Ice Dams, Black Mold, Insulation Failure
In homes that have a steeply pitched roof and are over 20 years old, a 98% probability exists that the attic has excessive heat-and even newly constructed homes often have serious attic heat problems. In snow climates, excess attic heat will lead to ice-damming and subsequent water damage. Most people attribute the problem of ice damming to the roofer. However in most cases ice dam and water leakage are not a result of unsatisfactory roofer workmanship, but rather are caused by one or more subtle sources of heat created by the electrician, insulation contractor, HVAC contractor, GC or architect.
Your response may be, "I do not live in a snow climate, so I do not have to be concerned about the heat that causes ice dams, right?" Wrong! High attic temperatures increase stress on the roof decking material (plywood or OSB) and will short-en the life of asphalt shingles, as heat is the primary aging mechanism of asphalt. Poor attic ventilation can lead to moisture condensation causing wood rot, attic insulation failure, and the growth of mold and mildews that can lead to health problems for the resident.
There are two very important questions that must be asked of, and answered by, every construction design-whether it is for new construction, for additions or for renovations:
1. How does heat enter the attic, and how can those heat sources be reduced?
2. How does heat exit the attic?
The rule of thumb is that properly vented attic air should never be more than 15 degrees hotter than the outside air. A temperature difference of greater than 15 degrees is an indication of additional heat sources that should be eliminated and/or an indication of poor attic ventilation.
Proper attic ventilation is calculated by comparing the ratio of attic floor space to the total amount of "net free air ventilation area." The most beneficial ratio is 1/150; that is 1 SF of net free air ventilation for every 150 SF of attic floor space. All ventilation sold for this purpose has an assigned "net free air" square footage rating that is roughly the total opening size of the vent, minus the various areas of block-age from louvers and screens. Ideally, the total of inlet air at the soffit will equal the outlet air at the ridge for a balanced and efficient airflow.
The following are pictorial examples of some ways unwanted heat enters the attic space and some reasons why heat cannot exit the attic space.
