Healthy Indoors | 51
with relative humidity you have to know
both the RH and the temperature. Relative
humidity makes it hard to compare indoor
and outdoor humidity levels, where dew
point makes it easier.
You know what dew point is already
though! If you have noticed condensation
on a lemonade glass or beer can, that tells
you the surface of the glass or can is be-
low dew point. The air around that surface
can't hold any more moisture, so the mois-
ture gets sucked out onto the glass or can.
As the beverage and container warm up,
they go above dew point and the moisture
disappears. The same thing happens in
homes, often in unseen places.
The generally agreed upon target is
to stay below a 55-degree dew point. For
more on that look up the EPA Moisture
Control Guide by Lew Harriman. Let's look
at that same house, how much is it below
55F dew point?
The answer this time is almost never,
the dew point was higher than we like to
see the entire month of September 2019.
Why is this important? Check out this
result from dpcalc.org (short for dew point
calculator.)
This is a pretty typical shoulder season
day. It's 70-75 degrees out, and the dew
point is a rather sticky 70 degrees. Still, it's
nice out! Windows are open, and moisture
is pouring inside through the windows like
The Blob.
Note the "Days to Mold" number of 6. If
those conditions continue for 6 days straight,
something bad is going to be growing some-
where inside your house. The good news is
that those conditions don't continue that long
in most cases.
Or do they? It's usually cooler in your
basement or crawlspace, right? Let's see
what's happening there on the same day:
Yikes! 2 days to mold! If your basement
or crawlspace smell musty, this is probably
what's going on.
When we dry out that cool basement or
crawlspace to a 55-dew point, what happens?
Look ma! No risk!
This is a pretty typical shoulder season day. It's 70-75 degrees out, and the dew
point is a rather sticky 70 degrees. Still, it's nice out! Windows are open, and mois-
ture is pouring inside through the windows like The Blob.
Yikes! 2 days to mold! If your basement or crawlspace smell musty, this is probably
what's going on.
