Humidity - think I’ve finally cracked it 👍

[Purrfecttune]

Tried a humidifier but made very little difference. Most of what I have read mentions ventilation.

So this week I tried locking all the windows on ‘vent’ including the overcab window and opened the toilet roof window onto the first setting. We have a MH cover for it, so there’s very little chance of detritus or rain getting in but plenty of air.

Last week I was getting readings of 70%, +/- 5%
This week I’m getting readings of 49%

So this appears to be the answer for when it’s not being used.

 

[PaulW2]

Is this while you're not in the van?

 

[Purrfecttune]

Is this while you're not in the van?

Yes, when not in use. I’ve never actually checked while in use.

(Using the Swift Command/Sargent app to obtain readings)

 

[PaulW2]

So you would expect the internal relative humidity levels to essentially be the same as outdoors?

In summer that should be in the region of 50%. In winter it could however be 80% or 90%.

Reducing it in winter may presumably then rely on having less air circulation from outside and reducing internal relative humidity, either by extracting moisture or increasing temperature (warmer air can hold more moisture and so relative humidity drops).

 

[Purrfecttune]

So you would expect the internal relative humidity levels to essentially be the same as outdoors?

In summer that should be in the region of 50%. In winter it could however be be 80% or 90%.

Reducing it in winter may presumably then rely on having less air circulation from outside and reducing internal relative humidity, either by extracting moisture or increasing temperature (warmer air can hold more moisture and so relative humidity drops).

Only time will tell

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[manners]

So you would expect the internal relative humidity levels to essentially be the same as outdoors?

In summer that should be in the region of 50%. In winter it could however be 80% or 90%.

Reducing it in winter may presumably then rely on having less air circulation from outside and reducing internal relative humidity, either by extracting moisture or increasing temperature (warmer air can hold more moisture and so relative humidity drops).

Unfortunately the moisture suspended in the warmer air will then condensate on any cold surface, unless you can heat the van and maintain it so that no surface is cold it's better not to have any heat, and keep the blinds shut so you don't get any heat from the sun if that's such a thing in the UK winter

 

[pappajohn]

Dehumidifiers are great ... In a sealed room and that will never happen in a motorhome.
When i worked as a sparky the plasterers redoing rooms in renovation houses would always shut window and doors, block all air vents and put a towet etc at the bottom of the doors at the end of the day and start the dehumidifier.
If they didnt the new plaster would take an age to fully dry.

 

[PaulW2]

Unfortunately the moisture suspended in the warmer air will then condensate on any cold surface, unless you can heat the van and maintain it so that no surface is cold it's better not to have any heat, and keep the blinds shut so you don't get any heat from the sun if that's such a thing in the UK winter

Are you sure about that?

If one is sleeping in a van and adding moisture that is one thing. You wake up to condensation on the windows.

But if one isn't in the van, and one heats it, then the amount of moisture in the vehicle remains the same. It's just that the warmer air can hold more moisture and so relative humidity drops, reducing the likelihood of it condensating out.

 

[whisky walker]

My humidity is showing 59%at the moment on my swift command and van has been shut up all day on site. The last thing you want to do is run a dehumidifier in an unsealed room because it will just drag moisture in from the outside. Fresh ventilation is the best and realised that from living in a 300 year old stone cottage. I've heard some horror stories from people using cover's remember using one on a land rover for about three months when I took it off there was more growth on the inside than a botanical garden

 

[Purrfecttune]

My humidity is showing 59%at the moment on my swift command and van has been shut up all day on site. The last thing you want to do is run a dehumidifier in an unsealed room because it will just drag moisture in from the outside. Fresh ventilation is the best and realised that from living in a 300 year old stone cottage. I've heard some horror stories from people using cover's remember using one on a land rover for about three months when I took it off there was more growth on the inside than a botanical garden

I guess it depends on the quality/breathability of the cover and how long it’s on for. We generally only put the cover on when we have finished travelling for the year (i.e. December - April). Although I’ve put it on now as we won’t be using it again until December.

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[PaulW2]

My humidity is showing 59%at the moment on my swift command and van has been shut up all day on site. The last thing you want to do is run a dehumidifier in an unsealed room because it will just drag moisture in from the outside. Fresh ventilation is the best and realised that from living in a 300 year old stone cottage. I've heard some horror stories from people using cover's remember using one on a land rover for about three months when I took it off there was more growth on the inside than a botanical garden

In the UK I have no condensation problem in my house in winter whether I'm in it or not.

