Vapour control layers

[Richie866]

Currently doing a timber frame bungalow conversion - the whole of the upstairs has been knocked down and timber frame erected with full fill rafter insulation and rockwool in the walls. With hardiboard on the outside and slates on the roof.

Question is - the spec says vapour control layer throughout the walls and roof. My builder is severely against this, saying where is all the moisture going to go, it will create condensation etc etc. I've read all the gumf on vapour control layers, saying it protects the building fabric. But I can see both sides of the argument.

If I did VCL the whole lot where does all the moisture go? All I've got is trickle vents and a bathroom fan!

Any advice would be appreciated

 

[ban-all-sheds]

My builder is severely against this, saying where is all the moisture going to go, it will create condensation etc etc.

So he'd rather it went into the fabric of the building, got through the insulation to the cold parts?

And he thinks that there it won't create condensation?

Not a very competent chap, is he.

Has he adopted Accredited Construction Details? In the Building Regulations application, how was compliance with Parts F & L promised?

I've read all the gumf on vapour control layers, saying it protects the building fabric. But I can see both sides of the argument.

So you can see an argument in favour of allowing warm moisture-laden air to get into the cold parts of the construction?

If I did VCL the whole lot

I really don't think there's an "if"...

where does all the moisture go?

Out via adequate mechanical ventilation. Possibly with heat recovery.

All I've got is trickle vents and a bathroom fan!

Probably inadequate.

Any advice would be appreciated

Speak to the person who did the design, and ask him the questions, particularly about the ventilation and insulation/energy performance requirements.

 

[Richie866]

Thanks for your thoughts, Thats my only worry 'adequate ventilation'

Like I said there is no heat recovery - don't want to splash out x thoushands as budget is getting hammered. I've voiced my concens with the architectural tecnologist and he says its fine, but I'm trying to get me head round a few trickle vents and a bathroom extractor to provide adequate ventilation. Can you appreciate the situation?

 

[ban-all-sheds]

Indeed.

I really don't think you have any alternative to a VCL if you don't want the building to rot.

Either the ventilation will be enough, or it will not, and the laws of physics will not be influenced by your budget.

If you have written confirmation from the designer that the ventilation will be sufficient, that all relevant Building Regulations will be complied with and condensation will not be a problem then there's not much else you can do.

Be wary of the builder who seems to have missed the move away from leaky houses.

 

[Richie866]

and the laws of physics will not be influenced by your budget

I like that quote, Just not he cost of it!

 

[jeds]

It isn't the presence of water vapour that causes condensation. You can't avoid water vapour. It's everywhere around us, within all the layers of a building, all the time. The problem comes when internal water vapour - which is at a higher temperature than external vapour - migrates through the building fabric, from internal to external, cooling as it goes. As it cools its capacity to hold water reduces until it reaches 100% - known as the dew point. That usually happens somewhere within the insulation fabric. So the VCL layer is vital. It prevents internal water vapour entering the system. Without it the insulation will be saturated in no time and that will eventually get into the timbers and they will rot.

 

[Richie866]

Thanks Jeds, really good explanation. What I'm trying to get my head round though is if there was no ventilation at all (which there is with trickle vents etc) what would happen? So to elaborate further, if my proposed ventilation is inadequate what's the worse that could happen.

Also when the vcl gets penetrated, by fixing the plaster board etc. I'd that something you just have to live with and is factored into the design?

Cheers

 

[jeds]

Vapour just needs to be restricted. A small amount of vapour - via screw holes etc - won't be a problem. In reality some vapour always gets through. Even with a VCL. As long as it's only trace amounts it will stabilise with the local vapour and not cause a problem.