Damp clay in crawl space

[James Howard]

I live in a 1930’s detached house with a 2ft crawl space under timber joists, but the crawl space floor is mainly clay and is quite wet/damp. There are a couple of air bricks around the sides of the house, but I am going to install some more to increase air flow.

The previous owners have had a sump pump installed with drainage paths dug into the clay that drains into the rainwater system and works quite well, there’s some standing water where the ground/clay is uneven where people have knelt down etc. but it’s only a couple of cm.

When it rains, water is seeping in through the brick work at the wall/floor joints and then the pump drains it away.

My issue is the clay is always damp even after periods of dry weather and it can take a long time for any standing water to eventually soak/evaporate away. Is this a problem that could cause moisture to get onto the timbers?
At present none of the joists have any damp/rot issues and all look ok

I was thinking of adding a couple of inches of shingle/crushed rock on top of the clay then a damp proof membrane and then another inch of gravel to help the water soak away and hopefully stop any moisture spreading to the joists.
Or would this cause more problems?

 

[Notch7]

is the ground level under the house lower than around the house?

I would be wondering where the water comes from first rather trying to deal with it once its in.

Clearly the previous owner had issues......

Good air flow is certainly important so more and larger air bricks is good -what you want is cross ventilation so there is a good air current

 

[James Howard]

Yes the ground level under the house is lower than outside. And the outside levels are well below any current air bricks.

It’s comes when it rains, I think because the cavity is full of soil, that is higher than the dpc, that the dpc has been breached, possibly tracking on the outside brick of the cavity and then through the soil and out through a crack at the foundation level. I’m currently in the process of removing bricks and cleaning out the cavity, it’s a slow and painful job, but needs to be done!

 

[Notch7]

Yes the ground level under the house is lower than outside. And the outside levels are well below any current air bricks.

It’s comes when it rains, I think because the cavity is full of soil, that is higher than the dpc, that the dpc has been breached, possibly tracking on the outside brick of the cavity and then through the soil and out through a crack at the foundation level. I’m currently in the process of removing bricks and cleaning out the cavity, it’s a slow and painful job, but needs to be done!

I personally dont think a dpm would be a good idea. Theres a risk of trapping moisture below and above the dpm. Theres then the risk that might build up and work its way to the edges of the dpm and into the brickwork.

If the joists dont have rot, they should be fine.

What you want to minimise is warm moist air from the house getting down -althoughnif you currently have finished floor finishes theres not much you can do.

If the joists are exposed, then a vapour barrier across the floor would be good.

 

[James Howard]

Sorry yes that’s what I meant a vapour barrier on top of the clay, followed by some crushed rock or pea gravel to hold it on place.

Currently between the joists under the floorboards is polystyrene insulation that is fixed from below, so half of the solid joists are only visible

 

[conny]

If you cellar level is below the outside street level, (it usually is but not always), then the water you are finding may be run-off from the surrounding ground.
I don't think there is a lot you can do about this except keep it to a minimum with the sludge pump. Trying t get a fall on the clay surface so it all drains towards the pump could be a pain in the neck so if it's just a few pools here and there, well below the joists, I wouldn't worry.

 

[Harry Bloomfield]

Might some forced ventilation help, in the form of a fan mounted in place of an airbrick?