HELP: Mould in Loft insulation

15 Jun 2009
Reaction score
United Kingdom

I'm new here, but need a lot of advise and help. Here's the story:

I've just laid a lot of flooring in the loft, over the top of the fibre glass insulation. The fibre glass insulation meets the joists.

After completing about two thirds of the flooring I moved over to the last section of loft. This is where I found the problem: A couple of old doors where laying across the joists and fibre glass and as I lifted one of the doors there was mold.

The loft, has ventilation as I can feel it blowing across. It is very hot in the loft and humid.

Will the flooring cause more mould to form underneath the floor boards? If yes, is there anything I can do to prevent the mould from coming?

Many thanks for your advise in advance


Sponsored Links
if the loft is actually humid, then moisture is getting into it

the most common sources are:
Draping wet washing about the house
Using bath and shower with no extractor fan to the outside
Holes in ceilings (including those for downlighters, trap-doors and pipes) allowing warm moist air from the house to get upo into the loft
Water leak (pipes, tanks and roof)
Overheated water tanks in loft getting hot and steamy

Have you observed any of these?

Is the loft felted? Describe too us where the ventilation is getting in and out (when the hatch is closed).

there also needs to be a ventilation gap between the insulation and the flooring, with an air current
Hi, thanks for the quick response. :D :D

We do tend to drap wet washing about the house and where the doors were laying in the loft is directly above where we tend to hang the washing...

We do tend to shower without the extractor fan on too :cry: I'll have to replace that.

The loft is felted and after I closed the hatch I did find it very difficult feeling any ventilation. I then looked around the loft and on the roof for any visible ventilation and couldn't see anything.....

You state that there needs to be a ventilation gap between the insulation and the flooring: Does this mean I need to take up the flooring and replace the insulation with something alot thinner to create the gap required?

Many thanks again for your help
you could also lift the flooring, and lay counterbattens across the existing roof timbers, at right angles. this will have the advantage of letting the air flow fairly freely under the flooring, and also prevents the flooring pressing on cables on top of the old timbers. you should screw them down, and lay them so there is one under each end of each board, and one in the middle.

you need to stop draping wet washing about the house. It is a dreadful source of damp. save up for a washing line. alsp buy an extractor fan and have it connected so that it runs whenever ther light is on.

have a look at the eaves and see if there is a gap, it might have been blocked by insulation, or you may have soffit boards to close the gap.

if the roof has gable ends, it is possible to make a hole or two at each end, through the brickwork. You can hire a core drill to do this neatly.

Sponsored Links
Brilliant information :) I'll get the counterbattens sorted for the flooring and the washing is no longer being hanged inside the house as of tonight, I've also got an extractor fan on order :D

I have no soffit boards on the eaves, so it looks like its the core drill solution.

Many thanks for your advise with this issue and for the link

If you can think of anything else that may help I'd be greatful to hear what you have to say

Kind Regards


DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local