GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Why not Rockwool for basement brick walls? Warm moist air won’t hit cold brick because insulation reduces temp before air meets wall. Condensation in batt?

Melville2 | Posted in General Questions on

I’ve read the articles Three Ways to Insulate a Basement Wall, “Insulating Old Brick Buildings”, and “How to Insulate a Basement Wall”, and most of the comments. 

It still seems to me (perhaps I missed something) that rockwool batts installed between studs, floor to ceiling, would be fine because even on very cold days the dew point would be somewhere in the middle of the batt, so there’d be no condensation on the wall. (Right?)

On the other hand, I’ve never understood why it’s okay for the dewpoint to be in the middle of the insulation.  Doesn’t that mean there’d be condensation in the insulation?  I don’t know if it would get so heavy as to distort the insulation, but I think it would lose some of its R-value, and although the material itself is not a food for mold, if dust has gotten into it thru air flow then there could be some mold on the dust – unless when it’s warm enough for mold the moisture dissipates.  

When my walls had to be torn down, I found the bottom two feet had zero insulation, so heat / cold moved between the interior and the 5″ airspace behind the insulation with ease, rendering the insulation of little-to-no value.  Is it okay to have batts floor to ceiling?   (on top of the wall frame’s bottom plate)

Toronto = zone 5   


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. maine_tyler | | #1

    Dewpoint is not a blackhole for water, nor a wall. Moisture can, if allowed, continue 'past' a location which is at dewpoint if there is an adjacent area with lower vapor pressure (often lower temperature).

    Dewpoint temperature is the temperature at which condensing action exceeds evaporative action. BUT, evaporative action does not cease entirely. If there is something colder (in your case, the wall being colder than the center of insulation) the evaporated moisture will be driven towards that colder spot.

    What you are probably thinking of with dewpoints being 'in' the wall is when it is foam. In this case, the face of the foam (which should generally NOT be at dewpoint) is acting as a vapor retarder/barrier to slow the migration of moisture into the parts of the wall that are at dewpoint temperature of the interior air.

    Some of these concepts also help to explain why the *relative* permeability of warm side to cold side vapor retarders matters. If moisture is being driven from warm to cold, you want to slow the drive on the entry side more than the exit side. If you're filling a sink with water and you don't want it to overflow, you need the drain to allow flow out faster than the faucet flows in.

  2. Expert Member
    BILL WICHERS | | #2

    Nope, the *moisture* still makes it through the batt, where it will hit the block as the first condensing surface and condense out into water. That's a Bad Thing, unless you're mold, in which case it's just the perfect damp space you were looking for to grow! The issue is that batts are not air or vapor barriers, so both things can migrate through the batts with relative ease. Ridig foam is an air barrier, and usually a vapor barrier or pretty tough vapor retarder, which keeps moisture away from the block, avoiding the condensation issue.


  3. Expert Member


    It's about the First Condensing Surface. Here is Joe Lsiburek explaining it:

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |