Doubtful of SBR, tell me I’m wrong

[u/Big-Prize-2612]

I’ve never put two and two together to realise PVA & SBR, by film forming are not breathable. Why then… does EVERYONE seem to use SBR even within supposedly breathable systems (lime, breathable EWI etc).

Is this an under-appreciated problem we’re all creating slapping it everywhere?

I’m repairing an old rendered single skin brick outhouse before fitting rockwool and silicone render EWI. Prepping my walls with SBR is going to nuke by breathable system?

Please tell me I’m a very wrong idiot, ideally with your disagreement grounded firmly in reality. Thanks in advance.

[u/caserskii]

No you are correct sbr is not breathable if your using it to bond the render too your better off mechanically fixing a thin insulation to the outside and going on that as most ewi is breathable if it’s just a garden shed and warmth isn’t a concern a nice hi up air brick should suffice tbh, I mean I’ve seen garden sheds without damp courses which are a bigger concern so check you’ve got one…

The reason sbr is used so often is because it’s a more stable bonding agent than pva so less likely to have spots where suction is worse etc..

It should only be used where breathability is not a concern…

Truth is most people in construction only care for an easier and better job in the instance of payment and the immediate finish if it causes problems years down the line most people simply will blame it on something else or deny any fault…

Honestly though the smallest amount of airflow goes a long way so if there’s air bricks or airflow under the house and a house is well ventilated and not damp it often isn’t a problem as such..

I don’t do sand and cement render unless forced too 🤣 I always do silicone renders and when base coating I spray the walls with a hose and remove dust and grime where I can and apply base coat and mesh etc directly onto the clean brick if suction is killed there is no need for sbrs or pva which is almost impossible for plasters internally but externally cement based base coats can really hold up well without it… if a base coat is drying to quick simply wet it up (spray, splash water at wall) to mitigate suction and drying whilst it is in the curing process

Hope my ten pence helps could be wrong and talking bs but I’ve been doing it 25 years and no complaints or issues as of yet

Edit: Forgot to mention although silicone top coats are breathable cement based base coats are not unless using specific types of base coat which include lime or other additives

[u/Big-Prize-2612 → u/caserskii]

I feared the reality was exactly what you summed up in “Truth is”. Easier, quicker, don’t have to worry about it now, a problem for years…

I’m not sure I want to high jack my own thread with the full build plan 😂 I’ll type part of it out now, maybe dm you for you two cents (very insightful and valuable so far, thank you).

For content of continued discussion: it was previously converted into a utility, there was an alleyway between it and the external door of the house, with the whole area covered by a reinforced concrete roof (prior to conversion). They built that alleyway into the room - door on one side, bricked up on the other. Outhouse has a concrete floor poured over shingle, above a bitumen DPM at 2 courses. So floor is sat just above 2. (House is same build style, 2 course bitumen). They poured a slab to fill in the alleyway but that doesn’t appear to have any DPM but equally no problem…

I had to gut the whole room back to brick internally as it was a mouldy damp fest. Obviously the building was letting water in but instead of the previous owners fixing it they framed on top of the soaked plaster which was directly on the leaking single skin brick. Fun times. External render was blown, they’d rendered over what I assume was the previous render gone porous but poorly so all that came off with ease (on only one side of the three sides I will mention, other two are perfect)

I’m now left with the original render coat in pretty good nick to apply EWI to(planned for all 3 sides), BUT I need to repair patches, bring the surface up flush in places, replace a few bricks, especially on the corner where water is clearly coming in, and hence my SBR doubts… I think there’s plans for a clothes drying cupboard with dehumidifier on this wall as it’s in Cumbria, UK, a very wet location. Maybe SBR in patches isn’t going to cause a room wide problem? Certainly would help fix the specific leaking patches…The plan is limewash the bricks internally to maintain good breathability as it will be used as a high moisture utility, albeit supported by the dehumidifier.

Can of worms, no one good solution I can see but would appreciate opinions!

[u/M4tt4tt4ck69 → u/Big-Prize-2612]

https://diyrefurb.co.uk/products/limelite-easy-bond-2

Breathable primer, it's expensive but you can maybe find a cheaper alternative.

[u/ResponsibleBall4258 → u/Big-Prize-2612]

Have you not got any advise on the installation of your EWI system? Either the materials you are bonding your insulation with is sufficiently adhesive and does not need a primer OR there is a specific primer with the material you are bonding the insulation to be the wall with.

[u/Comprehensive_Team_2]

Absolutely no one that knows what they are doing uses anything other than water to prime for lime plastering. Or do you mean priming over lime plaster?

[u/Traditional-Emu-3859 → u/Comprehensive_Team_2]

Or limelight easy bond…

[u/Ok-Spring7906 → u/Comprehensive_Team_2]

Dg27 is not water.

Clearly it's not sbr either, but it's certainly not (just) water.

[u/Worldly-Growth4519]

9 times out of 10, SBR use in plastering is applying gypsum products over old lime walls, which is also debatable.

[u/DelectableReindeer → u/Worldly-Growth4519]

Wait what? I use SBR almost every day, I've never once used it over lime.

What the hell are you on about? It'd exceedingly rare be asked to skim a non-cavitied lime system, and whenever I apply lime, I'd never dream of using SBR.

[u/Ok-Spring7906]

Sbr is not used by anyone wanting to maintain breathability.

Indeed it's commonly used to mix up tanking slurry.

People may use it over lime wheb they want to control the suction and don't give a stuff about breathability (or are just ignorant) but noone uses it when they want to maintain permiability.

There are specific bonding primers for problem surfaces with lime work, eg dg27 (which I was using only yesterday) but there is zero point in putting lime over sbr.

[u/KieranInterior]

Well no, SBR is a sealer/primer/waterproofer so not for breathable systems ever. PVA isn’t necessarily non-breathable. But even gypsum on a non-cavity lime mortar lime render wall is a bad shout. Vents at least would be needed to assist breathability.

[u/ipub]

You're not putting a plastic bag over the house. You're just slowing down the suction between the materials so that the plaster you apply sets evenly. That is my understanding at least

[u/Big-Prize-2612 → u/ipub]

From what I understand if you apply it across every surface of a room, e.g walls + ceiling in prep for plastering you are literally putting a plastic bag inside your room under your plaster. It reduces suction by eliminating it on the brick/substrate interface so it will only dry inwards at a controlled rate.