Under floor ventilation confusion

[Bob49]

Hi,

I'm confused about under floor ventilation.

With a suspended timber floor the void underneath is ventilated with air bricks. So this must mean that the temperature/humidity of the void must match closely to that of the outside. There can't be much heat from the rooms above to lower the relative humidity.

In June the average humidity is 70%, in December it is 89%, so the under floor void must be similar. This in December would be an EMC in wood of 20%.

So in Winter is the void really at this level of humidity and are all joists really at this level of moisture content?

What have I missed?

Thanks

Bob

 

[John D v2.0]

yes but if you keep a bit of treated timber outside under cover and off the ground, it won't rot. You only need to worry if the water is dripping onto the wood or soaking in from something wet.
You can get those prong meters to find out the moisture content of the wood within and outside the heated part of the house, when it gets up to 40%+ then you have to worry a bit.

 

[big-all]

its about 19% moisture in wood that causes rot
remember air weighs virtually nothing so 80% moisture is not a lot
where as wood is heavy so perhaps 5% in wood will be the same amount as perhaps 70% in a room

 

[oldbutnotdead]

Ventilated is the key. Sit in your front room, put one leg in a sealed plastic bag (doesn't have to be clingfilm, a bin bag will do long as there's an airtight seal), leave the other leg exposed to room air. Wait for a few hours. The leg in the bag will be dripping wet- the perspiration has nowhere to go so condenses and turns into water (simulating a damp piece of timber drying out). The other leg will be dry.

Or another example- drive past your local builders merchants- you'll see stacks of structural timber sat out in the rain. But look closely- they aren't on the ground, there are bearer blocks below the stack and usually every foot or so- the wood gets wet, it dries, no drama. Wood sitting in water- another matter entirely.

 

[John D v2.0]

I think what people have missed is the op has understood that timber is hygroscopic and that 90% humidity in the air will balance out at around 20% in timber.
I don't think he's suggesting blocking up the ventilation, i think he's just wondering is the wood safe at outside humidity levels.
The answer is yes it's fine in normal circumstances.

 

[Bob49]

I think what people have missed is the op has understood that timber is hygroscopic and that 90% humidity in the air will balance out at around 20% in timber.
I don't think he's suggesting blocking up the ventilation, i think he's just wondering is the wood safe at outside humidity levels.
The answer is yes it's fine in normal circumstances.

Yes that's it exactly. Thanks.

 

[Leofric]

As well as cross ventilated underfloor void you might have missed suspended timber floor joists should be on dpc on honeycomb sleeper walls

 

[Leofric]

Sit in your front room, put one leg in a sealed plastic bag (doesn't have to be clingfilm, a bin bag will do long as there's an airtight seal),

- but don't put the bag over your head