Crawl space and vapour barriers

[Jemster]

We live in a 1920s detached house, red brick construction and rendered. Ground floor is suspended timbers over a 1-1.5m high crawl space. The ground is compacted earth/clay and would be about 60cm below outdoor ground level for the most part. Over the years the house has had an injected DPC although there seems to be a variety of slate and bitumen DPC installed in a variety of places I’ve checked.

The ground is continually damp to the touch = not puddled, only seen a very small amount once in extreme weather in one small patch in the middle, so that was a temporarily raised ground table. I’m concerned about the moisture levels in the area as I’ve no desire to have the wood timbers decay due to damp. They are wrapped and treated where they meet the outer walls but they are reading in the high teens at the moment for damp content.

I’ve increased ventilation with a couple of extra holes strategically placed and the original airbricks and vents are clear. What I’m thinking now is to lay a vapour barrier on the floor and extend a foot or so up the walls. Using a clear 1000ga sheeting overlapped and jointed, tacked back to the walls with a butyl tape. However I can’t find many online references on this side of the pond that says this will help.

I asked on another forum and was advised by the only person who answered to throw down a 50mm concrete screed over the entire (rough) ground area. This seems a fairly irreversible step and not something I’d be inclined to try. Question is, will the vapour barrier help keep the humidity down and would there be any unwanted side effects (extra moisture in walls for instance?)

Thanks for any advice!

[wes56]

The vapor barrier is your best bet to protect against condensation.
The pros outweigh the cons.
Through ventilation from air bricks - spaced c/c 1500mm to 1700mm - from front to rear elevations is also necessary.

Given that this has been going on for a 100 years then why not closely inspect all the suspended floor joisting and woodwork - esp any trimming around any hearths and chimney breasts?

[Jemster]

The vapor barrier is your best bet to protect against condensation.
The pros outweigh the cons.
Through ventilation from air bricks - spaced c/c 1500mm to 1700mm - from front to rear elevations is also necessary.

Given that this has been going on for a 100 years then why not closely inspect all the suspended floor joisting and woodwork - esp any trimming around any hearths and chimney breasts?

We think there’s enough through ventilation now - it’s a double-fronted building, left side of the house has 3 vents, right side 3 vents, front has 1 vent in 1 bay, 2 in the other bay. It used to be that there was only 1 vent on the right and 1 in the right hand bay, you could tell the dead spots and the stale atmosphere but that’s now pretty much gone.

Thing is, the house was fine for probably the first 50 years then some bright spark extended out the back and closed off the rear door to the coal cellar and blocked whatever air bricks there may have been along the rear of the house. Then at some point maybe 30 years ago, there was an outbreak of rot (dry or not is unknown) and a fair few of the joists have been cut and buddied up to new joists. Then additional supports have also been placed below the two largest living rooms. Drill holes are present in the brickwork indicating a treatment course was injected everywhere thoroughly. All woodwork is currently in good condition.

So there’s this constant worry that whatever caused the outbreak then may happen again. And of course the urge to improve the humidity down there to help prevent it ever happening again.

I like the vapour barrier idea as I can see why it would help, and it’s not impossible to reverse if I had to.

[Jemster]

Sorry, posted too soon. I meant to add that my concern with the vapour barrier is whether it could cause more moisture to be retained in the walls - I’m not over confident about the DPC effectiveness and the last thing I want to do is bring more dampness into the block work, even if it may be helping the timbers out.

I plan on keeping an eye on it for any pooling but as far as I know, the only moisture is from the ground, the lower walls below ground level and damp air, not leaks. I just wish I could find a local damp ‘expert’ I could trust to evaluate the job...

[wes56]

More pros than cons - but fwiw, If needed i'd put one in my house. V/B's were recom by D&T surveys for many years.