Solving Ice Dams & Icicles - Attic Design
What causes Ice dams and icicles when the temperature is below freezing and how do you minimize the risk of getting them on your home?
Most likely, the origin is heat from the home, escaping the building envelope and melting the snow from underneath. Typically, the frigid melted snow re-freezes at the eve. Freeze and thaw cycles and orientation will always play a role, but many sources are caused by poorly performing homes.
Factors that will weigh in a solution would be the usual combustion safety and ventilation prerequisites and an analysis of the most cost effective treatment. Duct location plays a huge role here. If both heat and A/C ducts are in the attic, then a spray foam "hot roof" (Conditioned) attic may be the best choice.
Architecture, client preference and conditions "on the ground" determine the best choice. A conditioned attic, and properly engineered floor joists, allow for vast amounts of "conditioned" storage space. To many homeowners, this is a great reason to incur the added cost of a properly done, conditioned attic.
It is imperative, that dew point is completely prevented in the conditioned attic. Dew point equates to "issue" in home performance, we hate "issues"!
I would like to see some studies on smoke performance of an air tight attic, sprayed with foam. Although modern foam performs extremely well, it only barely passes smoke developed requirements. I would suggest a hard wired smoke detector in the attic to provide the most possible time for occupants to escape, should a fire occur. Of course, detectors are imperative in all projects, I recommend adding an additional one with foam in the attic. The bottom line, I would have no issue at all spray foaming my own attic.
On homes with hydronic (water or steam) heat, even with A/C ducts in this climate, conventional air sealing, insulating and ventilating usually makes the best economic sense. We typically see an approximate 10 CFM50 air sealing reduction per 100 sq ft just by blowing 6" of cellulose of more. (Pretty decent air sealing numbers due solely to insulation choice). If you air seal properly, reposition the old fiberglass, (Pressure boundary and thermal in alignment) install wind-wash baffles, provide adequate ventilation and blow cellulose on top, it is a considerably less expensive proposition which performs very well. Vents, such as dryer and bath fans, should be wrapped in insulation while implementing this remedy.
Cellulose is also an ideal candidate for insulating the hot roof, and it performs immensely better in a smoke developed regard.
Cellulose is a "green" product. Green meaning really sound ecological choice, not "green-wash" advertising. It is made from recycled newspapers, it requires less energy to manufacture, is a phenomenal insulation and air sealing product and preforms exceptionally well in smoke and fire testing. Thomas Jefferson, man ahead of his time and author of the "Declaration of Independance"; insulated Monticello with an early form of cellulose.
The basic rules always apply. Test safety, determine ventilation requirements, provide a solid pressure boundary, and a substantial thermal boundary to assure dew point is not reached in the structure, and minimize conductive heat loss.
Following either of these methods should help minimize ice dam and icicle issues.
Heavy snow pack is a great insulator. In areas where snow pack is expected over 24" or more (that number is approximate), the roof deck is typically installed over a 3/4 inch ventilated air gap to alleviate the issue. Personally, a steep roof and a partially enclosed attic with the upper half properly ventilated, would be a better choice, under those conditions, in that kind of climate.
Client feedback is enormously important to assure properly performing buildings. A dedicated home performance contractor is enormously interested in how "their" buildings perform over time.
Weatherization isn't brain surgery, but if done without taking all the factors into consideration, can create issues. If issues are present, they can usually be solved by simple measures done in a timely fashion.
Steve McKenna
Director – Building Efficiency.
Murtha Construction.
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