Prevent Mold Growth Under Fiberglass Insulation
Written by Todd Fratzel.
Missing Vapor Barrier Leads To Mold Growth
Quite often we get emails from folks asking why their walk-out basement walls have mold growing on the inside of the exterior wall sheathing. Some folks even find frost under the fiberglass insulation on extremely cold days. The following is such an example and why this has happened.
Insulation Vapor Barriers Are Critical
If you’re going to insulate a wall with fiberglass insulation it’s absolutely necessary to understand and properly install an effective vapor barrier. In cold climates water vapor will move from warm damp areas to cooler drier area.
In basements similar to the on in this example that means the water vapor in the basement wants to migrate outwards from the basement to the outside of the house. When that happens the vapor can get trapped behind the fiberglass insulation at the backside of the wall sheathing. Once trapped there it can lead to mold and mildew.
The photos were taken from a house with a walk-out basement. The framed walls along the walk-out side of the house were insulated with un-faced fiberglass insulation with no vapor barrier. This particular basement is not currently heated but does maintain a minimum temperature of at least 55°F.
No Vapor Barrier = Mold Growth
It may be hard to see in the 2nd photo but under that fiberglass insulation was a layer of frost and also a small area of mold that has begun to grow. This particular basement is only a few years old and it looks like the process has slowly started to take hold on all of the framed walls.
This is a perfect example of why vapor barriers are so crucial to insulating with fiberglass insulation. Clearly the water vapor was trying to escape the basement and it was trapped between the OSB wall sheathing and the fiberglass insulation. When the vapor hit the cold wall it condensated and then froze into the frost that’s currently on the wall. It’s safe to assume that once warmer weather came the damp fiberglass insulation helped promote the growth of mold that we see in the photo.
How To Fix This Problem
For this basement we recommend that all the fiberglass insulation be removed and disposed of. Next the walls need to be dried out. Once the walls are dry they should be scrubbed with a mixture of water and soap detergent. Next it’s a good idea to wash/scrub the surface with a mixture of 1/4 cup bleach to one gallon of water. After the surface dries another step that’s recommended is to apply a borate -based detergent solution without rinsing it off. Some laundry and dish washing detergents contain borate which is listed in the ingredients.
Obviously if you’re going to do your own mold removal you should wear proper safety equipment and seal off the remainder of your home. This article does not provide sufficient safety information on safely removing mold and we recommend you seek additional information. In some severe mold cases it’s best to hire a professional.
Once the mold is removed, new fiberglass insulation (better yet a new product such as dense-packed cellulose insulation or spray foam insulation) can be installed in the stud bays. Be sure to use either a kraft-faced insulation or a proper vapor barrier. Kraft faced insulation works well if you’re going to cover the walls right away with drywall and paint them. Most paints today provide a decent vapor barrier so the combination works well. If you’re not going to drywall right away then we recommend a 6 mill poly with all the seems taped and sealed really well.
This is such a good example of why vapor barriers are so important for fiberglass insulated walls. It doesn’t take very long for a potentially serious and expensive problem to develop if your walls arean’t properly sealed against water vapor. A simple step like installing a vapor barrier can save so much time and money in the future. Take your time and do it the right way the first time.
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