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Community and Q&A

Rockwool basement insulation

Dmitry R | Posted in General Questions on

Hello GBA community, 
I just watch this video
and spoke to Rockwool Customer Service technical department to confirm this application for Climate zone 3a. 
According to them they recommended this assembly for my walkout basement. 
When I. asked about potential condescension between insulation and cold FND wall due to insulation being vapor perm. , representative told me that insulation will warm up the foundation wall moving this cold condensing surface deeper in the wall. 0_O

I have one wall 100% underground.  One wall 100% above ground. 2 walls that start as 100% underground to 100% above ground where walk out is. 

In my zone I need R5/13  for basement walls  and R8/13 for walls that are 50% above ground. 
I don’t want to use rigid foam due to potential health and termite concerns  
After addressing exterior grading and drainage basement is dry even during heavy rains. 
Walls are made of CMU blocks. 
Basement will be conditioned during summer and heated during winter. 

Does installation application shown in the video makes sense ? 

I already have walls that are framed with 2×4 that are  1.75″ to 2.25″ away from the walls.

In my area I see. a lot of basement insulated in the cavity only with that air gap left behind studs.

My 2 options are following :  
1) Comfort board 80- 1.25″ to slide behind studs. for R5 all over. And add comfort bat where wall start getting 50%above ground.

or  2) Comfortbat in a cavity only. with 2″ gap behind studs. 

Which one makes more sense?

Am I setting myself fo potential mold problem? 

I will be talking to Roxwool science department tomorrow, will see what they say about it .

Also found this :

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  1. Expert Member
    Zephyr7 | | #1

    > ". asked about potential condescension between insulation and cold FND wall due to insulation being vapor perm. , representative told me that insulation will warm up the foundation wall moving this cold condensing surface deeper in the wall."

    That's backwards, unless you put the insulation on the OUTSIDE of the wall. From the rest of your post, I'm assuming you are insulating the INSIDE of the wall. Insulating the inside surface of your foundation wall will result in less heat getting to the basement wall, resulting in a colder wall.

    Some comments on that video also mention they have it backwards. In physics, heat is energy. Cold is defined as a lack of energy. Cold doesn't move, since it's not energy. Heat does move, as a transfer of energy, from a warmer area to a colder area as the energy attempts to equalize temperatures of two adjacent areas.

    You generally want a non-vapor permeable insulation where there are moisture concerns. Rigid foam is really the best option here. You don't mention what your specific concerns are, but there are three main types of rigid foam: EPS, XPS, and Polyiso. EPS and Polyiso are probably the safest in terms of any outgassing, so perhaps one of those would work for you?


  2. GBA Editor
    Brian Pontolilo | | #2


    I believe Bill is correct here and that the company rep gave you bad information. Interior basement insulation is preventing heat from getting to your foundation walls. Insulation doesn't warm anything; it prevents heat transfer. Most builders prefer rigid foam for interior basement insulation. Here are a few articles that should be helpful:

  3. Dmitry R | | #3

    Thank you for your replies . I have read quiet a bit on a subject and for the most part agree with rigid insulation concept. However I can’t imagine company like roxwool with all their money and research would recommend something that would create a problem. But who knows ?!? I’ll contact their science department and report back here .

  4. Jon R | | #4

    Of your two options, #1 with MemBrain on the interior side of the studs is the better choice.

    1. Expert Member
      Dana Dorsett | | #5

      Agreed #1 with a smart vapor retarder on the interior side is the better choice, but it's less than ideal. The problem is that 1.25" thick rigid rock wool isn't going to be a snug fit in 1.75" -2.25" deep gap, and there is guaranteed to be bypass air currents undercutting thermal performance. To mitigate against that, install the rigid rock wool snug to the foundation, and the batts snug to the rock wool, even though it's leaving a 0.5-1" gap between the batt and the gypsum board.

      On the other hand just the R15 in a 2x4 wall already meets or exceeds IRC code min for zone 3, even when derating the rock batts for not having an exterior side air barrier.

      The video did not discuss the Flexgard Aspire housewrap installed between the rigid rock wool and foundation. That particular housewrap is on the vapor-tight side compared to most housewraps, guaranteed to be over 3 US perms, but tested at 332 ng/(Pa·s·m2) to 373 ng/(Pa·s·m2) , or 5.8 - 6.5 US perms in this third party evaluation:

      That's solidly in the middle of Class-III vapor retardency, comparable to that of standard interior latex paint. The housewrap also would be providing at least some amount of capillary break to limit the amount of wicking of ground moisture into the fiber. That may or may not be important to the moisture performance of the assembly in your local climate and soil conditions, but don't discount it.

      1. Dmitry R | | #7

        Dana, to clarify :
        You recommend- to install house wrap against foundation wall before installing insulation ? What for ? To limit moisture traveling from CMU wall to insulation ? Isn’t we are trying to let the wall assembly to dry to the interior? Then 1.25 “ insulation and then Roxul Bats tight against comfort boards ? Then Drywall with latex paint ?

