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Vapour Barrier required?


Kernow
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Hoping someone can help with a bit of advice as to whether we need a vapour barrier in our roof construction. 
 

Roof make up is raised tie trusses, 150mm pir between trusses, 60mm pir on the underside of trusses, taped to form a vapour barrier, then plasterboard. 

 

On the flat sections of ceilings we will have 350mm of Rockwool type loft insulation. 
 

My dilemma is do we require a vapour barrier fixed to the underside of the ceiling timbers, and taped to the pir on sloped ceilings, before we fixed plasterboard?

 

Is standard green vapour barrier suitable?

 

My thoughts are using this will help with air tightness and limit moisture travel upwards?

 

Any opinions would be appreciated. 

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8 hours ago, MikeSharp01 said:

How will you achieve air tightness - plaster?

There’s never an issue 

 

Back in the 80s every slope had polythene 

In the 90s much of this was stripped out 

I’d say about a quarter of jobs that we do Mainly housing associations do 

I’m starting 4 houses today with lots of slopes All well over a million pounds each 

No VP specked 

The Jury is out 

I wouldn’t put one in my own 

 

As above Airtightness can easily be achieved with plaster and paint 

You would need a lot of air pressure to force through a Thermal boarded and skimmed ceiling 

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18 minutes ago, nod said:

Back in the 80s every slope had polythene 

In the 90s much of this was stripped out 

I’d say about a quarter of jobs that we do Mainly housing associations do 

I’m starting 4 houses today with lots of slopes All well over a million pounds each 

No VP specked 

The Jury is out 

I wouldn’t put one in my own 

Interesting, just about to order ours - looks like I need to do some more research before I do!

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2 hours ago, nod said:

There’s never an issue 

 

Back in the 80s every slope had polythene 

In the 90s much of this was stripped out 

I’d say about a quarter of jobs that we do Mainly housing associations do 

I’m starting 4 houses today with lots of slopes All well over a million pounds each 

No VP specked 

The Jury is out 

I wouldn’t put one in my own 

 

As above Airtightness can easily be achieved with plaster and paint 

You would need a lot of air pressure to force through a Thermal boarded and skimmed ceiling 


Woulld you say this is also true on flat ceilings with loft insulation as opposed to sloped ceilings with PIR?


I know it’s a highly debated topic

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2 hours ago, nod said:

There’s never an issue 

 

Back in the 80s every slope had polythene 

In the 90s much of this was stripped out 

I’d say about a quarter of jobs that we do Mainly housing associations do 

I’m starting 4 houses today with lots of slopes All well over a million pounds each 

No VP specked 

The Jury is out 

I wouldn’t put one in my own 

 

As above Airtightness can easily be achieved with plaster and paint 

You would need a lot of air pressure to force through a Thermal boarded and skimmed ceiling 

 

Is there a reason for the polythene being ripped out and presumably not reinstated? 

 

I guess we all want to stop rising vapour from getting to the timbers and being trapped, as well as the airtightness aspect.  If you wouldn't put a vcl in yours, is that cost saving and just don't see it as necessary or have you seen issues where it's been used in the past.  - Genuinely interested in your viewpoint as you've been in the game a long time.

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1 hour ago, Kernow said:


Woulld you say this is also true on flat ceilings with loft insulation as opposed to sloped ceilings with PIR?

The enclosed void above your raised ties is unvented and will be even more thermally decoupled if you insulate it with 350mm rockwool. So what happens to water vapour if it's not prevented from getting up to the ridge? Even without the rockwool the lack of airflow will result in a significant temperature drop leading towards the dewpoint. It never ceases to impress me how much mildew can develop behind freestanding cupboards or inside fitted wardrobes in poorly insulated homes. The lack of conditioned air circulation in these spaces is all it takes. 

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13 hours ago, Kernow said:

Roof make up is raised tie trusses, 150mm pir between trusses, 60mm pir on the underside of trusses, taped to form a vapour barrier, then plasterboard.

I'm not sure my previous reply makes sense without further info: Does the 150mm pir stop at the raised tie level or go on all the way up to the ridge? From what you described the space above might even be ventilated.

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16 minutes ago, Radian said:

I'm not sure my previous reply makes sense without further info: Does the 150mm pir stop at the raised tie level or go on all the way up to the ridge? From what you described the space above might even be ventilated.


