A warm front of moist air will come in off the Atlantic. All of the relatively cold contents of your unheated and ventilated workshop will get covered in condensation, regardless of how well ventilated it is.

 

[dlockers]

Is it always that high, or just temporarily due to the weather?
(high humidity happens even in my new build if it's high outside)

It is pretty much always this high. I often have doors open, windows open etc. I guess weather dependent to the extent that during summer it was lower:

I have very similar readings for temp/humidity in all of my rooms, similar situation with insulated external walls (75mm).

I also have a PIV in the loft, which brings humidity down marginally (a few % at best), and have read that the "average" humidity should be less. Not sure how that is meant to work as outside its 86% humidity at the moment and without a dehumidifier that is just not going to happen. Open the windows and again it will drop the internal humidity slightly due to the breeze, but still not below 75%.

Seems to be another of those numbers people get hung up on, providing you have enough air movement and surfaces insulated to prevent them dropping below the dew point then it should be fine. Even in the height of summer when it was 28 degrees in one of the rooms, humidity was still 61%.

Yeah doesn't seem like anything I do makes an ounce of difference. Bathroom windows open, closed, doors open throughout day. I guess I could heat the house really high every week or something? :S

I find dehumidifiers are less efficient in cold, heat the air up to 20-21 first. As mentioned above, then air has more water in it to remove.
For me, 18C and 70+% humidity is panic mode. It means there will be condensation around cold corners and windows overnight. Means mould in hidden spots.

Airing with 8C outside should help, that air will be drier than indoors. But heating the air up is the best humidity soak

I've put the dehumidifier on in here; we'll see how it goes. This room is generally at 20 odd anyway.

 

[danlightbulb]

I too have tried to understand the humidity in my house, to no avail.

One time (at band camp), I measured the humidity and temperature in my house. Then I heated up the house to 20 degrees and measured the humidity again.

Using an online calculator, I had worked out what the new humidity should have been, at the increased temperature.

So basically if relative humidity was 70% at 14 degrees (cant remember the exact figures) then this calculator says at 20 degrees relative humidity should now be under 50%. https://www.lenntech.com/calculators/humidity/relative-humidity.htm

However that isn't what happened. RH only dropped a few percent to 60 something. So completely defying the laws of physics and so Im at a complete loss trying to understand what the issue is.

Im unable to get RH below 60 something percent naturally in my house. With no dehumidifier on its sitting around low 70's, possibly high 60's when heated to 20 degrees. Dehumidifier on, door to room closed, I can maybe achieve high 50's after several hours.



I was thinking back some time ago to when there would have been open fires/chimneys in the house. My house would have had two open chimneys (one in front room, one in back). When in use, there would have been a) high localised heating of the air in the room, and b) a huge draw of this air up and out the chimney, with air being replaced through leaky flooring from the sub floor and through drafty old windows etc. So it would have acted as a huge air replacement cycle. The removal of chimneys means this cycle no longer happens. So ventilation is key I completely agree with this, but still doesn't explain to me why the numbers Im seeing don't make sense.

 

[Raymond Lin]

I too have tried to understand the humidity in my house, to no avail.

One time (at band camp), I measured the humidity and temperature in my house. Then I heated up the house to 20 degrees and measured the humidity again.

Using an online calculator, I had worked out what the new humidity should have been, at the increased temperature.

So basically if relative humidity was 70% at 14 degrees (cant remember the exact figures) then this calculator says at 20 degrees relative humidity should now be under 50%. https://www.lenntech.com/calculators/humidity/relative-humidity.htm

However that isn't what happened. RH only dropped a few percent to 60 something. So completely defying the laws of physics and so Im at a complete loss trying to understand what the issue is.

Im unable to get RH below 60 something percent naturally in my house. With no dehumidifier on its sitting around low 70's, possibly high 60's when heated to 20 degrees. Dehumidifier on, door to room closed, I can maybe achieve high 50's after several hours.

You need to run it for like a week in 1 room in order to get all the moisture out of every surface. Everything is a sponge. If you just run it for several hours, and then turn it off. It is not going to be effective.

