Which treatment will reduce or prevent mould on cement screeded walls

Mould is forming on the inner walls of a bathroom and a room adjacent a stone walled barn located in southern France. The barn walls are stone and approximate 2.5 feet thick but the bathroom and adjacent room have been formed from a larger room and a section of the outer original wall replaced with a 6 inch concrete block wall and windows. The exterior and interior finish over the concrete blocks is a portland cement screed No DPC was added under the new concrete blocks. Before the changes to the room there was no evidence of mould on the walls or on the adjacent floor

The tiled roof is sound as is the guttering. The terrain is largely sand and drainage should not be problem.

No natural or forced ventilation has been added. The barn living accommodation is occupied for a short time only in the summer, otherwise closed but the door to the rooms mentioned remains open. The absence of mould on the exterior surfaces of the concrete blocks suggest that the natural ventilation of an outer surface prevents mould growth

The problem is growth of mould on the inner cement screeded walls. In addition there is mould on that area of the room floor close to the outer wall. The bathroom floor has been remade with a DPC under a concrete floor and is 2 inches higher than the adjacent room, no mould on the floor but a little behind the loo.

The question is that of treatment.

Ventilation by air bricks would seem to be the first step but would only overcome the presumed higher RH of the two rooms Further ventilation by a fan may be necessary but for 50 weeks the barn is empty and energy expended to reduce mould would be out of proportion to the gain.

If the cement screed was removed from the concrete blocks and replaced with a lime mortar plaster and skimmed with lime mortar/silver sand would that allow the damp in the concrete blocks to be dissipated leaving the plastered surfaces dry enough to avoid mould ? Or would it be necessary to remove the concrete blocks and replace with original stone then face both sides with lime mortar screed ?

If it was just a matter of ventilation what type could reach a satisfactory surface moisture level ?

Comments if you please

Is moisture only generated when people are using the room? if so then just need a extract fan running when in use!

sorry about bad spelling can't edit as I wasn't logged on by misstake that's breeze blocks

Hi Joatex (Queenstown, little Auckland!)

I know very little about lime plaster but it has got to help! I would still be interested in where the moisture is coming from to cause the problem to start with?

<blockquote><cite>Posted By: Jeff Norton (NZ)</cite>Hi Joatex (Queenstown, little Auckland!)

I know very little about lime plaster but it has got to help! I would still be interested in where the moisture is coming from to cause the problem to start with?</blockquote>

These old building are permanently wet, moisture rising from the floor (no dpm in the old days) but this wasn't a problem as the structures were breathing (no vapour barrier either). People have problems when they insert non breathing materials into these buildings (even paint can do it)... good luck with your remedial work Joatex, please let us know what works.

I'd say lime render with a non-film making paint (calcium silicate) would assist, as there was no mould before then I'd say that permiable is good, sealed is bad.

Matthew Stanford