Brand new house with condensation in loft

[spencetj]

My family moved into a brand new house in summer 2012. Being new it should be built according to current regulations, the loft has about 300mm insulation laid in 2 layers between the joists and the 2nd layer at 90 degrees. The only thing I have changed is to lay boards over about 1/3 of the loft, on top of battens to avoid crushing the insulation, which is used for storage. Last winter I noticed the loft felt damp and there is a fine layer of grey mould growing on the fabric suitcases. I queried it with the builder but he said it was all as it should be. I think he is wrong!

I think it is an unventilated loft,I can't feel any air movement and I can't see any gaps at eaves level, the insulation seems to go right up to the wall. Initially I thought this was the problem but further reading indicates this is probably normal for a new house. I can't see if there are any vents in the roof apex or elsewhere. I understand for an unventilated loft to work it needs to be well sealed. I have no idea if there is a membrane under the insulation but I have never seen one. Also all the lights upstairs (about 35 of them I guess) are low energy spotlights set into the ceiling so that is lots of holes which may or may not be sealed. The first layer of loft insulation has been cut around the protruding light fittings but they are covered by the top layer of insulation. I presume (hope!) there is no problem with heat since they are low energy!

What can I do about the condensation? I don't want to have to pay someone to fix a problem that should not be there but the original builder is not very helpful and I think there is no chance of him doing anything about it or acknowledging that there is a problem.

Taking note of the sticky about loft condensation, all of the bathroom and kitchen have extractors which we do use. Washing goes in the tumble dryer when possible but some stuff cannot. However we did the same in our old 1950s house with fewer extractors and a well insulated but ventilated loft and that had no condensation.

 

[freddiemercurystwin]

The membrane beneath the tiles should be 'breathable' eradicating this problem. so begs the question what membrane do you have under the tiles? Is it non breathable felt or is there a trade name you can see?

 

[spencetj]

I don't know but I will have a look and see if there is a name on it, from memory there is something written on it. But even if it's breathable, don't you need to stop moisture entering the loft space by having it properly sealed?

 

[AronSearle]

If it's a cold roof, you ventillate, this is basic stuff.

When you say you can't see any gaps at eaves level, what do you mean?

Do you mean you can't see a gap at the soffit when looking at it from outside, or that the insulation butts up to the eaves leaving no gap, if it's the latter, re-jig the insulation to create an air gap.

And effing ell, 35 downlighters!

Pull some insulation aside and have a look at them, see if they are sealed or have some kind of loft cap on them, they are likely the cheap nasty ones that let loads of air through, if so you basically have 35 holes in your ceiling, I'm sure you can guess that makes the thermal value of your roof somewhat ****.

 

[AronSearle]

Actually, thinking about it more, the downlighters may be part of the problem.

You should have a VCL layer in the plasterboard, that will be compramised by 35 downlighters, It's a safe bet they are the cheap unsealed type.

 

[spencetj]

I had a look at the roofing "felt", I can't see a name on it. The side facing the loft is a cream colour, the other side is black and does have something written on it in huge letters but I can't see enough of it to read it. It feels like really thick slightly fibrous paper, I expect it is the breathable type not the waterproof type.

With regard to ventilation, what I saw here http://www.greenspec.co.uk/loft-insulation.php indicates that with a breathable membrane there is not supposed to be an air gap at the eaves. Maybe I need the high level vent instead? But if that is the case surely the builder should have fitted one in the first place instead of landing me with a bill to repair a problem that shouldn't exist?

 

[OwainDIYer]

as well as the downlighters, it would be worth checking that all the extractor fans vent to outside and that the builder hasn't vented them into the loft.

Such dastardly shortcuts are not unknown amongst the more speculative of the speculatively building classes of builder.

 

[freddiemercurystwin]

The whole point of having a breathable membrane is that no other form of ventilation should be required. That said if excessive moisture is allowed to enter the loft then such a membrane will have its limits as to how much vapour can pass through. It is not unheard of that breathable membrane's have manufacturing faults reducing their breathability.

Or maybe its not breathable and the builder has just cocked up.