Lime mortar + sheeps wool do I need a VCL?

[ajay16]

I'm about to insulate a room in a circa 150 year old outbuilding, lime mortar and brick construction, walls are about 30cm thick. The size of this part of the building is 10m sq, nothing huge.

I'll be using Celenit acoustic wood fibre boards on warm battens as the finished walling to keep it breathable, and sheeps wool insulation behind that.

Do I need a hybrid type VCL membrane sitting behind the Celenit boards before the sheeps wool?

I wouldn't have thought so as the whole system will be breathable anyway, with low moisture levels in the room, but any thoughts?

The room is going to be used as a workshop/studio/office.
Two small UPVC windows are going in with trickle vents.
Wooden main door with half height single glazing.
Ceramic electric radiator for heat.
DPM is already in the concrete floor (put down before my ownership)
Floor will be XPS, hardboard, carpet tiles.

So the build up of the wall would be:

*Outside*

-External lime mortar/brick wall
-Sheeps wool insulation
-Celenit wood wool boards

*Inside*

 

[fernicarry]

I don't think you need it since the whole structure is vapour open. Making it vapour closed. even part time, would seem to defeat that advantage.

I've expressed this opinion before that the use of VCL's in renovations can be pretty sketchy. You won't get the continuous envelope and the detailing you need to make them work properly as they should (might) in new builds. Any defect or a bit that was too tricky to do properly becomes a conduit to funnel warm, damp air into the building fabric.

I also wonder whether or not those hybrid/intelligent membranes actually balance out over the course of 4 seasons. I think it would depend very much on your situation. If you spend a lot of the year waterlogged like we do then the membrane is going to spend more time closed than open so you will get a build up of dampness. Really not for me, but others might have different experiences or opinions.

If you are looking for something with a bit of give for the layer against the brick then look at Indinature products. They perform as well as sheep's wool but are quite a bit less spendy. No connection other than I use a lot of it.

 

[ajay16]

Cheers for the input!

I know these hybrid membranes are meant to slow down the transfer of moisture etc but as you say, trying to create something so airtight in such an old building seems counterintuitive (and tricky), even if it is 'breathable'.

An external wall in a bedroom had a melamine built in wardrobe right on the brick, so I took that out and went with sheeps wool directly against the brick then the Celenit boards. So I think i'll do the same again.

 

[Kearn]

Definitely no membrane required. Your materials are hydroscopic, they can do their thing if they get wet from vapour or liquid…