Insulation Challenge

My house is roughly 140 years old of post and beam construction. When we bought it in 1989 it had just been rehabbed with new plumbing electrical, mostly new floors many new walls etc. The exterior walls were insulated with cellulose fiber and the attic has fiberglass. However, it is a story and a half so it has slanted ceilings in the second floor, which have no insulation, so I get ice dams whenever we get a lot of snow, not to speak of the heat loss. The cavities are approx. six feet wide by about eight feet down to the knee walls, and they are only about three inches from the plaster lath to the roof boards with a lot of nails sticking through both surfaces. Ten-fifteen years ago I managed to fit some two inch foam board down into one of the cavities, but it doesn't make much difference in the ice daming. I need to get the insulation down more onto the plaster lath, and I also need to maintain a better cold air passage under the roof boards. Today I cut through the plaster knee wall in a closet to see if I could get at the bottom of the cavities from there but I found an outer board wall to the outside of the insulation so there is no access to the bottom of the slanted cavities from there. Has anyone ever encountered and solved a problem like this?

Foamed in place will work if there is no firestop between the rafters. If the bottom of the location has access, the lower end can be blocked off. if not there could be some foam leakage into that space, but not much. Jim

I have insulated such places with blown-in cellulose fiber or blown-in fiberglass.

There are companies around that will come in and put a few small holes in your ceiling and inject expansion foam inside the space to insulate it.

I solved my problem which was similar to yours. I used a stud finder to locate places that would accept blown in insulation. My method was to use a hole cutter to create holes in the walls from the inside. I then blew insulation in the walls where it was indicated. I am very pleased with the results.
I saved the plugs that I cut from the walls and used dry wall paste to insert them back into the wall to plug the openings and restore the walls. When we were finished it was impossible to tell where the holes had been cut.

I would pull the material off the slanted ceilings, put in the vent inserts, foam them good and be done with it.

I do not think that any other insulation will work well enough.

You could build rafters down inside the room and insulate from the inside making sure to overlap a
vapor barrier down over the knee wall and onto the ceiling. Six mil poly works good for a vapor
barrier. This would mean cutting into the ceiling and knee wall but you could easy get 6" of
fiberglass in if you wanted. It would reduce the size of the room a bit if that matters. A vapor
barrier is important to keep warm moist air from the house from making it's way to the cold space
and condensing in the unheated space. It cuts down on drafts too.

I got into it some more today, and found that Instead of vertical rafters I have purlins that are about four feet apart. There are vertical boards on the underside of the purlins for the lath to be nailed to, so the mid and bottom space is pretty much inaccessible except perhaps by foam, but then I wouldn't have a means of leaving a cold air space under the roof boards.