Quote:
Originally Posted by DickR
In both concrete and plaster curing, part of the water is tied up by chemical reaction and becomes part of the solid result. The excess water must be removed by evaporation. Ultimately, that water should be removed from the air by mechanical dehumidification or by ventilation. One should not expect or want all that moisture to go out through the walls.
Nearly all cases of ice damming at the edges of roofs are due to warmth getting to the roof surface. Much of the time this is due to heat carried by air leakage from conditioned space into the attic (vented attic) or the roof cavity in the case of a cathedral ceiling. Can lights in the upper floor ceiling are notable heat leaks, even those rated "air tight" (AT), and from a building science point of view are generally a bad idea in those locations. But any air leak that carries heat from conditioned space to the roof deck contributes to conditions that lead to ice damming.
More and more attention is being given to building science issues, and some builders have learned how to get it right, but sadly some still just build the way they've always done it. Insistent homeowners who prefer form over function also are part of the problem.
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I was building one of the 'green' homes back in 2010. I had a lot of contractors dispute RBC 2010 codes and architect/engineer specifications mostly because they had a tried and true method of building. One of them was dehumidifying the space between the roof and plastic sheeting even during the addition of insulation. This particular build is a closed system where there are no air circulation under the roof. Many contractors walk away from the project. The home is into its 3rd winter without any problems. Roof has never been raked and there are many valleys and gables.