Overcoming Problems
Facts About Condensation |
1. Introduction
1.1 What
is humidity, anyway?
1.2 Where
does the moisture come from?
1.3 What's
relative humidity?
1.4 Just
what is condensation?
1.5 What
causes condensation?
1.6 What
does condensation on windows mean?
1.7 How do
I know if I have excess indoor humidity?
1.8 What
does excess humidity do to my home?
1.9 You mean
moisture can actually go through walls?
1.10 What
happens if moisture goes through walls?
1.11 Is
condensation more prevalent in any geographical region?
1.12 Does
condensation occur only in winter?
1.13 Does
condensation depend on whether my home is new or old?
2. Controlling Indoor Humidity
2.1 How
do I measure indoor relative humidity?
2.2 Isn't
high indoor humidity healthy in winter?
2.3 Then
what are some recommended indoor relative humidity levels for
winter?
2.4 How can
I reduce inside humidity in winter?
2.5 Will
reducing the humidity in my home during winter help control condensation?
2.6 How can
I reduce humidity in my home during the summer?
3. Ventilation
3.1 How
can I ventilate my home?
3.2 Does
structural ventilation include attics?
3.3 What
harm can attic condensation do?
3.4 Are
some kinds of attic ventilation better than others?
3.5 Just
how much attic ventilation should I have?
3.6 What
about the crawl space? Should it be ventilated, too?
3.7 How
much ventilation should I have in the crawl space?
4. Windows and Condensation
4.1
Do windows cause condensation?
4.2 Why
do I see condensation on my windows and sliding glass doors first?
4.3 Are
windows the only place condensation is forming?
4.4 What
causes condensation on windows and sliding glass doors?
4.5 Do
curtains and window shades cause window condensation?
4.6 What
causes condensation on the inner surfaces of storm windows?
4.7 What
damage can excess window condensation do to windows?
4.8 Are
there any cases where window condensation is only temporary?
4.9 Can
windows help control moisture in my home?
4.10
Is there anything I can do to the windows to eliminate condensation?
4.11
Is window condensation really reduced that much with double-pane
insulating glass?
5. Building a Low-Condensation
Home
5.1 I'm
building a home. What steps can I take to prevent excessive condensation?
6. Summary
6.1
What steps can I take to reduce excessive humidity in my present
home and control window condensation?
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Condensation is the fog that suddenly appears
in cold weather on the glass of windows and sliding glass doors.
It can block out the view, drip on the floor, and freeze on the
glass. It's annoying.
While it's natural to blame the windows,
you shouldn't. Window condensation is the result of excess humidity
in your home. And the glass only provides a visible cold surface
on which humidity can condense.
The important thing is your foggy windows
and sliding glass doors are trying to tell you to reduce indoor
humidity before it causes hidden, costly problems elsewhere in
your home.
Problems like peeling paint, rotting wood,
buckling floors, insulation deterioration, mildew, and even moisture
spots on ceilings and walls.
Foggy windows and sliding glass doors
are the indicators, the warning signs, that humidity could be
damaging your home.
What to do? We've compiled this Condensation
Answer Book to answer that and other questions you may have about
indoor humidity and condensation. We'll start with the basics,
and offer solutions and alternatives along the way.
However, some solutions to window condensation
problems pose a different kind of problem that of energy efficiency.
Some ventilation techniques (e.g. opening windows to air out a
home; running fans more frequently) may add slightly to a homes
energy bills. It is up to the reader to decide which solutions
are most practical for him. This, of course, will depend on the
severity of the condensation problems, both on windows and in
other less obvious places in the home.
Finally, as you read through this information
and learn the causes and cures for condensation, remember: it's
not the windows, it's the humidity.
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Humidity is water vapour, or moisture,
in the air. Usually it's visible, but sometimes, such as with
steam or ground fog, it's concentrated enough to be seen. But
see it or not, all air contains a certain amount of moisture.
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There are many things that generate indoor
moisture. The normal perspiration and breathing of a family of
four adds about ½ pint of water to the air every hour.
Cooking three meals a day adds four or five pints of water to
the air. Each shower contributes ½ pint. In fact, every
activity that uses water (like dishwashing, mopping floors, doing
laundry) adds moisture to the air. The truth is daily living activities
of a family of four can add more than 18 gallons of water a week
into the air in their home. And more water vapour in the air means
a higher indoor relative humidity.
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Air can hold only a limited amount of
water vapour, and that amount depends on the air temperature.
