Major condensation in loft space
Discussion
Sam_68 said:
Busamav said:
I suspect it is due to a lack of ridge ventilation , particularly if you have a steepish roof pitch .
What leads you to this conclusion?Make sure there is access for air to enter the roofspace at the eaves.
Sam_68 said:
Busamav said:
I suspect it is due to a lack of ridge ventilation , particularly if you have a steepish roof pitch .
What leads you to this conclusion?Make sure there is access for air to enter the roofspace at the eaves.
andy43 said:
Pleeeease tell me I wasn't wasting my time and my timber framed dormer walls aren't now dripping wet.
OK. Don't say I didn't warn you... long post, so if you don't find it interesting, don't complain, just don't read it! 
Ironically, the limited air space within a cold flat roof construction can prevent it being a serious issue (where conventional wisdom would lead you to think that the limited space for air circulation would cause a problem).
What I'm pretty sure is happening on modern houses (though I am still researching and compiling data) is quite simple:
- In the old days, when we used 2-3" of loft insulation, the air space within the loft was actually kept fairly warm by the heat being lost from the house below.
- The 'traditional' problem was water vapour from the warm, moist (people breathing, water vapour from cooking, bathing, etc.) air inside the house 'leaking' into the loft where it would condense out in (or on) the loft insulation. But since the loft air space was kept fairly warm and the insulation quilt was fairly thin, it didn't take too much ventilation across it to dry the moisture out again; so it was fairly easy to control the problem using adequate cross-ventilation from eaves and ridge vents.
- In modern house, by and large, it's not the moisture from the warm, damp air within the house that's causing the problem; extract ventilation gets rid of most of the moisture from cooking and bathing and the construction of the house itself is usually pretty air-tight, so very little of this air finds its way into the loft space (how do I know? I have pressure tested houses that have exhibited problems with damp in the loft and they've been as tight as a drum).
- The loft insulation is much thicker, though, which means that less heat leaks out into the loft space above it, hence the loft space is much colder.
- Because the loft space is much colder, each night, the air within the loft space itself cools to dew point and a dew settles out on the insulation quilt, just like a dew forming on your lawn (how do I know? I've been up into effected lofts on freezing nights and you can feel - and test with a moisture meter - the moisture on the top surface of the quilt).
- This 'dew' soaks into the insulation quilt which, because it is much thicker (anything from 11" to 18" is now typical, instead of the 2-3" we had in the old days), will never dry out, no matter how much air you allow to circulate over it from eaves/ridge ventilation.
- Remember that the air will only ever be circulating at a fraction of the external wind speed (you don't want a howling gale blowing through your loft, after all), is circulating over the top of the insulation quilt (down at ceiling level, buried deep at the bottom of the insulation, there no air flow at all) and while fibreglass insulation is pretty good at letting the 'dew' trickle down the strands under the influence of gravity, each individual strand is pretty impermeable to moisture so doesn't 'wick' it back up again to allow it to evaporate in the air that's circulating over the top of the quilt.
The amount of 'dew' that settles out of the air space in the loft is obviously related to the volume of air contained. Hence with the quite limited volumes of air within a cold flat roof (or any dormer construction, for that matter - since the majority of the volume within the roof structures is taken up by 'warm' living accommodation), you're not likely to suffer problems.
Solutions?
- Increasing the air flow within the loft and ensuring there is no leakage of warm, moist air from within the house (downlighter fittings, loft hatchjes etc.) will help, but may not be enough on its own.
- Reducing the thickness of insulation quilt will help, both by raising the average temperature within the 'cold' loft space (thus reducing the amount of 'dew' that condenses out) and by giving the air circulating over the top of the quilt a better chance to dry the moisture out when the relative humidity of the outside air falls again.
- Reducing the volume of air in the loft will help, thus it would be better if housebuilders uses lower roof pitches (though these are unfashionable at the moment and Planners often don't like them).
Simpo Two said:
In some countries they have loft fans - a small extractor fan that helps circulate the air and get rid of excessive heat. I guess it would also help get rid of excessive humidity by drawing in fresh air...?
The problem is that if the fresh air is high in humidity, then, as Sam68 explains, you're just going to get dew settling in loft in same way as it does on the grass or on your car.Simpo Two said:
I guess it would also help get rid of excessive humidity by drawing in fresh air...?
Not much, because the fresh air it's drawing in is equally humid. Moisture falls out of 'fresh' air as it cools, just the same... that's why you get dew forming on grass and why roofs and pavements are often wet in the morning, even when it hasn't rained.The difference is that the roofs, grass and pavements dry more quickly because they have wind blowing over them and sun heating them up directly. Insulation is in a loft space with (comparatively) low levels of air circulation, no matter how much ventilation you provide, and the insulation is specifically designed to minimise the heat transfer that would otherwise help dry it out.
eta: Thank you Deva Link - cross posted 'cos you answered more concisely than me!
