Basement Insulation – Mold Problem

By Todd Fratzel on Basements, Insulation

I recently received an email from a home owner in Pennsylvania that’s having trouble with mold in their basement insulation. They are living in a new home built in 2006. The house has a walkout basement and the building inspector forced the builder to install insulation along the basement walls. So the builder framed 2×4 walls up tight to the concrete block walls and insulated the stud bays with foil faced R13 fiberglass insulation. During the winter months there are visible signs of water and ice along the walls. The real clues here are shown in the attached photos.

Why Did Mold Begin To Grow Here?mold-on-basement-wall
It’s hard for me to say exactly what has happened here because I’m not there and I don’t know all the facts. However, I can certainly make some assumptions and give you my advice on this disturbing situation.

Mold needs moisture and food in order to grow. Food for mold can come from paper, wood, carpet and many other things. So it’s easy to see that if you trap moisture between a cold damp concrete wall and a blanket of fiberglass insulation that you’re likely to have a mold problem.

Where Is The Moisture Coming From?moldy-basement-insulation
The home owner stated that the relative humidity in their basement is approximately 40%. This doesn’t seem like a very high number to me. However, I do know that concrete walls (even concrete block) are very damp and full of moisture. In addition to that, the soil on the other side of the concrete all is typically at a constant temperature of 50 degrees F. This means that the concrete is most likely also at a temperature of about 50 degrees F.

Most likely warm air from the heated basement space is infiltrating (probably in between studs) into the stud cavities and coming in contact with the cold concrete where it condenses. The newly condensed water then gets trapped in the fiberglass insulation. Once the water gets trapped in this space it becomes a breading ground for mold.

Better Way To Insulate Basement Walls
There are much better ways of insulating basement walls to help prevent mold growth. There are several ways that I like to see basement walls insulated (in order of least expensive to most expensive):

  • Use a combination of polystyrene foam board insulation behind a framed wall that’s insulated with fiberglass. You can read more about how I insulated basement walls in our storage room. This is a good example of using a combination of foam board and fiberglass.
  • Use a combination of polystyrene foam board insulation behind a framed wall that’s insulated with spray-in-place cellulose. you can read more about how we insulated basement walls with cellulose on a new Energy Star Home at work.
  • Use spray-in-place closed-cell foam sprayed directly onto the concrete wall and filling the stud wall cavity. It’s a good idea to frame the wall so that it’s not in direct contact with the concrete, this way the foam can seal behind the wood. This is the best solution and also the most expensive one.

I don’t have all the answers but what I do know is it’s not a good idea at all to place fiberglass insulation directly in contact with concrete. Concrete is never dry enough for this type of condition. I also know that it’s extremely important that you prevent moisture (water) from coming in contact with materials that promote mold growth such as paper, wood, carpet and other building materials. The bottom line is be VERY careful how you insulate a basement. Make sure you place materials such a foam against damp concrete to prevent mold growth.

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the Editor-in-Chief and Founder of Tool Box Buzz. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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  1. D says:

    Can you clarify your third bullet? You say, “Use spray-in-place closed-cell foam sprayed directly onto the concrete wall and filling the stud wall cavity”. You then say, “It’s a good idea to frame the wall so that it’s not in direct contact with the concrete, this way the foam can seal behind the wood”. If the studs are not against the concrete then what will the foam adhere to? I imagine the studs on the left and right sides of the bays but what will prevent it from ‘reaching’ back towards the conrete wall?

  2. Todd - Home Construction Improvement says:

    @ D – What I was trying to say is this: Frame the stud wall an inch or two away from the concrete. Then spray foam the entire stud cavity, let’s say 3-1/2″ stud plus the inch for 4-1/2″ total foam depth. This way the foam creates a foam barrier between the stud and the concrete.

  3. Anonymous says:

    What about framing the wall so that it is off the concrete and using fiberglass insulation with an air gap between the insulation and the concrete? For example: frame stud wall so the front is 7 inches from the wall, and fill the cavities with R19. This would leave a 1 inch gap between the insulation and the concrete and allow for airflow.


  4. Todd - Home Construction Improvement says:

    I fear the gap won’t be able to stop moisture from being absorbed in the fiberglass. During the summer the concrete will give off lots of moisture. I still don’t recommend fiberglass unless you create a proper moisture barrier first.

  5. Jan says:

    Thank you for sharing it. I am a new mom and I am always looking for house tips.

  6. john says:

    Just to clarify..after you install the dow blue the 2×4 wall touching the blue board or did you leave a gap ? I was looking at website and your approach seems simliar to what they recommend..

    • Todd says:

      I put the framing up tight against the blue board. The reason I do that is to make sure the blue board stays in place. I don’t think it matters much.

  7. Kathleen says:


    What about fiber glass installed int he ceiling of a basement, is this OK or could problems result? Also, which direction should the paper face – the ceiling/floor above or into the basement facing the floor?

    Thank you!

