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Icicles -Whats wrong?

| Posted in Energy, Heating & Insulation on January 7, 2003 09:39am

I am finishing the 2nd floor on my 1938 brick cape. It is a full-height 2nd floor that I have insulated and heated this past season.

I have 3.5 ft high of knee wall w/ R-25 encapsulated ins. and 8 Ft of sloping cathedral ceiling w/ 2 layers of 2″R-14.4 rigid plus 1 layer of 1/2″ R-4 rigid. There is a 1″ air space running the length of all rafters. For the flat ceiling I have faced R-38 batts installed. There is a continuous air space from the air vents (3″) in the soffit to the ridge vent in the roof, also there is a gable vent on one end of the house. Everything has been sheetrocked w/ 1/2″ board, except for aprox. 20 sq.ft in the bathroom where I’m using Durock under the tiles.

We received about 14″ of snow this weekend and I was shocked to see water dripping off the roof line, forming icicles along the gutters. This tells me that heat is getting to the roof deck and melting the snow. How can this be with a continuous air space? Are the 3″ soffit vents 2ft on center too little? Or am I over-reacting?

Any feedback would be appreciated while I can still fix things.

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  1. 4Lorn1 | Jan 07, 2003 10:50am | #1

    What are the temperatures like there? Is the roof in direct sun during sunny days? Has it been sunny?

    Seems to me that if the temperature has been close to freezing, mid to high twenties, the small amount of heat loss, no insulation system is perfect, could tip the balance. In this case there might not be a problem.

    Also the sun might kick the temperature up a few degrees and help melt the snow. Any water might take some time to migrate down to the roofs edge. If during that time the sun went down the dripping water could freeze and form icicles.

    On the other hand if the temperatures have been in the teens for days and overcast you have a heat leak. Recess cans, electrical boxes in the ceiling or any other ceiling penetration, are a common source. Unsealed electrical boxes in walls can sometimes allow air, and heat, to migrate up and flow through unsealed holes drilled into top plates. These then vent into the attic.

    1. Haole27 | Jan 14, 2003 07:12pm | #2

      which side of the house is this area? probably nw as it gets juuuuust enough sun to melt a small amount of the snow but not all...so the heat is probably coming from the sun not your living space leaking

  2. BarryO | Jan 19, 2003 06:52am | #3

    Is the snow blocking the ridge vent?

    1. donpapenburg | Jan 19, 2003 07:55am | #4

      Snow is sorta clear and lets sunlight through . It will heat up the roofenough to melt snow an the melt reaches the open air it freezes again. Snow will melt off of a blacktop road at 25* . If your roof is dark green or black or brown it will heat up fast.

  3. xMikeSmith | Jan 19, 2003 08:20am | #5

    capes are really tough... the area behind the kneewall is very difficult ..

    the floor behind the kneewall is usually suspect..

    encapsulated insulation has a lot of edge leaks..

    the break point where the rafters sit on the kneewall is another bad area..

    all in all... anytime we do capes now... we use cellulose dens-pak.... and  1" foam on the interior....

    the ceiling /floor  behind the kneewall is also denspak between the joists from 12" inside the kneewall  all the way to the band joist.. and another 12 " on top of the floor behind the kneewall.. the kneewall is usuall an 8" space with denspak.. the slant ceiling is furred down with full propa vent from the soffits , denspak  in the rafters , and foam with furring on the slants... the little flat attic is 20 " to 24" of cellulose... we have a 6000 degree day design temp.

    Mike Smith   Rhode Island : Design / Build / Repair / Restore

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