Edited by Sam_68 on Wednesday 16th December 19:30
M005 said:
* Persimmon built the house.
Not one of mine, then. 
In that case... complain loudly to Persimmon and the NHBC; by all means quote my explanation above and make it clear to them that you know that the technical bods from all major housebuilders are aware of and regularly encountering this problem; threaten to go to the national press.
If you do this:
1) You ought to be able to frighten them into doing everything they can to cure the issue in your instance.
2) The more people that complain to the major house builders, the NHBC, the BRE and the Local Authority Building Control Departments, the quicker the industry as a whole will acknowledge the scale of the problem and make the switch to more appropriate construction techniques (at the moment, we're all sticking our heads in the sand because in the current economic climate we don't want to be the first company to switch to costly warm roof systems, because we don't want to put ourselves at a commercial disadvantage compared to our competitors.
You'll realise that I feel quite strongly about this issue and have been doing all I can to solve it on the houses that I build (hence my knowledge that reducing air leakage into the loft and maximising ventilation of the loft space still wont completely cure the problem).
As a result of my efforts (I hope), the houses built by my employer are amongst the best available in this respect (very high levels of airtightness and a belt-and-braces approach to roof ventilation, with both additional eaves/ridge/gable ventilation area and permeable roofing felt), but even these measures haven't made us totally immune...
...as an aside, the level of complaints on this problem goes through the roof (pardon the pun) at this time of year because it's when everyone goes up into their loft to recover their Christmas decorations and finds that their insulation quilt is sopping wet.
Sam_68 please excuse my layman's ignorance, but isn't there a problem with over-draughtproofing and condensation in the living spaces of modern houses, never mind the roofspaces? It seems to me that the warmer we make our houses and the more airtight they are to achieve this, the worse the condensation problem will get. What is the long term effectiveness of heat exchange ventilation that extracts as much heat as practical from the exhaust air, and returning it to the house in incoming fresh air? Have you any plans to offer condensation free houses in future? We are under such pressure to insulate more and more, as you've already indicated, and this trend is leading us into uncharted waters vis-a-vis damp generation and dangerous mould spore inhalation. Dehumidification is an energy hungry remedy that I, for one, would try to avoid if at all possible. Your evident knowledge would be valuable...
Sam_68 said:
Simpo Two said:
I guess it would also help get rid of excessive humidity by drawing in fresh air...?
Not much, because the fresh air it's drawing in is equally humid. Moisture falls out of 'fresh' air as it cools, just the same...My loft is uninsulated (apart from the floor of course) and there is no condensation issue. However it does get bloody cold in winter and bloody hot in summer, which makes it unsuitable for storing certain things.
My parents never had a problem in 20 yrs with condensation in the attic until they had some free survey for cavity wall and loft insulation
As they are pensioners they had it done for bugger all money ,the loft was reinsulated with more of the fibreglass stuff an extra 6 inches I think it was
Now everything gets wet in the attic
As everyone knows condensation is warm moist air against cold surface ,no matter how much insulation is in the roof space warm air still gets in , vents in the roof tiles was the only thing that cured it ,bit daft really as it's now
freezing in their attic
As they are pensioners they had it done for bugger all money ,the loft was reinsulated with more of the fibreglass stuff an extra 6 inches I think it was
Now everything gets wet in the attic
As everyone knows condensation is warm moist air against cold surface ,no matter how much insulation is in the roof space warm air still gets in , vents in the roof tiles was the only thing that cured it ,bit daft really as it's now
freezing in their attic M005 said:
- No CH items up there, just the cold water tank & header tank, all pipes are well insulated.
stick a hand in it, is it warm, if so then this will be the cause and your heating system is backing up the vent, ie over expanding, as stated earlier
ps venting the ridge tiles or high level vent tiles (close to ridge and cheaper to install) would also solve the problem if not the tank
Edited by x 7usc on Wednesday 16th December 21:08
motco said:
Sam_68 please excuse my layman's ignorance, but isn't there a problem with over-draughtproofing and condensation in the living spaces of modern houses, never mind the roofspaces?
Yes, there's a problem if it isn't managed correctly.The Building Regulations have steadily inccreased the insulation requirements and also now include requirements for air pressure testing to prove air tightness (and secondary standards like Code for Sustainable Homes and future revisions to the Building Regs will increase both insulation and air tightness still further).
The increasing insulation/air tightness standards have been coupled to enhanced requirements for management of ventilation, however (eg. extractor fans in sanitary accomodation & kitchens, background ventilation of living rooms etc.) which, in theory, should manage the problems of condensation and breathable air quality.
As air tightness (in particular) is further increased, it will become the norm to use the MVHR (Mechanical Ventilation and Heat Recovery) systems that you mention. They are effective in the long term, provided they are maintained correctly (ie. filters changed on schedule). Look upon them like the air conditioning system on your car: 10 years ago, air con was a luxury, now it is the norm and cars are designed to work most efficiently (aerodynamically) with the windows closed and the climate control managing fresh air and temperature. Houses will become the same... but just as your car's air con needs the filter changing occasionally to stop it getting manky and smelly, and maybe the system re-gassing, so the systems on houses will need periodic routine maintenance.
motco said:
Have you any plans to offer condensation free houses in future?
