Condensation Is All Relative: Humidity That Is:
It frosts up your windows on little cat's feet
and mists your walls with a delicate fine sheet.
Outside your house the moisture is trying to go,
but cold surfaces condense it giving mildew
places to grow.
With unusually cold weather, the moisture on the
ground and high winds, we are seeing more window
frostings and much more mold and mildew in
houses than usual. The reasons are
straightforward enough; the increased soil
moisture contributes extra moisture into houses
and the colder air and strong winds create a lot
of very cold surfaces on which the extra
moisture can condense.
Side Effects:
In extreme cases moisture can be seen running
down windows and walls with mildew and fungus
rapidly covering entire walls or attic roof
sheathing. It ruins paint, warps wood trim and
nearby furniture, and damages carpeting and
flooring. Clothing in closets becomes covered
with fungus, and often must be discarded.
The mildew and mold smell foul, and the huge
concentration of spores in the air trigger
allergic reactions. Some houses can become
impossible to live in.
First Some Basic Facts:
The warmer air is, the more moisture it can
hold. If air at a certain temperature is holding
all the moisture it possibly can, it is said to
be at 100% relative humidity. If it is holding
half of what it possibly could, it is at 50%
relative humidity. Air at 100% relative humidity
would have to contact something only slightly
cooler to condense some moisture while air at 50
% relative humidity would have to contact
something a lot cooler to condense any
moisture.
When the weather isn't too cold, the ideal
indoor humidity of 35% can't contact anything
cold enough to condense any vapor; but let the
thermometer drop enough and the glass of window
panes (even sealed insulated glass ones), metal
frames on sliding glass doors, and some cold
spots on outside wall corners will condense
vapor. For some houses with particularly cold
window frames like the steel casement type,
routine moisture condensation changes to icing
when the biter cold weather strikes. If you have
a power humidifier attached to your forced air
heating system, be sure to lower the humidistat
setting when the weather gets really cold. A
temperature/setting scale is usually included
right beside the humidistat.
The Solution Is Simple: You can either
lower the interior humidity level or warm up the
surfaces on which the moisture condenses. Since
the latter is often difficult in the case of
sliding doors or quite impractical in the case
of un-insulated exterior walls, it is usually
best or easiest to work on lessening the indoor
relative humidity. An inexpensive combination
hygrometer/thermometer can be purchased for less
than $10 and will let you know when high
relative humidity is the cause of your
problems.
Simple Cases:
Throw out excess moisture. If your kitchen or
bath windows or walls sweat a little after
cooking or showering, that is to be expected.
Use your exhaust fans (re-circulating ones in
kitchens won't help) if you have them and
consider installing them if you don't.
Warm up windows and frames. Install exterior or
interior storm windows and consider applying
wood trim over metal trimmed windows. Bed the
trim in caulking to ensure continuous contact
and prevent condensation between the metal and
the wood.
If your windows already have storms over them
and the interior sash frosts up, the exterior
storm window is too loose. Check the caulking
around the storm window frame (be sure to leave
the two weep holes along the lower edge open)
and the felt along the sash edges. If the storm
sash gets frosted, the interior window parts fit
together too loosely. Use 3M's "V" strip or rope
caulk or felt around the edges. Be sure the
individual sashes are pulled tightly together
when the lock is secured.
Tougher Cases:
If you find frost on the underside of your roof
sheathing, it means that there is insufficient
ventilation in the attic to discharge the
moisture from the living space below. (This is
common on older houses which have had siding
applied. The installer frequently covers the
gable louvers over with perforated siding
pieces.) The general rule of thumb is that you
need one square foot of free (screening reduces
air flow and effectively cuts the rough opening
size in half) ventilation for every 300 sq. ft.
of attic floor space if there is no vapor
barrier under the insulation and 150 sq. ft. if
there is. Increase ventilation by recapturing
your gable vents, adding soffit vents, or
installing a ventaridge system along the peak of
the roof.
Condensation high up on wall ceiling
intersections usually results from outside air
currents through the framing and against the
wall surface. This is usually due to sloppy
framing or caulking or insulation that has been
skipped or moved out of position, or some
combination of all of the above. Caulk any
outside cracks and survey the ceiling from the
attic to see if any insulation has been missed
or blown back. Go behind knee walls or walls
that back to inaccessible attic spaces or
garages since wall insulation here is
notoriously poorly installed and frequently
spottily done. Always inspect behind the gutters
in attics with loose fill insulation since wind
currents will frequently push the insulation
toward the center of the house. Lay short
sections of blanket insulation in the voids.
Darkened wall or ceiling areas are usually dust
collections due to moisture condensed over
colder (poorly insulated) areas. Occasionally
darkened outlines of the wall studs or attic
truss chords will show as darkened vertical
lines running up the walls or across the
ceilings. The wall lines are typically 16 inches
apart (reflecting the stud spacing) while the
ceiling lines are usually 24 inches apart
(reflecting the truss placement) The cause is
usually a dirtier surface due to dust collecting
in the somewhat moisture surface areas. Confirm
by washing or erasing the dirt with a pencil
eraser.
Impossible Cases:
Occasionally a downdraft furnace with slab ducts
will get water in the ducts and become one giant
humidifier. The water must be drained from the
ducts (usually with a small sump pump under the
furnace) and kept from returning and usually
with proper landscaping outside. Occasionally
the slab duct system must be abandoned for a
duct system in the attic or along the walls.
Solid masonry buildings with plaster walls that
are retrofitted with tight fitting windows
sometimes get condensation and mildew problems.
The new windows being tighter than the old ones,
cut down on air infiltration and build up
moisture condensation. The plaster walls and
ceilings are good vapor barriers, so the
moisture stays inside to condense on the
un-insulated walls. These buildings usually
respond well to air-to-air heat exchangers, a
device that promotes ventilation without losing
much heat energy.
Here are some common sources of increased
humidity that often go undetected. Crawl spaces
under houses without plastic over the dirt. The
earth's moisture rises right up through the
floor.
Downspouts that empty close to the foundation
and poor grading away from foundation walls.
Basements with sump pumps usually have water in
the sump pit and back under the slab itself.
Moisture can rise through the slab and into the
house. A piece of plastic left on the floor
overnight will usually show vapor on the
underside in the morning if there is a problem.
Uncontrolled humidifiers without humidistat
controls.