Damp loft

I went up into the loft yesterday to search out the Christmas Decs, and discovered that the roofing felt on the underside of the roofing tiles has condensation on it - to the extent that there is a line of drips along the loft floor boards where the condensation has dripped.
The loft is well insulated with about 8" of rockwool, except where the loft boards are, where there is about 4" underneath the boards. The soffits were replaced last year with ventilated soffits and I can see light at the edges of the roof when in the loft with the lights off, and the air bricks in the gable walls are clear - free from blockage/obstruction.
What do I need to do to stop the condensation? The only source of warm air I can think of is from the loft hatch which has draught excluder around it as it can't be insulated due to the loft ladder.
Do I need to improve the air flow? If so, how? or are there other things that I need to do?
Ta
Steve
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Steve Bilton wrote:

Live with it I reckon. We currently have a small puddle under the velux window but it only happens in very cold weather
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More ventilation required.
I have been to a number of properties,one just occupied for three months where the wardrobes are mouldy/ back of leather suite mouldy. Client put a dehumidifier in and got 6 pints of water from the bedroom in one day. To solve problem open the windows and put the heating on ! The country has gone made with insulation insulation insulation and this is the result ! Not to mention plastic windows and no open fireplaces.
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Steve Bilton wrote:

Steve,
It's interstitial condensation and more ventilation is required (possibly a couple of tile vents on either side of the roof [in the middle] to create a cross-flow of ventilation).
Cash
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Cash wrote:

So a cross flow of freezing air at 90% relative humidity will dry it out?
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stuart noble wrote:

yes, if the temperature inside is enough to lift that temp even sightly.
But in this case vapour barrier under the insulation is probably at least as advisable.
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I don't have any felt or underlay under the tiles on my roof and it blows a gale up there. I never see any condensation at all, even in the coldest weather.
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Peter Taylor wrote:

condenation is the result of warm vapour ingress exceeding the ability of ventilation to remove it.
You can solve it by increasing ventilation or reducing vapour ingress.
or both.
Generally the less ventilation you have, the better the roof will stay warm in icy blast conditions.
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stuart noble wrote:

Yep - and in fact specified in many contracts where heavy loft insulation has been done and where interstitial condensation is present and a breathable (Tyvek) sarking felt has not been used under the roof covering.
As a matter of interest - leaving loft insulation short of the fascia/soffit space causes a barely sufficient cross flow of air to prevent condensation (especially of there is a 'heat leak' into the attic) - even when soffit vents are fitted on occasions.
Cash
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I'll take your word for it.
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1) Ventilate, ventilate, ventilate the attic. One code calls for 0.3% of the area (that's 3 sq feet venting per 1000 sq. feet area.) and must allow 'cross-ventilation'. 2) The moisture is coming from somewhere. For example warmed heated house air infiltrating into attic/loft! Seal up as much as possible. 3) Make sure bathroom and kitchen fans are used and/or window open slightly when cooking. Ventilate house from time to time (open unused chimney etc.). 4) Proper ventilation healthier for house and inhabitants. 5) At an extreme install an air-heat exchanger and run it either continuously or frequently.
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stuart noble wrote:

he's right. I have more hole in my soffiits and ridge than there are in Gordon Browns economic policy. Totally wrecked the insulation effectiveness till I boarded over it.
But that's what they said was the regulations.
Dry as a bone too!
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The Natural Philosopher wrote:

My point is that freezing air with 100% relative humidity, such as we have now, can't, by definition, dry anything. I wouldn't go looking for a solution to a problem that may exist only on odd days in the year.
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Well, your first point was actually referring to 90% humidity which will dry slowly. You have now switched to the safer 100% which, of course, won't dry anything.
Ventilation is always the key as it will stay dryer in the lead up to days like we have now and will dry quicker afterwards so the incidence and persistence of such conditions in the loft will be much improved. If there are freezing, wet conditions with no wind for an extended period AND you introduce warm damp air into the loft, you will have a problem whatever, granted, but, over a year, a well ventilated loft will have fewer problems.
--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)
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It suddently occured to me that there is a toilet vent going into the roof space from the upstairs toilet which comes on automatically with the light, and children being children it tends to get left on. If I turn this off and cap the vent pipe while it is turned off this will reduce the amount of warm air getting into the loft, as when it is off the fan is a clear straight route into the roof...
A job for tonight.
Thanks all for the help - and as always the additional "discussion"
Cheers
Steve
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Steve Bilton wrote:

I think you've found the source of your damp! Are you going to admit who bodged it? ;-)
Tim
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wrote:

All I'll say is - it was like that when we bought the house... it helps keep the unpleasant smells at bay... but for no longer!
Steve
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Bob Mannix wrote:

Can't argue with that, but most lofts have "sufficient" ventilation by design i.e. a flow of air under the roof tiles
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stuart noble wrote:

If there is enough heat to carry in condesnable moisture, that air wont be freezing when its in the loft. And neither therefore will it be at 100% humidity.
And in any case, that air will be whisked out of the roof as a stream of steam, before it can settle on the beams.
And even if it does, the first dryer day, it will evaporate again.
If you seal the roof, that doesn't happen. Water vapour builds up
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warm moist air infiltrating will cause condensation, or lack of venting.
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