Also, if you are trying to just have 1 running, best place to put it is the landing, upstairs. Cloud rises, so upstairs will be more humid than downstairs. If you remove the moisture from upstairs, then it will force the more moist air from downstairs to travel up.

 

[danlightbulb]

If you just rin it for several hours, and then turn it off. It is not going to be effective.

If we're isolating a specific room, closing the doors and running the unit, I disagree here. Should be able to test the response of the air in the room relatively quickly.

I agree the items in the room and the fabric of the room act like a humidity soak. However the humidity is very quick to rise, but slow to fall. That doesn't make sense because the soak IN should be at broadly the same rate as the soak OUT.

Lets say my room is 20 degrees at 55%, and I turn my dehumidifier off. It only takes a very short time for the room to return to 70% RH. Yet it takes hours to dehumidify the room from 70% down to 55%. All whilst maintaining the same temperature. So that doesn't make sense.

If I start at 14 degrees, 70%. Then heat the room to 20 degrees rapidly, I should see the RH fall quite sharply, because the furnishings and fabric of the room will only release humidity at a much slower rate than the air in the room changes from being heated.

 

[Scuzi]

If we're isolating a specific room, closing the doors and running the unit, I disagree here. Should be able to test the response of the air in the room relatively quickly.

I agree the items in the room and the fabric of the room act like a humidity soak. However the humidity is very quick to rise, but slow to fall. That doesn't make sense because the soak IN should be at broadly the same rate as the soak OUT.

Lets say my room is 20 degrees at 55%, and I turn my dehumidifier off. It only takes a very short time for the room to return to 70% RH. Yet it takes hours to dehumidify the room from 70% down to 55%. All whilst maintaining the same temperature. So that doesn't make sense.

If I start at 14 degrees, 70%. Then heat the room to 20 degrees rapidly, I should see the RH fall quite sharply, because the furnishings and fabric of the room will only release humidity at a much slower rate than the air in the room changes from being heated.

What hygrometer are you using to take the measurements? Some have a noticeable lag.

 

[danlightbulb]

What hygrometer are you using to take the measurements? Some have a noticeable lag.

It was only a cheap one from Amazon. However i tested it by putting it outside, and it responds relatively quickly in both directions. I want to get one like dlockers has which has a data recording function, they're available on Amazon in 3 packs so I can take more measurements around the house like dlockers has done.

But, can't explain the response of the one I have which is very frustrating.


@dlockers your garden room should be a perfect test for this, as its almost completely sealed.

Open up your doors, flood the volume with outside air. Take your temperature and humidity measurements. Then close it up, vacate the room (so moisture in your breath doesn't add humidity to the room) and heat it rapidly using an electric heater. See if the humidity change is as per the calculator says it should be.

 

[alec]

Lets say my room is 20 degrees at 55%, and I turn my dehumidifier off. It only takes a very short time for the room to return to 70% RH. Yet it takes hours to dehumidify the room from 70% down to 55%. All whilst maintaining the same temperature. So that doesn't make sense.

If I start at 14 degrees, 70%. Then heat the room to 20 degrees rapidly, I should see the RH fall quite sharply, because the furnishings and fabric of the room will only release humidity at a much slower rate than the air in the room changes from being heated.

well, there is no magic. Water will go from air to items and back
it takes a while to evaporate some water, but condensation on a cold surface is almost instant

 

[danlightbulb]

well, there is no magic. Water will go from air to items and back
it takes a while to evaporate some water, but condensation on a cold surface is almost instant

Yes because glass/metal is non-porous and surfaces like glass and metal are able to maintain a large temperature gradient to the room, so the moisture hits it and condenses immediately.

Substances like brick are porous so they absorb and release moisture. If the temperature gradient is large enough (if the walls are very cold compared to the room), water will condense on walls too as well (which is what causes the mould). But a small temperature gradient means it doesn't, it instead will absorb and release moisture over time.


What happens with moisture in a house is that the substance of the house will absorb and release it according to the vapour pressure gradient between the air and the material.

So a room in a house is essentially a brick container with air inside. The air will absorb and release moisture according to its temperature, which is what relative humidity is.

