Hi,

I've got a 1925 solid walled semi by a canal (so pretty humid). After much deliberation, we decided to go for breathable Ewi - and to take off the cement based render and internal gypsum plaster.

However, a (thoughtful) builder has pointed out to me that not only do we have sand and cement mortar, which I knew, but the bricks are apparently london brick company flettons - or commons...but definitely very hard and arguably fairly impermeable.

Said builder noted it was worth checking out whether there was much point using more breathable (& more expensive and less insulating) insulation (probably wood fibre), as well as lime based render, hacking off plaster, replastering with lime plaster etc when we have relatively impermeable bricks and mortar.

So, this is me checking...are lbc commons/flettons sufficiently impermeable to negate the advantages of breathable Ewi? It might be worth noting that we're planning on replacing wood suspended floors with insulated concrete slab, with have a slate dpc that's been bridged with render and there's still. I sign of rising damp, we do have mouldy issues ATM but hope to achieve sufficient airtightness for mhrv to be effective.

Really appreciate any advice/comments/suggestions or experiences,

Thanks,

Tania

AFAIK engineering bricks are about the only bricks that are vapour closed (don't take up water) I would suggest that any brick that can get damp (internally) would allow the passage of water vapour.

IMO the EWI to go for would be the standard EPS of your chosen thickness with the standard reinforcing mesh and thin film acrylic render to a colour of your (or the planners) choosing. No point in going for the more expensive graphite EPS unless you really do want the extra 'u' value and you are stuck for thickness (Boundary line, eves /sofit overhang etc.).

I probably would leave the existing render on - unless it is falling off - but I would sort out the DPC bridging by the render (but EPS is not a problem bridging the DPC)

The vast majority of houses in the UK that are rendered out and plastered in have a cement based render and gypsum plaster without problems so I would be cautious about citing these as the demon of the problem without real evidence that this is the case.

Mould is often a problem of poor ventilation / air movement and cold walls. EWI will help as will powered ventilation (either MVHR or PIV)

How close is the canal and what is the ground water level?

When you do the EWI I would suggest that you carry the EWI down to about 500mm below ground level if possible or down to just above the base of the foundations (budget dependent - but don't go past the bottom of the foundations) This will get rid of the cold bridge at the base of the wall and if you put in a french drain instead of backfilling then this will help lower the water level around the house and help alleviate any issues there may be with the slate DPC

Thanks all – I knew you'd have some useful comments.

Canal is something like 20 foot from the house? I am not sure how high the water table is, but we are on made up lands on an bank meant and I have never noticed it being a problem when digging various holes in the garden ;-)

Yes, we don't have details for Ewi yet, but I would aspire to going below ground level and the French drain is a good idea.

I agree that it would not be sensible to blame cement render etc for time – I don't, but if we do go round the more breathable Ewi Route, my understanding is that cement render et cetera is relatively impermeable and therefore rather negates more breathable AWI – by which I mean woodfibre tight. We have spent far too long organising over more breathable / less breathable and I thought we actually made a decision, but the issue of the bricks slightly wrong footed me on this one. To be honest, I am still not utterly convinced that more breathable Ewi Will be sufficiently better to justify the worse u values and the increased cost.

On the bright side, one builder seemed very Sanguine about extending the eaves of the house and as we only have about 10 cm under the soffits, it will be great to put on a thicker layer of insulation..

So, no one here a great fan of woodfibre Ewi?

Thanks,

Tania

I am a great fan of woodfibre (though I mainly use it for IWI), and have helped on a couple of WF EWI jobs - one with ('solid') Diffutherm and one with Udireco, which is 40mm (I think) Diffutherm with flexible woodfibre attached to the back. It is splendid stuff to use, though it takes a little getting used to. It is available in a variety of thicknesses.

The implications of 'getting it wrong' with WF (in terms of waterproof detailing) are more immediately 'mushy', I guess, but our detailing on every sort of EWI should prevent ingress and therefore, if it is 'done tight' it should offer no more in the way of risks than any other EWI material.

On the Udireco job I helped with, the (180 deep) plinth layer was platinum EPS, and extended below ground. It came up to about 300mm above external ground level, which I think is a general recommendation from WF providers.