I have a split level home with a part of it on a slab. We started remodeling our family room that is on the slab and after removing the paneling and exposing the studs, I found that the sill plate on the interior walls was rotted in a few places. The exterior wall sits on a 18" brick wall and was not affected. The interior wall has a sill plate on top of a block foundation with the slab poured around it. There is a kind of fiber board between the sill plate and the block about 1/2" thick. It appears that this fiber board is holding moisture either coming up through the block or where it is contacting the slab. I replaced the rotted sections that I could find with PT lumber. So far this is about 1 1/2 feet on either side of an interior door way. Only the top half of the sill plate is visible but the remaining sill plate looks solid. The house is 40 years old. The question is this, would it be worth the time and effort to replace the remaining sill plate on all the interior walls? I am drywalling the room but will leave a gap so I should be able to see the sill plate and do some inspecting but not much. I guess if I replace the remaining sill plate I would do it in sections and nail them together.

I'm more concerned with the moisture that might be trapped within your new walls, and what it might do over time, even if you did replace the whole sill plate. Are you also removing that fiber board as you replace the sill? You can replace it with pressure-treated plywood of the same thickness. I'm also concerned that there's a vapor retarder under your floor slab. In a 40-year-old house there probably is one under there, but that foundation wall which penetrates the floor slab is a breach of the vapor retarder, and allows moisture to enter your living space. Probably it isn't much moisture, and probably the problem comes from the fiberboard retaining the moisture. Exactly what can be done about it at this point isn't known, except to get rid of the component that retains the moisture, and maybe try to place a true moisture barrier between the block and the new sill plate.

Thanks for the reply. There is plastic protruding from the slab that I could see when I removed the sill. There was no vapor barrier under the sill however. In the section I replaced, I put plastic sheeting under the PT lumber. I probably will try to replace the rest of the sill and do the same with it. Perhaps caulking around the new sill would be a good idea to try and keep the moisture out of the wall.

Do I undestand that the slab is poured against the framing with the fiber board acting as a buffer between the cement and wall? Or is the framing layed on top of the slab with the fiberboard being placed under the sill.
Does this fiber board seem to have a tar appearance?
Something does not sound correct.
The fiberboard may be cement expansion boards used as a material between sidewalks and large slabs as a control for cracking. Because the house is older they may have used this material to prevent wood contact to the cement and dampness from wicking up into the wood sill.
If the damage is localized and the area did not show signs of water stains when opened. This may have been this way for many years. If water signs are seen, Find out the cause of the moisture before closing up the walls.

I think I understand that the block foundation protrudes through the floor slab, and that the black fiberboard material (which Home Care Club probably correctly identifies as expansion joint filler) was laid on the top of the block, between the block and the sill plate of the partition. Whoever did it probably thought it would act as a vapor retarder. Since there is foundation under the wall, the wall is very likely to be a load-bearing wall...I wonder why the original builder did not use a thickened slab footing for this wall, since it seems to be an interior wall? Well, that's neither here nor there, now. The plastic under the new sill plate is good, bit there is still a small breach of the vapor retarder, which may lead to moisture and mold problems if the wall is closed. I can't say much more without seeing clearly what the exact conditions are.

Actually, the fiber board is under the sill. It keeps the sill from resting on the top of the block. The block is about 1 inch below the top of the slab. It looks like they built the footer, put the sill on top then poured the slab around it. The damage does seem to be localized. The fiber board (I'm not quite sure what to call it) was used all around the house under the sill plates and it appears that it was used on the exterior in place of plywood under the siding. It is a dark brown material about a 1/2 inch thick. Kind of like a really heavy cardboard for lack of a better description.

I am debating about replacing any more of the sill plate. I have replaced the rotten sections and the remainder seems solid and should last for quite a while since the existing damage was over 40 years. There are no problems with water damage, the moisture appears to have come from the soil over time.

Replacing the whole sill will be a major undertaking. The wall is an interior load bearing wall and would probably require extensive temporary supports and jacks. Also, behind the wall is a stairway to a basement which further complicates adding temporary supports. At this point I am leaning towards leaving it as is.

You are on the right track. The material used is called Homosote. A very common building material during the time when the house was built. No you do not need to change any more then that was rotted. However you still need to figure why the area that was rotted did so. Could there be a leak or past leak around the door frame? What condition is the flashing? While you have the wall down, take a hose and spray and wet the outside wall for a while. See if you experiance any interior moisture issues. Better to find out now then to do repairs and find out later when it would cost more to fix.