They are two separate animals, different and not to be combined (or
confused).
Old buildings use soft fired brick and lime mortars and were designed
to breathe moisture, relying on natural laws of evaporation.
Dampness in old buildings is normal, and, except in cases where there
may be roof or drainage problems, the dampness would come and go,
without mold developing, since the inherent nature of the building
followed the natural law. The wall, as an assembly, was designed to
breath.
New buildings use high-temp fired brick and cement mortars and are
not designed to breathe, rather to isolate moisture. Outside. They
also contain vapor barriers behind the brick to prevent mitigation of
moisture to the building inside. And, they contain vapor barriers on
the inside to prevent mitigation of interior moisture into the
wall/insulation. When they leak there can be significant health
risks. They are designed for watertightness, and when water is able
to penetrate, they are designed to be self-draining.
Go downtown to find an old brick building and look around on the
joints and you will probably see where someone attempted to mix the
two systems, using a newer masonry cement mortar to patch the old
system. Or they tried to apply a sealant to the old brick face, or
they screwed up the tempered face of the brick by sandblasting. What
you will also see, if not now then soon, will be areas of brick
spalling around the patch. The newer mortars in addition to keeping
water out will also hold water in. The result is a freeze-thaw spall
of the brick face shell. Kerrrrpop !
In my opinion you would not use modern high-strength mortars, usually
raked and tooled for watertightness, with materials like papercrete
which breath more like an old brick. At least not without some
modification to the mix - make the papercrete waterproof, or add a
lot of lime to the modern mortar to make it breathe. The structural
qualities and physical performance of the two should be similar as
they perform together as an assembly and should function as a team.
Either you design the wall to breath as an assembly, or you design it
for total watertightness. There is really no inbetween.
The other component at play here is old lime vs new lime. Old limes
were not pure and developed hydraulic strength from a host of
impurities. New limes mined today are more pure and require a
pozzolan, fly ash or portland to develop the hydraulic/adhesion.
So the correct answer on which to use, is...well, it all depends.
-Duane
--- In papercreters@yahoogroups.com, "Neal Chabot" <sire@...> wrote:
>
> This post seems at the very least to be one-sided. All the points
seem to come from a UK strawbale site which quotes only two books as
its sources. One of the books is only 32 pages long, out of print,
and was published by "Black Dog Press" (whatever that is or was).
The other book "Building in Lime" has only one review at Amazon and
is published by "Practical Action". At the very least this should be
balanced with articles from Britannica.com and wikipedia.com.
>
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