Logic check please - humidity calculations

[Chris Hogg]

I volunteer at our local Archive. The Trustees have recently been getting concerned at the high levels of relative humidity in the main document room, which is unheated. I have been running some tests for them, using a little data-logger to record temperature and %RH over extended periods of time, with and without a domestic dehumidifier running.

I have also done some simple calculations. It is the logic of those calculations I'd like opinions on, in case I've got something wrong.

The volume of the document room is near enough 500 cu. metres (12.2*8.5*4.8 metres, l*w*h; it's a large, tall room). The temperature in there at the moment is around 12°C (as I said, it's unheated). From published tables of saturated moisture content of air vs. temperature, I see that at 12°C, air will hold approximately 11g water per cu. metre.

This means that the total amount of moisture in the air in the document room at saturation would be

5,500g (11 x 500), i.e. 5.5 kg, or 5.5 litres.

The National Archives recommend a relative humidity of between 35 and

60% for storing documents, call it 50%. But the relative humidity in our document room is around 75% (sometimes over 80%). This means that the total amount of moisture in the air is 75% of 5.5 litres, i.e. 4.125 litres.

If we set a target of 50%RH, the total amount of moisture in the air has to be reduced to 2.75 litres (50% of 5.5). That in turn means removing a mere 1.375 litres of moisture from the air (4.125-2.75).

It is last figure that worries me. It seems a trivially small amount for such a large room, especially as with a small dehumidifier running continuously and condensing about 30 litres of water over a week, the relative humidity only fell by a few %.

Have I misunderstood something in my interpretation of relative humidity and my calculation of the moisture contents? To only have to remove 1.375 litres of moisture from a room that size in order to bring the relative humidity down from 75 to 50% seems a gross underestimate to me, but I cannot see any misunderstanding. Where else might my logic be at fault?

The other explanation for the relatively small change in humidity with the dehumidifier running is that moisture is continuously bleeding into the room from somewhere. Possibilities are the slow drying out of the structure and documents, or draughts from outside around windows and doors.

[Theo]

Are you sure the humidity is just in the air? Surely the room is full of other things beyond air, that also absorb moisture?

If the room has lots of paper in it, I'd expect that to absorb more water per unit volume than air and so, when you dehumidify the air some of that water will be released from the paper.

The same might go for a laundry room full of wet washing: you can dry the air, but until the washing is dry it'll continue releasing moisture - effectively being a buffer for the humidity. Obviously clothes can hold a lot more water per unit volume than air can.

Eventually you reach an equilibrium where the amount of humidity seeping into the room (is it well sealed?) balances the amount you can take out of the air.

Theo

[Chris Hogg]

Thanks for your input Theo. That was my thinking also, see my last paragraph. Lots of documents, all having reached a moisture equilibrium with damp air over a long time. But I was surprised and concerned about the small amount of moisture theoretically needing to be removed.

[Brian]

Why not invest in a humidity meter or several?

It is unlikely the room is ?sealed? - unless design to be. Even modern homes with double glazing etc aren?t air tight. I expect your room is somewhere in an older building.

[Andy Burns]

Where is the humidity being topped-up from, how often are doors opened, what surrounds it, would it feasible to dryline it incorporating a vapour control membrane?

[Richard]

Maybe this will help you a bit:

[Robin]

On 26/02/2022 21:20, Chris Hogg wrote: <snip>

Given the dimensions of the room, does it have solid walls? Old, solid walls can store a lot of water. The amount varies naturally over the year but in winter it can be a lot.

And /absence/ of draughts can be a problem in a room where draughty doors, windows, floorboards and chimneys originally got rid of moisture that wicked in through the walls.

[Brian Gaff (Sofa]

In the clean room we used to have when I worked, the incoming air was dehumidified to some extent, indeed one could control it so it did not promote static problems but was as low as it needed to be for the job that was being done. There were also extraction systems, and the pressure in the room was always kept higher than the pressure outside to stop anything coming in but the dust and humidity controlled air, and of course there were co2 monitors and ionisers in the system as well. This type of climate control is expensive and often rather nosy and expensive to run though. Brian

[Chris Hogg]

I already have, one of these

but it won't answer my question or bring the humidity down!

