Cure for high humidity

7 Mar 2013
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United Kingdom
My daughter recently bought a 1920's semi-detached house. I noticed that the inside humidity was higher than you would expect, typically higher 60's to early 70's. This high humidity produces quite a lot of condensation even though the fanlights are kept slightly open.

The house itself is not damp and there was no sign of mould growth but the previous owner had had the lounge floor joists and flooring replaced.

The bathroom has had an extractor fitted and when cooking she always has a window open.

Recently whilst changing the radiator in the hall I noticed that the floorboards were a bit soft and when I started to remove some to get access to the CH pipework I found that the joists had suffered decay.

Whilst this flooring was up I noticed that the underfloor void was quite deep, about 1200mm. I could also see that the floor of the void was wet and in some places there was running water. This water was clear and appeared to be surface water especially as the area is known as a Spa and has underground water streams.

When I got down into the void the oversite was covered with mud and sand which was about 100mm deep and was sodden. Thankfully there was no rising damp in the walls.

The outer walls to the building have air bricks but we intend to fit a couple more, but I suspect that our high humidity is down to the running water and the sponge like effect the mud and sand on the oversite is having.

My question is; knowing this how can I get the humidity down to an acceptable level.

My thoughts are:-
1. Remove the mud/sand on the oversite to reduce the amount of water held under the floor.
2. Add some more air bricks to allow quicker drying of the void.
3. Lay a vapour barrier over the floorboards and seal to skirtings before carpet or laminate is laid.

Is there anything else which would help?

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Airflow and ventilation in void.

I would think insulation in between joists, say celetex.

Vapour barrier on warm side. Some underlays have a vapour barrier built in.
Is it OK to have say a small extractor fan fitted under the floor for times when the humidity does get too high?

When you fit Celotex between the joists is there a danger of not allowing the joists to breath and risk more damage in later years?

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When you fit Celotex between the joists is there a danger of not allowing the joists to breath and risk more damage in later years?
No. Fit Celotex between the joists, at the top, and they can still breathe at the bottom

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