Logic check please - humidity calculations

[Andrew]

Assuming those windows are single-glazed, they couldbe given some 'double glzing' with double-sided tape and some clear heavy duty polythene sheeting.

[Chris Hogg]

Can't tell what it's made of or how. Probably rendered concrete blocks. The single-story flat-roofed building with the high windows in the centre of this Streetview image houses the archive.

[Robin]

Location matters: coastal Cornwall tends to have high RH.

If those windows open I'd try using them to get the air circulating whenever the local forecast is for low RH. A lot cheaper than running a dehumidifier (assuming the electricity ain't free).

I also wondered if you'd considered curtains, blinds or just paper over the windows to reduce the risk of damage to documents from the light and temperature fluctuations.

[newshound]

My compressor based dehumidifier just dumps liquid water in a bucket (to which a drain hose can be attached). My portable air conditioner on the other hand has an elephant's trunk.

[Chris Hogg]

Thanks. A few of the windows are already done like that, using bubble-wrap for added insulation. I've suggested to the trustees that they extend coverage to all the windows.

[Martin Brown]

I think the paper will be acting like a reservoir of water and will hold a lot more water than the same volume of air would (a good few % by weight). "Dry" paper of the worst possible sort (newsprint) can be 10% water content but 5% is a more normal situation in good conditions.

Remember that people in the room by breathing will be adding new humidity all the time. Increasing the temperature slightly whilst running a dehumidifier will probably be the lowest cost solution.

Also ventilate the room on dry sunny days when possible. I know how hard managing humidity in such spaces can be in mid winter. There is a compromise between heating and dehumidifying to be had somewhere.

If the humidity is too high the books and paper will degrade. Leather bindings are particularly susceptable to going mouldy. Sudden changes in humidity are not good for archives either so whatever you do do it gradually (you may not have much choice about that).

A fan to just move the air around might also help. Worth looking to see if there is a particularly bad wet wall with eg penetrating damp.

[Animal]

It's not quite that simple, RH & temp both matter equally. Absolute quantity of water vapour per volume is the thing to compare. As a drying plan, it's likely to not be sufficient.

OTOH solar gain would help with temp which lowers RH.

[Robin]

Well possibly, although the TNA advise "It is preferable to have as little constant light in a repository as possible, both to reduce direct damage to the documents through fading and to minimise the heat introduced by light energy. Sunlight is particularly damaging due to its intensity and the corresponding rise in temperature accompanied by localised drying of the air."

[Dave W]

As others have said, it's not just the air that needs dehumidifying. I think you would need to put heat into the room to evaporate the water absorbed in the walls, ceiling and floor. There's probably no damp-proof course so when the heat stops the dampness will return.

[Chris Hogg]

Virtually all the documents are stored either in files, folders or filing cabinets. None are exposed to direct sunlight.

The Archive is a voluntary organisation, and relies on charitable donations for it's income, either large donations from businesses or organisations, or smaller donations from individuals who we've helped with their private research (family history, property history, old photographs etc). Funds are short at the best of times, but especially over the last two years due to Covid, and heating is expensive and getting more so as the weeks progress. There is a lot of window area, single glazed. Comprehensive insulation is just not practical or affordable. I've already mentioned the bubble-wrap on some of the windows, which will probably be extended at minimum expense.

[alan_m]

Could that be part of the problem - no airflow in the filing cabinets or folders.

I have a fire safe* for documents and noticed after a while that there was a musty smell when the safe was opened. This was cured with a "bulk" purchase of of silica gel which was put in cloth bag and left in the safe for while. I now just have a small amount of silica gel in the safe which is rejuvenated in a low heated oven approx every 12 months.

Have you tried to find out how much moisture is currently in your documents? Perhaps put a sample amount with the de-humidifier in something like a (sealed) wheelie bin and extract x amount of water to see the resultant RH.

*The safe is only for limited fire protection as these are easily opened with a bit of Youtube knowledge :)

[Robin]

+1

It's no accident that TNA uses cardboard boxes on open shelves.

[Animal]

warming & drying would be a desirable thing in this case. But it's all fairly trivial. The op's situation is a very long way from ideal document storage.

[Animal]

No, that is not causing 80% ambient RH. Fanning the air can delay mouldering, but only to a limited degree, and there's already a dh fan running. A ceiling fan could help delay mouldering a but I'd put effort primarily into finding the cause of the damp, that's what really matters.

Running a sample through an oven is the usual way to do it. But I can't see it giving any info useful to the op.

[Andrew Gabriel]

I created an environmental control system for a large shed which is used to store furniture (i.e. finished timber and soft furnishings and steel which could rust). The requirement was for this to remain dry and free from damp and mold damage. A further requirement was for storage of water-based chemicals not to suffer from frost damage.

The first thing was to understand how an unheated shed gets damp, as that was far from obvious to me to start with. I logged the inside and outside temperature and humidity for a while and then learned from that how a shed gets damp, why metal in it rusts, how condensation forms in it, etc. So I got to understand how condensation forms, and that I had to prevent any of the stored items dropping to the dew-point temperature. (Dew-point is effectively the measure of absolute humidity, i.e. the temperature at which the relative humidity becomes 100%.)

Having done this, the logging system was extended to maintain the relative humidity below a configurable level. The aim was to prevent it ever reaching 100%, but with a configurable margin to allow for some variation in conditions across the shed, and some margin to cope with sudden temperature changes which can easily cause a rise to 100% humidity without changing the absolute humidity on the shed.

