Insulating a pebbledash solid wall and damp concerns

[Ben Weston]

Hi all,

 

We have an 1870s cottage built with 9 inch solid walls. We're adding an extension off the back which will be built to new cavity spec but we need to get the 3 walls of the original house up to standard.

 

There seems to be so much conflicting advice and 'old wives tales' when it comes to managing damp, solid walls and insulation. The current red brick solid walls are pebbledash rendered on the outside. There are no signs of rising or penetrating damp (though we are looking at an injected DPC as part of our renovation works) and, despite there being no damp issues, I believe the cementitious render won't help the breathability?

 

It's been suggested we use EWI for the outer three walls and marry up with the cavity of the newer extension. This would be complemented with foil-backed plasterboard internally. I'm concerned that applying EWI on top of what we currently have might introduce damp issues by locking that water in the solid wall with nowhere to go.

 

The original plan was to chip the pebbledash off, render in lime so it can breathe and insulate internally instead but I'm open to EWI if it's suitable in this situation.

 

Can anyone help advise?

[SimonD]

Yes, lots of old wives tales here.

 

If you already have a cementitious render, then the walls can and will dry to the inside of the house if they're allowed to, which means both a hygroscopic and vapour permeable buildup to the inside, including paints. This may be easy for you to do, but may not be. If you are to ewi the pebbledash, you'll probably need to first apply a levelling coat before applying the ewi (unless the ewi system manufacturers confirm it's okay to have the air gaps created by the pebbledash plus the ewi adhesive mortar behind the ewi. The advantage with ewi is that the wall is on the warm side and thus interstitial condensation risk moves outwards whereas with iwi, it moves to the interface between the iwi and outer wall. In the iwi instance, you then need to ensure that you either have a vapour control layer and ventilation cavity between the iwi and exterior wall, or use a breathable insulation material like woodfibre.

 

The other option as you say is to remove the pebbledash and apply ewi to the wall. This is probably the best option rather than lime render and iwi. But again that depends on where you are and if you're in an exposed location?

 

With ewi, you could just run that around the whole building, including your new extension, which could make it more seamless.

 

What you'll really need to do is get a condensation analysis done on each of your options.

 

The cementitious render isn't great for breathability of old walls and there's a different in 'strength' and 'flexibility' between the two. But I would also suggest caution in the selection and application of injected dpc if there isn't any sign of damp. Many buildings cope just fine without a dpc. Just have a careful look at the existing dpc if it has one - if it does it will probably be slate, and if not, have a look at the guidelines produce by SPAB and others as they can guide you in alternative material selection, for example using capillary and vapour permeable insulation materials on most of the walls, but instead using only a vapour permeable materials where there is more moisture risk.

Edited January 28, 2022 by SimonD

[SimonD]

ooops double post

Edited January 28, 2022 by SimonD

[Ben Weston]

Thank you for such a detailed response, Simon; hugely appreciated.

 

No existing DPC but also, as I say, no signs of rising damp either. The house sits low in the plot (the internal floors are level with the ground outside) but we are installing French drains around the house to mitigate any potential issues there.

 

I agree that the house has sat for 150 years without damp issues so why interfere with it by adding an injected DPC? Main issue is mortgage lending criteria. Although it's not currently mortgaged (it's not mortgageable!), it will be and almost every lender demands proof of a DPC in older properties. Infuriating as the requirements of the lenders can often be contrary to what's best for the property. That's a separate issue for me to square though.

 

I tested removing a small section of the pebbledash and the bricks behind are very soft – it's highly likely that removing the pebbledash would lead to a much more uneven surface. Probably best to save the labour (not a dealbreaker if required) and apply a levelling coat in that case?

 

It sounds like you're confirming my understanding that EWI may well be the best thing for the building as it keeps the original wall 'warm' and protected. Presumably the walls would still dry to the inside of the house meaning we'd still need a moisture-control layer internally? Battens and plasterboard unlikely to be sufficient?

[nod]

If you install EWI You will need a lightweight pebble or wet dash as apposed to traditional sand and Cement 

[Wil]

59 minutes ago, Ben Weston said:

it will be and almost every lender demands proof of a DPC in older properties.

 

Really? I've not had this with either our last (G2 Listed red brick Georgian with a terrible injected damp course) or current old farmhouse with no visible damp course. Lenders didn't ask any questions about damp courses. More interested in wall and roof construction.

