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    • CommentAuthorsweevo
    • CommentTimeJan 25th 2009 edited

    We've got really bad condensation in our loft. This space is "supposed" to cold vented but there's lots of condensation on the vapuour barrier that sits over the joists and it's been like this since I can rememer [out of sight.....].

    Our home is a dorma bungalow with two bedrooms and a bathroom in the roofspace which leaves the last 3-4 feet of the roof for the loft space itself. There are soffits that run the length of the roof at the bottom, with fibre insulation running between the joists all the way up to the upstairs ceilings where it then runs over and along the ceiling and then back down the other side. The insulation is supported by netting between the joists and has a vapour barrier over the top which runs up and over the ridge and back down. Of the four roof vents on each slope, two are at the lower part of the roof , the other two around a foot above 1st floor ceiling height.

    Last week we had a builder around who we'd used in the past and he suggest a vented ridge tile system which he said would mean cutting the vapour barrier along the ridge (think He also said removing the cut section too), placing felt over in it's place and then using plastic holders to suspend the ridge tiles to assist the current venting.

    The work was done on tuesday so I thought i'd give it a few days and then check out the loft. I've examined the loft space today as best I can (very cramped in there!) and the space is as full with condensation as it ever was. I had a good look around and I've got a few questions regards venting of roof spaces as things didn't look the way I was expecting them to.

    First up, the original roof vents don't seem to pearce the barrier to allow any kind of direct flow of air into the loft, in fact today I struggled to find any sort of tear/cut in the barrier where I thought these vents would be when I went for a closer look. Should these vents pierce the barrier like i'd expect a typical house vent to, ie unrestriced air flow between the interior and exterior?

    Secondly I had a good look at the ridge and I was expecting to find the membrane cut the full length of the ridge but all I found but for a few feet between two joists I could see no tears/cuts on either side whatsoever. Should I have expected to be able to put my fingers through a cut membrane and touch the new felt? For adequate ventilation I was expecting to see a clear cut the full length of the roof ridge on both sides but I saw nothing for 99% of the roof.

    Obviously i'm going to call the builder tomorrow but I'd be interested to hear a second opinion on this as for what I paid it's made 0% difference.
    • CommentAuthorchuckey
    • CommentTimeJan 25th 2009 edited
    I am having a problem with your "membrane", which you say is underneath the rafters and over the joists. If its a real vapour proof membrane then it should be under the plasterboard as close to the habitable side as possible. Thats why silver foil backed plasterboard is used. If the construction is plasterboard with insulation between the joists with a membrane over the lot, this is incorrect and you will get condensation within the insulation, causing the timber to rot and the plaster board to decay.
    From scratch, within the habitable side, cooking, washing and people breathing release a lot of water vapour into the air, which is warm. As this air migrates through the plasterboard (still with its water vapour content), it then migrates through the insulation and gets colder, as it's colder, some of the water vapour will actually condense into water droplets. On the outside of the insulation it's at its final lowest temperature, which is the outside temperature. So the last thing you want to be doing is to trap this wet vapour and and the water droplets under a plastic membrane. The membrane should have been next to the plasterboard or at least under the insulation, to stop the water vapour at source, while allowing any dampness in the insulation the evaporate away.
    A cold roof is normally ,tiles on battens then a layer of nothing (pre-1960s), or tilers felt (1960-1990) or breathable membrane (post 1990). Then the actual roof rafters. The problem with this is the tilers felt, it is not "breathable" so it stops the flow of air. I had this in my loft which I cured by fitting soffit vents (5 for £5). If you are saying that you have a layer of some plastic stuff underneath the joists, this is a sure way of trapping the moisture and impeding the air flow.
    If your pitched roof is a warm roof, with insulation between the rafters and /or underneath them, its just like a habitable room so to stop condensation you have to heat it and to ventilate it just like you would do in your kitchen.
    So what I would do is to make sure the original loft insulation is good, make sure it goes properly up and over the box which is your loft extension and remove any plastic membrane(s) you can see , except for the stuff over the top of the rafters and under the tiles. If this does not cure the condensation problem install soffit vents (buy a set of rotary hole cutters ~£8 from a big box DIY store to drill the 60mm diam hole) every 5' around the building's perifery.
    • CommentAuthortony
    • CommentTimeJan 25th 2009
    There should not be vapour barrier anywhere on the cold side of the insulation -- you sure you mean vapour barrier?

