Folks I have a friend who has a 200 yr old home with rather soft and porous bricks finished inside with plaster. Their is no cavity to add insulation and I am informed that the bricks require "breating" to keep from moisture deterioration in our heating dominated climate (MI). My friends floor plan does not accomodate interior stud walls and fiberglass through out (due to impact on sq').
What would be the best low profile way to improve R value of walls.
Thanks,
Joe

Converting it into a cavity wall can be done by creating a 1/4-12" cavity maintained by a moisture tolerant furring (metal furring recommended) mounted to the brick on which foil-faced rigid iso is mounted:

outside air|soft brick wythes| metal furring/air| foil faced rigid iso | gypsum or metal lath+ plaster| interior air

Micro vents need to be drilled from the exterior into the mortar on the verticals at the top course, and should be at an upward angle such that condensation or rain on the exterior won't funnel into the cavity. Weep holes should be drilled into the mortar at the bottom course to allow any condensation or bulk water incursions to drain toward the exterior.

The foil facers on the iso should be FSK taped on the seams & foam sealed on the edges to minimize interior air or water vapor migration into the cavity and condensing on the (now much colder in winter) interior face of the brick.

There may still be condensation issues where structural timber rests on the brick. If the wood itself is checked & split it may need to be carefully sealed with epoxy or something, and foam sealed around to keep it from becoming an air & moisture path into the micro-cavity.

The thickness of the iso & depth of cavity determines the total loss of floor area, but the soft brick is giving you at best ~ R1.5-R2, and even 1/2" of iso would more than double that. To that add ~ 3/4" for blueboard & plaster + 1/4-1/2" for cavity depth. For comfortable wall temps it's best to have at least R8 walls, which would be achieved with 1" iso (R6 of iso + R2 of soft brick.) If you go minimal on the micro-cavity & 1" iso the interior finish wall would be ~2" in from the face of the soft brick- or about half that of a 2x4 wall depth, with performance similar to "typical installation" R11 batts w. 16" o.c. studs.

With the weep holes & vents the humidity of the cavity will track the outdoor humidity, whereas in it's original state there is a constant transfer of indoor humidity through the brick to the outdoors during the winter months, and transfer of humidity inward during rain & dew wetting events. With a micro-cavity the inward moisture drives are blocked by the foil facers on the iso, but with a cavity + sufficient vent & weep holes the moisture won't be retained in the brick. Outward vapor drives in winter are now blocked by the foil facers, reducing the risk of spalling on the exterior.

Insulating cavity walls as a retrofit by filling the cavity can be a dicey proposition- results & longevity are highly dependent upon materials used and the local climate. In the UK it's becoming popular to blow cavities full of adhesive-loaded EPS beads, which results in a breathable ~R3/inch (a ~50% improvement over poured perlite), but that approach can still create issues in significantly colder climates than the moderate winters in that area. Filling cavities with slow rise foam has proved problematic, and should only be done after a very careful analysis of the building construction & climate.