New roofs, with a combination of plywood roof sheathing, felt paper, ice and water shield and roof shingles are simply “tighter” than old roofs. Old roofs were more able to “breathe”. By that, I mean air was able to move in and out of the attic space through gaps or spaces in the roof. What you will see now is that with the lack of air movement, water vapor can no longer escape. The water vapor or condensation is created when the warm air from within the home comes in contact with the cold surface of the roof. What use to escape is now trapped in your attic, typically right below your roof sheathing.
This can lead to issues for the house itself. The condensation can cause the rafters and plywood to be compromised and potentially rot. The inability of the heat to escape can cause significant problems with the roof shingles. You may see the roof shingles curl or deteriorate. If that happens, due to a lack of proper ventilation, the manufacturer’s warranty for the shingles will be voided.
In addition, it can lead to significant health concerns for you and your family. If mold does develop over time, it will spread and it can become air-born and have hazardous respiratory effects.
So, what’s the solution? Install proper ventilation. The best bet is to ensure that your home has both soffit and ridge ventilation. What this will allow is for air to enter the attic space through the soffit, travel up the rafter bays, and out through the ridge vent. This will allow for proper ventilation and will prevent condensation and mold from forming.