At some point, a previous owner had kept cats in the house. This was particularly obvious in one upstairs room which reeked of cat urine. Throughout the renovation process we referred ot this room as the “cat pee room.” It is now the guest room, which I find rather fitting.
We attacked the problem in small, escalating steps, each time hoping it would resolve the problem and each time failing.
At first, we sprayed the entire room with a liquid that was supposed to neutralize the odor.
No luck.
Then, At first, we ripped off the ugly hardboard paneling that covered the plaster walls.
No luck.
Then, we stripped the plaster from the walls, figuring that it might be saturated with urine.
No luck.
We sprayed again, this time on the exposed lath and underlying cellulose insulation.
No luck.
We then ripped up the flooring and subflooring. Upon removing both layers of flooring, we discovered that the cats had been so prolific with their pissing that the urine had seeped through the floorboards and onto the underlying joists, which in spots were actually stained black.
Even spraying these joists had no effect, so we ended up painting them with a heavy-duty paint designed to seal odors. This, at last, worked.