Perhaps the attached photo makes more sense?

pir between trusses stops just above ceiling height. 
 

Loft space to be unventilated

 

 


 

image.jpg

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This is exactly what we have done, but we have added a vcl to all ceiling areas with 50mm service cavity.

we made sure we took the PIR on the sloping elevations far enough up into the loft to meet the top of the 500 mm loft roll, also maybe not necessary but felt better with it like this.

Regards. James

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I never like the idea of large volumes of air trapped in unventilated roof spaces. Yes you can apply vapour barriers to keep out the moisture from occupied spaces below but air infiltration from outside will be much more problematic. Without insulation the inside temperatures will follow the outside far more rapidly than the RH levels, bringing the structure close to the dew point on occasions. Most of the year this will not be a problem but sometimes it may lead to condensation that has no safe way of being dissipated. I'd be interested to know what the experienced builders think about this because the inexperienced ones I've discussed this with seemed oblivious to the notion.

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To be fair hygrodynamics is something of a mystery to most people - including me and some construction professors of my acquaintance! About the only working out on our build I didn't feel comfortable doing was the WuFi analysis, partly because I could not lay my hands on the full version of the software, but mainly because interpretation of the output is not remotely simple unless you have a lot of experience. One is left falling back on the old adages of good ventilation (if it does condense it can dry out) either outwards or inwards and if the latter might be useful then the correct membrane / structure is needed - hence perhaps the move from polythene to Pro Clima Intello Plus or similar and of course plaster breaths both ways I guess.

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6 hours ago, crispy_wafer said:

 

 

Is there a reason for the polythene being ripped out and presumably not reinstated? 

 

I guess we all want to stop rising vapour from getting to the timbers and being trapped, as well as the airtightness aspect.  If you wouldn't put a vcl in yours, is that cost saving and just don't see it as necessary or have you seen issues where it's been used in the past.  - Genuinely interested in your viewpoint as you've been in the game a long time

Yrs brown patches and mildew on ceilings 

 

Pictured is one of six 3.6 million pound houses I’m doing at the moment 

70 mil insulation between joists 

100 mil insulated plasterboard 

No polethene 

As I’ve said Very few want it 

37EFFF9C-259F-4003-8166-4DA1687FABEA.jpeg

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9 hours ago, nod said:

As above Airtightness can easily be achieved with plaster and paint 

You would need a lot of air pressure to force through a Thermal boarded and skimmed ceiling 

You are totally missing the point here.  If you rely on the plaster and skim for your air tightness, then EVERY penetration, light fitting (especially downlights) switch, socket etc is a hole in your air tightness layer.

 

The point of an air tight layer and then a service void, is it makes a completely air tight building and then ALL your services run INSIDE the air tight layer, without penetrating it. so it matters not one bit what holes you cut in the plasterboard for downlights etc, it will not degrade your air tightness and there is nothing to seal up to try and stop leaks.

 

I am convinced that the air tight membrane and the decision to have a warm vaulted roof are the 2 best design features that made out house so energy efficient. Of course that and a good level of insulation, decent doors and windows etc, and MVHR.

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28 minutes ago, ProDave said:

You are totally missing the point here.  If you rely on the plaster and skim for your air tightness, then EVERY penetration, light fitting (especially downlights) switch, socket etc is a hole in your air tightness layer.

 

The point of an air tight layer and then a service void, is it makes a completely air tight building and then ALL your services run INSIDE the air tight layer, without penetrating it. so it matters not one bit what holes you cut in the plasterboard for downlights etc, it will not degrade your air tightness and there is nothing to seal up to try and stop leaks.

 

I am convinced that the air tight membrane and the decision to have a warm vaulted roof are the 2 best design features that made out house so energy efficient. Of course that and a good level of insulation, decent doors and windows etc, and MVHR.

Read my answer Dave 

Im not missing the point 

I’m just stating a fact 

The vast majority of new builds don’t use plastic on the slopes

There is a reason and it isn’t cost 

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6 minutes ago, nod said:

Read my answer Dave 

Im not missing the point 

I’m just stating a fact 

The vast majority of new builds don’t use plastic on the slopes

There is a reason and it isn’t cost 

Let the mass builders churn out whatever rubbish they want.  For your OWN house, take the time and trouble to do it properly is what I am saying.

