Do you have a humidity sensor in your house?

[JP_]

We got one to monitor levels to help combat condensation / mould issues. But, I am not sure it is giving the right reading.

I currently have all the windows open, so humidity should be similar to outside, which it is (I put it outside earlier) but at the moment (8.43am) it is reading 81 - with clear blue skies over Essex.

Although, I may have just answered that by looking at http://wow.metoffice.gov.uk/ ... apparently humidity is higher at the moment?

What sort of figures are you getting in your home at the moment?

 

[Mr Chibs]

Yes, today 67% window closed, upstairs bedroom and 16 degrees...
It's only a cheapo one, so possibly not that accurate.

Just checked your link and within a couple of miles of us are three stations, reporting the humidity is 63, 73% and 80%

 

[IWalkLikeHimaMartian]

Humidity outside is regularly and normally above 80%, often above 90%. You need to read up and understand Relative Humidity.

To reduce your humidity inside, simple ventilation, without heating is pointless. All you are doing is swapping cold air with high RH, with more cold air with high RH. It is why empty houses, with no heating, suffer condensation/mold/damp problems.
You need background heating, so that cold air with high RH coming in, is heated so that the RH falls, allowing it to 'absorb' more moisture, thus raising the RH. The air is then ventilated out, to be replaced again with cold air. Thus the cycle is repeated ad infinitum.

In a house with high RH, the fabric of the house (walls, floors, furnishings, etc) will have absorbed some of the moisture from the air. It will take some time for the fabric of the building to release this moisture to the air. The greater the hygroscopicity of the fabric materials the quicker and easier will the moisture be drawn out of those materials.

A little riddle / conumdrum:
Is the dew on the ground, in the morning, moisture from the air condensing on a colder surface?
Or is it moisture from the ground being released and meeting the colder environment, then condensing?

To answer your question: 18°C, 69%RH
Heating was on 7.00 - 9.00 but only on low setting (17°C). Then heating off and passive ventilation opened.
Building is 1930's with half of it old solid walls, no insulation in walls or floors, and the other half modern standards. All double glazed with vents. Temp and RH readings taken from the old half of the house.

I consider the early morning heating, then the ventilation to be good practice to reducing the RH internally.

I would guess the house is unoccupied for about 6 months each year. The heating/venting cycle is maintained, but with a reduced higher setting in the evenings.

 

[EFLImpudence]

We got one to monitor levels to help combat condensation / mould issues.

How do you think it will achieve that?

I currently have all the windows open, so humidity should be similar to outside, which it is (I put it outside earlier) but at the moment (8.43am) it is reading 81 - with clear blue skies over Essex.

...and do you have any (problematic) condensation inside?

 

[JP_]

How do you think it will achieve that?
...and do you have any (problematic) condensation inside?

By highlighting areas of the house where humidity is high, and indicating when measures taken to reduce humidity are working.

Not yet, but there is black mould on walls in 3 rooms, I assume from last winter.

 

[EFLImpudence]

Not yet, but there is black mould on walls in 3 rooms, I assume from last winter.

Apply neat bleach.

Next winter, keep the house heated AND ventilated and avoid introducing exessive moisture.
I know that is difficult but that's the only way to stop problematic condensation.

 

[noseall]

Insulate the walls with 50mm Celotex, plasterboard and skim.

 

[Ihavenojob]

Get a dehumidifier , got one of these, brilliant.
https://www.google.co.uk/search?q=d...&ved=0ahUKEwjo0beTg6DWAhUkQJoKHQ71DlQQgTYItAU

 

[JP_]

Actually already got one! Picked this one: https://www.amazon.co.uk/dp/B01GRM302E/ref=pe_1909131_77697001_tnp_email_TE_AMZLdp_1

Just found this website: http://www.dpcalc.org/

So, am I understanding this correctly? Tool says, if room temperature is 16c, and RH is 70%, dew point is 11C - i.e. if my walls drop to 11C at night, condensation likely to form.

 

[JP_]

Insulate the walls with 50mm Celotex, plasterboard and skim.

