I have an older home with a stone and mortar foundation.  It appears to be limestone, but I am not a geologist.  I purchased the house about 18 months ago.  My plan is to reparge the entire interior wall surface.

The foundation appears to be in decent shape – no loose stones or cracks – but the parging is all over the map – it has probably been done 5 times over the past 98 years.  The wall/floor joint is the only area where moisture is visible after a rainy period – never any standing water though, but effervescence is present in a few areas.

After doing a considerable amount of research I planto use either a type N or a homemade type O (no one seems to sell a premade one) by adding 1 part type S hyrdrated lime by volume to the off the shelf Type N cement mortar and of course sand. Since I am working on the inside on a natural stone I am not concerned with excessive compressive strength, and since the existing mortar is in relatively good shape (i.e., not a repointing job), will not be exposed to the elements or any freeze-thaw action, my primary concern is vapour permeance and then durability.

Will a type N (1:1:6) or type 0 (1:2:9) (c:l:s) mortar have enough permeance to avoid the often discussed problem of trapping water in the wall when the original mortar appears to be a pre-portland mix (i.e., lime and sand only).

The other side of the wall is (unseen) but likely not dampproofed.

Thanks in advance for any help.

I have talked to a couple masons on different sites and they look at me like I have two heads when I mention lime mortar; both have recommended type s with an admixture to increase adhesion. Is it possible that given the sheltered relatively good shape, that something more durable like a portland sand mortar would be good enough?

sorry for all the long post