Internal wall insulation 600mm masonry wall - condensation risk

[Hastings]

@Mr Punter

3 hours ago, Mr Punter said:

 

A VCL is impermeable to vapour and water and can be fitted to the inside (warm side) or the insulation.

and sometimes fitted to the cold side, depending on the system design?

Edited March 20, 2019 by Hastings

[Miek]

45 minutes ago, Hastings said:

@Mr Punter

and sometimes fitted to the cold side, depending on the system design?

Not usual for a heating dominated climate like the UK. In a cooling dominated climate you might want an external VCL  to prevent external moisture in the hot humid environment condensing within your walls. I can't see the reason for it in the UK. Happy to be corrected though. 

[A_L]

6 hours ago, Hastings said:

I will be using a vapour barrier NOT a breather membrane, to stop moisture travelling from outside into the insulation.

 

We have to distinguish between liquid water (moisture?) and water vapour. If liquid water is getting to the vapour barrier/breather membrane layer from across the cavity this needs to be sorted by fixing the wall away from the water vapour/breather membrane layer.

 

Correctly chosen breather membranes have considerable resistance to liquid water penetrating them. They are 'non-tenting' (tenting is where touching the inside of a tent allows liquid water to come through the tent from the outside).

 

In substantially warmer climates than ours in summer it is possible for external water vapour to condense within the wall structure, this is known as interstitial condensation. This is highly unlikely to occur, particularly given the breathable nature of your insulation/lime plaster. In our climate the problem is likely to be interstitial condensation occurring from water vapour moving from inside the building during the heating season. Your breathable construction allows the water vapour to transfer through to the vapour barrier where due to the cold environment it is likely to condense to liquid water. I strongly recommend that you get a condensation risk analysis done for your proposed wall build up, perhaps the insulation manufacturer will do one for free.

 

6 hours ago, Hastings said:

but if using breathable insulation combined with MVHR then VCL on the cold side is the way to go.

 

do you not think using a non-breathable layer (i.e. vapour barrier) in an otherwise breathable build-up odd?

[ADLIan]

A VCL always goes on the warm side of the insulation - in the UK this is internal to the insulation. Some studies of upgrading existing masonry walls suggest the use of a totally breathable insulation system (no VCL and careful choice of insulation) to allow the wall structure to dry to both the external and internal environment. See Society for the Protection of Ancient Buildings website for more info.

[Hastings]

13 hours ago, A_L said:

 

do you not think using a non-breathable layer (i.e. vapour barrier) in an otherwise breathable build-up odd?

 

 

You are right - my mistake - I just talked to the Pava distributor and the VCL is not a total barrier but a breather type, (sounds very similar to roofing membrane) which makes sense. They also confirmed it goes on the outside (cold side) of the insulation.

[jake]

5 hours ago, Hastings said:

 

You are right - my mistake - I just talked to the Pava distributor and the VCL is not a total barrier but a breather type, (sounds very similar to roofing membrane) which makes sense. They also confirmed it goes on the outside (cold side) of the insulation.

what is the product you mention here??can you add a link please?

 

[Hastings]

@jake Pavatex is the system. Natural Building Technologies is the UK distributor.

[jake]

ok thanks

[paulmb]

I have a very similar challenge to several if the posters above, to covert a small 1930’s church with solid brick walls to a residential dwelling. Located on the Isle of Skye externally rendered with a cement based wet dash (ie non permeable - except when cracked and blown!). 

 

Like the OP I’ve read widely and found much sometimes contradictory opinions from ‘experts’. One approach suggested by a local architect that isn’t so widely discussed is to use a tanking membrane fixed mechanically to the wall an fix rigid insulation over it, with battens over (warm battened). Ventilation and/or drainage is needed behind the membrane which is sat off the wall by the dimples in a the membrane, creating a slim (7-8mm) gap. The insulation is butt jointed and taped and cold bridging is minimised.  That’s the claim. I spoke to one joiner who had used this approach locally without the battens and sounded like it was tricky as he said he’d only do it again with battens.

 

Has anyone else had experience of this approach? I spoke about it to the tanking membrane supplier who indicated that they would suggest now using rigid insulation but a Rockwool instead to help the the cavity breath, but think that suggestion was to place the batts between battens fixed directly to the membrane (cold battened)

 

still wrestling with with which way to go - but am leaning towards metal battens fixed direct to the wall and insulated plasterboard over, with vents top and bottom to allow moisture from below and the wall to escape.

[jfb]

If it has a cement render why can you not use EWI over the whole building? Would have a similar finish to how it is now, much better for long-term condition of the wall and if the existing render is mostly ok you can just apply the EWI straight in to it.

Edited March 27, 2019 by jfb

[paulmb]

3 hours ago, jfb said:

If it has a cement render why can you not use EWI over the whole building? 

I agree that EWI could result in the best solution if one is willing to make changes to the roof so that it overhangs the top of the EWI. I appreciate there are alternatives but don’t feel confident in them. Also there aren’t EWI specialists locally so bringing them from afar adds to cost and project management complications.

 

Another contraindication against EWI is the lack of a DPC. The thinking being that moisture rising in the wall needs to escape somewhere and that a ventilated internal cavity will provide that

Edited March 27, 2019 by paulmb