Attic Heat Loss

Ice Dams, Black Mold, and 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 damming 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.

  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. 


Illustration of Ice Dam


What causes ice dams?
There is a complex interaction among the amount of heat loss from a house, snow cover, and outside temperatures that leads to ice dam formation. For ice dams to form there must be snow on the roof, and, at the same time, higher portions of the roof's outside surface must be above 32° F while lower surfaces are below 32°F. For a portion of the roof to be below 32°F, outside temperatures must also be below 32°F. When we say temperatures above or below 32°F, we are talking about average temperature over sustained periods of time.

The snow on a roof surface that is above 32°F will melt. As water flows down the roof it reaches the portion of the roof that is below 32°F and freezes. Voila!—an ice dam.

The dam grows as it is fed by the melting snow above it, but it will limit itself to the portions of the roof that are on the average below 32°F. So the water above backs up behind the ice dam and remains a liquid. This water finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling finish.

Nonuniform roof surface temperatures lead to ice dams.