When air at a certain temperature contains all the vapour it can
hold, it's said to have a relative humidity of 100%. Thus, when
it holds only half the water vapour it can hold, the relative
humidity is 50%. Cooler air is capable of holding less vapour
than warmer air. So air at 30º F and 100% relative humidity
actually contains less water than air at 70º F and 100% relative
humidity.
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Fog on windows is a form of condensation.
So is the water that forms on the outside of a glass of iced tea
in the summer. It all comes from water vapour in the air.
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Excess moisture in the air. When warm,
moist air comes into contact with cooler surfaces, the moisture
condenses. That's because the cooler air surrounding cooler surfaces
can't hold as much moisture as warmer air.
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Window condensation can be a danger sign.
It may mean that excessive indoor humidity could be doing unseen
damage to other parts of your homes.
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Check for damp spots on ceilings and room-side
surfaces of exterior walls, particularly closets. Look for water
and ice on windows. Even water-filled blisters on outside paint
surfaces indicate excessive indoor humidity.
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Excess humidity contributes to the deterioration
of a home. Excessive humidity can pass through walls and freeze
in the insulation. In spring it melts, damaging your ceiling and
walls. Or, humidity can force its way out through siding to form
blisters under your exterior paint.
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You bet. It's because of a force called
"vapour pressure". Moisture in wet air tries to flow
toward drier air to equalize itself. This flow acts independently
of air currents. In winter, inside air is much more humid than
colder outside air. So, the vapour pressure, or equalization process,
actually forces the inside moisture through cement, wood, plaster,
and brick, toward the outside.
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Because certain varnishes and paints block
the flow of moisture, condensation can occur between the inside
and outside walls, or under exterior paint surfaces. It can rot
a homes wood frame and blister the paint.
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Yes. Condensation is more apt to occur
in climates where the average January temperature is 35º
F or colder.
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Usually, but it can occur during cold
weather anytime, and occasionally it will form on the outside
of windows on hot, humid summer days, when your air conditioner
has cooled the glass.
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Generally, yes. Years ago, before the
concern with energy efficiency, homes were built with less weather-tightness
than homes today. Insulation concepts were not as advanced as
today. Walls and ceilings were built with much more porous materials.
Water vapour could easily flow in and out of walls.
Todays homes are much "tighter".
Windows and doors are built to substantially reduce air leakage.
Weather-stripping, modern insulation, vapour barriers, and construction
techniques, which are intended to keep out cold air, lock moisture
inside. As a result, moisture created by bathrooms, kitchens,
laundries, and occupants no longer flows to the outside, unless
provisions for mechanical ventilation have been made. So it's
very easy to build up excessive, even harmful moisture levels
in todays homes.
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To get an accurate reading, you can buy
humidity-measuring instruments, called hygrometers, or sling psychrometers.
Otherwise, watch your windows for symptoms of excess humidity.
When excessive moisture collects on the inside glass in a living
room or bedroom, you're approaching the humidity danger level.
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That's a common belief, but there's little
evidence to support it.
Many believe that dry air increases the
susceptibility to colds. But humidity in a heated house, whether
it's high or low, is not an important health factor to a normally
healthy person. And there's little scientific evidence that the
health of most people will be adversely affected if humidity is
kept at a level that prevents excessive condensation.
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The University of Minnesota Engineering
Laboratories performed a series of long and careful experiments
on that subject. The following table shows the maximum safe humidity
for your home, paint, insulation, and structural members:
| Outside air temperature |
Inside relative humidity for 70-deg.
F Indoor air temperature |
| -30º F or below |
not over 15% |
| -20º F to -10º F |
not over 20% |
| -10º F to 0º F |
not over 25% |
| 0º F to 10º F |
not over 30% |
| 10º F to 20º F |
not over 35% |
| 20º F to 40º F |
not over 40% |
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There are at least two steps you can take
to reduce indoor humidity in winter:
Control the sources of humidity. Vent
all gas burners, clothes dryers, etc. to the outdoors. Use kitchen
and bathroom exhaust fans.
Ventilate your home. Because outside air
usually contains less water vapour, it will "dilute"
humidity of inside air. This takes place automatically in older
homes through constant infiltration of outside air. But again,
in newer "tighter" homes, the only way outside air can
get in is by ventilation. More on ventilation in the next section.
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It's the most practical way. Condensation
indicates excessive humidity. Eliminate the excessive humidity,
and you eliminate the condensation.
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Air conditioners or dehumidifiers are
your surest ways of reducing humidity to desired levels. However,
in most cases, outside air will be more humid in warmer weather,
and condensation on the inside of windows usually isn't a problem.
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There are basically two types of ventilation:
interior and structural ventilation.
