Stone-wall : vapour-barrier needed ?

[mointainwalker]

One wall ( stone and in-fill) of my barn gives onto a weather-proof annex.

In order to both add space and expose a very nice wall, I intend to insulate ( 200 mm fibre-wool) the exterior face of this wall ( 60 cm thick)

I started off by thinking I would put a vb on the outside face of the wall but then thought that the earth/lime whatever in the wall would certainly absorb any moisture and then re-vent it to the inside as and when.

Are my second thoughts correct ?

 

[jeds]

Personally I think a VP in that position will be serving a useful purpose. The space between the wall and inside of the insulation will be quite stable and, as you say, will happily breath in both directions. But some outward pressure is inevitable and in the right conditions that will allow condensation within the outer insulation layer. That may or may not cause an actual problem but since I can see no reason to not have a VP my feeling would be to fit one and eliminate the posibility altogether.

 

[alittlerespect]

Hi

If not aware, then you need to be aware that without the inclusion of a vapour barrier you are going to have water vapour affect the integrity of the insulation material and a worst case scenario would be that the insulation material becomes so damp that it provides little or no value, and in fact could add rather than reduce the heat loss of the building.

You may wish to consider using a PIR board which has a higher vapour resistance. By a numbers comparison - Vapour Resistivity (MNs/gm) of some common building components:-
Air: <1
Brick: 2.6
Mineral Wool: 5
Polythene Sheet: 120
PIR Insulation: 300
Aluminium Foil: 4000

Regards

 

[mointainwalker]

@ Jeds

Thanks for your comments , John .

@Alittlerespect

I entirely understand the need for a theoretical vb but must ask if you understand the reality of the situation ?

One m3 of random stone wall is 1000 kg +/- and so the same wall at 60 cm thickness is 600 kg. If we assume that 25% of that wall is infill, then that represents 150 kg of earth etc to absorb just the few litres of moistures ( inc baths etc ) generated by the inhabitants.

I don't find your proposition very convincing, can you correct/improve ?