Basement Insulation – Mold
Problem
Submitted by Todd on January 7, 2009
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.
Conclusion
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.
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