Vapour checks and breather membranes

[Tom Stopp]

Hi

I am building a pod - small summerhouse type thing - and I am trying to get it airtight and well thermally insulated as well as long lasting. It is all timber frame and sits up off the ground on 300mm stilts. I was thinking of wrapping a vapour check all around the inside and a breather membrane all around the outside. Would this work?

Many thanks,
Tom

[Ebs]

Vapour check as I understand it implies something akin to a VCL or in other words a plastic or other non pervious membrane.  Personally I would not use a VCL but there is merrit in just using a bresather membrane but like all things in construction it is not the material that you use but how it is installed.

[Jean-Marc Bouvier]

Tom, the method you stated is the proper one.

[Ebs]

Sorry Tom

You have listed components and materials but not a method.

What happens if the vapour check is not continuous?

What happens when services are run through the vapour check?

Why not just install vapour check plaster board, that is  if you are not worried about cutting holes in it?

Sorry Tom but your post is so construction, list a set of materials that can work together but inevitably don’t.  On site interpretation as to what each component is for and quality of workmanship often rendered a "shopping list" approach null and void.

By just stating that your approach is correct is more than a bit naive.

What does the term airtight mean to you?

Good luck by the way.

[Jean-Marc Bouvier]

Tom, I stand by my statement that your approach is correct.

[Ebs]

Jean

Thanks for the email Jean, I can not immagine why you felt that you had to send me this and not put forward your point of view on this thread.

Getting back to the subject:

If Tom is going to construct this Pod using a typical timber frame construction system then it is a fair assumption to assume that the Sarking Board used would be either OSB or Ply, both of which in their off the shelf state are impervious to the passage of air.  Of course they will not completely eliminate all air movement through them but Tom is building a Summer House and not a CAT 5 Laboratory. 
Install both of these materials into an air permeability test rig and measure the air permeability rate through both and then cover them in a plastic sheet and re run the tests (plastic sheet = VCL I am sure that you understand that).  Do you think that you will be able to measure much difference in respect to air permeability?  The VCL is just a visual safety net for those who have trouble building to a high quality.

We have just finished a warm roof constructed vaulted ceiling with insulation between and on top of the rafters.  The insulation in question is a typical hard foam board with a low emissivity coating
(again you should really know what I talking about).  Is there any vapour/air movement through this type of insulation?
On the surface of it and consulting the insulation manufacturer’s data sheet then a resounding  NO can be heard across the country, but hang on how about the detailing and tricky little interfaces (gaps and holes to the un educated). 
It is not the product per say but  it is how it is installed that makes it work or not!  That was the point that I was making but you seemed to take offense to.

Getting back to the warm roof, which has 120mm of said type insulation as previously described, on top of this I have had to install (according to building regs) a very expensive breather membrane.  Perhaps you can enlighten us all Jean with your superior knowledge of construction exactly what this breather membrane does?

We are both guilty of not asking Tom more questions about exactly how he proposed to construct his summer house and then advised him accordingly having first collected this information!  In my defence my reply was more aimed towards people who would default to the standard VCL/Breather membrane approach to timber frame construction without actually thinking or considering WHY!

Just because someone has been involved in construction for a number of years does not mean that their advice is correct or should be given, is anyone that good that they can sum up an entire construction methodology in one sentence?

I stand by my reasoning that we would need more information from Tom before any advice were to be given.
 

[Jean-Marc Bouvier]

EBS, I will leave it to you

[Lewis Taylor]

EBS - Not a warm roof if there is insulation between the rafters/joists, it is a cold roof!

Sorry, after your rant about getting it right etc, I couldn't let that go!

[David Freeborough]

I agree that it is not a true warm roof because all the roof structure is not at room temperature. However, it is not  a cold roof either as the roof void or loft cannot be ventilated as this would bypass the insulation. So can I suggest we call it a hybrid warm roof?

For flat roofs the distinction is an important one because it determines whether the structural deck is at room temperature (& so immune to condensation) or at outside temperature (& so requiring ventilation). It is critical because the watertight roof covering is also vapour tight & so getting it wrong is certain to put the deck in the condensation zone leading to premature failure.

For pitched roofs with a tile finish the distinction is not so clear. The roof covering is vapour open &, if a breather membrane is used, so is the sarking membrane. There isn't usually a structural deck or sarking boards, so we are only concerned with the rafters. Putting half of the insulation above the rafters is generally accepted to keep the rafters at close to room temperature & out of the condensation zone. So gives the benefits of a warm roof without the additional height & structural risk of putting all the insulation over the rafters.  

Calling it a cold roof is likely to lead to misunderstandings & someone somewhere will end up ventilating the space below the insulation. Calling it a warm roof better explains the approach & encourages anyone fitting ventilation to do so above the (breathable) sarking membrane. Calling it a hybrid warm room would seem like a sensible compromise.

David

[Lewis Taylor]

David

Yes I agree that neither 'warm' or 'cold' works well enough on its own.

But for the sake of simplicity (in my mind) if a roof isn't a true 'warm' roof (ALL insulation outboard of the structure), then it is a cold roof and ventillation and condensation control must be at least considered.