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?
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?
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.