 

Please don't do the injection job- it's terrible. Some old mill buildings don't have damp courses and sit in water all day long! Many references online to not damp proofing by injection.

 

 

Edited January 28, 2022 by Wil

[Ben Weston]

Thanks all.

 

9 minutes ago, Wil said:

 

Really? I've not had this with either our last (G2 Listed red brick Georgian with a terrible injected damp course) or current old farmhouse with no visible damp course. Lenders didn't ask any questions about damp courses. More interested in wall and roof construction.

 

Please don't do the injection job- it's terrible. Some old mill buildings don't have damp courses and sit in water all day long! Many references online to not damp proofing by injection.

 

 

 

Yeah, I agree Will. I don't see the need to disturb something that's been working fine for 150 years. The whole point of my enquiry about EWI is ensuring we don't introduce damp problems so I'm not keen on potentially doing so with an injected DPC if we don't have to. I'll look into it again.

 

31 minutes ago, nod said:

If you install EWI You will need a lightweight pebble or wet dash as apposed to traditional sand and Cement 

 

Excuse my ignorance: what constitutes a 'lightweight pebble dash' as opposed to a cement-based one?!

[nod]

If you look on the Webber Rend site or K Rend site It comes with its own backing and receiver coats 

Much lighter than San and Cement 

Ive learnt from experience that EWI

and sand cement arnt comparable 

[Ben Weston]

Thanks nod, good to know. I'll assume from now on that the pebbledash has to be removed in any eventuality, EWI or otherwise.

 

Does anyone know the advisable route doing IWI instead? Simon mentioned a vapour control layer which makes sense.

 

Presumably we'd render in something like K Rend on the outside so the wall could still breathe, introduce a cavity inside by creating a studwork inner skin and fill with woodfibre with insulated plasterboard on the studwork? Presumably this would reverse the current situation and allow the damp to travel out rather than in?

Edited January 28, 2022 by Ben Weston

[SimonD]

2 hours ago, Ben Weston said:

and almost every lender demands proof of a DPC in older properties

 

as @Wil says, I'm not sure this is a lending requirement.

 

2 hours ago, Ben Weston said:

Presumably the walls would still dry to the inside of the house meaning we'd still need a moisture-control layer internally? Battens and plasterboard unlikely to be sufficient?

 

Air movement is your greatest enemy when it comes to interstitial condensation so an airtightness barrier is your primary concern, which in older properties and masonry walls is typically  either a parge coat or full plaster. Lime plaster is an ideal candidate here You'll also need to look at floor to wall and ceiling to wall junctions and detail those appropriately. Plasterboard and battens would not be sufficient on their own.

 

30 minutes ago, Ben Weston said:

I'll assume from now on that the pebbledash has to be removed in any eventuality, EWI or otherwise.

 

Not necessarily. @nod is referring to the type of render applied to the outside of the EWI, which is typically a thin-coat render of about 8mm. You can apply ewi to the outside of bad existing render, but you may need to level coat so that you don't end up with excessive voids and thus air movement behind the ewi. This can easily be dealt with by using the correct ewi system.

 

Where you mention soft bricks, that may not be too much of a problem as again, if you did end up with a wall full of voids, this would receive a levelling coat prior to applying the ewi.

 

27 minutes ago, Ben Weston said:

introduce a cavity inside by creating a studwork inner skin and fill with woodfibre with insulated plasterboard on the studwork? Presumably this would reverse the current situation and allow the damp to travel out rather than in?

 

With this suggested buildup you introduce a non-breathable layer on the inside - the insulated plasterboard. If you then fill the cavity with woodfibre, once moisture gets into this area, it's likely to condense against the cold wall, which can lead to problems. You need to either have a breathable buildup internally, like woodfibre/hemp etc. which is then lime plastered, or use a non breathable buildup which has a fully ventilated cavity (top and bottom vents) between the insulation and the wall - it used to be the case that non-breathable iwi was place against the wall without ventilation but this is no longer recommended practise.

 

With iwi you also need to carefully consider cold bridging at all internal walls, ceiling and floor junction, as well as the loss of space to your rooms. With ewi this is less of a problem.

