- Q.How can I get rid of
mildew growing on joists in an insulated crawlspace?
The mildew is only on the bottom edges and sides of the
joists where the batt insulation doesn’t cover
them. I suspect it’s from warm, humid air
wafting through the basement in the summer. The soil is
basically dry.
A.Paul Fisette
responds: Mildew needs a local relative
humidity of around 70% to grow. Even though the
earth feels dry to the touch, it isn’t.
That’s probably where the moisture is
coming from. (I’m assuming there are no
other obvious sources of moisture, such as a
reverse grade outside, or gutter downspouts dumping
water against the foundation.)
As a first measure, you should install a
polyethylene ground cover throughout the
crawlspace. There is quite a bit of research
showing that ground cover eliminates up to 75% of
the moisture that ends up in the air as humidity in
a crawlspace. The mildew itself will probably not
cause any great problem in the crawlspace, but this
moisture eventually moves up into the walls and
living spaces through wiring and plumbing holes,
and other bypasses.
Take some humidity readings in the basement
before you put down the poly and after, when the
basement has had a chance to dry out. If this
doesn’t make a big difference, you will
need to search harder for the source of the
moisture.