Ask Jon Eakes

Last Updated: Friday, October 15th, 1999, Created: Friday, October 15th, 1999

WHY DID MY CEILING LIFT OFF THE PARTITION WALL AFTER I INSULATED THE ATTIC?

Many houses today are constructed with pre-fabricated "trusses" for the ceiling joists and roof rafters combined. Some, but not all, of the wood used for this reacts in a special way to temperature and moisture changes. As long as the entire triangular truss remains at the same temperature, nothing happens. When you add heavy insulation to an attic, the bottom cord of this assembly is wrapped in insulation, and is therefore in a very different environment than the two roof rafters. Given the right conditions this can cause the bottom cord to bow upward, cracking your plaster at the ceiling or even lifting the partition wall up and cracking the joint at the floor -- and certainly wreaking havoc with the doors and the attic air barriers. It may happen only once with a new house, or the ceiling may go up and down every year.

You cannot nail this bottom cord down, it will simply lift the wall off the floor. The forces involved are very strong. The true answer is to use a different truss design, one that is not a simply triangle -- such as a raised heel truss. This gives a bit of flexibility to the assembly and avoids this lifting action. If you are changing ceiling drywall, the ceiling is to "float" the joints toward the middle of the house -- screwing to the ceiling no closer than 18 inches from the partition wall, and then attaching the edge to the wall, not the ceiling. The truss can move above and bend the ceiling panel without cracking the joint. If you are not replacing the ceiling drywall, then you should consider installing a large cove molding on the wall about one to two inches below the ceiling, leaving a shadow design between the top of the cove and the ceiling. This shadow space will change with time, but not be noticeable below. (Put a rope light inside the cove and you have just justified the whole thing as a great decor concept.)

This problem is often confused with foundation shifting, which also cracks plaster and jams doors. Run a string across the ceiling in line with the trusses; if the center of the ceiling bows upwards, you probably have truss uplift (unless you live on Gumbo clay in the Prairies and the centre of the basement floor is pushing upwards).

Keywords: Attic, Cracks, Insulation, Molding, Truss

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