I own a historically significant 1898, 3 floor town house in San Francisco with 1 inch separation from the two nearest neighbors. There is no insulation in the exterior walls, and the code in my area requires introducing insulation into the exterior walls when remodeling costs exceed ~$15K. The walls (outside to inside) consist of 9 inch wide, old growth redwood shiplap siding, 4 inch rough saw stud walls, finished with lath and plaster interior walls, all with the original wood finish in the public rooms and most bedrooms. There is no sheathing, house warp or even tap paper to inhibit water infiltration. The remodeling I want to do is at the back of the house where I can install insulation in the remodeled rooms. The problem is the major, public rooms that will be untouched and are also un-insulated.

The issue is what insulation to use? Anything would have to be sprayed in via ports from the inside. Closed cell, slow expansion foam seems like a good choice, being both a vapor barrier, insulation and causes less damage to the plaster walls. But another complication is the Dutch Gutters that connect the roofs of the two neighboring houses to my exterior walls. If these Dutch gutters leak, and there are gaps in the (hidden) shiplap siding between the buildings, then any insulation could become wet, and eventually rot the load bearing walls, original lath and plaster walls, oak trim, etc. There is at least one example of an exterior wall leak where a strip of shiplap siding shrank ~1 inch longitudinally Luckily in a place it was exposed, accessible and could be repaired from my neighbors roof. Such a leak in the hidden parts of the exterior walls underneath the â?oprotectionâ? of a leaky Dutch Gutter could be fatal to the insulation and eventfully cause dry rot, mold, etc. Dutch gu tter leaks are hard to detect, itâ?Ts already too late by the time you find one.
The neighboring houses provide significant insulation, with only a 1 inch air gap between buildings.

Any Suggestions for retrofitting the insulation? Gutting and rebuilding six major public rooms is not is not a option, when I am only altering 3 other rooms at the back (exposed side) of the house (Kitchen, Master Bath Suite and main bath). The plaster walls are covered with “painting paper” so any patches for interior access holes will show, unless I re-plaster.

I am inclined to think the best solution is no insulation (i.e. air) rather than a blown-in product in the old work. The Attic is both accessible and insulated with blown-in boronated, shreadded paper (and it still does not ignite.)

Insulation advantage:
1. Lower heating cost (~ $500/yr. I do not have or need AC. Use gas heat 3-4 months/yr)
2. Meet local building code requirements.
3.(Potentially better adhesion of plaster to lath from foam insulation adhesion to both?)

Disadvantages
1.Cost
2. Visual damage to original interior lath and plaster walls and original woodwork & finish.
3. Potential long term water/vapor damage to structure from leaks
4. Opening of holes in exterior walls from expanding foam insulation, allowing easier water & critter penetration.

Richard Kadel