Can you Insulate a cinder block basement foundation with Rockwool?

Victor B June 19, 2019Updated: Aug. 4, 2020

I am looking for advise on how to insulate CMU walkout basement with Rockwool insulation. 

I have one wall fully above ground (walkout side) one wall fully below ground (opposite side) And 2 walls that grade slopes down from top of the wall (100%underground ) to bottom of the wall (100% above ground) when it gets to walkout side . 

I am in climate zone 3A. 

I already have stud walls framed with 1.5”-2.5” gap between back of studs and CMU wall.

I was planing on the following assembly : 

  • CMU Wall (cinder blocks)
  • Rockwool comfortboard 80 1.25” to remove few studs and slide behind the framing and fix to CMU wall with concrete nails. 
  • Comfortbat in studs tight against comforboard 
  • Airgap of 0.25”- 1.25 due to stud location
  • Drywall 
  • Membrain or Intello smart vapor retarder 
  • Latex paint 

In my climate zone I can get away with 1.25” comfortboard R5 for walls that are 50% or more underground . That is half the basement. For other half i can either add batt insulation to 1.25 or try to fit 2” instead of 1.25 “ if space behind studs allows:

My Questions are the following : 

Will there be any issue in having only 1.25” with empty wall cavity ? 

Now Is it better to install smart vapor retarder or just use latex paint ?  Code doesn’t require any vapor retarder in my area. But I am also trying to avoid getting condensation on cold foundation wall in the winter during heating season and want to allow wall to dry in the summer . 

I am also a bit confused about smart retarder and how they would work for basements. 

Basements and basement walls work differently in summer and winter than walls exposed to air . 

Basement walls are always cold but touching earth on the other side that could be 99% humid . So the wall needs to dry to the interior right ? 

How would smart vapor retarder work in winter   If wall gets wet from heavy rain will the membrain open up to let the wall dry to the inside? But if it opens up when it senses moisture how come does it block interior vapor moisture at the same time? 

Responses (6)

Ecohome 3 years ago

Hi Victor,

Word on the street is that mineral wool insulation is fairly termite-resistant since the stone fibers cut their exoskeletons, so they aren't big fans apparently. However, there is not a lot of hard evidence we've found to conclusively back that claim.  

Are you able to insulate from the exterior? If so - you might want to consider mineral wool rigid insulation board all the way up to the above grade wall then perhaps cover them with a cement board to further protect them from termites as well as keep them dry. You could parge the cement board as a finish.  If that turned out to be proud of the above grade wall then you could install a flashing to direct water to the exterior of the cement board. There is of course more to it than that and you would be wise to include drainage as well, but first let us know if that's even a posibility. 

As for the inside, perhaps you could do mineral wool in the stud cavities then cover them with rigid foam insulation boards and seal the joints (see below). That would offer you a possible termite resistant layer, then airtightness and vapour protection, and you could maybe then cover them with drywall? 

Rim joists insulated with mineral wool and foam


Victor B 3 years ago

Thank you for this Idea. I will try to to do that. Another question I have : do you see any issue with having only rigid 2". rockwoolboard (R6) agains the wall in the locations where wall more than 50% below top of soil and having no insulation between studs?   Meanin there will be approx 3.5" of air. space between back of drywall and insulation for half of my wallkout basement foundation wall length 

Mike Reynolds 3 years ago

Hi Victor, 

I don't quite understand the assembly you are describing, can you re-word that? 

Carlos E
Carlos E 1 year ago

Hey Mike following up on this thread as I have a similar situation, I had to cut open my basement walls 4' from the floor to redo my walls due to hurricane Ida coming through and my sump pump going out. I took advantage of the walls being open and had a french drain installed so now I have that 1"-1.5" gap between the slab and my CMU walls. I cut out a bunch of rotted studs about 2' up to replace the sections with scab. Originally there was no insulation or vapor barrier behind the old drywall.  The studs are about 1/4" from the CMU wall. I wanted to add insulation since the walls are open now and was thinking of just installing Rockwool between the studs.  The drain should fix any moisture issues as I've only had weeping on a couple of occasions prior to the drain during heavy rains or rain and snowmelt mix.  I run a dehumidifier and there is no musty smell.  Last and not least I live in north NJ.  Any advice is appreciated as I've searched this topic to exhaustion and am still not 100% sure as to what approach to take.  Thanks in advance.