Vapour Barrier required?

[nod]

29 minutes ago, Gus Potter said:

@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.

My problem with polethene is

Ive removed a lot of plasterboard over the years to find it wet through behind the plastic 

I don’t think it’s anything to do with quality 

I think many architects are aware of the risks 

 

 

 

 

 

 

 

[Radian]

21 minutes ago, Gus Potter said:

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.

 

I don't know if this is restricted to particular regions in the UK but it certainly isn't uncommon here on the South coast. Cornwall, South Wales and Counties adjacent to the English Channel see more of the Atlantic airstream and this occasionally carries in mild moist air, on the prevailing SW winds, during the Winter. We had a particularly good example for a few days in early January and I noted a number of posts on these forums where people were suffering from unexpectedly high amounts of condensation. I suspect we will experience more of these types of condition in the future and would suggest everyone factors it into their design stage to some degree.

[Gus Potter]

14 minutes ago, nod said:

Ive removed a lot of plasterboard over the years to find it wet through behind the plastic 

I don’t think it’s anything to do with quality 

I think many architects are aware of the risks 

Yes, have seen this myself Nod. Have seen a couple where you can actually wring the water out of the insulation into a bucket.

 

But when you investigate further you often find that this is a symptom of a problem that lies else where. For example, water vapour is getting in round the sides, leaky pipes, sparly ducting conduits sloping back into the wall.. it's a long list.  Also take a timber frame, if the trickle vents to the cavity are blocked then the moisture is more trapped. Sometimes the breathable membrane is installed to wrong way round.. !

 

Yes designers are aware of the risks and in the short term if you let the building breathe both ways then it will probably have more of a chance except in say family bathrooms or in heavily used kitchens. Probably the stir fry is you friend as the grease will seal the gaps?

 

24 minutes ago, Radian said:

 

I don't know if this is restricted to particular regions in the UK but it certainly isn't uncommon here on the South coast. Cornwall, South Wales and Counties adjacent to the English Channel see more of the Atlantic airstream and this occasionally carries in mild moist air, on the prevailing SW winds, during the Winter. We had a particularly good example for a few days in early January and I noted a number of posts on these forums where people were suffering from unexpectedly high amounts of condensation. I suspect we will experience more of these types of condition in the future and would suggest everyone factors it into their design stage to some degree.

 

Interesting you cobservations. Yes, this cropped up a couple +  decades ago when timber frame really caught on in Scotland but at that time while we knew about the "reverse dew point" we discounted it as a serious threat. Now however the insulation levels are significantly increased and air tightness is now a consideration. In the early days we got away with it as although the timber frame houses were comparitively much warmer than the existing housing stock they were still draughtyish compared with what we are building now.

 

But even back then we know that the more insulation you introduced and the more you controlled the air flow then an increased risk of condensation would follow as a consequence.

 

[nod]

At the end of the day 

It’s individuals choice It’s pretty obvious that the professionals can’t agree on this 

I can’t complain Ripping the stuff out over the years has kept me in work 

[SimonD]

12 hours ago, Gus Potter said:

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.

 

Studies in Sweden back in the mid 2000s found that new build airtightness reduced in the first year simply due to settlement of the building and materials. It was proposed that airtightnes tests should be carried out something lie 12 months after construction as this would be more accurate. And this is from a country that has far higher building standards than your UK developer.

 

I think this thread is giving the green plastic a bad rap because that's where the moisture is being found. Simply removing this yet using an impervious insulation material behind the plaster isn't going to fix that at all so I'm not sure why it's even being suggested as a better way to build to avoid this.

 

20 hours ago, MikeSharp01 said:

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.

 

I agree. The problems as I see them are to do how we build wall systems with impervious materials and fail to ventilate properly - a recent government study found that the majority of new builds did not even meet building regs for ventilation requirements. I'm also still surprised the option of moisture open wall systems almost never enters into discussion.

 

 

Edited February 25, 2022 by SimonD

[ProDave]

Well my air test was done at completion, something like 4 years after all the air tight membranes and tapes were applied, so it had plenty of time to settle.

[TonyT]

If you also fold the joins of membranes over and then tape the fold is compressed by the plasterboard helping in some situations

[ProDave]

7 minutes ago, TonyT said:

If you also fold the joins of membranes over and then tape the fold is compressed by the plasterboard helping in some situations

For most of mine, if the taped joint did not line up with one of the battens forming the service void, then I screwed a strip of OSB offcut over the taped joint to maintain it squashed together.

[MikeSharp01]

10 hours ago, ProDave said:

Well my air test was done at completion, something like 4 years after all the air tight membranes and tapes were applied, so it had plenty of time to settle.

And the reading was?

[ProDave]

Just now, MikeSharp01 said:

And the reading was?

1.4, which frankly I was disappointed with, but the tester was nearly wetting himself with how good it was.

 

What i am finding interesting is this winter, now the house is finished and the last few bits that might have been resulting in a bit of heat leakage are finished, I am finding my heating use is less than previous winters, and lower than the heat loss spreadsheet suggests it should be.  I am of course not complaining.

[MikeSharp01]

7 minutes ago, ProDave said:

1.4, which frankly I was disappointed with, but the tester was nearly wetting himself with how good it was.

Sounds good - where were the leaky bits? We are just about to install the Intello and would be interested in hints one where it goes wrong easily. I like the OSB crusher idea I have put OSB backing where I think the joints will come outside the frame studs / rafters so I have something to push against.  

[ProDave]

25 minutes ago, MikeSharp01 said:

Sounds good - where were the leaky bits? We are just about to install the Intello and would be interested in hints one where it goes wrong easily. I like the OSB crusher idea I have put OSB backing where I think the joints will come outside the frame studs / rafters so I have something to push against.  

The landing window had been missing it's glass. that was originally planned to be a stained glass unit until the reality dawned and we shelved that.  It had been for3 years boarded up with OSB and a slab of insulation.  Getting a 3G glass unit properly fitted in it no doubt reduced a bit of heat leakage.

 

The wall to the sun room had been incomplete and although sheltered from rain was still open to the cold.  Getting that complete and the sun room to encase it no doubt saved a bit more heat loss.

[Kernow]

Thanks for all the feedback and thoughts guys. 
I know it’s a hotly debated topic so it’s interesting to hear so many different opinions from experienced builders and self builders with such vested interests in there homes.