# dehumidifier - window open or closed



## chrisblack (Sep 10, 2007)

with the damp evenings , i've just put a dehumidifier in the van, to keep it fresh...

I thought I read somewhere to keep a rooflight or window open slightly?

is this correct?

chris


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## aultymer (Jun 20, 2006)

Well that depends on whether you want to dehumidify the van or the surounding countryside.


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## finyar (May 25, 2008)

Hi Chris, 
keeping the skylight open slightly helps to keep air circulating and this is what stops mildew from forming.

if you are using a dehumidifer( i assume crystals) it would seem to me that you would keep the windows and skylight closed

Finyar


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## chrisblack (Sep 10, 2007)

no idea about crystals - I borrowed it from a neighbour - an electric job, the size of a cool box...

chris


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## hymerowner (Sep 10, 2006)

We stick an electric one in and empty it regularly - keeps the interior dry. All windows etc are kept closed - there are lots of ventilation ducts, afterall. We do leave lockers and internal doors open and lockers propped open as well.


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## chapter (May 1, 2005)

open the lockers and lift the seats and if it has a out let pipe, place it in the shower tray and leave the waste gate open 
chapter


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## Spacerunner (Mar 18, 2006)

As above, close all opening widows and vents. There are enough permanent vents to keep air circulating.

Checked the humidity in my van yesterday and it is holding at approx 10% under the outside humidity level, so it definitely has an effect.


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## DABurleigh (May 9, 2005)

Interested in what it is indoors (say unused bedroom), garden shed, etc by comparison.

Dave


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## Spacerunner (Mar 18, 2006)

DABurleigh said:


> Interested in what it is indoors (say unused bedroom), garden shed, etc by comparison.
> 
> Dave


Give me 24 hours, and I'll let you know.


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## annetony (May 1, 2005)

I've used one in my front room aq couple of times a week, as we dont use the room very often and can get 2 litres of water out of it in about 8 hours, if my van was damp I would certainly use it in there


Anne


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## Zebedee (Oct 3, 2007)

If you run a dehumidifier continuously, it continuously fills up its container with water. This continues for as long as it remains switched on, even with all the doors and windows tightly closed.

So where does all that water keep coming from? :? 

Is it worth the bother? :?


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## aultymer (Jun 20, 2006)

> Is it worth the bother?


that's the million dollar question. (or should that be billion to take account of inflation)
We have one running in our bedroom on a 1 year test to see if we can keep clear of condensation induced mould on an outside wall in a 'still air' area of the room, ie behind a bookcase.


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## TR5 (Jun 6, 2007)

Zebedee said:


> If you run a dehumidifier continuously, it continuously fills up its container with water. This continues for as long as it remains switched on, even with all the doors and windows tightly closed.
> 
> So where does all that water keep coming from? :?
> 
> Is it worth the bother? :?


It certainly is! You will never reduce the humidity to the extent where there is no moisture in the air, in fact you would not want to, otherwise this would cause other problems such as shrinking wood, etc.

The prime reason for using a dehumidifier is to reduce the % moisture in the atmosphere to a degree where the internal fittings and furnishings do not become damp. Drier air will draw moisture AWAY from these fittings, rather than deposit it into them, thus avoiding mildew, mould, etc.

There is some permanent ventilation in the vehicle, as well as through the heater vents, etc. so moisture will always be extracted from the air, as some of this will be replaced by ventilation. However, by lowering the moisture levels (and keeping it lowered) will much reduce (or aleviate) the risk of damp damage.

I have a static caravan as well, which is shut up for 2 months in the winter, and the difference of using (or not using) a dehumidifier during this time is immense. I say that with experience!


