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help with attic insulation

I posted several months ago about help insulating an attic that is partly refinished. We have ice dams on the south side of the house and it makes for treacherous walking. Our house is in WI and it's 100 yrs. old. We have 2 rooms in the attic that were not refinished and only have insulation underneath the floorboards. We use these rooms for storage but we have frost on the nails and on the wood underneath the shingles. What to do?

Re: help with attic insulation

To determine a solution, we first have to understand WHY we have the problem.

Ice dams form because the snow on the roof over the attic space melts (because the attic is above freezing), and when the water hits the eave overhang, which is below freezing, it refreezes.

The solution, then, is to ensure that the underside of the roof remains at the same temperature as the outside.

You say that there is insulation under the attic floorboards. This insulation is apparently insufficient. Ther are a couple of possible solutions: add insulation to the floor or add it between the rafters under the roof. Either way, you must have exterior ventilation to the underside of the roof to maintain the temperature below freezing.

If you can't add insulation to the floors, you may be able to add insulation directly under the roof between the rafters, BUT you must leave an air gap between the insulation and the roof. This air gap must be open to the outside, usually through "bird block" or soffit vents at the bottom, and roof or ridge vents at the peak. If it's not ventilated, it's a pretty good bet that the roof will warm up above freezing.

The frost you have is an indication of poor ventilation. Moisture is being trapped in the attic and not vented to the outside. If the existing insulation really is sufficient, adding ventilation -- to keep your attic below freezing -- may do the trick.

Re: help with attic insulation

Thanks for the help!

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