        1. Expert Member
          Dana Dorsett | | #10

          > "You recommend- to install house wrap against foundation wall before installing insulation ? What for ? "

          I'm not recommending it- only speculating as to why they used in the project in that video. They didn't address it in the video, but might be important, especially since the product used is one of the most vapor retardent housewraps currently on the market, and comparable in vapor retardency to latex paint on wallboard.

          >"To limit moisture traveling from CMU wall to insulation ? Isn’t we are trying to let the wall assembly to dry to the interior?"

          Concrete & CMU foundations don't need to dry toward the interior. Those types of foundations function just fine even when fully saturated with water, and are not degraded by moisure unless it's constantly moving through the concrete at a high rate taking minerals with it. It's perfectly fine to put a true moisture barrier on the interior side of a concrete or CMU foundation. If there is a capillary break between the foundation wall and wood foundation sill it's fine to even use sheet EPDM or heavy polyethylene between the foundation and insulation. (EPDM that comes up and over the top of the foundation, under the sill makes a good capillary break for both the sill and insulation.)

          Rock wool on the other hand will wick moisture toward the moisture & mold-susceptible finish interior. The rock wool needs to dry toward the interior, the foundation does not. So putting a drain plane material that is also vapor retarder between the concrete/CMU and the moisture-wicking fiber doesn't hurt, and may even be necessary if there is much ground moisture making it into the foundation wall.

          >"Then 1.25 “ insulation and then Roxul Bats tight against comfort boards ? Then Drywall with latex paint ?"

          That would work.

  5. Nick Welch | | #6

    Rockwool is also claiming that their new coal-fired plant will have no negative health or environmental impacts. That plus their known flawed basement insulation advice makes me pretty suspect of anything they say.

  6. Dmitry R | | #8

    Ok, I spoke to Rockwool science person. He said what John R said. He advised to install 1.25 comfort board behind studs for R5. then Membrain. then drywall. for locations 50% above ground to add cavity insulation tight against comfort board. then MemBrain. then drywall. He was not concerned about 1" airspace between bats and drywall or empty cavity.

  7. Dmitry R | | #9

    I did some reading and I think I understand how these smart vapor retarders work.

    This is what tech brochure says:

    "Mixed-humid climates are defined as regions that receive more than 20 inches of annual precipitation, with approximately 4,500 heating degree days and a monthly average outdoor temperature below 45°f in the winter. this winter temperature is critical for building design since air at 70°f, 35% relative humidity (rH) has a dew point temperature of approximately 40°f. as a result, interior air at 70°f, 35% rH (a realistic condition in a tightly constructed home) will condense on a surface that is 40°f or cooler, thus leading to homeowner comfort issues.
    How much water will condense on the interior surface of the exterior sheathing (the first condensing surface) is
    a function of the dew point temperature of the interior air, the amount of air leakage into the wall, and the vapor permeability of the materials in the wall. Small amounts of condensation will not be a problem if the wall is allowed to dry to the exterior.
    Moisture is more of a significant problem in this climate than in areas that receive 20 inches or less of rain per year
    in a single climate area. That’s because the ambient air in mixed-humid climates tends to have significant levels of moisture most of the year. In addition, since air conditioning is installed in most new homes, cold surfaces are present on which condensation can occur.
    Since both heating and cooling occur for extended periods in mixed-humid climates, it can be difficult to determine the correct method for moisture control. To add to the confusion, The American Society for Heating, Refrigeration and air Conditioning engineers (aSHrae) and the energy and environmental Building association (eeBa) recommend that wall systems be allowed to dry toward the interior or exterior environment in mixed-humid climates and, if possible, allow some drying in both directions."

    Can they ever fail due to different RH conditions than those described ?
    It says that it remains moisture tight in winter when humidity
    in the cavity is low. and Increases permeability when RH is above 60% in summer to let moisture escape when needed.
    For my basement wall can there be a situation where RH is high in the cavity in winter when I am heating inside basement? How does MemBrain is going to work in this or any other scenario? Is it going to open up ? If so aren't we are trying to block interior vapor from condensing on foundation wall in winter?

    1. Jon R | | #11

      In the Winter, your interior relative humidity should be low and with the MemBrain close to the interior, it will also be dry. So it will let little vapor pass from the interior to the concrete.

      You should be far more concerned about air movement (for example, from the interior, to the cold concrete wall and then back to the interior).

      Wetter concrete walls have two potential issues. a) an increased chance of dumping water on the floor when there is a little bulk water wetting from the exterior and b) an increased chance of mold and the odor from it escaping to the interior. Drier concrete (which occurs with some inward drying) is better.

      I'd use unfaced EPS.

  8. Al Cobb | | #12

    Rockwool is a multi-national foreign corporation that puts their manufacturing facilities in the poorest States with the least stringent pollution controls. Their newest plant in West Virginia is slated to burn 80+ tons of coal and 1.2 million cubic feet of fracked gas per day. If health concerns are really at the forefront of your decision making, stop believing the green-washing from this company as their air pollution numbers are on par with the worst polluters in the world.

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