Prevent Mold Growth Under Fiberglass Insulation

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• i was hoping to avoid all of the expense of the blue board
  (or foam board) glued against the basement wall, instead
  using a 4 mill plastic sheathing up against the basement wall. then studding that same wall and adding a faced fiberglass
  insullation and then finally dry wall over it all. but will this
  promote moisture and mold between the wall and the drywall?
  thanks, tim in ohio

  • Tim - The problem with this approach is 2 fold.

    1. You will create a double vapor barrier which in itself is a no no for fiberglass insulation which will most certainly cause a mold problem.
    2. When you install the plastic, the concrete behind it is still cold. So you create a cold plastic surface. If any moisture and I mean any gets into the stud cavity it's going to hit the cold plastic and condensate. Once that happens it's only a matter of time for the mold to start growing.

    I highly recommend using some sort of foam product or combining it with fiberglass as a slightly cheaper alternative.

• I am really confused. I live in RI & have a 6 yr old house with an unfinished walk out basement. The wooden wall portion of the basement is insulated with fiberglass with a plastic vapor barrier. I have found mold growing on the interior side of the exterior walls and also on the insulation side of the plastic vapor barrier. I have also found instances where the fiberglass insulation is soaking wet, as is the exterior wooden wall. I have a dehumidifier running constantly. Any ideas.

  • Steve - This problem is seen very often with walkout basement walls. It's actually a pretty complex subject that has different issues resulting in different approaches. The following are issues that may have lead to your problem.

    - Pour sealing of the plastic vapor barrier. If the vapor barrier is not sealed well than warmer, damp air (during the winter) can move towards the cold sheathing, hit it, condensate and then freeze. Later on the ice melts and mold can start.
    - In some situations water vapor can reverse the direction during the summer months. Warm, damp outside air can penetrate the wall sheathing, enter the wall cavity and hit the cool inside surface of the vapor barrier (basements are typically quite cool in the summer).
    - Another problem can potentially come from the pressure treated sill plate. If you've sealed up the outside well, the inside well and now water vapor from the drying PT sill can get trapped in the wall cavity.

    As you see there are many problems with these wall types. In fact, the problem can be so difficult to solve that I recommend only using foam in basement walls (at least on the exterior surface, sometimes a combination of foam and fiberglass can work).

    Make any sense?

    • Thank you so much for answering my question. I'm thinking of trying the following, plese give me your opinion.

      1. Since most of the mold and wet spots appear towards the top of the walls on the side of the house which receives the most sun during the day, I'm thinking of placing a piece of styrofoam insulation about 2 feet in length between the exterior wall and the fiberglass insulation.

      2. In addittion I'm also thinking of leaving the top 2 feet of the fiberglass insulation exposed and only covering the remaining portion (approximately 7 feet) of the interior walls with the vapor barrier.

      I find it strange that one section will have mold or be wet and the section nest to it will be dry and mold free?

      Thanks again for your help.

      • Steve - Insulating basements is VERY difficult. I think your solution should help but I'd be nervous it may push the problem lower. I would give it a shot but keep a close eye on it.

        • I guess this really is a complex problem & I appreciate your help. Just a couple of more questions if you don't mind.

          1. Do you think it would be beneficial to put foam the entire length of the walls on the sunny side of the house? I'm especially worried about the particle board making up the exterior walls rotting, as I've already discovered one small area. I would then cover the foam with fiberglass and replace the vapor barrier.

          2. Do you see any advantage to leaving a space at the top of the vapor barrier?

          Thanks again.

        • Steve - If it were my house I'd use foam the entire height and seal the edges with spray foam. I think in your case I would consider installing fiberglass with paper vapor barrier as it's a bit more forgiving when it comes to breathing. Not sure the space is a good idea although with foam in the cavity it may not be that big a problem.

