Mould on ceiling eaves

27 Jun 2011
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United Kingdom
Hi I am having a problem with mould on my ceiling eaves (as in the picture). This is affecting my first floor bathroom at the rear of the property and my first floor rear bedroom.

The property details are as follows

Built in 1930s
Solid brick construction
Pitched roof
The property does not had eaves vents
100mm of loft insulation
Ali double glazed windows
One radiator in room
Extractor fan with 10 mins overrun

The mould appears to be affecting the two rear rooms which are north east facing. In the bedroom there are currently 3 isolated mould patches and in the bathroom 6 patches (across the width of the bathroom)

Please advise on experiences or tips you may have.

Thanks in advance
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Is the mould only on the sloping areas? If so my suggestion would be to insulate the sloping sections. As I'm guessing your mould is caused by all the water condensing on the un insulated colder sections of the roof as the main ceiling is warmer due to your roof insulation. It may just be this side due to the way the building faces, is this on the windy/north elevation.

I would also look at possibly getting a humidity controlled extract fan and see what causes there are in the bedroom for extra water being there and try to regularly ventilate the room to remove some moisture.
This is a typical and common problem and is down to the fact that this was an often overlooked part of the build regards insulation.

What people fail to realise about raked ceilings is that there is usually only a layer of plasterboard, a 3" timber, the roof material, then the outside!

To combat the cold bridge you will need some hefty thickness celotex/kingspan etc, nailed to the internal face of the rafters. 100mm will prolly suffice along with some pretty long screws to fix the plasterboard afterwards.
Good call noseall,
i was wondering about that post - when i've encountered similar in the past, i guess that i noted it and did nothing beyond the usual, more heat and venting advice.

AAMOI: i typically try and dissuade customers from having full height tiling in bathrooms. And to install 150mm ext's.- humidistat or run-on.

How, and if, the cavity is sealed can sometimes have a cold bridging effect.
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FWIW: A look at the drawing of an "Eaves with soffitt unventilated voids" on roofers guide, might show possibilities, depending on what construction detail the OP has?

It would, of course, involve some opening up from above or below, i would imagine.
Is it a bad idea to insulate the eaves?
No its not.

It's just a bad idea to prevent a flow of air on the cols side of the insulation, so an air gap needs to be maintained.

What this means is if you decide to insulate between the rafters don't fill right up to the underside of the felt/tiles, leave 25-50mm air gap.
noseall is bang on. Condensation due to lack of insulation in the void between ceiling and undertile membrane. The patchy mould is known as pattern staining. It is sometimes possible to slip rigid insulation into the void from the loft space but you must not cut off the ventilation completely. Normally you should leave a minimum 50mm gap above the insulation but for such a short run a 25mm gap would be fine. Alternatively you could fix insulation to the ceiling and overboard.
Is an air gap really that important considering there is no eaves vents. Roof is rather old and does not have membrane
No. If there's no membrane don't worry about the gap.

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