The central heating essentially takes car of it. Cold air in, heated, relative humidity drops, job done.

In Portugal, where my place doesn't have central heating, and where I'm often not in residence in winter, it's a different story. Significant problems with humidity in winter.

 

[whisky walker]

I guess it depends on the quality/breathability of the cover and how long it’s on for. We generally only put the cover on when we have finished travelling for the year (i.e. December - April). Although I’ve put it on now as we won’t be using it again until December.

The one I had said it was fully breathable and was in the region of 400 quid but I had my own thoughts of it's breathability after the couple of months.

 

[whisky walker]

In the UK I have no condensation problem in my house in winter whether I'm in it or not.

The central heating essentially takes car of it. Cold air in, heated, relative humidity drops, job done.

In Portugal, where my place doesn't have central heating, and where I'm often not in residence in winter, it's a different story. Significant problems with humidity in winter.

If we have our central heating on in the winter in the conservatory the condensation on the windows is terrible due to the warm air hitting the cold glass.

 

[Purrfecttune]

The one I had said it was fully breathable and was in the region of 400 quid but I had my own thoughts of it's breathability after the couple of months.

Used ours for 3 winters so far and it’s been brilliant

 

[whisky walker]

Another thought do you have the floor covered in the garage?

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[Purrfecttune]

Another thought do you have the floor covered in the garage?

No garage

 

[manners]

Are you sure about that?

If one is sleeping in a van and adding moisture that is one thing. You wake up to condensation on the windows.

But if one isn't in the van, and one heats it, then the amount of moisture in the vehicle remains the same. It's just that the warmer air can hold more moisture and so relative humidity drops, reducing the likelihood of it condensating out.

Warm air as you say holds more moisture so the humidity is higher think tropics. Cold air cannot hold much moisture so is less humid. Unless your van is totally airtight and the moisture is removed from the warm air by a dehumidifier, it's better to leave a small amount of ventilation unheated.

 

[whisky walker]

No garage

No worries it was just a thought because I covered mine with sponge matting and had condensation under after a while so drilled holes in the matting here and there that cured it

 

[Purrfecttune]

No worries it was just a thought because I covered mine with sponge matting and had condensation under after a while so drilled holes in the matting here and there that cured it

The Swift escape has a long narrow external access panel to the area under the rear lounge seating.

 

[whisky walker]

The Swift escape has a long narrow external access panel to the area under the rear lounge seating.

I thought it was like the Kon tiki? Like a small garage with access from either side? Not specifically a garage of sort

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[PaulW2]

Warm air as you say holds more moisture so the humidity is higher think tropics. Cold air cannot hold much moisture so is less humid. Unless your van is totally airtight and the moisture is removed from the warm air by a dehumidifier.

Ok, my reasoning has been almost exactly the opposite. The key thing isn't absolute humidity, but relative humidity - the risk that the humidity will leave the air and cause trouble. (As opposed to being hot and bothered in the tropics because our sweat won't evaporate.)

Back in England we have an amount of air which is fixed. A certain number of cubic metres in the room.

In this air is water moisture. Under certain conditions this will be at risk of condensating out.

Specifically, if the air temperature is 10 celsius, then the air can only carry 9.4 grams per m3 (at 5 degrees it's about 7 grams). So if the air has, say 15 grams from when it was warmer then, as the temperature drops, some will condensate out.

But if we heat the air to say 25 degrees then it can hold 23 grams per m3. It won't condensate out.

So now we're in winter. The outside temperature is say 5 degrees. This outside air holds almost 7 grams per m3. Its relative humidity - its humidity relative to the maximum it can hold - is therefore pretty much 100%.

We now let this air into the house, and we heat it. Say to 25 degrees. The relative humidity now drops to less than 33%. Lovely.

We also air out the house periodically, but this cycle essentially just repeats itself. Colder air in, heated, relative humidity falls.

 

[whisky walker]

Ok, my reasoning has been almost exactly the opposite. The key thing isn't absolute humidity, but relative humidity - the risk that the humidity will leave the air and cause trouble. (As opposed to being hot and bothered in the tropics because our sweat won't evaporate.)

