Ideal Humidity Level

[TonyH1]

i know there's a host of knowledgeable people regularly peruse these pages and I'm hoping one of you can tell me the ideal humidity level inside a caravan during storage.

I'm fortunate that I am able to store my caravan at home and I went inside the other week and found the bathroom door and fridge door (both left open) covered in mould. My first thought was that there must be water getting in somewhere but having damp found over the last three services I already have a damp meter and all checked out around 14% so I put it down to the extremely wet weather we've had for such a long time.

I washed the bathroom door off but left the fridge and purchased a de-humidifier and this has now been running for about a week. It has 2 fan speeds and I've been running it on the slow speed, plus it has a regulator, which works a bit like a thermostat but regulates humidity not temperature, which has been around half way. So far it's collected about 1 ltr of water and I'm happy to say the mould has gone although I'll still be washing it down.

I've got a humidity meter coming in a day or two and I'm wondering if anyone can tell me what the maximum humidity % is that I should be looking for.

 

[SirRogerFFS]

Tony H. I havnt got the humidity levels for you for the inside of your van but I do believe that some time back this subject was covered and De Humidifiers were not recommended as they draw in from the outside as well.Im not sure how long ago this was but the Techie bods will come along and give you a better reply than mine.
Sir Roger .

 

[Hutch]

Hi Tony, have a look at Leaving the heat on in winter , in the technical threads, a bit about humidifiers.
I Have 5 water traps in my van, and today they were all full, needed more chemical. But I am now undecided.
Hutch.

 

[Damian]

Humidity cannot be given a benchmark value as it varies day to day, hour to hour and what the weather is like at the time.
Very cold, dry days are less humid than roasting hot sunny days for example.

The light coating of mould as you describe it is quite normal given the weather and high wetness we have been experiencing, and is normally due to not cleaning down properly when storing and making sure after washing surfaces down that they are properly dried.

Dehumidifiers may make you feel better, but as the air is changing all the time due to the high and low fixed ventilation, all you are doing is drying each change.

As long as you don't have a water leak the van should be perfectly OK just leaving it empty and letting it acclimatise to the ambient temperature.

 

[00buzz]

we run a Prem-i-air 20l dehumidifier constantly in the house (its 250 years old and ventilation wasn't high on the agenda back then) and they recommend a setting of 50%....be aware though, a good dehumidifier can be destructively effective to the natural water content in wood if set to low, causing cracking and warping (took my guitar collection months to recover after SWMBO decided that she would use the continuous setting!!!!)....if you are going to use one make sure it has a regulated hygrostat so you can regulate the humidity.

 

[TonyH1]

Hi and thanks for the replies so far. I new there was more than one school of thought on de-humidifiers but decided that I would try one to find out whether it could be effective in preventing mould growth and I think the answer is yes.

I'm reluctant to leave heating on unattended long term even though it's at home plus I think the de-humidifier is low power compared to heating. I fully appreciate that natural humidity levels are constantly changing, I guess what I was hoping for was someone to be able to say that mould is less likely to grow with a humidity level of X and more likely to grow with a level of Y.

My dehumidifier does have a hygrostat (thanks 00buzz, that's the word I was looking for!) so I've set it to cut in and out at the current level until my meter arrives.

 

[ProfJohnL]

As Damian has stated the natural air humidity of the air is highly variable.

Just a few pointers about humidity. Warm air will hold more moisture than cold air which is why when warm air is cooled down it cant hold as much moisture so it condenses out and forms condensation on cold surfaces. Any absorbent surfaces (such as wood) will tend absorb the moisture.

Whenever we use a caravan, we generally keep it warmer than the outside, so the warmer air inside will absorb more moisture. We naturally breath out quite humid warm air so that will raise the humidity level inside a caravan, Add to this whenever we use an open gas flame (such as the cooker hob or oven) a significant part of the product of combustion is water vapour, and of course when water is boiled it releases super saturated water vapour, and all of these contribute to the raising of the humidity inside the caravan.

Consequently when we first store a caravan after use it tends to have a higher level of humidity. as it cools down over few days that moisture starts to come out and produce condensation. This initial stage provides prime conditions for the development of any mould spoors left on surfaces.

Again as Damian suggests a thorough clean should help prevent the moulds from growing, and after a few days when the humidity has come down towards the ambient level then further growth is likely to be limited.

The use of a dehumidifier is most beneficial during the first few days of storage, after the levels have dropped to ambient, there is little if any benefit to continuing. Except - if a caravan is stored in a particularly sunny spot, the solar heat gain inside the caravan will raise the internal air temperature and that of some surfaces, encouraging the fabric to release any moisture to the air and with convection currents the slightly warmer air will circulate and may find a cooler surface where it will again drop its moisture as condensation. But this will be short lived when the sun sets or shadows cross the caravan.

So the continued use of a dehumidifier may still produce some water collection, but it will quickly be replaced by the air exchange through the fixed ventilation required by the caravan construction regulations.

 

[RayS]

Humidity is a bit tricky to understand and the word usually missing is 'relative,
Figures quoted are usually stated as percentages without using it.
Relative humidity is the amount of water in a given volume of air as a percentage of what the amount which would be in the same volume of ait at the same pressure and temperature.