I do plan to do that in time, but cannot happen yet. Need to build a new room to put everything in first!

 

[IWalkLikeHimaMartian]

Actually already got one! Picked this one: https://www.amazon.co.uk/dp/B01GRM302E/ref=pe_1909131_77697001_tnp_email_TE_AMZLdp_1

Just found this website: http://www.dpcalc.org/

So, am I understanding this correctly? Tool says, if room temperature is 16c, and RH is 70%, dew point is 11C - i.e. if my walls drop to 11C at night, condensation likely to form.

Nice tool, but it is designed for purpose built storage, especially for books and documents.
It is probably not correctly showing the dew point, for domestic buildings.
I suspect it is for unheated storage facilities where the ambient temperature will be the same as all the surfaces and the stored articles.

This sentence is nonsense " If your building does not have humidification or dehumidification, the indoor dew point is the same as the outdoor dew point. "
The dew point is dependent on temperature, not just RH, or whether the building has or does not have humidifiers!
Unless the sentence assumes that the building is unheated.

If the surface of your walls falls to 11°C, there might be a tiny bit of condensation, but so little you will not notice it, it will be absorbed by the fabric of the building.

 

[JP_]

Interesting - so for so much black mould, the previous owner must have had the heating off completely for a period during winter? Is avoiding more mould just a matter of leaving the heating on at around 16C over night?

 

[IWalkLikeHimaMartian]

Just to clarify what I said last night: "A general rule of thumb is that the RH halves or doubles with approximately a change of 10°C".
So that theoretically, if you go from 16°C at 70%RH down to , say 6°C, the RH will double to 140%, obviously well past the dew point. So theoretically the tool is correct.
But in a heated domestic building the surface of the walls also gain some heat from the air., thus they are unlikely to fall much below the ambient temperature, depending also, of course, on the insulation, if any. But even uninsulated walls have some R-value.
That is why the tool is not applicable to domestic buildings, because the domestic building has more mass than any museum artifacts, etc, and therefore retains more heat. Thus the dew point is hardly likely to arise in a heated domestic building.

Once the mold has been cleaned, the key to preventing a re-occurrence is to maintain a lower RH. A temperature above the dew point is one way, but as I said, it is unlikely in a normal heated domestic environment for the temperature of the internal fabric of the building to fall to the dew point, unless the RH is particularly high. However, if the room is unheated, it is very possible for the air temperature to fall to the dew point, especially with high RH.

So the mold depends on the RH (moisture in the air or the fabric) to grow, not the temperature. An RH below 65% is sufficient to prevent mold. But I suspect many domestic rooms regularly have RH as high as 75% without any mold appearing. The key point may be "regularly" having high RH, not consistently.
Mold spores are everywhere and it is impossible in a domestic environment to prevent them entering that environment.

So the answer to your question is "yes". But occasional (during the daily cycle) heating of the room, along with a routine of ventilation, is usually sufficient to lower the RH, and prevent the surface of the fabric falling below the dew point.
Obviously, as others have said, moisture generating activities will require more effective heating/ventilation than a normal lounge/bedroom type room.

 

[EFLImpudence]

It's difficult to say.

For mould to form the walls must have been continuously moist so, presumably the rooms were never heated AND ventilated enough to dry them.
Or
There are leaks - water running in or into the cavity.
Or
You are introducing too much moisture -
Lots of steam in a closed kitchen,
Lots of plants,
Drying clothes indoors,
Wiping up condensation and leaving the sopping cloth lying around,
Even just lots of people and/or animals,

When I used to look after several flats, I would get calls complaining of condensation. I went to one once where the bath was left full of cold water. Don't know why; didn't bother to ask.

Condensation is how the world works. Moisture is produced or introduced and it will condense on cooler surfaces.
It must be allowed to escape or it will remain inside (for ever).

Put a sopping wet cloth in the car tonight, then see what it's (the car, that is) like in the morning.

 

[^woody^]

We got one to monitor levels to help combat condensation / mould issues

Then you are wasting your time.

Humidity is just a factor, and is not something to be dealt with in isolation hoping to prevent mould