Is better to vent a roof that terminates at a wall or leave it unvented?
We purchased a house last winter and experienced an ice dam leak. The ice dam is a three-fold problem. 1. Improper placement and amount of insulation in the ceiling of the room in question. 2. An unvented bath fan. 3. A double wall insulated chimney that terminates on the steep portion of a mansard roof with an open cavity in the living space in which the sheathing was exposed. I have enclosed the open cavity of the chimney and vented the fan out the gable end of the house.
The roof in question needs to be replaced because of water damage. From the reading I’ve done, there appears to be a debate as to whether one should or should not vent a roof that terminates at a wall. Can you offer some advice?
Thanks
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Wow! The builder of that house deserves the IgNobel Prize.
It makes no difference where or how a roof terminates, except in the manner in which it would be vented. Vented roofs are almost always more durable and less problematic than unvented roofs. Almost all the building scientists agree.
For a roof that terminates into a rising vertical wall, use a Flash Filter Vent (half a ridge vent). For a roof that terminates at a descending wall (a shed roof), use a Peak Filter Vent. Both of these, and any upper exhaust vent, must be balanced by at least an equal net free vent area of intake (soffit) vents.
http://www.airvent.com/homeowner/products/ridgeVents-specialtyFilter.shtml
http://www.airvent.com/homeowner/products/intake.shtml
Nate,
For once, I will avoid addressing the question of whether roof ventilation matters much.
I will, however, note that you shouldn't neglect air sealing between the conditioned space and the attic (or roof assembly). Air leaks from the conditioned space are the most common cause of ice dams. And you should certainly beef up the insulation -- ideally with some type of insulation that isn't air-permeable.