Unfortunately, we screw up this approach on a regular basis. This is because ventilation only works to remove small amounts of heat. In ice dam regions, where ground snow loads are greater than 30 lb/ft 2 (146 kg/m 2) you need 2 in.
(51 mm) minimum airspace is recommended under the sheathing. So, if the wall is R-30, you want at least R-30 directly over the top of the top plate. Also, it is a good idea that the minimum thermal resistance directly over the top of the top plate be greater than the thermal resistance of the wall. Exterior walls are like chimneys-you don’t want to vent the tops of them into your roof. Note the air seals at the top of the exterior wall. And then third, flush away any heat that gets to the roof deck by ventilating the underside of the roof deck with exterior air. Second, insulate the top of the lid with lots of insulation. First, construct an airtight “lid” or ceiling plane. 2įigure 2: Classic Control Approach-Keep heat from the interior from getting to the roof deck and then remove any heat that gets there. First, construct an airtight “lid” or ceiling. For this to work, and it does work, we have to do three things extremely well. Keep heat from the interior from getting to the roof deck, and then remove any heat that gets there using ventilation. The best approach, the classic approach, to ice dam control is the vented roof ( Figure 2). This does not prevent the ice dam from occurring it only limits the damage.
#Querious ice belt install#
Photograph 4: Insurance-It is not uncommon to install a water protection membrane on the entire roof to keep ice dam melt water out of roof assemblies. Heating cables are still a pretty common approach to dealing with ice dams. Photograph 3: Heating Cables-Throw energy at the problem. And, then we add insurance-the belt and suspenders-we add a water protection membrane to the entire roof to keep the melt water out even if we get the ice dam because we have become conditioned into accepting that we can not avoid getting the ice dam, so at least we can limit the damage ( Photograph 4). Heating cables are still a common approach to dealing with ice dams ( Photograph 3). When that doesn’t work, we throw energy at the problem. Photograph 2: New England Slip Surface-Older buildings in New England use metal “slip” surfaces to get the snow to slide off the roof, preventing the ice dam from occurring at the roof edge. In New England where I live, older buildings use metal “slip” surfaces to get the snow to slide off the roof, preventing the ice dam from occurring at the roof edge ( Photograph 2). Historically, we have tried pretty much everything except the right thing to control ice dams. Pretty easy you say? Not necessarily, as we shall see. The strategy to control ice dams is fundamentally straightforward: keep the roof deck below freezing when the outside temperature is below freezing. Ice dams are big problems because they often lead to water leakage into building assemblies, and more seriously, to falling ice that can be fatal (not kidding here) and to the weight of ice leading to structural collapse of roof overhangs and the shearing of deck assemblies when large masses of ice fall on them. At the roof edge and roof overhang, the deck is much colder, and the drainage gap freezes solid causing the water to back up. As more snow melts and the liquid water phase exceeds the storage capacity of the snow, it runs downward under the ice layer via gravity. As the liquid water migrates upwards, it gets colder and freezes. Folks die every year when big ones fall on them, or when they fall off roofs trying to get rid of them.įigure 1: Melt Water-When the snow layer adjacent the roof deck melts, the liquid water is wicked upward into the snow pack by capillarity away from the roof covering. Photograph 1: Ice Dam-Beautiful, but bad, and sometimes dangerous. 1 The warm roof deck causes the snow on top of the roof deck to melt, and the melt water runs down to the edge of the roof where the water freezes leading to a buildup of ice and a backup of water, hence the term “dam” ( Figure 1). Ice dams ( Photograph 1) happen when the outside temperature is below freezing, the roof deck temperature is above freezing, and there is snow on the roof.
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