Nemesis
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Really? Then I suggest the owners get them fixed.
- Thread starter Mark
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Really? Then I suggest the owners get them fixed.
Well, I always thought the requirement for a vapour barrier was based upon the condensation risk and the amount of ventilation on the cold side.
Is you're saying that water vapour can pass through Thermafleece, the risk of condensation on cold timbers is the same (IMO) as if you insulate with anything else that's permeable. Water vapour passing through glass fibre is precisely the reason why vapour barriers are employed.
I don't see how a damp rafter cares too much if the fluffy stuff next to it can tolerate interstitial condensation.
How can you be sure that the moisture content of the rafters won't be raised rather too much?
But if you do use it between the rafters (Ian) make sure that air gap is always maintained (how I wonder when you can't see) and make sure there is plenty of outside air flowing across it.
Also be aware that in some quarters breather membranes are suspected to "clog up" over time.
Even if I decided to take the chance with their design detail, I wouldn't rely on breather membrane to provide sufficient airflow
Is you're saying that water vapour can pass through Thermafleece, the risk of condensation on cold timbers is the same (IMO) as if you insulate with anything else that's permeable. Water vapour passing through glass fibre is precisely the reason why vapour barriers are employed.
I don't see how a damp rafter cares too much if the fluffy stuff next to it can tolerate interstitial condensation.
How can you be sure that the moisture content of the rafters won't be raised rather too much?
But if you do use it between the rafters (Ian) make sure that air gap is always maintained (how I wonder when you can't see) and make sure there is plenty of outside air flowing across it.
Also be aware that in some quarters breather membranes are suspected to "clog up" over time.
Even if I decided to take the chance with their design detail, I wouldn't rely on breather membrane to provide sufficient airflow
Lime
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There still seems to be a great deal of confusion.
Let's be specific:
If insulation (no matter what type) is placed directly under roof tiles with no ventilated gap (with or without breathable membrane under the tiles) then the likelyhood of timber rot is high.
The ventilated space between the insulation and the tiles or membrane is essential.
It does not matter whether moisture can pass through insulation or not, the timbers will not be able to dry out properly without a proper air flow across them.
Let's be specific:
If insulation (no matter what type) is placed directly under roof tiles with no ventilated gap (with or without breathable membrane under the tiles) then the likelyhood of timber rot is high.
The ventilated space between the insulation and the tiles or membrane is essential.
It does not matter whether moisture can pass through insulation or not, the timbers will not be able to dry out properly without a proper air flow across them.
I must admit to be a little confused.
We laid the sheepswool insulation directly onto the loft floor as the fibreglass insulation had been. Is this ok? There's nothing else there except for wool on lathe and plaster ceilings...obvisouly we have a loft space and roof above it!
Presumably you are talking about instances where the insulation is attached to the roof itself???
We laid the sheepswool insulation directly onto the loft floor as the fibreglass insulation had been. Is this ok? There's nothing else there except for wool on lathe and plaster ceilings...obvisouly we have a loft space and roof above it!
Presumably you are talking about instances where the insulation is attached to the roof itself???
The problem is with insulating the closed space between the ceiling slope and the roof covering, where insulation is layed between rafters.
I can't see a problem with your insulation lying between the ceiling joists.
It sounds as though it will not be possible for me to insulate between the rafters, as there's insufficient depth to allow airflow.
I can't see a problem with your insulation lying between the ceiling joists.
It sounds as though it will not be possible for me to insulate between the rafters, as there's insufficient depth to allow airflow.
It seems some confusion persists.
Sheeps wool is breathable, so are all flexible wool-like insulations, ie fibreglass and rockwool too. All these allow water vapour to pass right through because of their structure.
Sheep wool is said to be able to absorb a little moisture, but the amount is small compared to the quantity of condensation that forms if incorrectly installed, so slight absorption is not significant here.
The air gap above the insulation should not be omitted. The properties of any type of insulation make no difference to this.
Woolly insulation fitted to the roof should have a vapour barrier fitted below it, otherwise interstitial condensation can occur, which will rot timber as well as sheeps wool, and grow assorted unpleasant life forms.
Insulation laid on the ceiling below is a different business, and the requirements differ.
Unfelted old roofs often leak, but no problems occur because the good ventilation soon dries out the small amount of water ingress. In many cases the owners have no realisation that water is getting in then drying out again, its usually only if faults develop that water ingress becomes sufficient to become a problem. If you've ever sat under an old slate roof after dark and looked out through all the little gaps you'll see why water gets in. There are always many gaps between overlapping slates, because they arent perfectly flat and uniform, and wind drives a small proportion of rain up and in.
