everyday chemistry - Using table salt as a way to limit humidity in an unoccupied house

We own a static caravan in the UK (Snowdonia region, so rainy and wet all year round, moderately cold winters) and we leave it unoccupied during Winter (Oct-Feb).

We were advised of an apparently common practice among caravan owners which is to leave open boxes of table salt in each room, under the premise that they will capture moisture in the air and lock it away chemically in order to reduce the risk of moisture related decay in the building's structure.

I was skeptical that this would work but in the interests of science I did it anyway last winter. I took 7 plastic ice-cream tubs and put roughly 1 kilogram of domestic table salt into each and put one in each room of the home (two in the largest room - the kitchen/living room space).

I (stupidly) never took photos of this phenomenon but I have attempted to illustrate my findings...

Fig. B is complete speculation on my part as I did not return to the home during Winter to observe it. However in Spring when I returned, all of the tubs had experienced a change in their appearance.

All the tubs were now dry again, presumably down to evaporation due to increasing weather temperatures. And therefore releasing all that moisture back into the building again!

Three of the tubs were largely unchanged with some noticable "caking" together of the salt into crumbly, grainy lumps which returned to normal looking salt grains when crushed.

The most profound change from the remaining tubs was as you see in Fig. C of the diagram.

The salt had actually accumulated on the walls of the tub as a fine sediment. This suggests that water had accumulated in large amounts in the tub and had in fact risen higher than the original depth of the dry salt grains! I'd estimate that the tub would have had to accumulate about 0.5kg of water in order for the water/salt solution to reach the depth indicated by the dry sediment.

The salt had solidified into a single, large mass. The volume seemed to have increased noticeably but the density had also decreased accordingly, so the salt had basically expanded in it's container and solidified. It was crumbly and brittle and some of it had been reduced to a very fine sediment.

The home is a single storey, about 12m x 4.5m x 2.5m in volume.

My questions then:

1. Is this a valid technique for capturing excess moisture over Winter?


2. Are my observations and presumptions reasonable... Is Fig. B what really happened?


3. What is the chemistry / physics process that caused the salt to be transformed from Fig. A to Fig. C?


4. How many times did the tubs cycle between states B and C? Was it a single cycle that lasted all of winter, or a daily cycle following ambient weather temperatures? I could not tell just by looking at C on the last day of the experiment.

Answer

According to Transportation Information Service: Salt:

At up to 74% relative humidity and 20°C, salt does not absorb any appreciable quantities of water vapor. The critical water content of sodium chloride (NaCl) is 0.5% at 74% relative humidity, which is the flow moisture point at which salt begins readily to absorb water vapor, and increases such that, at 75% relative humidity, the salt dissolves.

See this graph for better understanding.

So at high humidity, salt can absorb lots of moisture, enough to become liquid.

When it is liquid, it will creep up the walls of the container by adhesion.

Then, when humidity decreases, the salt will lose almost all its moist, and contract, but leave residue up to the height of the former meniscus.