Anyone else had issues with damp creeping in under the floor panels over winter?

Had this exact nightmare last winter with my workshop build. Found a lovely carpet of green mould under my OSB subfloor in February — absolute gut punch after all the work I'd put in.

The culprit for me was condensation rather than actual water ingress. Ground moisture was wicking up through the membrane I'd used (cheap stuff from a builders' merchant, not a proper DPM). The thermal bridging where my joists sat on the base made it worse — cold spots collecting moisture like magnets.

What sorted it for me:

• Ripped everything up and laid a proper 1200-gauge DPM, lapping it up the walls properly
• Added 50mm Celotex between the joists — massive difference to floor temp
• Left a small ventilated gap at the base of the walls rather than sealing everything airtight

The irony is I'd been obsessive about the roof and walls but completely underestimated what comes up from below, especially sat directly on a concrete slab which mine is.

One thing I've seen mentioned elsewhere is painting the concrete itself with a tanking slurry before laying anything over it. Tempted to try that on my van conversion floor too actually, similar principle.

Anyone running their garden office through a full second winter now — have the fixes held up, or is it a constant battle? Curious whether those on timber post bases have fewer problems than those on slabs.

@ValleyOffGrid gutting when that happens. Worth considering a vapour control layer above the insulation rather than relying solely on what's beneath — moisture migrates upward through the floor stack and that's often where people get caught out.

Also, are you running any heating over winter even on a low background setting? An unheated space with intermittent use is basically a damp trap. I keep a small 150W tubular heater on a frost stat in my narrowboat workshop space — dramatically reduced condensation issues.

For remediation, a good phosphoric acid wash followed by a boron-based treatment like Boracol will deal with the existing mould without the nasty fumes of bleach-based products. Then restack with proper ventilated air gaps between panels if you can.

What's your floor construction — ground contact slab, joists on pads, something else?

@ValleyOffGrid been there with my garden office build — condensation under the floor is a proper pain.

One thing that helped massively was getting the structure up off the ground properly. I've got 150mm clearance underneath with treated timber bearers on padstone footings, which gives decent airflow rather than trapping moisture against the ground.

Also worth checking your insulation isn't sitting directly on a membrane that's collecting condensation on the cold side. Rigid PIR (I used Kingspan TR26) between the joists with no air gaps around the edges made a big difference — thermal bridging at the joist edges is where you'll get localised cold spots and that's where mould starts.

Have you got any ventilation under the floor void itself? Even a couple of airbricks on opposing sides can completely change the game if you've got a suspended timber floor.

@ValleyOffGrid had this same issue in my shepherd's hut build — the stack effect in small structures pulls surprisingly damp air through any gaps in the floor construction.

What sorted it for me was addressing the underfloor ventilation properly rather than just throwing more vapour barriers at it. Continuous cross-ventilation through the void, keeping the insulation itself off the ground, and a proper DPM beneath the whole structure made a significant difference.

Also worth checking your OSB grade — OSB/3 is moisture resistant, but it's not magic. If it's sitting without adequate airflow it'll still fail.

One thing I'd question: how was your insulation specified? Rigid PIR (Celotex, Kingspan etc.) will tolerate a damp environment far better than mineral wool, which holds moisture and becomes almost useless thermally once wet. Might be worth investigating what's actually under there before rebuilding.

@ValleyOffGrid had nearly the same discovery in my garden office last March — genuinely demoralising after months of work.

One thing nobody's mentioned yet: ground clearance made a massive difference for me. My original build sat too low and airflow underneath was basically zero. Raised the whole thing on taller joists with decent ventilation gaps at each end and the problem dropped off significantly.

Also worth checking whether your DPC is continuous — I had a gap where the membrane lapped incorrectly and moisture was wicking straight up into the timber frame.

Are you on a concrete slab or timber frame piers? That changes what options are realistic. My setup is pier-based which made remedial work much easier than it would've been on a slab.

Really feel this — had a similar scare with my garden office last autumn, though I caught it earlier than @ValleyOffGrid thankfully.

One thing nobody's mentioned yet: are you leaving any gap between the floor insulation and the ground membrane? I made the mistake of packing everything too tight with no airflow whatsoever underneath. Read somewhere that even a 25mm ventilated void can make a massive difference.

Also curious — what insulation are you using between the joists? I've got Celotex GA4000 in mine but I'm now wondering if I should've gone with something more moisture-tolerant like Rockwool instead. Does anyone know if rigid foam is actually worse for trapping moisture once it gets in?