the subjects says it all.
I want to plaster a natural stone outside wall because it is not beautiful and as it is a very old house (200 years) the rain washes out the old lime/sand mix. I thought of mixing in vermiculite to make a insulating plaster .. But I can't find info on it so I'm very much in doubt if this is a good idea. Is or isn't it ?
Thanks in advance
I agree with Peter. Vermiculite loosely filled has an r value of about 2. Mixed with lime/sand and applied in a pretty thick layer, you are not looking at a great increase in insulation. You are better off making a strong lime plaster that won't need to be reapplied anytime soon.
Offering my .02 cents if I may. If the rain is in fact washing out the old lime/sand mix, then applying ANY plaster is placing a bandage over a bullet wound...your wall will most certainly come crumbling down at some point. If your wall is also 200 yrs old then what you have in your favor is very likely a well built wall with Natural Hydraulic Lime that could certainly be made beautiful (and strong) again simply by repointing...you may just fall in love with it. Might I recommend a peek at www.limeworks.us , or better yet http://www.buildingconservation.com/articles/pointing/lime-pointing.htm