First I'd get one of these to confirm your assumptions https://www.amazon.co.uk/dp/B086YYL439 then I'd probably just get a dehumidifier and run it overnight. I bought one of these for a student flat and it has worked wonders. https://www.amazon.co.uk/Meaco-Portable-Dehumidifier-DD8L-EXCLUSIVE/dp/B077T4QPTZ/. Downside is, you might find the humidity drops too low during the day. In which case your only solution is to maintain a higher temperature overnight.

Fresh air might not help. It is the warm moist air from the day (folk sweating) that condenses overnight. The outside air might actually have more moisture in it.

We have a VA Tempra that sounds perfect for your needs, or something similar, we use it to dry washing indoors. You could put it on trickle through the day and boost overnight (noisy) with the bathroom door left open . Cheap and easy to fit, slot in replacement for a 100mm bathroom fan so no regrets if you just try it.

If it's 20⁰C/50%RH in the day, then when it drops to 12⁰C it will be 80%RH which is bit too high (lots of psychrometric calculator widgets online).

At this time of year, even when it is chucking down rain and snow outside, the cool outside air contains much less moisture than the warmer indoors air. That is why ventilation is so effective now and ventilation systems can be turned right down to low flow rate. Different in high summer, but then the room won't get cooled overnight and you can open windows.

As a workplace, you probably need to have plenty ventilation (eg avoid sharing germs and everyone goes off sick).

Agree in principle,

in VE's case that's about 1-2 ACH during weekday hours.

From the description of the factory, and the very acceptable %RH measured during the day, it wouldn't be surprising if there is already more ACH happening than that, due to air leakages, and people coming/going through doors.

Sounds like the problem is more at night when the temperature drops off (what about over the weekend?).

Posted By: borpinTest for CO2 to confirm what the air quality is like.

I agree with this.

Will, how can relative humidity change? The whole point it is relative....

I don't think fresh air is an issue either, I have access to several dehumidifiers. I think the issue is of the dewpoint. This appears to condense on the 4" cold main running through our office space. As this is the coldest point in the room this is why.

Could insulate it but I think the condensation will then go elsewhere. It is the black mould that is a concern

Posted By: VictorianecoWill, how can relative humidity change? The whole point it is relative....

The relative humidity (%} changes when the air cools down overnight. The absolute humidity (grammes of water) stays the same.

For example, your room during the day would have a capacity to carry up to 17 grammes of water in each m³ of air before it hits it's dewpoint. You measured that it is only using up 50% of its max capacity, so that's 50% of 17 = 8.5g per m³ of actual humidity is floating around the room.

Overnight, it"s still got the same actual humidity floating in it, 8.5g/m³. However it's cooler so it has much less capacity to carry water, the max capacity limit is reduced to 11g/m³. So it's creeping much closer (8.5/11= 80%), relative to its new dewpoint limit.

You don't want it to be that close to hitting its dewpoint limit because mould can grow. So either keep it warmer, or replace it with air from outdoors which is drier (less actual grammes of humidity) at this time of year than your room is.

Together, your team breathe out several kilogrammes of humidity during each work day. If you wipe up what condensed on the pipe and weigh it, it will only account for a tiny bit of those kilogrammes. The rest is going somewhere else - either condensing on unseen surfaces somewhere (bad) or being ventilated away (good). You likely only need to top up the ventilation a little, to get rid of the rest of the water that you're all breathing out, a continuous-running bathroom fan mhrv might be enough. If its not, then at least it's a cheap first step.

(Do you have a kettle, or hang up wet site jackets? Could be quick win to move that stuff somewhere else.)

Only if there is heat transfer into/out of the IBC. If the IBC were just sitting there, and remained a couple of degrees warmer than the room, the surface heat resistance would allow about 50W of heat to leak out of the IBC. Might help in a passivhaus, but not enough to be worthwhile in the building described here!

Walls and floors have much bigger surface areas, so are more useful for thermal inertia.

(Edit to add: offices are occupied for about a quarter of the hours in the year, so is good to have a setback temperature by a few ⁰ the rest of the time, so low thermal inertia helps with that. But a large area of hygroscopic materials would help stabilise the RH overnight - maybe a pile of straw bales? )

The VentAxia Tempra Will mentioned; a heat recovering extract fan