[A/S] Condensation

["George Butler" <gbb132000@xxxxxxxxx>]

An earlier poster indicated that condensation forms on surfaces 10
degrees below ambiant temperature.  Having worked many years for a
window and door manufacturor, I wanted to clarify...

Condensation is moisture from the air that condenses on a cold
surface.  How cold the surface must be depends upon the relative
humidity of the air and the ambiant temperature.

Relative humidity is an expression of how saturated the air is with
water.  If the relative humidity is 100%, then the air is holding all
the water it can for its' present tempurature.  If relative humidity
is 50%, then the air has only half the moisture content that it is
capable of holding.  Warmer air can hold more moisture than colder
air, just as hot water can dissolve more sugar than cold water can. 
If you take 70 degree air with, say, 60% humidity and cool it to say,
35 degrees, the air won't be able to hold the moisture anymore and
condensation will result.

For a given temperature and humidity, there is a "dewpoint"
temperature.  This is the temperature at which moisture will start to
condense out of the air.  At typical room temperature and humidity, a
cold drink will present a surface temperature below the dewpoint, and
thus moisture will form on the surface.  If it is cold outside, then
window glass will be cold, and if there is very much humidity in the
air inside, then there will be condensation on the window.  Of course
many variables come into play - HOW cold is it outside?  HOW humid is
it inside?  Do the windows have insulated glass (and thus HOW cold are
they inside?)

Regarding ventilation,  living (breathing, bathing, cooking, sweating)
adds humidity to the air.  Without ventilation, the living space gets
more and more humid, and eventually you will have a problem.  Bring in
 outside air, and that will usually help.

People get confused sometimes when they hear that the air outside has
high humidity.  If it is thity degrees and snowing, then the relative
humidity outdoors may well be 90%.  Be aware that when you bring that
air inside and heat it to room temperature, the relative humidity will
drop like a rock.  You won't have removed any water from the air, but
warmer air has much more cpacity to hold water, and thus with the same
water content and a higher temperature, the relative humidity drops. 
The water is still present, but it won't cause a problem.

Sound confusing?  That's because it is!!  Attemps to simplify it often
lead to errors in fact because it just isn't a simple subject.

Ventillation and moderate temperature are the keys.  Try to get as
much fresh air into the living space as possible.  Don't cover window
glass with interior window treatments - let fresh air get to the
glass.  Ditto any other surface that shows condensation.  Run interior
fan(s)to keep air moving around within the living space.  And try not
to keep the living space unreasonably warm.  I'm not saying you have
to freeze, but be moderate.  Usually these steps will help a lot.

I don't have experience with campers in cold weather, but I bet that
our campers are not anywhere near as well insulated as a well
constructed residence... so this issue is liable to be a challenge for
those who boondock in the snow.

George Butler
19ft Bambi due at the dealer this week!