One possible explanation is a problem called Truss Uplift. This can happen in regions of the country where there are dramatic changes in temperature from season to season. Truss uplift is more commonly seen in the winter months. At this time of the year attic spaces become very cold, while the living space below is kept at a warm comfortable temperature. The lower trusses are close to the ceiling, covered by insulation and kept warm and dry while the roof supporting trusses are exposed to the freezing air in the attic. Air in the home’s attic is often very humid. This moisture laden air tends to condense on the support trusses and roof decking. The combination of moisture and cold temperatures makes the wood expand and puts pressure on the lower warm trusses. This pressure forces the lower trusses to bend upwards lifting them and the drywall which is attached. Even a small amount of uplift will pull the ceiling away from the walls causing cracks along the edges.
There's a way to prevent this problem. If you're building a new house, roof trusses should not be nailed directly to any interior walls. Instead, truss clips should be installed. These L-shaped clips are simple, inexpensive devices that attach directly to the top of interior walls. The bottom of the clip is nailed to the top wall plates. Then a nail is driven through the slot into the side of the bottom chord of the truss. The head of the nail needs to be driven so that it is short of touching the metal clip. The idea is that you want the truss to be able to flex up and down without pulling up the wall and attached drywall.
Care needs to be taken when hanging drywall in the home. Drywall should not be nailed or screwed directly to the trusses within 16 inches of an interior wall. Drywall can be fastened to pieces of wood that is fit between the trusses or metal clips that attach to the top of the interior walls. Because the upward truss movement occurs over a period of weeks or months, the drywall will flex at the corners.
When attic ventilation is properly sized and installed for the home, it works just as effectively moving air through the attic in the summer as it does in the winter. Moving air through the attic helps to relieve heat buildup in the summer which can help to save on energy costs and extend the life of the roof. In the winter, proper ventilation helps to relieve the home of excess humidity and escaping heat. Air flow is critical in preventing condensation (moisture collecting on cold attic surfaces) from forming. Left unchecked, condensation can lead to wood rot, mold growth and a shortened lifespan for your roof.
The idea that air should enter from below the eaves and exhaust at the roof's peak is not a new idea in ventilation, but it is often not achieved. Soffit vents are sometimes added to increase air flow. But merely cutting holes in the soffit does not necessarily mean air flow will be increased. With the recommended attic insulation R value (measure of thermal resistance) of R-44 in the (Mid-West) climate, insulation can be built up to the point that there is no gap for the air to flow through at the eaves. To prevent this issue baffles should be used. As can be seen in the above diagram, baffles were added to keep the insulation from blocking air flow. Baffles are typically made of plastic or styrofoam and are low cost. If not installed when the home was constructed they can be installed from inside the attic. They should be of the same width as the space between the roof rafters and attached to the bottom side of the roof decking. Keeping the air flowing in your home's attic year round will help you to save energy and extend the life of your roof. By: Chip Starkey