Ventilation Advice needed please.

I have an old terraced house, 1950s vintage. It has rubble walls. I only bought it 2 years ago. There was no mould or damp, despite it being unoccupied for over 12 months. There was no insulation at all. I dry lined the interior walls and insulated the attic with 4ins of closed cell high density spray foam.

The 3 bedrooms had small fireplaces which were no longer needed and so I blocked them off and capped the chimneys. Now mould is appearing in these rooms. There are no wall vents and I'm thinking that the open fireplaces must have acted as vents. I think with rubble walls it would be a difficult and messy job to drill holes for 4in wall vents. The bedrooms are small.

Should I put cowls on the chimneys and uncap them and cut vents in the plasterboard where the fireplaces were covered? Or put trickle vents in the windows? or do both? I don't intend to keep this house so I am looking for a reasonably cost effective solution. Installing a mechanical ducted extraction system would be a huge job here. Any advice would be appreciated.

Thanks in advance,

Ballina said:

put cowls on the chimneys and uncap them and cut vents in the plasterboard where the fireplaces were covered

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This I think.

Maybe use a dehumidifier until ventilation sorted, whilst it won’t help with ventilation it should discourage mould growth by reducing humidity

We use one in a house that’s vacant a lot and makes a huge difference

removed - its already been posted.

Ballina said:

I'm thinking that the open fireplaces must have acted as vents

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The technical term for a chimney is a 'passive stack ventilator' so yes you affected the overall house ventilation by closing these. There is also likely a buildup of moisture in the stacks also because of the combustion deposits left in the flue from decades of burning and a likely lack of flue liner.
If looking for a short term fix then re-opening these will definitely help once the occupants don't block them up again of course (I'm presuming you don't live there yourself).
I would be recommending a different approach if you were looking for a longer term solution.

Thank you all for your replies. Much appreciated. One last thing. The attic floor is plywood. There is 4-5 inches of foam over this. The inside of the roof is bare slates, no membrane of any description. I can see light and feel a small breeze coming from under the eves front and back. Just an idea, would it work to put ceiling vents in the rooms underneath venting through a 4in pipe out to the eves? It` d be a lot easier than climbing up on an old roof at this time of the year.

Ballina said:

The attic floor is plywood. There is 4-5 inches of foam over this.

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Wha...

Would the mould be appearing on the ceilings by any chance?

No, just on one wall behind the bed headboard. Upstairs bedroom.

Ballina said:

I think with rubble walls it would be a difficult and messy job to drill holes for 4in wall vents.

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I've done that before, use a good core bit and take your time. As soon as the pilot bit protrudes out the other side, switch to drill from that side to minimise break-out. Ideally fit an anti-draft vent.

Ballina said:

No, just on one wall behind the bed headboard. Upstairs bedroom.

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Is that an external wall? Is there any reason that that wall would be colder than all the other walls?
First thing I would do is clean down the affected walls with parazone. If it doesn't come back then problem sorted.
If it comes back, it's almost certainly due to excess moisture on those walls - which is either water coming from somewhere e.g. a leaking sink/tap/bath/gutter/rising damp Or (more likely) condensation forming on a cold wall.

If it is the condensation, then your options are to make the surface warmer (insulation or keep the room warm) or reduce the moisture in the air (dehumidifier, reduce water vapour (by using extractor fans during cooking and showering), ventilation.