At least pre-war and probably 1920's or 30's. Not even sure it has a cavity wall.

[Chris Hogg]

Yes, walls are certainly old and probably solid, 1920's to 30's.

OK, but I don't see that happening here, BICBW.

[Chris Hogg]

Thanks - I'll run my numbers through it, see if they agree.

[Chris Hogg]

Dry lining not really an option. The Archive only rents the document room from a local church, which has no money anyway. There are also lots of tall windows around the walls at a high level, like a clearstory. Some of them may not be well sealed, which could probably be fixed at no great cost.

But it was my logic in interpreting relative and saturated humidities that I was concerned about - and the theoretically surprisingly small amount of water needed to drop the humidity in there.

[Andy Burns]

It may be a surprisingly small amount, but if it can get back in easily, you'll be removing that amount over and over ... as you've already noticed.

[Theo]

If it was a sealed glass box, then your theoretical numbers sound plausible. For something dry and sealed, look at those silica gel packets you get in boxes of shoes etc. They're not big enough to absorb very much, but enough given the shoes are dry to begin with, the box is small and the packaging relatively (but not entirely) airtight.

But your room it isn't sealed, nor does it contain dry things. So you have a lot more work to do.

Theo

[John Stockton]

(1) Calculate and consider the amount of ordinary air at ordinary humidity that would have to leak in to explain the output of the dehumidifier.

(2) I assume that the room is useful, which means that people go in and out. While they are in, they will presumably breathe out damp air; estimate the effect of that. And when they exit, at least a corresponding volume of external air must necessarily enter.

(3) Routine fluctuation of external atmospheric air will cause air to go in and out of the room, without necessarily causing any noticeable draughts. |

[Spike]

Back in nineteen canteen, my little group reoccupied a building that had been standing empty for about six years. The main part of it was a

40'x20'x20' brick structure with a wooden roof and oilcloth covered concrete floor, having a fair amount of wooden furniture in it.

I took it upon myself to dry this all all out, as it was cold and had a damp smell. The chosen method using what was to hand was to put out every night three large trays of dried silica gel, and in the morning put them in an oven (in a separate room), by which time they had gone from blue to pink. On a rough estimate I took over the course of a year about a ton of water. Not all of this would have been from the structure, but it shows that stuff absorbs more water than one might think.

Your paper repository will be holding shedloads of water and it will take something like a year to re-equilibrate the paper to its new lower level of water content. A big clue as to how you are doing is to monitor that rate of humidity rise when you turn off the dehumidifier - as things dry out, this will slow down. Look on the exercise as being a slow process that takes time - you're drying out not only the paper. but everything that can adsorb water.

The building I dried out is now Listed and of historical interest, one of the ones next to it has only the masonry structure left.

[Animal]

80% means imminent large scale mouldering of documents & thereby their destruction. It also means something is going very wrong with the building. You need to get this down by more than a couple of percent. If the documents are worth keeping you could start with another dehumidifier, desiccant wheel type since the temp is low. 80% means there is water getting into the room somehow. I'd go round looking for all the usual causes of damp. I wouldn't try to make things draughtproof just yet, at 80% draughts will be taking more vapour out of the room than in.

[Chris Hogg]

Thanks for that. Yes, 80%RH is getting a bit high for comfort. A couple of degrees lower in temperature and you hit the dew point, and then we'd really be in trouble. Your comment about desiccant wheel dehumidifiers was also useful. They're not something I have any experience of, although there are plenty of descriptions available. What I don't understand is how the 'self contained' types lose the water from the 'used' desiccant into a collection tank or whatever, compared to others that seem to discharge warm moist air to the outside via an 'elephant's trunk', a bit like a clothes drier, which I can understand.

[Spike]

If you go the desiccant wheel route, get one with an economy mode and it'll run at 300W instead of 600W. Some have a hose connection ("7mm"?) that continuously diverts water via the hose to outside,

Getting the humidity down of everything in the room/building is going to cost money...

[Andrew]

Look at the external brick bond. While not totally infallible a solid wall will normally have some bricks sitting sideways across the wall binding the two leaves of bricks together.