January's Stump the Chump

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Permalink Reply by John Brooks on January 24, 2013 at 8:52am

It's hard to say without seeing the building,the orientation, roof overhang and venting.
One possibility ....the microclimate near the wall is warmed by the sun...the warm air is buoyed up by surrounding cold air....warm air enters the roof thru soffit vents...and melts the snow.

note to Bud Poll...I did not say that warm air rises ;--)

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Permalink Reply by Craig Bird on January 24, 2013 at 10:04am

The work scope does not provide enough detail.

Air sealing attic is good., but were the exterior wall top plates air sealed? That is most important juncture for ice dams.

Did the work scope include adding ventilation baffles? This is important for 2 reasons: 1) prevent thermal bridging of insulation from drywall to roof if the insulation was blown ceiling-to-deck, 2) to ensure ventilation to cool the roof deck from soffit vents.

Is there soffit ventilation? Without it, the other measures may still fall short.

How much insulation is above the top plate? Clearance issues to roof deck can leave this area with significant heat loss from ceiling to roof deck. 

If the above issues were not addressed, ice dams may still occur. For stubborn cases, I found a combination of baffles and spray foam work the best. You can get a high r-value at the low clearance soffit area and be sure that all air leakage is stopped from the top plates. This all assumes that soffit ventilation is present of course.

Interesting concept with the micro-climate by John with warm air rising from sun warmed wall - not sure how to address that, but I would try all of the above and see if the problem is solved.

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Permalink Reply by Bud Poll on January 24, 2013 at 12:42pm

Hi John and Craig,

This thread on Linkedin had an ice dam issue that was new to me and similar to your suggestion John of the wall being warmed by the sun.  Here the issue was the vent from his high efficiency boiler where the warm moist air is apparently hugging the wall and being pushed up and into the soffit above.

http://www.linkedin.com/groupAnswers?viewQuestionAndAnswers=&di...

Craig, John and I have been encouraging the energy professionals to avoid the phrase "warm air rises" to help everyone understand that warm air cannot rise by itself.  But you are forgiven if you haven't bumped into one of the many discussions :).  The issue is still debated by many.  But this is a good example of how that warm air, whether it be from a boiler vent or solar heating would be, not only pushed up, but pushed against the house where it would be directed straight into the soffit air flow.  This would not only supply heat to the bottom of the roof deck but moisture as well.

As for Ken's issue, this stumper, there are many details lacking so difficult to cover all bases.  I was waiting to see if more info would be posted.

Bud

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Permalink Reply by Craig Bird on January 24, 2013 at 1:26pm

I don't have a problem with "warm air rises", but I do have a problem with "heat rises". I just always follow up with "unless a difference in pressure is pulling it in a different direction" and explain how it can be pulled down from a hot attic if cold dense air is exiting the bottom of the building. Getting into the physics makes clients heads spin - I just want them to realize there are more forces at play. Its just easier than saying "air which is warmer than the surrounding air is pushed up by the cooler denser air exerting an upward pressure on it", which they won't understand. Really, how do you say it casually in a conversation or forum without a 20 word white paper excerpt? How about warm air increases its elevation? Sorry, I know...other threads

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Permalink Reply by George J. Nesbitt on January 24, 2013 at 12:45pm

Could it be that the attic is connected to other units that did not receive air sealing & insulation. So the heat loss from those units still melts snow.

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Permalink Reply by John Brooks on January 24, 2013 at 4:07pm

The inspiration for my guess comes from this Joe Lstiburek Video Starting at around 18 minutes in http://www.greenbuildingadvisor.com/roofs-video-part-2-ice-dams-lst...

course like Bud says...very few details have been provided....so only guessing

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