Expert: Bruce E. Johnson - 12/21/2011

Question
QUESTION: Hi ~ I rent a flat out which has a single skin bathroom and it is consistently covered in black mould, I have done all the usual things, applied special paints etc but obviously to no avail.
I would like to create a cavity and was wondering if I could build a second skin on the outside rather than disrupt the interior of the bathroom. Thanks in anticipation.

ANSWER: Hi Loraine, I'm not sure what you are calling the "skin"?  If you are referring to the drywall or gypsum board or some other type of covering on your walls?  First of all, if the walls keep turning black then you have a serious mold problem that needs to be remediated before all else.  Simply adding another covering to the wall will only forestall the problem until the mold finds its way back to the surface.  Have you discussed this with your landlord?  Something moisture related is going on behind the scenes and really needs to be corrected.  Bleach works well in killing mold and there are mold prevention products out there that will keep it from coming back..once the mold inducing problem is under control there are several new wallboards out there that are paperless and mold can't grow on them.  Denseglas is one of them.  I hope this info helps, please feel free to write again regarding this or other matters, sincerely, Bruce Johnson

---------- FOLLOW-UP ----------

QUESTION: Hi Bruce, many thanks for answering. I'm a bit ignorant in the building area I'm afraid. The bathroom extension doesn't have a cavity as far as I can make out. The interior walls are plastered. I know I have a serious mould problem and I have tried most available products on the market to kill it but they only seem to work temporarily (also tried bleach) I was hoping, if I am correct in thinking that there is only 1 layer of bricks (and not 2 with a cavity) that if I got a builder to build another layer of bricks on the outside thus creating a cavity it would cure the problem as the interior walls are covered in condensation, and again I'm assuming this is because it's cold outside and warm inside. I haven't heard of Denseglas, but I didn't want to disrupt the whole of the sanitary ware if possible. I am hoping if a new wall outside is built it will solve the problem. I have contacted a couple of local builders to look at it but of course now it will be after the holidays. I am the landlord and thankfully I have a patient tenant who is bearing with me over this problem, mind you I have reduced his rent by 25% to pay for extra heating until it is sorted out. Again Bruce thanks for your time ~ regards Loraine

Answer
Hi again Loraine,  making the exterior wall a "double wythe" with a weep space between may be the answer provided the exterior of the existing wall is cleaned properly and then a heavy tar based  mastic applied to it.  This will seal it from moisture penetration and allow any trapped moisture to weep out.  Hopefully the problems you are experiencing are external moisture penetration and not internal plumbing leaks otherwise the double wythe system won't solve the issue.  Please feel free to write again as needed regarding this or other matters, and have a Merry Christmas!  Sincerely Bruce Johnson ..bejohnsonconsulting.com