best substrate for lime render

ndon · Post by **ndon** » 08 Mar 2009, 22:13

thought that i would seek the advise of the assembled gurus...

I am embarking on externally insulating my house with 100mm of kingspan, (or other rigid board) then a 25mm cavity, felt and then the crunch, what would be the best form of boarding to use to allow for lime rendering onto?

all your thoughts would be appreciated, I am keeping my ideas to myself in order to try and coax up all the ideas from you.

Thank you.

ps ken, I hope you may be able to help...

contadino · Post by **contadino** » 08 Mar 2009, 22:37

I've only rendered onto tufa (sandstone) or cement blocks. The thing you have to be careful about is that lime render isn't waterproof, so if you render onto something and it's external. it'll 'blow' when you get a hard frost.

The only solutions I know of are either to mix a little cement in with the lime (which kinda defeats the object really) or seal it with nasty chemicals.

Post by **kenneal - lagger** » 09 Mar 2009, 03:06

Hi! Brandon

How are things going? Finished the thesis yet, or even started it? I've got to get mine in for July so I'll have to stay away from here for a while.

I've used Expamet Riblath on a job or the alternative is to render direct onto some of the German fibre boards.

Heraklith Tektalan-E-21 in thickness up to 125 instead of Kingspan but accepts render direct.

Pavatex-Unger-Diffutherm is another board that can be direct plastered/rendered, boards up to 100 thick and 0.040 Wm-1K-1

This page from Calch Ty-Mawr is useful http://www.lime.org.uk/products/p2490.asp

Another alternative that I am trying with an earth render is synthetic carpet (non foam backed) screwed with ss screws onto oak pegs at about 600 crs. The carpet is a problem in the waste stream as it doesn't rot so to find a use for old stuff would be good. It would have to go onto a solid backing, I would have thought.

Hope that helps.

Ken

emordnilap · Post by **emordnilap** » 09 Mar 2009, 16:46

Interesting.

What's your preferred solution to contadino's point about a hard frost?

Post by **kenneal - lagger** » 09 Mar 2009, 18:03

Lime will only blow in a hard frost if it's not sufficiently calcified when frosted. I suspect that the problem Contadino has is with the substrate. If the substrate is weaker than the lime render and is damp that will blow. Lime is virtually water proof but not vapour proof so it will breath and allow any moisture out.

A cement render cracks and allows water in through the cracks. This then gets frosted and blows. Lime does crack but not to the same extent as cement and the cracks tend to be smaller. Lime also self heals - the lime is attacked by the acid in rain which washes some of it down the face of the wall. This lime solution gets blown into the tiny cracks where the lime precipitates out of solution and eventually seals the crack.

biffvernon · Post by **biffvernon** » 09 Mar 2009, 19:23

kenneal wrote: calcified

carbonated
[/pedant]

Post by **kenneal - lagger** » 10 Mar 2009, 00:00

biffvernon wrote:
kenneal wrote: calcified
carbonated
[/pedant]

OK, I was in a hurry and it was the first word that came to mind.

contadino · Post by **contadino** » 10 Mar 2009, 07:08

kenneal wrote:Lime will only blow in a hard frost if it's not sufficiently calcified when frosted. I suspect that the problem Contadino has is with the substrate. If the substrate is weaker than the lime render and is damp that will blow. Lime is virtually water proof but not vapour proof so it will breath and allow any moisture out.

A cement render cracks and allows water in through the cracks. This then gets frosted and blows. Lime does crack but not to the same extent as cement and the cracks tend to be smaller. Lime also self heals - the lime is attacked by the acid in rain which washes some of it down the face of the wall. This lime solution gets blown into the tiny cracks where the lime precipitates out of solution and eventually seals the crack.

I don't think it's an issue of carbonation as this is a common problem, even with old lime. Also, lime is a long way from 'virtually water proof' IME - it soaks up water quite easily.

However, my initial post was just to say that both tufa and cement blocks provide a poor substrate. Other materials that wick the moisture away may provide a perfectly good solution.

Post by **kenneal - lagger** » 11 Mar 2009, 02:16

There is a saying that concrete and steel go well together and earth and lime do because concrete and steel expand with heat while earth and lime contract. It is probably this reason why lime blows off concrete blocks. Could it be the same with Tufa?