Impeameable render

[FreeD]

Would you agree with this general statement that internally and externally an impermeable render e.g 3 to 1 sand cement with SBR should be used on single skin buildings and on buildings with a cavity a 6 1 1 or similar should be used?

 

[flynnyman]

no i would disagree

 

[Chris W]

no, thats completely missing the point :rolleyes)

 

[irish_spread]

no i would disagree

I would agree to disagree :RpS_cool:

 

[FreeD]

Ok well my view is that on a single skin buildings water is more likely to penetrate through missing pointing etc so an impermeable render will help repel water outside and if it foes get through the internal render will prevent it coming through...the flexibility of using sbr will make it last longer less likely to crack.

On a building with a cavity where the cavity is the barrier to water a more breathable render with lime no waterproofer would be best as it can naturally dry out between exposures.

 

[FreeD]

what point am i missing?

 

[flynnyman]

Ok well my view is that on a single skin buildings water is more likely to penetrate through missing pointing etc so an impermeable render will help repel water outside and if it foes get through the internal render will prevent it coming through...the flexibility of using sbr will make it last longer less likely to crack.

On a building with a cavity where the cavity is the barrier to water a more breathable render with lime no waterproofer would be best as it can naturally dry out between exposures.

I agree

 

[FreeD]

But if possible probably best to use a modern factory render which is designed to be flexible and highly water resistant.

 

[Chris W]

what point am i missing?

strength of render should be dictated by strength of substrate..
theres more to it than simply banging a 3:1 render on the external and expecting it to be waterproof...
its not waterproof, its water resistant and by stopping the wall from breathing, you stop its ability to dry out properly... I'd prefer the 3:1 to be internal to be honest, cos that way any water that does get in will dry out to the external atmosphere and not the internal...
although you can take the whole thing a stage further and say that offering any waterproofing coating to a single skin water permeable wall is a bad idea and is what causes a lot of damp / condensation problems in older buildings...

so technically, what you should do is insulate the wall first either internally or externally utilising a vapour barrier if nessesary internally and make your external coating breathable and flexible...

to be honest i'd prolly just treat it the same as any other s+c render job...

all that said, these new thincoat and mono systems although water resistant do allow to wall the breathe, though to what relevance i have absolutely no idea... when people start stating figures like '98%' waterproof breathable coating its just sales pitch to chuck at the customer as far as im concerned...

to breathe or not to breathe? that is the question eh?

 

[Chris W]

oh, r.e. water getting in via poor pointing...

re-pointing it and giving it a coat of thomsons water seal will have a similar effect...

so whos gonna say 'water seal' is a bad idea externally cos it stops the wall breathing?

just repointing it will probably solve most of the problems...

 

[FreeD]

Yea older single skin buildings e.g Victorian weren't designed to be breathable were they? the bricks were designed to repel water. Render was put on to add further protection and to repel water and be decorative. Stone walls were designed to be breathable?

Thats a good point about insulating I guess you are creating a cavity with the vapour barrier? why do u say if necessary?

 

[Chris W]

depends on how you insulate...
a vapour barrier is designed to stop warm air (which contains moisture) penetrating through to a cold surface where it deposits its water content as condensation, so, if you use a porous insulation (even cellotex is porous where the joints are - think about it) then youll need a vapour barrier internally between the plasterboard and the insulation. That will stop the warm air reaching a cold surface.... gaffer taping the joints with cellotex will do the trick.
think 3:1 render internally on a single skin building. The room in question being a bathroom or utility room. The walls will be dripping with condensation. insulate the walls internally and the warm air remains warm, it doesnt touch a cold surface till it gets to the window. thats when the condensation appears, not on the walls where it tends to destroy plaster, paint and leaves a nasty black mildew.
providing the external bricks arent too soft, pointing the wall should solve any problems.
victorian buildings were built with lime mortar - breathable and flexible. sticking a 3:1 over that may well cause you problems.

 

[FreeD]

Chris surely a decent extractor fan would reduce build up of condensation in a bathroom. As to your comment about putting a 3.1 over a victorian property with lime mortar what sort of problems would you get? if the mix is SBR based I can't see it cracking?

As for insulation I had a brief read up about it recently and came to the conclusion that intenally it is very difficult to insulate in older properties unless the whole wall on every level and in between floor levels is insulated to protect against thermal bridging. For example I read somewhere that if you were to insulate just one external wall in one room it would only make about 20% difference most savings would be lost to bridging above and below. Because of this external insulation is probably best.

 

[Chris W]

the building will move more than the render will..

 

[FreeD]

So what mix with what additives would you use on a single skin brick property with high weather exposure?

 

[Chris W]

i think a better question might be 'what spec' as opposed to what mix...
and in which case i think your better off contacting weber with details of your substrate and offering whatever they say to building control bearing in mind the 25% rule and see what they say...