Back in England we have an amount of air which is fixed. A certain number of cubic metres in the room.

In this air is water moisture. Under certain conditions this will be at risk of condensating out.

Specifically, if the air temperature is 10 celsius, then the air can only carry 9.4 grams per m3 (at 5 degrees it's about 7 grams). So if the air has, say 15 grams from when it was warmer then, as the temperature drops, some will condensate out.

But if we heat the air to say 25 degrees then it can hold 23 grams per m3. It won't condensate out.

So now we're in winter. The outside temperature is say 5 degrees. This outside air holds almost 7 grams per m3. Its relative humidity - its humidity relative to the maximum it can hold - is therefore pretty much 100%.

We now let this air into the house, and we heat it. Say to 25 degrees. The relative humidity now drops to less than 33%. Lovely.

We also air out the house periodically, but this cycle essentially just repeats itself. Colder air in, heated, relative humidity falls.

The actual mass of water in a given volume of air is absolute humidity not relative?

 

[PaulW2]

The actual mass of water in a given volume of air is absolute humidity not relative?

The actual mass is absolute, the actual mass relative to what it can hold at the temperature is relative.

So an absolute mass of 7 grams is 100% RH at 5 degrees but about 30% at 25 degrees.

 

[whisky walker]

The actual mass is absolute, the actual mass relative to what it can hold at the temperature is relative.

So an absolute mass of 7 grams is 100% RH at 5 degrees but about 30% at 25 degrees.

Absolute humidity Highly dependent on temperature; cooler air can hold less moisture, so relative humidity increases as temperature decreases, even if the absolute humidity stays the same.

 

[PaulW2]

Absolute humidity Highly dependent on temperature; cooler air can hold less moisture, so relative humidity increases as temperature decreases, even if the absolute humidity stays the same.

That's what I was explaining with my example

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[whisky walker]

That's what I was explaining with my example

Yes but it was quite confusing if your not used to working with humidity

 

[manners]

Ok, my reasoning has been almost exactly the opposite. The key thing isn't absolute humidity, but relative humidity - the risk that the humidity will leave the air and cause trouble. (As opposed to being hot and bothered in the tropics because our sweat won't evaporate.)

Back in England we have an amount of air which is fixed. A certain number of cubic metres in the room.

In this air is water moisture. Under certain conditions this will be at risk of condensating out.

Specifically, if the air temperature is 10 celsius, then the air can only carry 9.4 grams per m3 (at 5 degrees it's about 7 grams). So if the air has, say 15 grams from when it was warmer then, as the temperature drops, some will condensate out.

But if we heat the air to say 25 degrees then it can hold 23 grams per m3. It won't condensate out.

So now we're in winter. The outside temperature is say 5 degrees. This outside air holds almost 7 grams per m3. Its relative humidity - its humidity relative to the maximum it can hold - is therefore pretty much 100%.

We now let this air into the house, and we heat it. Say to 25 degrees. The relative humidity now drops to less than 33%. Lovely.

We also air out the house periodically, but this cycle essentially just repeats itself. Colder air in, heated, relative humidity falls.

I thought we were referring to a motorhome not a house, and I certainly don't have the money to keep my house at 25 degrees nevermind a van, and the reason our sweat won't evaporate in the tropics is because the air has a high percentage of moisture.

 

[PaulW2]

I thought we were referring to a motorhome not a house,

As for a house so for a motorhome. Just trying to describe some of the physics based on something we all have experience of

and I certainly don't have the money to keep my house at 25 degrees nevermind a van,

It was a number to illustrate a point. Same logic holds between 5 degrees and 15 if that's more within your budget

and the reason our sweat won't evaporate in the tropics is because the air has a high percentage of moisture.

I know. You asked me to think tropics. That's what happens there. It's why the humidity is uncomfortable.

 

[manners]

It was a number to illustrate a point. Same logic holds between 5 degrees and 15 if that's more within your budget

I think that's a bit insulting.

 

[PaulW2]

I think that's a bit insulting.

Insulting? Why would it be insulting? You said

"...and I certainly don't have the money to keep my house at 25 degrees nevermind a van..."

So I assumed that you didn't mean that as a snide putdown (you didn't, did you?), but that you seriously didn't understand what I was saying because it was so far beyond your experience. The physics works for all budgets. And Carthagos are fortunately insulated.

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