    There should be one on the warm side of the insulation.

    With a cold vented loft it sounds to like you dont have enough vents and they must ventilate the loft all over Ridge and eves.
    • CommentAuthorsweevo
    • CommentTimeJan 25th 2009 edited
    Hi Tony, could be i'm getting mixed up but the builder who came did say it was acting as a vapour barrier. It's certainly cold side of the insulation.

    Regards the ridge, I can't see anwyhere along it where the builders who came this week have cut this membrane. I was expecting to see cuts in this membrane the full length of the ridge and I can see nothing apart from a 3 foot cut on one side near the loft hatch.

    Also regards the roof vents should there be clear air flow from them into the loft space, as in my ASCII drawing below !!!

    .......Tiles --->//##JOIST##//
    ................//#########// <-Membrane
    VENT->// ----------------------------> AIR
    ..........//#########// <-Membrane

    At the moment this is not the case, the vent is hidden behind the membrane. I would've expected the vent to break this membrane to ensure cross ventilation etc.
    • CommentAuthorMike George
    • CommentTimeJan 25th 2009 edited
    Hi Sweevo. My slant on this is that you need both low level and high level ventilation to effectively ventilate a loft.

    Ridge ventilation systems are great in my opinion, but need sufficient eaves ventilation to cause a stack effect.

    Some questions:

    Is the membrane black or grey plastic in appearance?
    Is the insulation at ceiling level stuffed well into the low level 'triangle' at the eaves?
    • CommentAuthortony
    • CommentTimeJan 25th 2009
    Can you show us photos or sectional plan and give trade names of the membranes?
    • CommentAuthorsweevo
    • CommentTimeJan 25th 2009
    Mike, the membrane is grey plastic in appearance, has a sort of square criss-cross part on it. As for the insulation, it pretty much fills between the joists down the eaves which won't help with the ventilation. The builder last week commented on that and I had my supsicions that was the case.

    Tony, i'll try and get some phot'os up in the coming days as it'll be of more use to you guys. Gonna call the builder tomorrow too regards the ridge ventilation.
    • CommentAuthorCWatters
    • CommentTimeJan 25th 2009
    Perhaps see these products and their fitting instructions..

    I think I know the type it is black one side grey the other. We used to call it Monfoil. It is pretty close to, if not a vapour barrier. I have seen several cases of bad condensation where this has been used.

    There are several solutions, depending on what you have at the eaves.

    1. If you have eaves or over fascia vents, then moving your insulation out of the way will allow airlow up to the ridge [Probably not a popular solution here]
    2. Put in some slate or tile vents, just above the level of the insulation at ceiling level, though don't align them from front to back
    3. Remove the bottom few courses of slates/tiles and use continuous over rafter corrugated vents [This relies on the soffit or fascia also being ventilated]

    Regarding the ridge, the membrane should be cut continuously along the length of the ridge [though the ridge system has to be installed with sufficient lap to avoid leaking] Don't be tempted to cut it yourself from the inside, I'd get the builder to do it if I were you.

    Hope this helps
    Nice pdf Colin. The corrugated vents I referred to are on page 20
    • CommentAuthorsweevo
    • CommentTimeJan 26th 2009 edited
    Thanks for the PDF Colin. Having had a look at that I can say 100% that the original builder did not cut the underlay [what ever it is] to allow clear ventilation into the loft/roof space as shown in the digrams on page 15. I can't say whether this was done further down the roof as I can't really get into the those eaves that easily but i'd bet money they're the same.

    Gonna call the builder regards the ridge shortly but from my [inexperienced] point of view I really think I need to get that membrane cut where the roof tile vents are regard less of what happens of the ridge venting. Anyone think this is a bad idea?

