Whats causing this mould/how to stop it happening?

[getafix the druid]

Pulled up the old floor boards and the wall plate and joists ends by the fireplace were a bit rotten. Basically the hearth was about 4 inches concrete sat on top of compacted sandy earth stuff. The old joists and plate were pushed up against the earth.

I replaced the timber and coated the new stuff with blackjack dpm stuff. Dug the earth back a bit so it wasn't in contact with the new timber. Cut some ply and laid it down then got on with life and other tasks for a few months.

Lifted the ply up today to insulated between the joists and then finally screw them down. Noticed there was furry mould both on the underside of the ply and the concrete layer of the hearth. Hopefully you can make it out in the pics

Doubt it's a ventilation issue as there wasn't ply down im the whole room so it's hasn't been sealed in or anything down there.

What do you think I should do about it? Don't really want to fix anything down just for it to end up going rotten in the not so distant future

Thanks in advance

 

[Kearn]

The bases of chimney stacks are notorious for pulling up ground moisture and are areas of extra caution. It’s a bit of a direct bridge from sub soil upwards.

Floorboards, unlike ply, would have had some between board airflow, not much but maybe it’s the difference.

 

[getafix the druid]

Thanks for the reply. Don't think its an airflow issue, there was a couple of plywood boards i hadn't laid down so it wasn't a sealed floor in the couple of months it took for the mould to grow.

At the moment im thinking of 2 options

1. Try to wrap some dpm around the hearth so its between the concret/earth and any wood. Feel this is a bit of a bodge

2. Bit more drastic. Dig out the hearth, put some dpm down, refil with concrete. Would rather not do this!

Any other solutions welcome!

 

[Kearn]

Moisture, warmth, no airflow, something to eat, darkness… all the things mould needs to thrive. And it can be very localised, so just closing that area with ply might have been enough.

I wouldn’t be doing either option, both will just push the moisture elsewhere. It won’t go away, so needs to be allowed to evaporate…

Will the fireplace be used? Some vents in the floor in front of the hearth would help both with draw but also forcing air movement in the subfloor

 

[IHateDamp]

How and where your airbricks?

 

[getafix the druid]

There's a few airbricks dotted around so should hopefully be a bit of a cross breeze.

Tbc if the fireplace is going to be used yet. I'd like to get a log burner in there but other half just wants to block it up sadly and she usually wins these sort of things. I know it hasn't been used for at least a year and a half as we haven't lit lt and thats as long as weve had the place.

The thing thats confusing me is that the mould seems to be growing on the concrete of the hearth and then spread onto the boards? That's the only place I can see it anyway. Just seems to have sprang up there in the few months since those boards have been down

 

[Kearn]

There's a few airbricks dotted around so should hopefully be a bit of a cross breeze.

Tbc if the fireplace is going to be used yet. I'd like to get a log burner in there but other half just wants to block it up sadly and she usually wins these sort of things. I know it hasn't been used for at least a year and a half as we haven't lit lt and thats as long as weve had the place.

The thing thats confusing me is that the mould seems to be growing on the concrete of the hearth and then spread onto the boards? That's the only place I can see it anyway. Just seems to have sprang up there in the few months since those boards have been down

Because the hearth is wet and will be as it’s in contact with the ground. Then the are has been covered with ply. Great conditions for mould.