 

At your 'dpc' level, you can overcome damp concerns by using, for instance, eps or xps ewi boards for the first 30-50 cm up the wall followed by a small break and then use woodfibre ewi above there. You can even run this insulation down into the ground and if you have solid floors, the depth of this (e.g. if it extends ca. 60cm below ground) can actually provide you with a small amount of heat mass advantage in the soil below the house.

 

Either way you do it, you really do need to talk to the technical department of a system manufacturer or distributor and get a condensation analysis done. That way you can make an fully informed decision on which system you're going to use and whether you need to remove the existing pebbledash.

[Ben Weston]

Thanks again for such a detailed response, Simon.

 

All the floors of the house are coming out (and new roof) so these three walls can be dealt with more easily than if we we were working around a finished house we're living in, if that has any bearing. In other words, we could minimise/eliminate any cold bridging but appreciate still less of a problem with EWI.

 

The house is being rendered anyway so the additional rendering costs of EWI are negligible. We are having all new windows and doors so the headache of that is irrelevant too. From what I can tell, the materials for EWI are relatively inexpensive – it's mostly a labour-intensive job? Unless I've missed something, I think EWI could end up being cheaper/the same cost as IWI but more effective and potentially at no more cost. Which doesn't square with everything I've read saying how much more expensive it is than IWI. Can only assume it's because of the other factors I mention which are already factored into the budget for us, or irrelevant.

 

 

Edited January 28, 2022 by Ben Weston

[Mr Punter]

With EWI you need scaffold and have to consider how to insulate window and door reveals and cills.  Downpipes need moving and roof overhangs need to be adequate.  If it is a detached property and not in a conservation area it would be my choice.

 

EPS seems popular  You could look at Rockwool to take up the unevenness of the pebbledash but it will need to be thicker and more expensive for the same performance. 

[Roger440]

Id suggest drilling a hole in a brick close to the floor. If it really comes out as dry dust. Great. But i doubt it.

 

That said, if you have no damp, even with the render, essentially the whole thing is at an equilibrium. If you change things, you may mess that up.

 

You dont say whats on the interior walls now, nor what the floors are that you say you are going to remove. Timber or solid?

 

As per other posts, DO NOT inject a DPC. It wont work, cant work, but again. may upset the balance of things.

 

For wall insulation systems, you do a lot worse than look here. Crap menu but its all there. https://www.lime.org.uk/

 

My walls were damp, properly damp. Enough to rot out the socket back boxes!

 

Plaster removed, concrete floor removed. 9 months later, totally dry. Drill into a brick, just dust. Staggering the speed its sorted itself out. Just put a limecrete fllor down. Lime plaster to follow.

 

As simon said, no plaster board! Its a moisture proof board. If you fit breathable products, then it has to be every layer, otherwise you are wasting your time. And money. And dont believe the bull on products that claim they are breathable. Ask for the actual data. They wont be compared to the likes of lime.

 

 

[Ben Weston]

Thanks both.

 

All floors are being dug up and relaid with underfloor heating and to current spec, obviously. First floor coming out and going up so that'll be new. We'll already have scaffolding. New roofs, doors and windows so no issues there. Pretty much only these three external walls are staying in the whole renovation (for a number of reasons I don't want to revisit). So the added complexities of EWI don't really apply here.

 

So if we use EWI, use a breathable render straight on to the internal brickwork? Such as lime?

Edited January 28, 2022 by Ben Weston

[Roger440]

9 minutes ago, Ben Weston said:

Thanks both.

 

All floors are being dug up and relaid with underfloor heating and to current spec, obviously. First floor coming out and going up so that'll be new. We'll already have scaffolding. New roofs, doors and windows so no issues there. Pretty much only these three external walls are staying in the whole renovation (for a number of reasons I don't want to revisit). So the added complexities of EWI don't really apply here.

 

So if we use EWI, use a breathable render straight on to the internal brickwork? Such as lime?

 

I would strongly recommend it.

 

But whats the ground floor made of?

[Ben Weston]

36 minutes ago, Roger440 said:

 

I would strongly recommend it.

 

But whats the ground floor made of?

 

It can be made of whatever we want! It's being dug up and started again. It'll have wet underfloor heating if that makes a difference but the construction just has to pass Building Control.

[Roger440]

43 minutes ago, Ben Weston said:

 

It can be made of whatever we want! It's being dug up and started again. It'll have wet underfloor heating if that makes a difference but the construction just has to pass Building Control.

