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Community and Q&A

Porch roof insulation over conditioned space

denyank78 | Posted in GBA Pro Help on

I have a house dating from 1897 in Westchester New York. The house has a wraparound porch of which about 1/3 is enclosed finished space ( by previous owner). The remainder is just a run of the mill front porch. The roof of the porch is original with Yankee gutters and slate. This roof is due for replacement. I would like to add exterior insulation above roof sheathing ( 4” of Rockwool). This would only be over the conditioned space. What would be the best way to make up this 4” difference with the rest of the unconditioned porch roof? I am currently leaning toward reslating the roof with metal snow bib and modern gutters. I am a little apprehensive of using built up battens. Preferring a solid wood deck of rough cut hemlock ( As called for by “Slate Roof Bible”).  Any help or ideas you can give would be much appreciated. 


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  1. Expert Member
    Michael Maines | | #1

    Rockwool insulation needs to be vented, so I would add 2x6 rafters over your existing roof, all the way around the porch. Then you can insulate the sections you want insulated with R-15 batts and leave the rest of the roof uninsulated.

    1. paulmagnuscalabro | | #2

      Seconding Michael here. Figure out how thick the buildup on the insulated portion has to be, and that will drive the overall thickness for the remainder of the roof.

      1. denyank78 | | #3

        Thanks to both of you

  2. otinkyad | | #4

    Just piggybacking on this question, as I was curious about the ventilation aspect for rockwool -- hope that's OK.

    If rockwool in the roof needs to be ventilated, what about rockwool in exterior wall cavities? I ask because I am considering rockwool for exterior wall cavities. A major reason I am considering it is because I thought it was damp- and mold-resistant, as mentioned here: and here:

    Any advice on exterior-wall-cavity rockwool? Is the main consideration that it should be able to dry to the interior..?

    Any thoughts greatly appreciated -- thanks!

    1. Expert Member
      MALCOLM TAYLOR | | #5


      Walls and roofs are very different creatures, and building codes reflect that. Roofs have a stronger moisture drive to the exterior due to the stack effect, and are clad in impermeable materials which preclude drying to the outside.

      The interdiction against using permeable insulation like rockwool or fiberglass against the roof sheathing isn't because those insulations will get wet, it's so that moist interior air doesn't make it's way through them and accumulate in the sheathing above, where (unlike an exterior wall) it has no path to dry.

      Rockwool works very well in wall cavities coupled with a good air-barrier, and the ability to dry to the exterior. Ideally the walls also include a rain-screen cavity.

  3. otinkyad | | #6

    Malcolm, thank you very much! (again! :)

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