    Mike, thanks for the suggestions also. I just lifted a tile to look underneath and this membrane is black the other side so I think it could be this monfoil stuff you mentioned. Will see if I can find a manufacturer name etc on it next time i'm up in the loft. I was think along the lines of option 1 short term but will first see what the builder says this morning.
    • CommentAuthorcookie
    • CommentTimeJan 26th 2009
    Sweevo, I wouldn't cut any of this yourself not until your absolutely clear. The new bit, you need to get your builder back, he may of simply forgot to cut it or just been lazy. If you cut it and its wrong, you’re going to have to pay him to correct it.

    Has he laid new ridge tile vents all the way along or just a vent at one point?

    Moisture permeates through your walls and condenses on the cold side of structure; this is called interstitial condensation it happens through out your structure (if it didn’t our houses would be very wet without massive amounts of extraction) its normal. (You’re not going to stop condensation forming within normal measures its just where it forms) The issue here is, are the existing ventilation measures adequate to remove the condensation to prevent damage to the building.

    I'm not entirely sure you’re using the correct terminology, which may be leading to some incorrect advice. I really think you need to take some photos or draw a simple sketch showing a cross section of the loft space (just do it in Microsoft paint doesn’t have to be great)

    There are other reasons why there is so much moisture in there that may be the cause, but first we need a sketch!

    • CommentAuthorsweevo
    • CommentTimeJan 26th 2009 edited
    Ok, here's my artists impression ! :bigsmile:

    The two spaces in the lower eaves marked "A" are accessible through doors/hatches in the plasterboard walls. The mineral wool is suspended in there with netting so it stays between the joists. The insulation is packed in there so there doesn't seem to be any air gap to assist the flow of air upwards.

    The insulation then moves up the slope, sitting on the plasterboad and pretty much filling all the spacing between the joists until it comes to the ceiling level, where it goes over the ceiling and then back down the other side.

    The vents are positioned approx like they are in the diagram but the membrane does not look to have been cut/pierced as far as I can see in the loft space. Can't comment on the lower ones as yet without removing the insulation.

    One of the guys from the builders came round this afternoon and said he was going to get someone to remove the venting tiles and make sure the membrane beneath them is cut to ensure air gets into the roof space. He's also going to get him to cut along the membrane on the ridge [from the inside] too so I guess the guys who came first time around didn't do the job right. He said that should rid me of the moisture issue in the loft but he expressed grave concerns regards the insulation blocking the airflow from the soffits all the way up to the loft. He mentioned the risk of damp to the interior walls from water running down the membrane and longer term dry rot to the timber. His advice was to remove the insulation asap and get the whole timber structure dried out. Then maybe think about trying to slide in rigid insulation longer term. He said it should not have got past building regs too.

    I've already booked this friday off work with the intention of removing this stuff so anyone care to comment on the above?
    Sounds like he is giving honest advice to me
    • CommentAuthorsweevo
    • CommentTimeJan 26th 2009 edited
    Me too Mike, think I need to get this lot dried out first and foremost.
    • CommentAuthorcookie
    • CommentTimeJan 27th 2009 edited
    Great sketch :o)

    Whats in area 'B' ? If its just air then that’s ok, if its insulation I'll change my comments to suit. So the following is assuming its just air and the roof consists of outer tile, apex of roof truss, then and this unknown membrane.

    How old is your house? Is it still under NHBC or Zurich? Area A is currently a warm space, which is unusual if it was a new house, but sometimes it happens (usually as a fix to get around building control because loft insulation was installed out of construction sequence i.e. bad planning)

    I would leave the membrane in place and cut it around the vent positions and tape it to the vent if it has an insert. The membrane probably was added later after the house was built I don’t know why, maybe the tiles were flapping around or some small leaks either way if there is no felt under the tiles there is plenty of air flow in area ‘B’

    The loft insulation may also be a later addition to the house and if so, well to be honest any simpleton can install loft insulation but for some reason loft insulation installers seam not to be able to do it (unfortunately they usually paid by the square metre and the more houses they get done in a day the more they make) of course this is not true of them all.

