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Insulating against brick wall in attic–Advice Still Needed

mfleck | Posted in General Questions on

Hi all,

I’ve put a lot of work into insulating my attic cathedral ceiling. Before I get a crew to drywall the attic ceilings and walls, I’m trying to figure out what to do with the two triangular brick walls at either end.  When I bought the home 2 years ago, there was 2×4 framing against these walls with rock wool batts between and drywall over that. The rockwool was right against the brick—which, as I’ve read here, is not advisable.

I removed the drywall and pulled back some sections of rockwool to look at the brick and didn’t see anything too concerning. The house is 1930s and the rock wool’s been there at least 50 years.

ive also looked closely at the brick on the outside of my home, especially on the chimney and parapet. Everything there seems to indicate that I have good bricks as there isn’t any visible evidence of damage from freeze/thaw. 

im wondering what to do about these walls in terms of insulation. Seems like the safest approach would be to pull the rock wool and cut and cobbled in some rigid foam between the 2x4s. The evidence above, however, seems to suggest that I could get away with leaving the rockwool, maybe adding a layer of rigid foam over the 2x4s to minimize thermal bridging through the 2x4s, and then Drywall through the rigid foam and into the framing.  This would save a lot of old insulation from a landfill too.

i should mention that the house is at the northern edge of NYC  


thanks, Matt

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  1. brendanalbano | | #1

    If you do decide to leave the rock wool in place (and hopefully some other folks will chime in with advice about that), you might consider adding a smart vapor retarder on the interior, like in figure 9 at the very end of this article:

  2. mfleck | | #2

    Hi all,

    Still seeking some advice from the experts here on this question.

    Can anyone advise?


  3. Expert Member
    AKOS TOTH | | #3

    Insulation directly against brick is an issue if you have water problems. Up in the attic, except around chimneys, this is rarely the case.

    Since that section has been insulated in the past and there is no signs of moisture problems, I would not change anything, even leaving the existing batts as is.

    If you want slightly higher R value, putting 1" of polyiso under the drywall would bump up your assembly R value to the same as 2x6 with batts. You hang drywall directly over the foam with longer drywall screws.

    Just make sure to get the new construction nice and air tight, most important part in making sure you don't have issues down the road.

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