Mouldy Rafters / Poor Ventilation in Loft

31 Jan 2009
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United Kingdom
We have a problem with white mould on our rafters, which has been put down to poor ventilation. I am hoping for some advice on how to improve our situation ...

This is a late 19th century terrace house with a London pitch or butterfly roof, which gives us two completely separate, shallow loft spaces divided by a central gully that runs from front to back and sits almost directly on top of the ceiling. These lofts have always been acessible until last Autumn when we had hatches and insulation installed, with boards put down for storage. The builder fitted wire wool type insulation under the boards and then stapled foil insulation rolls to the undersides of the joists - which was clearly a mistake.

After a couple of months we noticed damp patches on the ceiling and on further investigation we could see that water was collecting between the foil and underside of the roof felt, then running down the top side of the foil and dripping down near the gulley. A section of the foil was removed along the top and bottom of the roof slope to allow air circulation in January but the problem persisted and mould began to develop on the joists and underside of the felt. In March we completely removed the foil insulation and the joists were found to have high moisture levels. (It is possible that the loft has historically been quite damp but clearly the foil has magnified the problem.)

The situation improved over the summer but the mould is still present and we have decided to have it treated and removed. In order to restrict its return we need to improve the ventilation in the loft. At the moment, there is only one visible airbrick in one side of the loft. Airbricks that we thought had been installed 10 years ago, and which we can see from the front of the house do not seem to be visible from inside. (I don't think we have a cavity, but maybe it's a double brick thickness and the naughty builder only put the airbtick on the outside, where it can be seen but is doing no good.)

We will try to open up these airbricks from the inside, which hopefully can be done without scaffolding.

The slate roof does have some vent tiles but it seems they may be blocked inside by the felt. Is it advisable to cut the section of felt directly below the vent tiles, or could this lead to rain getting inside ?

Other suggestions have been to install humidistat extractor fans or PIV units in the lofts. Would this be advisable ? And if so, should they be vented through the end walls or through the roof ?

If anyone can offer any advice we would very much appreciate it.


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This is very similar to my situation so I feel your pain!

I have a late victorian end terrace house. When we moved in, it had almost zero loft insulation. We moved to rectify that immediately, installing glass wool insulation to current recommended levels (270mm), ensuring a gap to the eaves for ventilation (50mm). Since doing this, we've started to get white mould growing on the rafters.

One suggestion we have had is for felt lap vents such as those from Manthorpe, however the spacing of our rafters may not allow for this as the spacing varies a lot, sometimes as low as 30cm (lap vents are 37cm I believe). Would the lap vents be an option for you @earlgreaves ?
the spacing of our rafters may not allow for this as the spacing varies a lot
Spacing schmacing - all they do is hold the overlapping layers of felt apart to allow airflow. You could use almost anything to do this. I did buy a pack of them but when I realised, I just used some old ~25mm PVC pipe hacked into lengths to do this.
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Very valid point - and we have been looking into using home made alternatives :) I’ve seen examples of pipe insulation being used, wooden wedges…..

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