NT
Sheeps wool is breathable, so are all flexible wool-like insulations, ie fibreglass and rockwool too. All these allow water vapour to pass right through because of their structure.
Sheep wool is said to be able to absorb a little moisture, but the amount is small compared to the quantity of condensation that forms if incorrectly installed, so slight absorption is not significant here.
The air gap above the insulation should not be omitted. The properties of any type of insulation make no difference to this.
Woolly insulation fitted to the roof should have a vapour barrier fitted below it, otherwise interstitial condensation can occur, which will rot timber as well as sheeps wool, and grow assorted unpleasant life forms.
Insulation laid on the ceiling below is a different business, and the requirements differ.
Unfelted old roofs often leak, but no problems occur because the good ventilation soon dries out the small amount of water ingress. In many cases the owners have no realisation that water is getting in then drying out again, its usually only if faults develop that water ingress becomes sufficient to become a problem. If you've ever sat under an old slate roof after dark and looked out through all the little gaps you'll see why water gets in. There are always many gaps between overlapping slates, because they arent perfectly flat and uniform, and wind drives a small proportion of rain up and in.
NT
NT - the useful depth of the rafters is 3 1/2 inches max.
A vapour barrier would be difficult to install, due to the lath & plaster ceiling with purlins and principal rafters passing through it. I don't want to cover this ceiling if I can avoid it.
The attic has l&p walls to the eaves, with lath and plaster ceilings above the bathroom and beds. Condensation is likely to find its way into the eaves void via the attic walls, doors to the eaves, bathroom ceilings etc.
Moist air in the eaves void could also find its way into the rest of the roof, via the space between the rafters and the top of the eaves wall. You can't seal these gaps anyway, as you'd stop the airflow across the roof.
Is it not better to ensure that water vapour can escape, rather than trying to keep it out altogether, which is likely to fail? To me a vapour barrier sounds like a DPC for the roof.
A vapour barrier would be difficult to install, due to the lath & plaster ceiling with purlins and principal rafters passing through it. I don't want to cover this ceiling if I can avoid it.
The attic has l&p walls to the eaves, with lath and plaster ceilings above the bathroom and beds. Condensation is likely to find its way into the eaves void via the attic walls, doors to the eaves, bathroom ceilings etc.
Moist air in the eaves void could also find its way into the rest of the roof, via the space between the rafters and the top of the eaves wall. You can't seal these gaps anyway, as you'd stop the airflow across the roof.
Is it not better to ensure that water vapour can escape, rather than trying to keep it out altogether, which is likely to fail? To me a vapour barrier sounds like a DPC for the roof.
1.5 - 2" of insulation is a lot better than none. Each successive inch brings less heat saving.
How roof insulation works is quite different to how wall DPCs work, and yes you need a vapour barier or interstitial condensation is likely to occur. A vapour barrier does not stop breathing and drying out in this position, it stops water vapour condensing in the insulation.
Its possible to paint a vapour barrier onto the plaster, even if not ideal. Sticking foil on is also possible if necessary.
NT
How roof insulation works is quite different to how wall DPCs work, and yes you need a vapour barier or interstitial condensation is likely to occur. A vapour barrier does not stop breathing and drying out in this position, it stops water vapour condensing in the insulation.
Its possible to paint a vapour barrier onto the plaster, even if not ideal. Sticking foil on is also possible if necessary.
NT
biffvernon
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NT said:
Eh?
Lime
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biffvernon said:
The actual quote:
I have no difficulty in understanding that.
The "position" is a particular one of being near/next the roof tiles.
In general the house air will be more moist than outside air but not always.
The vapour barrier is to stop the almost unlimited interior moisture from penetrating the insulation and making it damp/wet by condensation on or near the often cooler roof tiles.
Periods of exterior moist air will also cause condensation to occur both sides of the tiles. However when the outside conditions change and the tiles dry the moisture can escape to the outside as the vapour barrier does not impede it.
The only time a vapour barrier positioned on the inside of the insulation is a problem is when the inside/interior is cooler than the outside (cool enough to condense out the moisture) and the air outside is very moist. Then it would be possible to get the insulation wet and the vapour barrier prevent drying.
However such a scenario is very rare IME and is only likely where mechanical cooling occurs as with air conditioning.
Usually when the inside is cooler the outside is hot with sun on the roof effectively heating the whole roof so that condensation does not occur and evaporating all condensate.
biffvernon
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Well I'm not as clever, Lime, because I'm struggling to understand a vapour barrier that doesn't stop 'breathing'.