The brick will also absorb and release moisture. If the moisture in the air is higher than the brick, the vapour pressure gradient will lead to moisture being absorbed into the brick until the moisture levels are equalised. If the moisture in the air is lower than the brick, the reverse happens and the brick will release moisture until the two substances are equalised.

So lets say we have a room with wet walls and low moisture content in the internal air. The walls will release moisture into the air, but it can't happen instantly - there will be a 'flow rate' of moisture from the brick to the air, which depends on the material properties and the vapour pressure gradient between the two substances. When the moisture content of the walls and air are equalised, the transfer stops.

If we then heat the air in the room, the air is able to hold more moisture again (because hot air has a higher moisture capacity than colder air). So then by heating the room, we establish a vapour pressure gradient once more between the brick and the air.

So under normal conditions we have wet air outside, brick, and warm air inside. There is a vapour pressure gradient between the brick and hot air inside which pulls moisture through the brick into the internal air, which depletes the moisture in the brick. There is then a vapour pressure gradient between the brick and the outside air, which means the brick absorbs moisture from outside. So we have the brick acting as a transfer mechanism from the outside air into the inside air because of the temperature difference (and hence ability to carry moisture) from the inside air compared to the outside air. That's why cavity walls work, as they break the outside/inside interface. Solid walls don't, they enable uninterrupted moisture transfer.

However the rate of moisture 'flow' through the brick isn't instant, its far slower than the rate at which we can raise the temperature of a room by heating it. So if we rapidly heat a room, we should get a fairly rapid humidity fall initially. Then what happens over time is that the now warmer air will re-absorb new moisture from the brick walls of the room until it is equalised once more.

All this is fine and I understand the mechanism. What it doesn't explain is why rapid heating of the air in the room doesn't see a temporary rapid reduction in humidity in my case. Because it should take time for the brick to respond to the new vapour pressure gradient, far slower than the direct effect of heating the air in the room. So we should see RH decline rapidly initially as we heat the air (and the humidity sensor is measuring the air humidity at given temperature, not anything else), then slowly rise again as the air in the room once more equalises with its building fabric. This should take time so you see a response curve. Im not seeing this, Im seeing almost no response curve, which doesn't make sense.

 

[Firegod]

I'm in a similar situation with the two lofts at each end of our property. Current Tapo readings are ~14ºC and 76% humidity. I was thinking about investing in a little dehumidifier with a 12L tank for both. Then just having Tapo control the on/off with a range %. But is it worth it for a loft? One of the two lofts has no windows and no ventilation. I have a nest thermostat in the main hallway and it is reporting 20ºC indoors and 64% humidity.

 

[dlockers]

I'm in a similar situation with the two lofts at each end of our property. Current Tapo readings are ~14ºC and 76% humidity. I was thinking about investing in a little dehumidifier with a 12L tank for both. Then just having Tapo control the on/off with a range %. But is it worth it for a loft? One of the two lofts has no windows and no ventilation. I have a nest thermostat in the main hallway and it is reporting 20ºC indoors and 64% humidity.

I've even considered this but I recently sold my Dehumidifer on marketplace, lol. Also, the house has been fine for over 100 years.

 

[danlightbulb]

I've even considered this but I recently sold my Dehumidifer on marketplace, lol. Also, the house has been fine for over 100 years.

The house is fine now?

It isn't a problem really as long as ventilation/humidity is controlled and you aren't getting mould. That's my understanding.

Its still very frustrating though, to be constantly dealing with 70%+ humidity levels when the optimum recommended is 40-60%. I get condensation on my windows in the morning under certain conditions. There are areas I have to keep an eye on for mould formation. I'd rather not have these problems but like you I don't know if its possible to stop it as its largely due to the construction of the house.

See my previous comment about chimneys and how they used to help drive ventilation and lower humidity. An open chimney (according to Google) can draw (depending on size) several hundred cubic feet per minute. 200 cfm would be 338 m3/hr. In a house with volume say 300m3 that is a whole air change every hour. There's no way we get anywhere near that in our modern homes with no open fire use and sealed up windows.