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1 hour ago, nod said:

Pictured is one of six 3.6 million pound houses I’m doing at the moment 

70 mil insulation between joists 

100 mil insulated plasterboard 

No polethene 

As I’ve said Very few want it 

37EFFF9C-259F-4003-8166-4DA1687FABEA.jpeg

With that build up you perhaps don't need it because you will have ventilation above the insulation under the breather membrane / slates and the plaster will breath either way but the PIR won't so it will be air tight. One assumes that whoever purchases these houses will have their legal team go through the hygrodynamic analysis to be sure there is no dew point risk - their mortgage companies surveyor (if they need one) will. 

 

On another note can I see past the frame to the outside on the mid left of the frame or is it a trick of the light?

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3 hours ago, nod said:

Read my answer Dave 

Im not missing the point 

I’m just stating a fact 

The vast majority of new builds don’t use plastic on the slopes

There is a reason and it isn’t cost 


What is used on the flat ceiling parts? I understand the lack of need for it on slopes with pir, it just adds an extra layer for condensation risk. 
 

I can see the benefit of both ways, I have come across properties in the past with excessive condensation on the breather membrane due to too much moisture travelling upwards, but also times when the vapour barrier has caused sweating. 
 

It seems there is no definitive answer on this one

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29 minutes ago, Kernow said:


What is used on the flat ceiling parts? I understand the lack of need for it on slopes with pir, it just adds an extra layer for condensation risk. 
 

I can see the benefit of both ways, I have come across properties in the past with excessive condensation on the breather membrane due to too much moisture travelling upwards, but also times when the vapour barrier has caused sweating. 
 

It seems there is no definitive answer on this one

There’s nothing on any of the ceilings 

The air test needs to come in under 3

We are doing a bunch of houses and apartments in the lake district 

We shot fire the plastic to to the steel ridge beam Then return it down the slopes and down to the floor 

Shot firing and acoustic sealing to the perimeter walls 

 

Obviously two chains of thought 

Edited by nod
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@Kernow It's a tricky one this.

 

Lots of alternative views from the likes of @Radian, @nod, @MikeSharp01 and others.

 

Nod, had a chuckle about the million pound houses....

 

For me, if you are aiming for a quality build then using the green plastic is a good backstop to guard against the odd bit of poor quality workmanship. It's relatively cheep and quick to install.

 

Going back to basics. The idea is to stop the water vapour (a gas) from inside the house entering the fabric and turning into a liquid at some point within the depth of the construction, when it does this is often called the dew point. Water gas does no harm, it's when it turns into a liquid (manifests as condensation) form that the problems arise.

 

Now most insulated plaster boards you see now are vapour checked.. they have a built in bit of material (could be a metal foil) that serves as the green plastic. Now these sheets have joints that can be taped and sealed.. all good .. until you get on site and the sheets don't get cut perfectly. Usually the place where they don't get cut right is high up and on angles.. just the place where the air is warm and humid.

 

Warm humid air has lots of water gas in it and this gas wants to migrate to areas where the humidity is less for example, just like a tea bag in a cup, it want's to diffuse. Also buildings move about so while all may be good for the first few months once the building dries and moves your joints may not be quite so well sealed. SE's for example often design for 10 - 20 mm movement on a timber frame house! You plaster board is connected to the underlying structure. Motto here is check the fine print on your moisture control spec, installation tolerances etc when buying the insulated plasterboard.

 

The green plastic is quite "stretchy" so that is a good property compared with your tricky "sellotaped" joints.

 

For me, for a qualty build where you risk moisture condensing near timber or other water sensitive materials I would stick a bit of green plastic under the insulated plaster board as a secondary barrier to prevent water vapour entering the timber structural zone. You have to be sure that you building is able to effectively breathe outwards everywhere! If not you will have problems.

 

It's not common but you do get a reverse effect sometimes in the UK where the inside of the house is cooler and dryer than the outside. This happens after a cold spell where the house cooler and dry.. then a warm weather front blows in carrying lots of water gas. The warm humid air tries to enter your house and you get a dew point in reverse. It does happen but not often enough to be a major issue in general.. but Radian point this out.

 

 

 

 

 

 

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