Check with city before retrofitting insulation
Q My 1906 graystone three-flat building has a troublesome flat roof which needs more insulation. During cold weather, moisture seeps up into the breathing space above the third-floor ceiling, freezes, and then forms icicles on the rafters that support the roof. When the weather turns warm, the ice melts and stains the ceiling. A thin layer of insulation already is in place, but there is no vapor barrier. I plan to place plastic sheets on the attic floor after removing the old insulation. Then I intend to staple foil-backed insulation into place. Finally, I plan to fill in the perimeters with the old insulation. Does my plan sound good? What R-value insulation should I use? Would it do any good to insulate with the foil-backed insulation under the roof? Peter B. Veerhusen Chicago A Retrofitting an old house with insulation calls for special awareness, area by area. I would recommend that you get in touch with the city building department for its suggestions. Then draw up written specifications item by item and step by step, following the city's idea.
Undoubtedly, the addition of a moisture barrier to the insulation should reduce the icicle problem and resulting stain on the third-floor ceiling.
You may wish to use R-29 or higher insulation on the attic floor. To insulate between the rafters sounds redundant, but again, ask the local inspector.
Some owners, if the roof configuration allows it, choose to install a second roof over the first in order to avoid the icicle problem at the eaves. If the new insulation does not get rid of the icicles, when the time arrives to reroof, consider a light second-roof system atop the first.