Green Building Forum - Ideas on prevention of mould on cement screeded walls

Ideas on prevention of mould on cement screeded walls

Joatex — Mon, 30 Apr 2007 21:06:52 +0100

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

Ideas on prevention of mould on cement screeded walls

funcrusher — Mon, 30 Apr 2007 22:11:00 +0100

Try a dehumidifier.

Ideas on prevention of mould on cement screeded walls

Jeff Norton (NZ) — Mon, 30 Apr 2007 22:19:35 +0100

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

Ideas on prevention of mould on cement screeded walls

Guest — Tue, 01 May 2007 06:05:57 +0100

I think as you have guessed itss the insertion of non-breathing structure and materials into a breathing one that is causing the problem. Your existing barn is wet but that dampness can breathe - go out of the structure, your bathroom is also wet but that dampness can't escape. Its not sso much the function of the bathroom as it is so rarely used.

I would do this by trial and error, first start with removing the cement, non breathing render replace with lime. My guess, definaately not an expert on lime, is that this would work (concrete blocks are not called breee blocks for nothing :-))

Then if problem persists ventilation, it doesn't ahve to be forced as you have guessed, the ideal would be a passive stack if you have the height and caan go through the roof...

Ideas on prevention of mould on cement screeded walls

Kieran — Tue, 01 May 2007 06:07:45 +0100

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

Ideas on prevention of mould on cement screeded walls

Joatex — Tue, 01 May 2007 07:46:14 +0100

Thanks Jeff (Auckland,Remuera?) and Kieran. No, the mould is a constant presence so it must be insufficient ventilation and no way to exhaust accumulating moisture. I'll try venting the blocks (air bricks) and replacing cement screed with lime mortar.

Ideas on prevention of mould on cement screeded walls

Jeff Norton (NZ) — Tue, 01 May 2007 23:12:27 +0100

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?

Ideas on prevention of mould on cement screeded walls

chuckey — Tue, 01 May 2007 23:35:37 +0100

Try using the special bathrom paint that contains a fungicide.
Frank

Ideas on prevention of mould on cement screeded walls

Kieran — Wed, 02 May 2007 07:14:58 +0100

Posted By: Jeff Norton (NZ)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?

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.

Ideas on prevention of mould on cement screeded walls

funcrusher — Thu, 03 May 2007 19:08:49 +0100

Check the Relative humidity with a meter. If over 70% routinely, then that is the root of your problem. An instant and cheap solution is a dehumidifier. Second-hand about Â£40; new about Â£120. Tearing the place apart to achieve better insulation/ventilation is probably not worthwhile.

Ideas on prevention of mould on cement screeded walls

Guest — Thu, 10 May 2007 11:18:49 +0100

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

Ideas on prevention of mould on cement screeded walls

Guest — Thu, 10 May 2007 21:03:08 +0100

Anyone heard of condensation ?