    If you’re going to buy new rigid loft insulation such as kingspan the red line shows the ideal position of the insulation. Personally I’d just buy enough to slide down the sloping roof of your rooms and the verticals. The remaining wool insulation I’d lay flat on the horizontal ceilings (areas ‘C’ & ‘D’). Use up all the material to fill completely between the depth of the joists, if there is more left after that lay it at 90 degrees across the top of it, modern standards is 250mm I think but more the better.

    At the point of the soffit and area ‘C’ ensure the insulation is tight up against the internal side of the internal wall and doesn’t cover the cavity between the internal and external walls (like the builder advises).

    From the top of the insulation there should be a 75 to 100mm air gap to allow air circulation.
    Make sure your loft hatch and access doors are insulated or stick some of the kingspan to them (if its not foiled insulation be careful what you stick it with, it melts with certain glues, silicone usually works).
    The insulation should be fitted as close to the external side of the habitable rooms as possible.

    Check where your bathroom extractor vents? Make sure its not venting into the roof space adding moisture. I’m not sure its necessary to remove all the insulation but I can’t see how wet it is perhaps lift a section up and check the bit against the plasterboard is dry.

    Personally I’d let the builder do his bit, check what’s in area ‘B’ if its just air, then cut the membrane around the vents (pull back the insulation on the lower vents), check the bathroom extractor is venting to outside the building then check it in a week.

    Another scenario is the membrane and the insulation was installed together, the lower vents are meant to be behind the building paper and vent the cavity there. The membrane is meant to drop into the cavity of the internal and external walls and only meant to go to the top of the sloping roof (loft hatch level) this ensures there is still a gap between the insulation and tiles, the membrane then should stop and be cut off here (but hasn’t been) This would allow area ‘A’ to be a warm room, the cavity walls to be vented and the loft area to be vented, but its just a guess.

    p.s. remember its normal to have some condensation in your loft!

    Health and Safety…
    Have a think of the risk and what you can do to reduce / remove them! Here are a few things to think about. (everything’s about risk these days)
    Falling through the ceiling
    Dust and fibre inhalation (you and your family)
    Poor visibility and restricted manoeuvrability
    Your physical fitness
    And any other risks you assess.

    Cookie, What makes you say it is normal to have some condensationm in aloft void? I would disagree, condensation is completely avoidable in new builds, and mostly avoidable in existing [roof voids]. Its about detailing, which in this case appears to be sadly lacking.
    • CommentAuthorsweevo
    • CommentTimeJan 27th 2009 edited
    Thanks for all your comments and questions Cookie.

    Nothing in area B except air. The membrane is this monoflo or whatever the name is. The builder confirmed this I forgot how he pronounced it! :oD

    The house is 14 years old so no NHBC now. :o(

    Got a guy coming tomorrow to look under those vent tiles and he'll cut them where necessary.

    I'll use rigid insualation once i'm happy the structure has dried out. Not sure how long that will be but i'll be applying where you suggested.

    Bathroom extractor is placed well away from the vents etc. I'll make sure it's venting correctly when I bath the kids next.

    With regards the platerboard the builder did comment he was amazed we'd not seen damp on it. Maybe we're lucky in this respect and all that excess insulation has soaked it up !!!!

    Getting under the floor will be a mare so i'll probably skip putting the unused insulation under there. Probably get rid on freecycle. :o)

    I know it's normal to have some condensation but my loft is soaked. I've got mold on the rafters, the membrabe has water droplets the full length just wating to drip or run down it into the eave sections where the hatches are etc. I was shocked when I saw the amount of water up there on sunday, I knew there was a problem but I'd not bothered to take a close look.

    Thanks for taking the time to post that lot mate, much appreciated.
    • CommentAuthorcookie
    • CommentTimeJan 27th 2009
    Mould is not good, just type a search in google for 'dry wet rot' and see what pops up and check to see if it matches.