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## Spacerunner (Mar 18, 2006)

aultymer said:


> > We have one running in our bedroom on a 1 year test to see if we can keep clear of condensation induced mould on an outside wall in a 'still air' area of the room, ie behind a bookcase.
> 
> 
> You wont stop this effect! With the bookcase on the outside wall you have actually moved the wall's cavity to between the back of the bookcase and the *inside* wall. I have seen the exact effect with wardrobes against an outside wall and the clothes inside badly effected by damp and mildew.
> ...


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## TR5 (Jun 6, 2007)

Also, if the damp and condensation is coming through the wall from the outside, reducing the humidity on the inside will encourage more moisture to be drawn through the wall. You would be better sorting out the cause, i.e. stopping the damp penetration at source.


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## Spacerunner (Mar 18, 2006)

DABurleigh said:


> Interested in what it is indoors (say unused bedroom), garden shed, etc by comparison.
> 
> Dave


Just checked both the van and the spare bedroom.

Unheated motorhome..........48% humidity

Unheated bedroom..............56% humidity


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## Spacerunner (Mar 18, 2006)

TR5 said:


> Also, if the damp and condensation is coming through the wall from the outside, reducing the humidity on the inside will encourage more moisture to be drawn through the wall. You would be better sorting out the cause, i.e. stopping the damp penetration at source.


Its not the damp penetrating the wall. Its the warm air meeting the cold air and causing condensation of the moisture already in the air.


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## aultymer (Jun 20, 2006)

Thanks TR5 but survey has confirmed that the outside wall is sound and there is no penetration. Funnily enough 2 neighbours with identical 1939 built houses have the same problem, in the same place.

As Spacerunner says, it is the meeting of warm moist air (our breath for example) meeting a cold surface and giving condensation. 
We could heat the whole room up until the wall was warm enough for this to stop but that would make the room too warm for sleeping! Getting air circulating in some areas is very difficult and our next plan is to install some small fans just to move air pockets.

If we could find someone willing to do a cavity wall insulation job on just the 3 walls of the original house I am sure the problem would vanish. (the rest of the house is a new extension and well insulated.)


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## DABurleigh (May 9, 2005)

Spacerunner said:


> DABurleigh said:
> 
> 
> > Interested in what it is indoors (say unused bedroom), garden shed, etc by comparison.
> ...


Thankyou. Perhaps I should have been more explicit. I was after a suite of readings in the various places at the same time on a given day, to draw conclusions as to the differences between them.

Dave


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## richardjames (Feb 1, 2006)

There used to be remedy for 'cold walls' in the form of a wall paper with a very thin layer of polystyrene - it works very well!
Richard


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## Spacerunner (Mar 18, 2006)

_Thankyou. Perhaps I should have been more explicit. I was after a suite of readings in the various places at the same time on a given day, to draw conclusions as to the differences between them._

Nah! I get out far too much for that!!.. :lol: :lol:


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## DABurleigh (May 9, 2005)

If the temperature in your bedroom one day is 18C and your motorhome on another day 20C, do you conclude your motorhome is 2C warmer than your bedroom? 

Dave


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## Spacerunner (Mar 18, 2006)

I can't sleep in both at the same time, so it doesn't bother me. I can adjust my personal settings, i.e. put an extra layer on to compensate.

What I can't do is to wring out the moisture in seat cushions and mattresses.

:wink:


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## Pard (May 1, 2005)

Returning to the question of dehumidifying a motorhome, I think it fair to say that a significant part of problems of mildew and mould growth arise from temperature change. If you can contrive to keep the temperature inside fairly even, humidity levels have less consequences for mould growth etc than if the space in question is hot in the day and cool at night. Because of temperature change summer can often be worse than winter for creating problems, but there is probably a slightly better chance of drying things (this summer excepted!). A consistent through-flow of air is equally important. It's when all this is impractical that dehumidifiers come into play.

Living as I do in a the damp south-western part of the country, with the coast half a mile away, I find that leaving roof vents half open (except in extreme winds or horizontal rain) has not induced any damp, and of course ventilates the van on hotter days. 

Best of all though is to use the motorhome all year....


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