My wife and I recently purchased a mid-1800's brick schoolhouse and are remodeling it from top to bottom. We stripped the walls down to the original brick (3 brick layers deep), and we then proceeded to stud the brick walls with 2X4's in preparation for adding Roxxul stone wool insulation. Our plan is to install the insulation and hang 1/2" drywall to finish the wall. My question is whether or not it is necessary to install a vapor barrier after putting the insulation between the studs. I've read conflicting opinions on the use of vapor barriers with brick. Some people have said installing one can create mold issues in the wall and will not allow the wall to properly dry, while others say it is essential as the exterior of the brick can get icy due to moisture escaping from the porous brick. I am seeking some clarification and advice on how to tackle this problem! We live just north of Cincinnati on the boundary between climate zones 4 and 5. Any advice and help is much appreciated!

Thanks for reading.

  • Exterior brick and concrete surfaces can get wet but at 3 thick I would not be concerned since you have the studs in place already. For me with below grade walls I don't use insulation as there is no air movement and the insulation doesn't help any more than an air space that is well sealed.
    – Ed Beal
    Commented Jun 20, 2018 at 13:54

2 Answers 2


NEVER use “expanding spray foam” on brick walls for four reasons: 1) expanding foam could put stress on the brick and crack the brick or the mortar joints, 2) too much insulation on brick could block heat transfer to bricks and cause the brick to crack, 3) moisture could get trapped in the brick, and 4) joists embedded in the brick wall could rot causing structural decay.

1) Spray foam is acceptable (and may be the best choice, although expensive) but EXPANDING SPRAY FOAM will crack the brick or crack the mortar joints. Rigid foam is acceptable, but must be carefully installed so as not to allow moisture to transfer into the wall, freeze and then crack the wall.

2) currently, warm moist air transfers into the brick and keeps the brick (and moisture) from freezing. Thickness of insulation must be considered so as not to make the inside of the brick too cold, freeze and then crack the brick.

3) you’ll need to determine if the type of brick will “absorb” moisture. Any moisture transferred from the exterior into the brick or any moisture transferred from the interior (via dew point) could freeze and crack the brick.

4) If joists are embedded in the brick, adding insulation could make the ends of the joists (on the cold side of the insulation) rot.

Some bricks are more vulnerable than others to freeze/thaw problems. This usually has to do with permeability. The thicker the insulation the greater the chance of cracking the brick wall during extremely cold times.

  • Oh, and never apply plastic sheet vapor barrier on the inside of your stud wall. It will trap moisture in your wall when the season changes and vapor starts moving from the exterior to the interior.
    – Lee Sam
    Commented Dec 24, 2017 at 9:25
  • Sam you really spend more time on spray foam. The op is planning on using Rockwood insulation, and nothing on the vapor barrier how is this answer helpful?
    – Ed Beal
    Commented Jun 20, 2018 at 13:46

Consider shooting the interior of the brick walls with expanding foam insulation. It will provide all the moisture barrier needed well as some fabulous insulation. It will plug all the cracks that no other insulation can match. You can then put the stone wool between the studs. Another heavy plastic barrier would be a good idea on top of this insulation. Then the sheet rock. People don't realize the tremendous heat loss incurred through infiltration. Nothing stops infiltration like expanding polystyrene.

  • Plastic as a vapor barrier is the fastest way to rot a wall that I know of, vapor bariers need to breathe.
    – Ed Beal
    Commented Jun 20, 2018 at 13:50

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