Airtightness and Vapour Control:

[ToughButterCup]

I'm working hard at understanding these two concepts thoroughly. And the Internet is probably the worst and best place in the world to sort it out.

Worst because of the shared ignorance everywhere, best because BH will sort it out .

 

Pull the following semi-formed thoughts to bits, please.......

 

Air-tightness

is about the restriction of air movement BOTH into and out of a building to values, at the very least, that are acceptable to BC. And air contains water vapour. So action to increase air-tightness might well also contribute to vapour control. Here's Keira Proctor of the Proctor Group saying that air-tightness layers can act as a vapour control. 

(Expanded webinar transcript, https://www.proctorgroup.com/news/vapour-and-air-permeable-membranes-explained#expand-webinar-transcript accessed May 2018)

 

Is it too naive to ask - well why bother with a Vapour Control Layer  when the Air-tightness Layer will do the same job?

 

Vapour Control

Many on BH have written  about interstitial condensation. I get it. Don't let warm air leak out into the fabric of the house so that it comes into contact with cooler bits and so - precipitates water which then pools there and does all sorts of nasty stuff, from mould to rust to other nasties.

Take a warm roof. Slates (say) will be cold, and water vapour exiting the house would normally condense on the underside of the slates. So you need roofing felt (vapour control) allowing the vapour to exit, but not return. A bit like a Gortex jacket (other jackets are available) . I have heard that referred to as Vapour Permeable Underlay, VPU (not VCL).    But it is acting as a Vapour Control Layer             [Thank Gaud its not shortened to  VPL ?]

 

Now, I'm under the impression that a house needs BOTH a VCL and an air-tightness layer. And IF that is correct doesn't it make sense (because it would be cheaper) to

• prevent vapour leaving the house by using the air-tightness layer as a VCL?
• prevent ingress of vapour in the same way?
• And put the air-tightness layer where there are least penetrations?

 

But then I'd be wrong, I wager.

 

Looking at whats available; the specs, the advice, the drivel, the spurious claims  on Tinternet, I feel like my roofer mate. It used to be so simple

 

[joe90]

My take on this from years of trying to understand it is, yes you need airtighness to stop drafts but a building needs to breathe so that if/when any vapour gets into the fabric it can escape slowly. For example, my roof make up is engineered joist with OSB clad inside ( glued joints), mineral wool insulation between joists with vapour permeable felt, battens then slate. I remember on another forum a long winded discussion about using OSB as the airtight layer and whether it was good enough, well I think so?.

[Dreadnaught]

Just to add to your thoughts, I found the whole chapter on moisture control in the Passive House Handbook very helpful in understanding these issues.

 

In that chapter, and for a sandwich layers of materials in, for example, a wall and from inside to outside the layers ideally need to have a progressively lower vapour permeability. It states that vapour permeability of the material on the outer surface should be 5x that of the internal surface as a rule of thumb. For example, wood fibre board is very vapour permeable and so well suited to being the outer layer.

 

It also helpfully states "Moisture within the fabric of a building is the most common cause of building failure".

Edited May 17, 2018 by Dreadnaught

[Ian]

18 minutes ago, Dreadnaught said:

In that chapter, and for a sandwich layers of materials in, for example, a wall and from inside to outside the layers ideally need to have a progressively lower vapour permeability. It states that vapour permeability of the material on the outer surface should be 5x that of the internal surface as a rule of thumb. 

 

That advice comes directly from British Standard BS5250:2002.  Although its essentially still correct the advice has since been superseded by the 2011+A1:2016 version of the standard which lowers the requirement to 2x vapour resistance. The exact wording is:

"....an AVCL, with a vapour resistance of at least double that of the [external] sheathing should be provided on the warm side of the insulation"

[A_L]

41 minutes ago, recoveringacademic said:

Is it too naive to ask - well why bother with a Vapour Control Layer  when the Air-tightness Layer will do the same job?

 

Vapour Control

Take a warm roof. Slates (say) will be cold, and water vapour exiting the house would normally condense on the underside of the slates. So you need roofing felt (vapour control) allowing the vapour to exit, but not return.

 

Now, I'm under the impression that a house needs BOTH a VCL and an air-tightness layer. And IF that is correct doesn't it make sense (because it would be cheaper) to

• prevent vapour leaving the house by using the air-tightness layer as a VCL?
• prevent ingress of vapour in the same way?
• And put the air-tightness layer where there are least penetrations?

 

Is it to naive to ask........... - Consider two means of transport of water molecules, 1) Bulk air movement 2)Vapour diffusion. These have to be handled differently. Bulk air movement caused by say a convection current from a radiator or by an air pressure differential, carries water molecules along. Vapour diffusion occurs when there is a concentration gradient of water molecules, in this case the molecules in the more concentrated areas move to the less concentrated areas in an effort to maximise their path lengths before another collision with another water molecule.

An air tightness layer therefore may or may not control both mechanisms. If it does not provide resistance to water vapour diffusion and it is inappropriately sited then interstitial condensation may occur in appropriate circumstances.

 

Vapour Control ..... So you need roofing felt (vapour control) allowing the vapour to exit, but not return.  - Not quite, you need to allow water vapour to exit but prevent the return of liquid water

 

needs BOTH a VCL and an air-tightness layer

 

prevent vapour leaving the house by using the air-tightness layer as a VCL?.............potentially, but this will towards the inside of the structure and you will probably need something, i.e. more membrane, to protect the external sheathing for example.

 

prevent ingress of vapour in the same way?........vapour ingress in our climate is not normally a problem as it is usually warmer inside than out and the incoming water molecules will not condense as the Rh falls as they move inwards.

 

And put the air-tightness layer where there are least penetrations? ..... preferable but case dependent, on the outside it must not be water vapour resistant or interstitial condensation may occur unless another VCL present on the warm side or preferably use a breathing construction which is not subject to interstitial condensation