    • Todd says:

      @ Kathleen – Fiberglass in the ceiling shouldn’t be a problem as long as your basement is fairly dry. The vapor barrier depends on a couple of things. Do you plan on heating the basement? If so don’t use any vapor barrier. If you’re not heating the basement then the vapor barrier goes on the warm side, up towards the heated space.

  8. Tom Chaplin says:

    Hi Todd,

    Love your website! Renovating the basement of our home, built 1950. Poured concrete foundation, no vapour barrier in the slab or footing. Entire foundation excavated (due to heavy hydrostatic pressure on the front side) and drains added at the footing with 3/4 crushed stone and geotextile installed over them. Also added a spray-on asphalt rubber membrane from the footing to just above grade. Installed 2 inch blue XPS 4×8 sheets with tyvek tape to exterior foundation from footing to top of sill. Had the above grade insulation stuccoed. Metal flashing covers the top of the insulation/stucco. There is a slight air gap between the XPS and the foundation.

    Preparing to renovate and finish the interior of the basement. In a nutshell our joists sit directly on the concrete sill with no capillary break. Chief concern is with the joists. Had 3 inches of spray-in foam insulation added to the top of the sill between the joists and onto the rim joist up to the subfloor. About 1/3 of the foundation has 4.5 to 7.5 feet of exterior foundation above grade with no membrane, just XPS and stucco. The other 2/3 has only about 2 feet of unsealed wall with XPS and stucco. I plan to use XPS and metal studs, with paperless drywall on the interior and heat the basement.

    1. If I seal the interior with foam and vapor barrier am I leaving a sufficient path to water vapor to dry to the outside?

    2. Should the vapor barrier be next to the foam to keep any vapour from entering the stud space?

    3. Is a vapor barrier necessary in addition to the XPS foam given the membrane on the outside of the wall?

    4. Do I really need fiberglass batts given my exterior insulation?

    5. What about water vapour entering the room from the floor? I was considering Delta FL
    as a vapour barrier. Are you familiar with it all or its competitors Subflor, DRIcore?


    Tom Chaplin

    • Todd says:

      @ Tom – Wow you’ve provided some great info and you’re certainly doing your project correctly. Not sure if I have answers for everything but I’ll give it a shot.
      1. Let’s just assume you’re not, does it really matter? Wet concrete is stronger….wet foam won’t grow mold….doesn’t seem to be a problem in my mind.
      2. If you use a thick enough layer of foam you won’t need a vapor barrier because the foam stops the vapor (some debate, the safe number is 2″).
      3. I wouldn’t think so.
      4. Where do you live? What is the energy code requirement? Probably not.
      5. This really depends quite a bit on your site. Do you have a dry basement? Signs of water? What type of flooring do you want to use? Depending on how you answer these questions you may or may not need a vapor barrier. Delta FL is a great product and DRIcore is another good one.

      Best of luck and thanks for the compliments.

  9. Ryan says:

    Thanks for the great info on insulating basement walls. I’m finishing off the basement of my 1920’s Milwaukee bungalow. I’m not sure if there is a moisture barrier underneath my cement slab. What advice do you have for preventing moisture from coming up through the slab?

  10. JKin says:

    Hi i have a new home and have noticed some mold spores behind my insulation as well. I also have not been able to keep humidity levels below 55% and this is with a dehumidifier in the winter in Iowa. Since my house was just built a year ago is it possible these high levels are due to moisture in the new construction materials? And if not how do i locate the source?

  11. JKin says:

    Well this really helped my understanding. The solution the home builder has came up with on our new house was to add a cold air return in the basement and two extra heat ducts to “circulate more air” and they also said i should turn the heat up in the home which i usually kept around 65 degrees. After researching your links it seems they are taking the cheapest route possible and im not sure if they’re route is going to work.

  12. C. K. Torrence, Jr. says:

    In re. to a suspended concrete slab over a basement crawl area : It`s a 2 1/2 inch concrete poured over a 22 gauge metal with a 3/4 ” form board cemented to it. A small bit of the form board has broken off. This is in a home built in 1966. 1/4 of the basement is walk in, 3/4 is a crawl area. The entire basement, about 4100 sq. ft. has been escapilated (sp.?) & treated for mold. My question is, should anything be done about the small amount of fiber board that has broken off . Preventing mold is the concern.

  13. Lisa says:

    Basement wall finished by contractor with visqueen against the concrete and fiberglass batt with the paper against the drywall. In the colder Michigan temps we noticed puddles on the unfinished floor against the wall. The basement is heated. Where we can feel in the wall cavity, the visqueen is moist as is the fiberglass. I believe the visqueen needs to come out with foam board against the concrete and then the wall studs +\- fiberglass batt. The contractors proposed fix is to pull out the fiberglass batt, leaving the visqueen with no insulation in the cavity because this is how they finished basement walls before foam board and the new air pocket will prevent condensation from forming?

    • Todd Fratzel says:

      Lisa – Time for a new contractor! You are correct…remove it all and install a continuous layer of foam (spray or board). This subject is so mis-understood by so many contractors. Good luck.

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