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Very bad condensation in loft

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from United Kingdom

Joined: 06 Jan 2010
Posts: 1
Location: Glasgow,
United Kingdom

PostPosted: Wed Jan 06, 2010 2:52 pm Reply with quote

I bought a new build house just over 2 years ago. we went up into the loft yesterday the first time in ages and was horrified to find that there is loads of condensation to the point that the floor is soaking and the timber frame of the roof is soaking wet. We also have some mould growing. I contacted NHBC who just said this is normal and if we were really concerned to get a roofer out to check for ventilation. NHBC said that this type of problem is not covered by them.

Do you think that lack of ventilation could be causing this much condensation?? The loft hatch seems to close securely so I dont think there is hot air getting up into the loft. Also if it turns out that there are few or no vents does anyone have any idea roughly how much it would cost to fit them. I know some people have done it themselves however I really wouldn't know where to start. Any help/advice would be greatly appreciated. Thanks
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from United Kingdom

Joined: 10 Dec 2009
Posts: 20
Location: Edinburgh,
United Kingdom

PostPosted: Wed Jan 06, 2010 2:56 pm Reply with quote

Definitely sounds like your loft needs ventilated.
Is the insulation laid on the ceiling of the rooms below - ie a 'cold' roof or a 'warm' roof where the insulation follows the roof pitch?
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Nige F

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PostPosted: Wed Jan 06, 2010 3:39 pm Reply with quote

According to Building Regs Approved document F2 ( which I googled icon_redface.gif ) there should be either 10 mm or 25 mm continuous ventilation gap @ eaves . Maybe the 200+mm of loft insulation has not been tucked in properly and blocked the vent tray icon_idea.gif I`m NOT a builder, just a Plumber who`s been in a few lofts icon_wink.gif
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from United Kingdom

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Location: Hampshire,
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PostPosted: Wed Jan 06, 2010 3:43 pm Reply with quote

This is very common (see "Similar Topics" below)

Read and see if you are doing any of the things that cause it

Then increase ventilation; of the house generally and especially the bathroom; and in the loft, perhaps by pulling back insulation in the eaves to improve airflow.

If you have gable walls you can hire a core drill for half a day and make neat 105mm holes at each side that you can line with plastic duct and fit cowls or grilles on the outside.

If you have wooden or plastic soffits you can drill holes in them and fit insect/bird guard mesh
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from United Kingdom

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Location: Surrey,
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PostPosted: Wed Jan 06, 2010 11:30 pm Reply with quote

Your problem is related to either not having a vapour barrier or having too much insulation over the ceiling in the loft space.

Insulation is good at what it doe's but has relatively little vapour resistance and severely insulating between/over the floor joists to the loft will make the loft space a hell of a lot colder in winter months, and the where there is no vapour barrier the water vapour will still travel into the loft space and because the loft is a lot colder (as the heat is being kept in the habitable areas the condensation will be a lot worse.

The easy solution would be to temporarily remove the insulation material and introduce a vapour control layer over the ceiling and across the tops of the joists and then reinstate the insulation material.

However, if you still have a problem after this then you will need to reduce the thickness of the insulation material to the loft floor to around say 150mm thickness and add some insulation either over or between the rafters, although you will need to maintain an air gap of around 50mm between the insulation and the roof covering, also you should use a PIR material for this part as the condensation level on the underside of the roof structure will from time to time be pretty high (or as bad as you now have) but the ventilation gap will allow the moisture to dry out before it becomes a real problem.
Also PIR has a high Vapour Resistance and will not be unduly affected by excess moisture in the air gap. As an added precaution (if not already there) you should consider adding eaves and ridge ventilators to vent the air gap.

Hope this helps.
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