06/03/2026
I've had a few inquiries about this type of problem and I thought of it this morning being a little cooler.
As can be seen on the left, the photo shows almost a wet stain .... It's not wet - it's cold !! .
This is a poor insulation problem or thermal bridging problem that results in this problem.
( We should have 300mm of continuous insulation above the ceiling and all cold bridge points should get proper attention to minimize cold/heat transfer) *Current Part L Technical Guidance Documents available on line will more info on this.
NZEb (Nearly Zero Energy buildings) is also a point of info in which I have had extensive training in NZEb principles and applications - this is all taken a bit further in an effort to minimize energy consumption within buildings by. Improving insulation , ventilation and reducing/ elimination of air leakage (heat loss) - all of this once fixed and corrected will lead to a more comfortable home and potentially lower bills* ( *usage reduction we have no control over provider price increases)
This room is an en-suite bathroom (notorious for this problem) where hot showers create warm humid air and then this air comes in contact with a cold surface and shrinks leaving the moisture behind on the surface (dew point) , now it allows the formation of spores as can be seen in the left photo. You may wonder how this is happening in an almost perfect "T" shape ? ---- Well this is exactly what is happening as the ceiling joist in the cold attic above sits directly centered to the staining travelling to the wall.
Solution is to Insulate and ventilate ( a trickle vent on a window will not suffice for this environment) . If you don't have an openable window you should have a mechanical vent with delayed shut off (15mins) when you leave the room .
Ventilation is important but can be a tricky one and all scenarios are covered in the current *Technical Guidance Documents part F these are available online at gov.ie publications as far as I am aware. These are the Bibles that building regulations are created from
I hope this helps somebody who may need a solution.