• hey todd,
  i am confused as hell. i am renovating my basement.i gutted all the exterior walls and exposed the block walls. i have started framing the exterior walls,after reading articles about vapor barriers i am worried that my framing/vapor barrier is wrong. my basement walls are cinder block;in some areas i see the previous owners applied tar on the bottom half of the walls.i am framing with spruce 2x4s installing,then applying a tar paper underlay(like they use for roofing) on the back side of the wall and staple it down. i have been building my walls on the floor and then installing them using this this detail and methods on all my renos. once the wall is built i install it in place against the block wall. i will then install some r20 pink insulation between the studs, then install a 6mil poly over the wall,staple tucktape joints and i'm ready for drywall.

  Have i screwed up?help me out.

  • Gino - I personally haven't taken apart a wall with that detail to know for sure what the outcome will be. The tar paper certainly is a different approach and may keep some moisture out. However, I would be worried that if water vapor gets past either the tar paper or poly that you would have a cool surface (tar paper against concrete) for the vapor to condensate on. Tar paper has been used for years very successfully in areas that get damp so it might just work. I'd also be concerned about having tar paper on the "inside" of the home as it surely off gasses some nasty stuff.

    I hope this helps.

  • What type of climate are you in? Are you using the tar paper as a vapor barrier or to keep the white wood from contacting the block wall? Also if you are putting an R 20 in a 3 1/2" cavity you may end up compressing the insulation and loosing much of the R value beond R13-15

    • Brian - This is located in northern New England, no wood contacting concrete and it's R19 insulation in a 5-1/2" cavity;

• We have a new home and are planning on finishing the basement. The basement is walk out, with 2 sides completely framed. It is 2x6 walls, and they have been filled with "Spider Spray". I do not know if there was a vapor barrier installed before the insulation was sprayed in.

  What would you recommend to finish this area?

  • Tom - I'm not familiar with Spider Spray. Can you tell me more about that?

    • Our builder only uses this. The only details I can find are from this link:

      http://www.specjm.com/files/pdf/bid0060_SpiderDataSMech.pdf

      It says it is mold resistant, but I am unclear what that really means.

      I was hoping to just put wallboard up over this, but want to make sure that it will not be being torn down in several years due to mold. The builder said I can just wall over it.

      Thanks

      • Tom - That product is a sprayed applied fiberglass insulation. It's probably fine to wall board over. However, I would caution you that once you do that it's best to be sure you condition the space and watch the humidity levels.

        • Thank you.
          Another question...

          As I said before, 2 sides of basement are completely framed. The third side is block that has been sided on the exterior. The back side is block that has 2" of insulation on the exterior side and is below grade for all but the last 12 ft. (Our contract called for 2" exterior insulation for all below grade foundation.) From what I understand from the reading of the Minn. study, there should be no interior foam board if there is also exterior foam board. Is that correct? If it is, it will save quite a bit on the foam board insulation since I will only need to do 1 wall plus 12 ft of the other wall. But, can I frame and put FG insulation directly against the below grade wall?

          Thanks again.

        • Tom - Forgive me if I'm confused..I'm on the road this weekend with limited access to internet.

          I'm not 100% convinced that foam can't be placed on both sides. However, at the very least I'd recommend the entire inside be foamed if there are areas on the outside that are exposed. I'm not entirely sure why someone would say NOT to do both sides. Concrete is full of water for it's entire life span and the presence of foam won't change that.

          The reality is you need to stop both cold and moisture from leaving the concrete and getting into your finished space.

          Make sense?

• I am not trying to be difficult or confusing, and I appreciate your comments and site. It is just that there is a lot of conflicting information out there.

  From http://www.eere.energy.gov/buildings/building_america/pdfs/db/35017.pdf "Basement Insulation Systems"

  It says on page 6,
  "Effects of Insulating Basement Walls
  From a moisture and thermal perspective, basement walls with insulation on the exterior perform better than basement walls with insulation on the interior. Walls with exterior insulation are “warm” and can dry to the interior. Since the walls are warm there is little risk of condensation of interior moisture. No vapor barrier should be installed on the interior side of externally insulated basement walls. In fact a vapor barrier on the interior would prevent the walls from drying should they ever get wet. However, exterior insulation is rarely installed because of perceived difficulties protecting it from damage during backfilling..."