 

It was clear to me in our house the concrete floor with plastic underneath was at least part of the damp issue. Moisture is effectively forced outwards, until it finds the wall.

 

So we used a system like this in ours to avoid any repeat issues.https://www.lime.org.uk/products/sublimer-insulated-limecrete-floor.html  Was it absolutely necessary? Cant be sure. But had we gone the modern route, then had damp again, bit late at that point!

 

This effectively returns it to how it was built. And, in theory, isnt likely to cause any issues, bearing in mind that for several months until 2 weeks ago, it was bare earth. And completely dry. However moisture is still rising. Leave a piece of cardboard for week and it will start to form mould where it touching the soil. Humidity even with no floor is 60%. 

[Ben Weston]

This is really interesting, Roger, and makes sense. Will look into it, thank you. I instinctively prefer natural materials and have always liked lime render. Wasn't aware of limecrete until now.

[Roger440]

8 minutes ago, Ben Weston said:

This is really interesting, Roger, and makes sense. Will look into it, thank you. I instinctively prefer natural materials and have always liked lime render. Wasn't aware of limecrete until now.

 

The good thing with the system is the insulating layer is also the sub base. Total depth is 225,,

 

Piccy of freshly installed floor.

[Ben Weston]

Looks great!

 

Could I ask how the pricing compares to a more traditional PIR + concrete slab floor?

[Roger440]

21 minutes ago, Ben Weston said:

Looks great!

 

Could I ask how the pricing compares to a more traditional PIR + concrete slab floor?

 

No idea! Didnt worry about it as using PIR and concrete was not ever a consideration.

 

But expensive i think is the right answer.

 

I prepped it, they put the glass and membranes down, i fitted the heating pipes, then they came back and laid the limecrete. With hindsight i could easily have done the glass and membranes too.

[Ben Weston]

Evening all,

 

I spoke to an architectural technician today who specialises in 'historic' buildings. He advised the Ty-Mawr EWI systems. He advised that the entire wall must be constructed from breathable materials. Fundamentally, he was saying to remove the cement render and, externally to internally: render in lime, woodfibre board, solid wall, lime plaster. Basically, this system: https://www.lime.org.uk/applications/retrofit-insulation-systems-for-old-buildings/external-wall-insulation-system/expanded-cork-insulation-system.html

 

He thought a land drain around the building might be a good idea so long as it didn't dry out the clay the house is sitting on too much and lead to subsidence. The reason for us considering this is that the cottage sits in a 'dip', the internal and external floor levels are the same, and it's been known to flood in the street.

 

This looks like it would get us to around 0.30 W/m2k, which isn't ideal if we wish to go for a heat pump (even though this only applies to 30% of the house - the rest will be new and up to current spec), but there may not be much more we can do about that? 

 

Any thoughts on this?

Edited February 1, 2022 by Ben Weston

[SimonD]

15 minutes ago, Ben Weston said:

Any thoughts on this?

 

Yes, a lot of words of caution here. The buildup you have linked to uses Secil Reabilita CAL AC as the render on Schneider woodfibre boards. However, if you look at the technical data sheet for the Reabilita, it is a finishing render for SecilTek's Cork EWI system. There is no mention of suitability for use on a woodfibre board. If you go to the Scheider woodfibre page and download their European Technical Assessment document for alternative renders on their woodfibre boards installed on masonry substrates, Reabilita CAL AC is not present. https://www.schneider-holz.com/en/service/the-plus-in-service-accessories/downloads/download/eta-15-0731-different-plaster-manufacturers-on-mineral-substrates/ Also, the Reabilita Cal RB listed in the buildup by Ty Mawr is not given to be suitable as a render basecoat on Woodfibre by SecilTek - it's for masonry substrates.

 

My experience has been that Ty Mawr have changed the finishing render they use on this system but other than a registered LABC detail, there doesn't appear to be any technical assessment as to the suitability of the buildup.

 

You will also find that there is scant information on the whole ewi buildup by Ty Mawr; missing are, for example, reveal details, corner beads, stop beads, mesh detailing around window and door openings etc. If you go to Schneider's website, the technical details can be found for their render system, but given that it is totally different from the one Ty Mawr are proposing, it is unlikely to be in line with Reabilita CAL AC/Cal RB. https://www.schneider-holz.com/en/service/the-plus-in-service-accessories/downloads/download/installation-guidelines-etics/ . As this is going to be a thin-coat render system, there is no way you want to be installing it without all these requisite details and reinforcements. In addition, the layers given by Ty Mawr for the Reabilita renders, take it to 15mm, which is not suitable for woodfibre, which uses lighter weight thin coater render systems at approx 8mm.

