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Old 11-15-2012, 10:10 AM   #16
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Ice dam in the making


Sorry I didn`t put it on a list for you ,but it most certainly is a part of proper insulating,,,for foaming at the plate,,there should be an air channel secured between the joists 1st,then the remaining area would be filled with the foam so airflow is continuos,,Although in some situations the foam can be installed directly to the bottom of the roof deck,,and would meet shingle requirements(Certainteed),,,and intake venting can be addressed with the use of smart vent installed just above the ice area

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Old 11-15-2012, 10:12 AM   #17
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Just a quickie,,But,,hey What do you think insulation is for,if not to prevent air leakage as well ? ,,,Insulation is used to prevent nthe transfer of heat/cold from one area to another,,,Air flow would definitely be a part of that equation
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Old 11-16-2012, 10:05 AM   #18
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Just a quickie,,But,,hey What do you think insulation is for,if not to prevent air leakage as well ? ,,,Insulation is used to prevent nthe transfer of heat/cold from one area to another,,,Air flow would definitely be a part of that equation
Most insulation does nothing to stop air leakage.

Blown in Fiberglass is the most often used insulation in new construction and you might as well not even put it in if you are using it as an air barrier.

Fiberglass batting, if low density, is not far behind.

Cellulose is much better once it settles and foam is the best.
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Old 08-31-2013, 12:17 AM   #19
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I would add to your list of 3 and point out that air leakage is just as important as insulation when it comes to preventing ice dams.

It is tough to get enough R-Value at the outside wall top plate and the air leakage at that location is usually as culpable for the ice dam as anything else.
Bringing back an old post here...

So spent the rest of last winter shoveling the roof off. Air sealed the attic as best I could but couldn't get in to clear the soffits. Finally got around to pulling them off today and clearing them.

Here's what I ended up with:



Previously, all there was for ventilation was the 3" black pipes every second bay. Those were supplied by 2" holes cut into the old cedar soffits and then covered by aluminum soffit sheets. Along with opening up the ridge vent a bit (sheeting was covering over about half of the useful area), removing the cedar and just leaving the aluminum soffits in, hopefully this'll make a difference.

Last edited by burnt03; 08-31-2013 at 12:45 PM.
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Old 08-31-2013, 08:16 AM   #20
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Ice dam in the making


I see you've gone to a lot of work and I hope it works out for you but I fear the people recommending soffit venting so cold can travel from soffit up along the bottom of the sheathing to a ridge vent are misinforming a whole lot of people. The reason being, cold air doesn't rise and any air entering the soffit travels along the top of the attic insulation until it mixes with resident attic air and warms sufficiently to rise.

So next is recommended is some sort of hog trough to direct the air from soffit to ridge. Same rule applies, cold air still doesn't rise naturally. The building suppliers are loving it though.
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Old 08-31-2013, 08:24 AM   #21
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I see you've gone to a lot of work and I hope it works out for you but I fear the people recommending soffit venting so cold can travel from soffit up along the bottom of the sheathing to a ridge vent are misinforming a whole lot of people. The reason being, cold air doesn't rise and any air entering the soffit travels along the top of the attic insulation until it mixes with resident attic air and warms sufficiently to rise.

So next is recommended is some sort of hog trough to direct the air from soffit to ridge. Same rule applies, cold air still doesn't rise naturally. The building suppliers are loving it though.
That's.... unfortunate . How is it supposed to be done?
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Old 08-31-2013, 08:40 AM   #22
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I see you've gone to a lot of work and I hope it works out for you but I fear the people recommending soffit venting so cold can travel from soffit up along the bottom of the sheathing to a ridge vent are misinforming a whole lot of people. The reason being, cold air doesn't rise and any air entering the soffit travels along the top of the attic insulation until it mixes with resident attic air and warms sufficiently to rise.
You're perhaps forgetting the temperature difference of the roof. There is certainly going to be convection. That and as heat from the interior is transferred (and it will be, even without leaks) that will also contribute the convection effect. It doesn't have to be some blustery blast of air in motion. But it does benefit from NOT being concentrated at the lowest point of the roofline, closest to the interior walls. Allowing for that radiated heat to move away from just that area helps prevent ice damming.

There are, of course, many different approaches to this.

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