We (the company I work for) already do. Don't take my comments above to suggest that every new house has a serious problem with loft (or internal) condensation. Our houses don't experience significant problems with internal condensation (so the extract fans and background ventilation appears to be effective for the moment) and as I said above, I take loft condensation very seriously and have been taking measures to monitor occurrences and enhance our specifications to avoid it; but my experience suggests that even with these measures, we're now right at the limit of traditional 'cold roof' design.We are already introducing MVHR and warm roof construction on some of our products, though (particularly where we are building to 'Code for Sustainable Homes' standards that are above current Building Regulations minima).
motco said:
We are under such pressure to insulate more and more, as you've already indicated, and this trend is leading us into uncharted waters vis-a-vis damp generation and dangerous mould spore inhalation.
I agree. I'm nervous about the pace of change that is being forced upon us and I'm sure that there will be mistages along the way, just as we made mistakes with system-built houses in the '50's, early cavity wall construction and early timber frame construction.
And if you're worried about the health issues related to airtightness, just wait until you get grey water recycling and rainwater harvesting...
Unfortunately, the Government has a definte agenda to move (rapidly) toward 'zero carbon', low water consumption, low energy consumption housing, so we don't have a choice in the matter.
Sam_68 said:
Unfortunately, the Government has a definte agenda to move (rapidly) toward 'zero carbon', low water consumption, low energy consumption housing, so we don't have a choice in the matter.
Are you not going to be required to build to something very similar to PassivHaus spec by 2016?Simpo Two said:
Yes, if the loft is colder relative to the fresh air.
No, it's not; even with masses of insulation, the roof space is slightly warmer on a night than the outside air.But the problem is just the same as the one that causes dew to fall on the ground: warm air that has been happily absorbing moisture all day cools down dramatically at night and can no longer hold that moisture, so it dumps it as condensation or 'dew'.
Ventilation helps a little, but its complex (for example, the air in the loft space remains cooler in the day time, so if left undisturbed would absorb less moisture in the first place... you could find that you're ducting in warm, moisture-laden air that then dumps part of its moisture out as it mixes with the cooler air in the loft part of the time, if you're not careful) and the net result is that you shouldn't expect ventilation, whether natural or fan-driven, to be sufficient to avoid a problem with a highly insulated 'cold roof' construction.
Deva Link said:
Are you not going to be required to build to something very similar to PassivHaus spec by 2016?
Possibly. We've designed them already (though we watered the design down a little so that it wasn't true PassivHaus by the time we built it, due to prevailing economic conditions).Though it has to be said that Code for Sustainable Homes is taking quite a different direction/approach to PassivHaus at present.
My thought is that personally you don't need to care what is causing this.
The house is 5 years old so by my reckoning is still covered by the NHBC guarantee. Get the original builder in or contact the NHBC if you can't find them or they have gone bust.
In otherwords get someone else to sort it out for free.
The house is 5 years old so by my reckoning is still covered by the NHBC guarantee. Get the original builder in or contact the NHBC if you can't find them or they have gone bust.
In otherwords get someone else to sort it out for free.
Sam_68 said:
Busamav said:
I suspect it is due to a lack of ridge ventilation , particularly if you have a steepish roof pitch .
What leads you to this conclusion?mrmaggit said:
Sam_68 said:
Busamav said:
I suspect it is due to a lack of ridge ventilation , particularly if you have a steepish roof pitch .
What leads you to this conclusion?Make sure there is access for air to enter the roofspace at the eaves.
Soft Top said:
My thought is that personally you don't need to care what is causing this.
The house is 5 years old so by my reckoning is still covered by the NHBC guarantee. Get the original builder in or contact the NHBC if you can't find them or they have gone bust.
In otherwords get someone else to sort it out for free.
That's not the case I'm afraid, if the OP has added another 6" of insulation into the house at ceiling tie level in the roof then the developer will (IMO) laugh at the claim.The house is 5 years old so by my reckoning is still covered by the NHBC guarantee. Get the original builder in or contact the NHBC if you can't find them or they have gone bust.
In otherwords get someone else to sort it out for free.
Personally (after a bottle of vino I might add) I think the OP has driven the dew point level up into the extra layer of insulation he has added in the roof.
The OP needs a vapour barrier on the warm side of the insulation methinks.
Edit: Sam said that ^^^^ in his longpost above that I didn't have the focus to read lol.
Incidentally Sam, I'm not sure increasing the air circulation in the loft will help much as the condensation may be interstitial....maybe replacing the ceilings with foil lined plasterboard would be cheaper than hacking about the eaves/ridges/tiles anyway?
Edited by jules_s on Wednesday 16th December 21:55
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