    Sorry Mike, I'd have to disagree, condensation will alway form on the cold side of an insulated room where moisture can pass through the structure, it happens in your cavity walls, it happens under your floors, its worse against rooms with higher moisture levels. If you want to know more have a search for Interstitial condensation. Cold surface + moist air = condensation again if you want more info look on any weather site about dew points. Granted with modern house design its going to be less, if we didn't get it we wouldn't need air spaces :o)

    Clearly your loft shouldn't be so wet, right now, this is the something you should do before you do the work. Even though your property is outside the NHBC period, if the defect is caused by original workmanship incorrectly installed they have still paid out! There is a couple of cases I could quote but first off get on the phone to NHBC tomorrow and get them out, keep moaning and keep calling, tell them you suspect you have dry rot. Next get on the phone to Rentakill and get them round to give you a quotation (they are the dearest but probably the least cowboy of the lot, its always doom and gloom from them, just be glad its not a pigeon infestation, lol they make it sound like your already in your grave). Then you need to think about ringing your insurance company.

    Don't touch any of the insulation, let your builder do his bit, but before he does get a shed load of photos taken of ridge from inside and if possible get your builder to take some photos while he's up on the roof. Keep all your receipts!!! Make notes of lost time off work etc. If NHBC won't come out or fob you off, write to them explaning you beleive this problem is down to incorrect workmanship, be susinct and to the point list it as bullet points, need any help just holla

    If the mould is dry or wet rot, you could in worst case be looking at part or all of your roof removed (I doubt it but last thing you want is to be paying for that yourself) so NHBC and or your insurance company need notifying.

    Posted By: cookieSorry Mike, I'd have to disagree, condensation will alway form on the cold side of an insulated room where moisture can pass through the structure,

    Okay agree to disagree:) But to clarify, I was referring to roof voids, in particular cold roof scenarios, which require adequate ventilation [in the UK]. This is what sweevo should have, but does not, mainly because of lack of ventilation, but also because of the use of what is effectively a vapour barrier as roofing felt.

    If the roof is ventilated at eaves and ridge, and has no obstructions, there will be very little if any condensation in this case, other than perhaps on very cold still days/nights

    If the felt were removed, and a breather membrane installed with the correct installation of counter battening above, then ventilation would be irrelevant, and condensation would not occur in most instances, unless of course the vapour is coming from outside.

    So mostly it depends on the relative conditions in the space, but there are other preventative measures which can be taken.

    The second line of defence as it were is the vapour barrier [as opposed to vapour check such as foil back plasterboard] The correct positioning of a Vapour barrier [on the warm side of the insulation] will stop the moisture from migrating through the fabric. This is why building regulations require such a barrier directly above the plasterboard ceiling in areas such as bathrooms. The same principle works in room in roof scenarios. I have done many, and never had a problem.

    Then there’s the third line of defence, which is extraction of moisture at source, ie bathrooms and kitchens. All the better in airtight houses where the heat can be recovered as part of the process.

    So to be pedantic, Condensation does not always occur in lofts, and is not 'normal' Get hold of some Condensation Risk software and you will see how easy it is to prevent in a cold roof. Surface Condensation on painted surfaces/ceilings for example is of course another can of worms especially where ‘skeilings are concerned

    So to wrap up [and I expect you hope I do] I have been working on buildings for more than twenty years and in my experience loft condensation is rare, if not very rare. On the other hand perhaps we just build better in South Wales :o)
    • CommentAuthorsweevo
    • CommentTimeJan 27th 2009
    Thinking back I don't think the house is NHBC after all..... I seem to remember speaking to the original architect about some issue , think it might have been drainage, and finding out it wasn't NHBC. Will have to dig out all the paperwork. Will get the builders opinion on the timber tomorrow.
    • CommentAuthorJackyR
    • CommentTimeJan 27th 2009
    Posted By: chuckeyAs this air migrates through the plasterboard (still with its water vapour content), it then migrates through the insulation and gets colder, as it's colder, some of the water vapour will actually condense into water droplets. On the outside of the insulation it's at its final lowest temperature, which is the outside temperature.

    So what happens to these water droplets in a cold, vented loft without a VCL?

    Sorry if this is stupid, but even after v helpful comments on a previous thread, I still don't understand what happens next. Do I end up with insulation soggy at the dew point, and just hope it dries out in the summer?
    • CommentAuthorCWatters
    • CommentTimeJan 27th 2009
    > So what happens to these water droplets in a cold, vented loft without a VCL?