 

The only pure lime render that I'm aware of that is technically developed specifically for woodfibre EWI is by Lime Green - https://www.lime-green.co.uk/warmshell/warmshell-exterior

 

I could of course be wrong here and the best way to test it is by asking Ty Mawr for the technical assessment and approval for the use of Reabilita CAL AC/ Cal RB on woodfibre and for their documentation on the system buildup detailing and specific installation instructions. Also, is there any warranty with their system?

 

What thickness ewi is your AT suggesting?

[Ben Weston]

Thanks yet again, Simon. 

 

This is very thorough and you've certainly looked into it in more depth than I have yet. To be honest, I hadn't really considered that their system wouldn't necessarily meet spec if it's registered with LABC.

 

If we leave their system aside for a second, is the principle of lime render > insulation > solid wall > lime plaster correct? It strikes me that there's no 'vapour control' layer though perhaps the insulation itself performs this function? What's stopping driving rain working its way all the way through to the lime plaster?

 

The AT I spoke to was referred and it was a general half hour non-chargeable chat. My own architect hasn't worked with EWI before: he was the one that suggested whacking EWI on top of the cement render which struck me as incorrect, hence the ongoing research.

 

Am I making a mountain out of a molehill here? It seems to be so difficult to get consensus on this but that may say more about the people I've currently spoken to about it.

 

Essentially, I have three solid walls and want to insulate them to the best possible standard/U-values without introducing, and preferably improving, moisture control.

Edited February 1, 2022 by Ben Weston

[SimonD]

2 hours ago, Ben Weston said:

If we leave their system aside for a second, is the principle of lime render > insulation > solid wall > lime plaster correct?

 

You're on the right track, but think modified lime render on the outside. Generally speaking it's mostly lime/cement with additives or the Lime Green render which is lime modified with various polymers.

2 hours ago, Ben Weston said:

It strikes me that there's no 'vapour control' layer though perhaps the insulation itself performs this function?

 

Your internal plaster performs this role together with providing airtightness.

 

2 hours ago, Ben Weston said:

What's stopping driving rain working its way all the way through to the lime plaster?

 

Driving rain in exposed areas is potentially an issue with EWI and needs to be considered carefully. If you are in an exposed location, then probably better to look at: lime render > render carrier board > battens > cavity > insulation > wall > plaster. Your render carrier boards for lime would be something like Celenit wood-wool exterior boards.

 

In other areas, then the modified lime render - across all its coats - contains resistant agents to prevent excessive moisture ingress. Final coating can be a silicate paint, silicon paint, or a silicon/mineral finishing coat. Depends on the system in question.

 

2 hours ago, Ben Weston said:

Am I making a mountain out of a molehill here?

 

Not at all, you're asking all the right questions and doing your due diligence. Totally understandable IMHO.

 

2 hours ago, Ben Weston said:

It seems to be so difficult to get consensus on this but that may say more about the people I've currently spoken to about it.

 

Yes, it is. Once you get into the realms of retrofit/conservation and the subject of lime, you can get caught in the shackles of lime purists as well as get loads of different opinions coming at you from all directions.

 

In terms of EWI the most common options are EPS and Mineral Wool. Woodfibre is still fairly niche. Then there is cork. Both EPS and Mineral wool are vapour permeable so with the right system, you would still have a 'breathable' wall, so I wouldn't totally discount them yet. It all depends on your budget and goals. Worth finding some people to speak to on this as all the major companies are selling EPS and Mineral wool systems, for example Weber, K-Rend, Baumit, EWI Pro etc.

 

If you are set on the natural insulation route, I'd suggest you speak to those who are supplying complete systems properly tested by the manufacturers first. Some options for companies to speak to are Back to Earth, Mike Wye, Lime Green (Actual Manufacturer), Ecological Building Systems (EWI Pro does have some woodfibre systems but not for solid masonry retrofit). I haven't included Ty Mawr for obvious reasons as I think their EWI system is a bit experimental, although I believe they would like to call it innovative ?

 

I'm sure you can find others in the business but hopefully this helps you on your way.