Warm air will hold more water for a fixed volume than cold air, so you can 'reduce' the humidity, say, inside a caravan by increasing the temperature ( providing you do so by 'dry' heat e.g. electricity rather than open gas flame.

Absolute humididity is not often used outside some specialised industries but is expressed in weight v. Weight e.g in old units grains of water per pound of air.

So keeping heat on will not actually remove any water from the air. The relative humidity will get less but the absolute humidity will stay the same. You need to physically remove some water to reduce absolute humidity.
The two common ways of doing this are by cooling the air such that it becomes 100% saturated and then dropping the temperature a bit more such that water vapour condenses to liquid water and can be drained off (as in portable and other dehumidifiers or by absorbing it in a material such as silica gel or on an industrial scale lithium chloride. These substances can then be trasferred elsewhere to have the water removed, usuallu by heating.

The temperature at which air becomes completely saturated and can hold no more water is call the dew point temperature.
Although having considerable exposure to industrial humidity control L have very little with domestic units so don't know whether they have a '%age RH' control setting knob by which you can ensure avoiding the over dehumidifacation damaging to timber fabric etc.

I appreciate all this does not really answer the original question but hope it may help in understanding the physics involved

 

[JamesH]

I remember when I fitted an extractor fan in our bathroom the enclosed instructions suggested that the humidistat should be set at 40%.

 

[Jaydug]

As I wrote in another topic on this same subject a few weeks ago, I use my caravan during the winter in Spain. During the last two or three weeks in October I transfer my dehumidifier from the house into the caravan and set it up there. For the first few days it runs almost full time and the tank quickly fills with water. But gradually as the days go by, the machine works less and the tank doesn't fill quite so quickly. I don't go along with the theory, to use that well-used phrase, "that I'm drying out the world". By the time I'm ready to set off for the ferry (usually the 1st November), I'm satisfied that the soft furnishings in the van are dry. Incidentally, I have one of these - http://www.amazon.co.uk/Smart-Weather-Wireless-Station-outdoor-sensor/dp/B000YAR5V8/ref=sr_1_1?ie=UTF8&qid=1452839252&sr=8-1&keywords=smart+weather+wireless - permanently fitted in the caravan. Yesterday, with door and windows open in the van, the humidity was recording 40%, although by evening it was 48%. Right now, having slept in the van and had the kettle on for coffee, it 55%.

 

[WoodlandsCamper]

I'm hoping one of you can tell me the ideal humidity level inside a caravan during storage.

As a comparison, I have a barometer hanging up in the hall of our house and one of the dials is a hygrometer. The dial is marked from 0 to 100 and is non-linear. There are two markings on the dial which suggest the 'ideal' humidity should be between the readings of 43 and 65. With the central heating on in winter the reading tends to be very near the lower limit. In the past (when the children were young and subject to croup) we have used ceramic humidifiers to moisten the air because it was TOO dry. hmy:

You can't win can you.

To answer your question it appears you need to be near 43%, as any lower would possibly over-dry the construction of the van, as others have said in the thread.

 

[Gafferbill]

.........well as everyone has said the air in a caravan constantly changes even with everything closed as there is fixed ventilation.
I have a digital weather station in my house (one upmanship Sir WC!) with a RH readout for inside and outside values.
The last few months here in West Sussex readings have been 45-55% inside and 65-98% outside.........mostly to the higher end of the outside range.
Today the readings are 47% inside and 49% outside....... a beautiful cold sunny day for a change!
Thus as my caravan is stored at home ......roof vents are fully open to get this dry air circulating and dry out any dampness.

 

[WoodlandsCamper]

I have a digital weather station in my house (one upmanship Sir WC!) with a RH readout for inside

Did you have to calibrate your oneupmanship station? When we had ours 35 years ago we had to set the upper limit by placing a damp dishcloth over the meter and adjusting it with a set screw to the 100 reading.

Perhaps we need to recheck its calibration or the Prof may say something. :evil:

 

[Gabsgrandad]

My "1 upmanship machine" is showing - outside temp 4° and 80% humidity, inside 21.3° and 43% humidity, again beautiful sunny day in 'Gods' county, my van now in the shade but been in the sun since early morning, but relying on it's own natural ventilation to keep it aired!

 

[Gafferbill]

The last few months here in West Sussex readings have been 45-55% inside and 65-98% outside.........mostly to the higher end of the outside range.
Today the readings are 47% inside and 49% outside....... a beautiful cold sunny day for a change!
Thus as my caravan is stored at home ......roof vents are fully open to get this dry air circulating and dry out any dampness.

Well today's readings are 45% RH inside my house and 93% RH outside........and the roof vents on my caravan are firmly shut.

Guess what hmy: :huh: ..............

 

[Jaydug]

Guess what hmy: :huh: ..............

Yes Bill. As I sat in my awning soaking up the sunshine whilst browsing the internet, I saw that Gatwick had a light covering of snow, and I thought to myself............................................................................ I'm in the right place B)

 

[Gafferbill]

Save some of that for me please