    If the loft is well ventilated the water vapour shouldn't condense out into droplets, it should escape out the vents. If you do get condensation it can occur at any time of year. I've seen the problem in a garage roof get worse in summer. I believe this is because (Contrary to popular belief) the air is more humid in summer than in winter. Hands up those who get chapped lips in winter?
    • CommentAuthorJackyR
    • CommentTimeJan 27th 2009
    Posted By: CWatters
    If the loft is well ventilated the water vapour shouldn't condense out into droplets

    How come?

    Are we assuming air in the loft can always take a bit more water vapour (ie RH<100%), even though temp will plummet overnight?
    • CommentAuthorsweevo
    • CommentTimeFeb 2nd 2009 edited
    Just thought i'd give you an update as to where I am with this. On friday I ripped out all the insulation as discussed previously. The membrane was wet the full length where ever I removed the mineral wool and the surface in contact with it was black with mould a lot of the time. Lucklily for me there was that much insulation in my roof that it seems to have acted as a sponge and the wood seems ok. I'll keep a close eye on this now anyway as I can see the whole structure down to the vented soffits which as it turns out were also pretty much blocked by all the insulation!

    By sunday afternoon 99% of the moisture in the loft and eaves was gone and most of the timber "looked" dry. Only thin now is the roof is no longer insulation and is covered in snow! :oD

    I've started insulating the plasterboard walls with some mineral wool in the eaves (area A) to help but insulating the pitched area of the living space is going to have to wait a while due to time and financial constraints. I'm planning to cut lots of sections of rigid insulation to slide up between the joists in this area while making sure I leave a 100mm air gap for continued air flow. Obviously these sections won't fill the full width but I'm hoping the region of 90% coverage is better than nothing. Will just resting sections of these insulation boards on the sloping plasterboard be enough as I won't be able to bond them to the plasterboard surface itself as i'd need 8 foot long arms! My way of thinking was to keep feeding them into the joists until the length is covered and then just fixing the last piece in place with adhesive/nails etc to stop them sliding back down.

    One other thing which is concerning me after removing all the excess insulation is that the first floor joists are exposed to the cold air now since freeing up the soffit venting. Think it's probably easier to do another pic here than babble on ;o)

    My concern here is that cold air will be cross venting between joists of the entire the first floor. I would imagine that this would draw out lot of heat from both floors of the house so please correct me if i'm wrong on this. I also wondered whether there might be any new condensation problems arise from this too. There is some insulation between these joists but it' doesn't fill the the full void depth etc. The space between the joist ends is now not sealed and is exposed to the cold air racing in through the vented soffits. Would sealing these sections off as best I can with either rigid insulation or mineral wool have any detrimental long term affect? Short term I've folded the insualtion at the ends over to in affect double it's hieght to fill more of the space.
    • CommentAuthortony
    • CommentTimeFeb 3rd 2009
    What ever you you should do an air tight job on the floor void -- we have discussed this lots before --see dormer bungalows.

    Mineral wool or rigid insulation fitted with gaps wont do.

    My advice would be to dry out the insulation you have and reuse it under the floor and up the vertical walls and across the ceilings and also seal up the "cold air flow" path 100%
    • CommentAuthorsweevo
    • CommentTimeFeb 3rd 2009
    Any reason why I couldn't use that foam sealer stuff to seal the ends of the joists air tight?
    • CommentAuthortony
    • CommentTimeFeb 3rd 2009
    • CommentAuthorsweevo
    • CommentTimeFeb 3rd 2009
    Great. Thanks for all the help Tony. I'll get myself off to Wickes to pick some up later today.
    • CommentTimeFeb 3rd 2009
    I second Tony's comment and would have been first but lost my post at 2am and couldnt be bothered to retype it.
    Anyway, you have the usual choice: patch up as best you can and face consequences later or commit fully and do belt and braces job.

    Definitely dry out the insulation and re-use what you can. Black mould is toxic so bin that - you may be able to peel the mouldy bit off and retain most of the material for re-use.
    You have to choose whether to rip out the internal finishing board so that you can properly fix insulation and vapour barrier.
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