I first ran the system set to 70%RH, and this worked fine and everything stayed dry. I did wonder if the margin might be larger than required, so I adjusted it to 80%RH, and this was also fine. It's been running now for 8 years, and nothing in there has got damp, smelled, or gone moldy or rusty. It's possible I could push the %RH even higher, but the costs of maintaining 80%RH have been very reasonable (although are going to double now with increased electricity prices, so maybe worth revisiting that).

There is frost-stat protection set at 5°C, also handled by the environmental controller. The controller itself is the very first Raspberry Pi model B, and has been in continuous operation for nearly 10 years now.

The original aim had been to use a dehumidifier, but I quickly learned from another project I was doing that compressor based dehumidifiers don't work much below 12°C - they ice up and may or may not know how to do defrost cycles (the ones I bought claimed to, but it never worked and the evaporator just turns into a solid block of ice). However, they still worked even when they were blocked with ice and not removing any moisture, because they were raising the shed temperature, and only a very small temperature rise is required to reduce the %RH significantly. Given I had an electric oil-filled radiator in there for frost protection, I simply switched to use it also to raise the temperature to reduce the %RH. So little heat was required for this, that I actually run it at half power (via a single diode) when it's operating for dehumidifying purposes, and if you go in the shed, the radiator barely even feels to be on as it needs only a very small duty cycle.

To raise the temperature sufficient to get %RH down to 50% would be a higher temperature rise and you'd need to check tables to ensure that was viable on, say, a hot humid summer's day. However, this would be a situation where a dehumidifier would work, so you might use both methods, deciding which based on if it's too cold for a dehumidifier to work or not.

Anyway, that's some more food for thought. In another project, I replaced the control board in a dehumidifier with a Raspberry Pi and part of the purpose of this was to enable the dehumidifier to operate at a lower ambient temperature, with my software able to correctly handle the evaporator icing up. The Pi being networked means the dehumidifier can be remote monitored and controlled over the Internet too (as indeed in the shed controller above, which means I monitor and look after it from 60 miles away).

Desiccant disc dehumidifiers may work at lower temperatures than compressor based dehumidifiers, but I've seen too many which have set light to themselves brought in to Repair Cafes for me to have one of those anywhere near anything I'm looking after. (Disc stops rotating and heater then burns it.)

Cheers Andrew

[Chris Hogg]

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Thanks for all that, Andrew. I shall read it several times to make sure I've got it all. Your shed sounds not too dissimilar to our archive.

We seem to be below 80%RH most of the time. There have never been any signs of actual condensation AFAIK, but it would only take a fall of a couple of degrees to hit the dew point.

I have thought about desiccant dehumidifiers. I have seen it said that they are more efficient at lower temperatures but more expensive to run, although it wasn't clear what 'more efficient' meant. I compared the actual efficiency of water collection (litres per kWh) for our compressor dehumidifier (DD-TEC10) versus the quoted rates for what seems to be a popular desiccant model (Meaco DD8L).

The compressor machine is rated at 300 watts and claimed to work down to 5°C and extract 10 litres per day under optimum conditions. Ours was only removing about 4.2 litres/day so clearly, conditions weren't optimum, and it regularly went into 'defrost' mode as the evaporator coils iced up. However it did use almost exactly the 300 watts claimed, giving an 'efficiency' of 0.61 litres/kWh.

OTOH the Meaco datasheet claims 3.5 litres/day at 330 watts (Fan speed

1) , which gives an efficiency of 0.44 litres/kWh, or at the higher fan speed 2, 7.5 litres/day at 650 watts, giving an efficiency of 0.48 litres/kWh. Both are less efficient than for our compressor model, even though the latter isn't performing at its optimum.

On that basis we'll stick with our (donated) compressor model, further encouraged by your comment about the desiccant models being prone to catching fire, which saves us having to invest money we haven't got much of!

Thanks again.

[Spike]

<very big snip>

This is such an insight into the issues surrounding RH, it's control and its effects on stored items, that perhaps something based on it should go in the wiki?

Many years ago I did read that the greater part of the damage caused to unused buildings and their contents was though humidity cycling.

[Thomas Prufer]

So look at environmentally controlled ventilation... i.e.

(which I have not used, but the manual is a good read as it shows some of the pitfalls -- such as freezing the wine cellar, running the fans at 4 a.m., ...)

There are a number of controllers that measure absolute humidity outside and inside (usually by taking relative humidity and temperature, and calculating the absolute humidity), and control ventilators to drop the humidity inside.

They do "ventilate when drier outside than in" plus: no colder inside than X, no warmer than Y, do not run during times Z, no drier than V, fire up the dehumidifier, minimum air change, etc.

Some of these where pretty cheap, 150 Euros or so, with the expensive parts being the holes in the walls, the ventilators, running the wiring etc, most of which can be diyed.

Advantage is that you are using dry air from outside when possible, at the running cost of a fan, which reduces the load on any dehumidifier.

Thomas Prufer

[Chris Hogg]

Thanks. Interesting possibilities, but a) we don't have a lot of money

- so paying for a controlled ventilation system is not really on the cards, and anyway I doubt if the church who own the place would be too pleased if we asked to knock holes in the walls, and b) living near the coast in west Cornwall means that the humidity is always high. The only time it drops is in the summer when the weather is warm and the humidity in the Archive would probably be OK anyway.

[Andrew Gabriel]

They all condense the water out. Some aircon units then run it over the condenser (hot) element to evaporate it into the exhaust air stream going up the elephant trunk. This not only conveniently gets rid of the condensate so no tank to empty, it also makes the condenser more efficient by cooling it further by the latent heat of evaporation it draws from it. Obviously the elephant trunk must exit the room, or you simply dump the moisture back in the same air, so this won't work with a self-contained dehumidifier that has no elephant trunk.

Cheers Andrew