I've had some really helpful advice on insulating the crawlspace, thanks.

Stepping back, I realised the relative humidity in there is too high, and that seems to be the first thing to solve. It's 80% today (the ground floor is around 40%, first floor 20-30%).

It's a suspended timber floor, and the base of the crawlspace is old concrete (1930s house) - no damp proofing. Two external walls are cavity walls, one is solid brick and stone. There are two good air vents on the back, a good one on the side, a good enough one and a weak one on the front (the higher ground where the Apr brick is squashed up against the joists and at external ground level, so just a single air brick with reduced flow on the inside).

The air bricks were improved in February, and a few cubic metres of problematic damp soil two weeks ago

I'd appreciate thoughts on 1. the acceptable level of humidity in this context
2. Whether I might need to just wait a few months for the work to have it's full effect (and if it's worth trying to speed it up with a fan or dehumidifier)
3. Once it's settled to its new normal, how do I work out whether to focus on further ventilation or a VCL layer on the ground as my next step?

Thanks

Mould & fungi spores germinate around 70+% relative humidity, e.g. dry rot. I assume pine/spruce/fir joists? which dry rot loves. High humidity over time is problematic; sort out the leak, then monitor the humidity. It would even be worth measuring the actual humidity in your joists, dry rot starts at around 25% from memory.

Posted By: DannySheffielduse an incense stick to see if I can discover anything about airflow (and then probably an anemometer)

Smoke is about the best way to see draughts or other air movement. An anemometer won't show much unless you can feel the breeze on your skin I think.

Things can take a long time to dry out, yes. You could try using a fan heater (or even just a fan) to see if you can get a bit of the surface to dry faster so you can see that's what the problem is for sure.

edit to ask: what does your moisture meter say about the joists?

Posted By: djh

Posted By: DannySheffielduse an incense stick to see if I can discover anything about airflow (and then probably an anemometer)

Smoke is about the best way to see draughts or other air movement. An anemometer won't show much unless you can feel the breeze on your skin I think.

Things can take a long time to dry out, yes. You could try using a fan heater (or even just a fan) to see if you can get a bit of the surface to dry faster so you can see that's what the problem is for sure.

edit to ask: what does your moisture meter say about the joists?

The smoke showed very little movement. I was thinking of getting a thermal anemometer which are more sensitive than the vane type, but maybe that's an unnecessary degree of accuracy as if the smoke isn't showing a clear direction of flow, it's clearly too low anyway?

The more I think about it, the more it feels like a ventilation issue. The number of airbricks is up to building regs, but on the front and side of the house (where the prevailing wind comes from) the two double and one single airbrick are all right at the top of the crawlspace as the ground is high there, and two of them are partially behind joists, not blocked but restricted, and too close to get a telescopic vent in.

I've been thinking about using something like an Airtitan T8 crawlspace ventilator to pull the air through to a lower point on the back wall (using a smart plug connected to weather station data so I only pull extra air through when it's less humid outside than inside).

I'll check the moisture in the joists out today.

I measured the moisture in the floor joists and it was 15-18% across the whole crawlspace. My understanding is that's pushing it, but manageable and I assume will drop once the wider humidity issue is addressed.

I don’t trust damp meters that use electrical resistance as a proxy for damp, so many ways for them to get confused.