Lean to roof - gable roof venting problem

Old 02-22-18, 10:13 AM
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Lean to roof - gable roof venting problem

I have a question regarding attic ventilation on a two roof lay out. We just added an addition to our cabin in northern Wisconsin, the original cabin has a beam ceiling with ceiling planking then a vapor barrier then 6" of foam insulation then 2" of vent space before roofing which is metal.
We just added a addition to the side of the cabin and attached a lean to roof to the existing gable roof. The original roof is a 12/12 pitch and the new lean to roof is a 3/12 pitch.
My question is what is the best way to vent the roof lines, since we had to cut off the original soffit to add a knee wall for the new rafters for the addition to sit on. New addition has full length soffit with ventilation going up the new rafters and dumping cold air on top of the original 12/12 roof. But remember we had to remove the original overhang with its soffit.
So do I add gable vents to the addition if I do how do I then vent the old original roof. Do I treat this as two separate roofs or one roof? We had a huge ice dam but figured out not enough insulation but biggest problem is not enough ventilation. Insulation has been addressed and we now have R 45 in new addition just seem we are not getting proper air flow.
We first added two 12x12 gable vents to the peak of the new addition, which seemed to bring differential from outside temp to inside temp to between 10-14 degree's. But still had no ventilation for my original 12/12 pitch roof. So I was told I needed to cut holes in original 12/12 roof line as high as I could where the two roofs met. After doing that my differential went up to between 20-30 degrees. But I am thinking I just opened a hole in old roof for warm cabin temps to escape to attic area. So do I now close my gable vents or plug my holes in original roof and leave well enough alone. My concern with if I leave the gable vents my cabin roof will not vent at all on that side?
Hope some one has an idea on how exactly we should vent this roof design to get correct ventilation for both roofs.
Fred Briggs

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