  It is saying nothing about the walls being "dry", but rather that the walls are "warm".

  I like your suggestions of how to insulate, but man those panels are not cheap.

  • Tom - There are some fundamental problems with what they are suggesting.

    First of all it's very difficult to insulate the exterior properly unless it's new construction. Insulating the exterior during new construction can work when the insulation can be carried up to the above grade framing.

    Anyone that thinks a foundation can be dried out completely doesn't understand concrete material science. Concrete contains millions of micro pores that trap water. I know this because I have a masters degree in structural engineering with a focus on concrete material science. The idea of drying out to the interior is such a bad idea (IMHO).....should that moisture be allowed to enter the wall cavity? I think not.

    If you want another perspective then I suggest reading more about this topic over at:
    http://www.buildingscience.com/documents/reports/rr-0308-renovating-your-basment/view?searchterm=basement
    You'll see a similar set of details.

    The reality is there are SO many basement insulated improperly in this Country and there's a TON of bad information out there.

    • Thanks again.

      Interesting that both reports cited were both done by Building Sciences Consortium. The one you refer to though is more recent (by ~4 yrs).

      I guess I will go with the full coverage.

      Thanks for all of the instruction, we never stop learning.

• I wanted to ask a couple questions. I bought a new house in Iowa and since it has become cold I have noticed condensation on the interior of all the windows and we do not have a humidifier on the furnace. Also I have noticed in the basement condesation surrounding the windows basically the concrete around the window is all soaked. Also i pulled back the insulation that goes halfway down the wall and also found random wet spots and frost. The home builder told me that i needed a dehumidifier because there is alot of moisture in new construction materials and to turn on my fresh air intake. I have done both with minimal results what should I do next? This home is still under warranty.

  • Your building is not wrong. Not just new homes but all homes have serious moisture problems. We build them so tight today that we trap moisture in the air from cooking, showering, plants, pets, etc. You should use a dehumidifier, also run your exhaust fans longer after showers. The wet insulation, does it have a proper vapor barrier?

    • I guess im not sure what the proper vapor barrier would be. The insulation is fiberglass directly against the wall which has like a plastic outside if that makes sense. The basement is roughly 1400 sq ft I have the biggest dehumidifier i could buy which has helped with the window condensation however the wet spots under insulation remain.

      • Is the insulation completely wrapped in that plastic? or is the plastic just covering it on the room side?

• The insulation only has plastic on the room side its not even really plastic id compare it to like a thin tarp like material. But yes plastic only on room side against the wall is just the insulation.

  • Sounds like the plastic (vapor barrier) isn't sealed well. Are the seams taped? If not water vapor is getting behind it, hitting the cold surface and condensing. You'll need to correct that or replace it.

• Well it only goes half way down the wall the lower half is exposed so there is actually about a one inch space between the foundation and insulation.

  • I'd start all over.

• I have a crawl space foundation that was poured in 2003 and left exposed to the elements inside and out. The exterior wall has been water proofed with the tar but has not been back filled inside or outside.

  What are you suggestions for insulating this. The county does require the crawl space to be ventilated.

  • Building Science . com has some great details for crawl spaces. Crawl spaces are very difficult to insulate and ventilate properly. I would follow their advice to the letter or you will likely be disappointed with the outcome.

    Good luck.

• I insulated my basement with a system found reading basement insulation systems on the building science site which includes:

  3/4" Extruded Polystyrene (seams taped)
  2x4 walls (bottom plate treated wood)
  R-13 Fiberglass insulation (faced)

  The info came from page 10 on their site:
  http://www.buildingscience.com/documents/reports/rr-0202-basement-insulation-systems

  I was not able to find un-faced r-13 which is what they recommend. I know this is not a perfect system per your articles. Should I be concerned with the faced fiberglass trapping moisture in the wall?

  I have not finished mudding the drywall and would be willing to remove it to tear the face off the insulation.

  Thanks

  • Jay - I wouldn't. My bigger concern is the 3/4" foam which will require you to really keep an eye on the humidity in that room. I always recommend a minimum of 1-1/2" to create a proper vapor barrier.