The vapour barrier acts, unless I've got it wrong, as a barrier to the passage of water vapour. 'Breathing' as I understand it, is the ability of a material to allow water vapour to pass through. You can't have it both ways.
Putting a vapour barrier on the outside, cold side, of roof insulation invites condensation and damp insulation and anything it touches. Bad idea.
Putting a vapour barrier on the inside, warm side, of roof insulation stops water vapour from in the house escaping. This invites increased humidity and condensation wherever there is a cold spot. This might be a tucked away bit of wall downstairs under the stairs and might then be claimed to be 'rising damp' and all that follows therefrom.
So I would suggest it is better not to use any vapour barrier anywhere.
(Shower curtains are perhaps best made of something waterproof.)
The vapour barrier acts, unless I've got it wrong, as a barrier to the passage of water vapour. 'Breathing' as I understand it, is the ability of a material to allow water vapour to pass through. You can't have it both ways.
Putting a vapour barrier on the outside, cold side, of roof insulation invites condensation and damp insulation and anything it touches. Bad idea.
Putting a vapour barrier on the inside, warm side, of roof insulation stops water vapour from in the house escaping. This invites increased humidity and condensation wherever there is a cold spot. This might be a tucked away bit of wall downstairs under the stairs and might then be claimed to be 'rising damp' and all that follows therefrom.
So I would suggest it is better not to use any vapour barrier anywhere.
(Shower curtains are perhaps best made of something waterproof.)
Of course a vapour barrier stops the wall from breathing, it's meant to. And if it invites increased humidity and condensation wherever there is a cold spot you can be sure it will be even wetter within the structure, where it's colder, and where you can't see it.
I could agree with you about not requiring a vapour barrier on a masonry wall, downstairs under the stairs and with lime mortar, but that's not what we're talking about.
I think it's telling that nowhere in the Thermafleece bumph does it mention vapour barrier, the diagrams omit it, despite carefully mentioning a breathable membrane, but the BBA cert. says: 7.5 A vapour control layer should be provided to the warm side of the product.
Even the Thermafleece diagram for a timber frame wall omits the vapour barrier whilst the BBA cert goes even further: 4.2 It is essential that the continuity of vapour control layers is maintained at laps and joints at wall/ceiling and wall/floor level. Perforations, eg for service outlets, should be kept to a minimum and be well sealed and taped. The recommendations of BS 5250 : 1989 should be followed to minimise the risk of condensation within the structure.
If Thermafleece performs in some way that negates the use of a vapour barrier they should qualify their publication of the BBA certificate and explain it properly. I'd like to bet that if they encountered a condensation problem when their product is installed according to their diagrams, they will cite the BBA cert. as evidence of non-compliance with correct detailing.
I could agree with you about not requiring a vapour barrier on a masonry wall, downstairs under the stairs and with lime mortar, but that's not what we're talking about.
I think it's telling that nowhere in the Thermafleece bumph does it mention vapour barrier, the diagrams omit it, despite carefully mentioning a breathable membrane, but the BBA cert. says: 7.5 A vapour control layer should be provided to the warm side of the product.
Even the Thermafleece diagram for a timber frame wall omits the vapour barrier whilst the BBA cert goes even further: 4.2 It is essential that the continuity of vapour control layers is maintained at laps and joints at wall/ceiling and wall/floor level. Perforations, eg for service outlets, should be kept to a minimum and be well sealed and taped. The recommendations of BS 5250 : 1989 should be followed to minimise the risk of condensation within the structure.
If Thermafleece performs in some way that negates the use of a vapour barrier they should qualify their publication of the BBA certificate and explain it properly. I'd like to bet that if they encountered a condensation problem when their product is installed according to their diagrams, they will cite the BBA cert. as evidence of non-compliance with correct detailing.
biffvernon
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Yeah.
I guess it demonstrates that those responsible for BBA certs like to cover their backsides and probably live in tupperware boxes.
I guess it demonstrates that those responsible for BBA certs like to cover their backsides and probably live in tupperware boxes.
Nemesis
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Please can I just say here that those who developed Thermafleece are genuine people, and it won a Country Living award for enterprising women, following the horrors of f and m.
They are not out to fleece people. (!)
I have talked to them on the phone, they are a friend of a friend etc. Hugely helpful.
Please any problems, give them a ring. Don't diss without listening.
They are not out to fleece people. (!)
I have talked to them on the phone, they are a friend of a friend etc. Hugely helpful.
Please any problems, give them a ring. Don't diss without listening.