Vapour Barrier

[Shultzy]

I'm confused - In most of the posts on workshop building the consensus seems to be that the vapour barrier goes between the outside of the frame and the cladding.

I've just read in a copy of the Woodworker, in an article called "Alan's Shed".

"The slight problem with insulation is that it can cause condensation, so its always best to install a vapour barrier between the insulation and the internal cladding. The rule is to apply the vapour barrier to the warm side of the insulation, so that any moisture is held back"

This seems logical. Any comments?

 

[JFC]

That is the correct way to do it but maybe in some of the builds they have not used felt on the outside but used the vapor barrier instead . A vapour barrier on an extension or loft conversion goes up just before the plasterboard .... and then you make thousands of holes in it fixing the plasterboard up :lol:

 

[Jake]

Usual (modern) timber frame construction is from inside out -

a) wall cladding (plasterboard, what have you)
b) damp proof membrane
c) timber with insulation between (and maybe a layer over the studs on the inside to stop cold bridging)
d) vapour permeable membrane
e) exterior cladding

He is talking about a) above, which stops humid warm air from condensing inside the wall as it meets the cold outside bits. Whether you need that in a shed is more debateable than in a house where you always have the heating on. Depends how much you are going to heat the shed, and what with.

The posts on here will be talking about d) which is there to keep the rain out.

If d) is vapour permeable, it also stops the humid air from being forced to condense on the inside face of the barrier, as water can pass through the membrane in vapour phase but not the other way as liquid rain. If you are never going to heat your shed, felt or a dpm could be just as good.

 

[mailee]

My vapour barrier is on the inside behind the internal cladding and I assumed it was correct from what I have read?? No problems so far :?

 

[Jake]

It'll be fine providing your cladding is waterproof enough.

All these things are to some degree precautionary, and just aimed at extending the life of buildings, which need to be much more permanent than sheds if you consider the length of the mortgages that are placed on them, etc.

 

[JFC]

Jake i think you have B and D the wrong way round but i could be wrong .

 

[Jake]

Try tyvek's information then...

 

[wrightclan]

Jake i think you have B and D the wrong way round but i could be wrong .

Jake's got it right. The damp proof membrane (vapour barrier) keeps warm, moist internal air from condensing on the cold side of the insulation. The vapour permeable membrane (such as Tyvek) allows any moisture within the wall out to the outside air, while not allowing rain in.

Brad

 

[JFC]

But if not using tyvek its from outside in ... tiles , felt ,stud with insulation , vapour barrier then plasterboard ?

 

[wrightclan]

But if not using tyvek its from outside in ... tiles , felt ,stud with insulation , vapour barrier then plasterboard ?

Not sure I understand your point. The vapour barrier (Jake says damp proof membrane--both accomplish the same thing in this instance) is in the same place either way. (between the plasterboard and studs):-k

Brad

 

[JFC]

Maybe its me but a DPM stops all water .

 

[wrightclan]

Maybe its me but a DPM stops all water .

So does vapour barrier, if it is "uncompromised." What I think may be happening may be a confusion of terminology. A vapour barrier
is supposed to stop moisture from escaping. A vapour permeable membrane is supposed to allow moisture to escape.


Brad

 

[JFC]

Im sure tyvek will come up with a levitation nail to make that possible :lol:

 

[wrightclan]

Im sure tyvek will come up with a levitation nail to make that possible :lol:

Tyvek is not a vapour barrier. It is a vapour permeable membrane. Vapour barrier is just plastic.

Brad

 

[JFC]

ops thats me not reading the replies properly

 

[wizer]

This had me scratching my head a while back. So in basic terms (for a basic person), before I ply line my workshop, I should cover the studs, insulation, electrics, etc with DPM material?

 

[Jake]

The vapour barrier (Jake says damp proof membrane--both accomplish the same thing in this instance)

I used DPM in an attempt to convey the idea that the inner layer is simply a sheet of polythene, to avoid any confusion between that and the permeable membrane. It didn't work!

 

[Jake]

Yes.

If you were doing a house, the role of the DPM is usually played by foil-faced insulation, taped over the joists with aluminium foil if you are doing a proper job, or with foil-backed plasterboard. If the insulation is rockwool and the internal cladding was something that can't be bought foil faced, staple sheet(s) of damp proof membrane over the studs before you put the cladding up. Overlap well, and taping the joints would be best practice.

Then you put the cladding up and punch loads of holes through the vapour barrier - but it still keeps most of the vapour out of the walls.

 

[stewart]

Hi all
Is a vapour barrier necessary in roof insulation too?
I've come into several rolls of rockwool insulation that I want to use to insulate the pictched roof of my garage - currently no insulation and cold! I intend to put a suspended ceiling in and put the rockwool on top (a la loft insulation). Is a vapour barrier needed between the rockwool and the suspended ceiling?
Many thanks,
Stewart

 

[wrightclan]

Hi all
Is a vapour barrier necessary in roof insulation too?
I've come into several rolls of rockwool insulation that I want to use to insulate the pictched roof of my garage - currently no insulation and cold! I intend to put a suspended ceiling in and put the rockwool on top (a la loft insulation). Is a vapour barrier needed between the rockwool and the suspended ceiling?
Many thanks,
Stewart

Yes, but you also need to leave a gap of at least 50mm between the rockwool and the roof for ventilation.

Brad