As a temporary solution to an acute problem,
open a window in each room for just a few minutes. Remember that
inside air continually gains humidity through daily living activities.
Opening windows allows the stale, humid air to escape, and fresh,
dry air to enter.
After a shower, for example, open the
bathroom window or turn on the exhaust fan, so steam can go outside
instead of remaining in the home.
Structural ventilation is slightly more
complex, but will save your costly repair bills in the long run.
Miniature louvers in exterior walls can be installed to prevent
moisture from condensing between the outside and inside walls.
This will keep paint from peeling as a result of indoor vapour
pressure.
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Definitely. Many homeowners cover all
attic louvers in winter in hopes of saving fuel. If the attic
is properly insulated, this practice can only do harm. Because
the indoor moisture penetrates ceilings, then condenses on the
cool underside of the roof and can even form frost. If the attic
were ventilated, moisture would be transferred to the outside
air.
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A lot. Moisture condensing in attics produces
mildew, or rotting conditions. Or it drips down to the ceiling
below to damage plaster or paint. Thermal insulation also becomes
wet and provides less resistance to heat loss.
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Yes. A combination of vents at the eaves
and at the gable ends is better than gable vents alone. And, a
combination of continuous eaves and ridge venting is best of all.
However, regardless of the type you have, there should always
be at least two vent openings, located so that air can flow in
one and out the other.
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That's a difficult question to answer,
because the size and number of vents depends on the location of
the home, wind direction, physical size of the building, quality
of workmanship and kinds of building materials used.
Consult a local heating and ventilating
contractor, listed in the Yellow Pages. He'll be able to tell
you exactly how much ventilation your attic should have.
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Yes. The crawl space beneath a house is
another place where ventilation is important. The crawl space
can evaporate gallons of water each day. When you seal the crawlspace,
that water penetrates the floor above and causes more humidity
problems in the home.
Providing foundation vents in the crawlspace
reduces the humidity, and a vapour barrier (like polyethylene
film) on the ground prevents moisture leakage into the house above.
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At least one vent should be located near
each corner, and should be placed as high as possible, but not
higher than the bottom of the joists. Plus, if you have a ground
cover, one 8" X 16" vent for each 350-sq. ft. is needed,
with a minimum of four vents.
Again, it's wise to consult your local
heating and ventilating contractor for number and proper placement
of vents. He's listed in the Yellow Pages.
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Windows are not a cause, per se. They
provide a ready medium on which the vapour can condense. But the
primary cause of condensation is excessive moisture in the air.
Windows are only indicators of excessive moisture.
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Condensation is generally seen first on
windows and sliding glass doors because they tend to have the
lowest temperature of any of the visible surfaces in the house.
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Possibly not. There's a point between
the exterior and interior walls that's just as cold as the inside
window surface. Chances are, if you can see condensation on your
windows, it's also forming between the walls, too. Room-side surfaces
of exterior walls are normally warmer, but occasionally condensation
occurs on cold spots such as nail heads and in the corners of
outside walls and closets. This is because insulation is weaker
and circulation is restricted in these areas.
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Recall that cool air is able to hold less
moisture than warm air. Therefore, when the warm, moist air of
the room comes into contact with the cool glass surface, some
water vapour that can no longer be held by the cooled air is deposited
on the glass.
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Curtains and other window coverings don't
cause window condensation, but they can contribute to the problem
by restricting the flow of warm room air over the glass surface.
Therefore, condensation is more apt to occur when curtains are
closed and shades are pulled down.
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This indicates that air is leaking outward
around the inner window, and is being trapped by a tight-fitting
storm window. Moisture is then trapped between the panes, and
condensation occurs on the outside pane. Tight storm windows should
have some ventilation to the outside to relieve this problem.
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It can cause the paint to peel from the
sash or stain the inside. Water can run down into the window frame
causing dampness around the frame. It can sometimes cause paint
to peel on the outside of the window.
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Yes, there are primarily three: new construction
or remodelling; the beginning of each heating season; and quick
changes in temperature.
New construction and remodelling produce
a lot of moisture. Wood, plaster, cement, and other building materials
contain a great deal of moisture. When the heating season starts,
this moisture will gradually flow out into the air in the home.
It will usually disappear during the first heating season and
not cause any further trouble.
At the beginning of the heating season
there may be a certain amount of temporary condensation. During
the humid summer your house absorbs some moisture. After the first
few weeks of heating your house will dry out, and you'll have
fewer condensation troubles.
Sharp, quick drops in temperature can
also create temporary condensation problems during the heating
season.
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Only in the sense that they can be opened
for ventilation. Otherwise, windows are only indicators of excessive
moisture in the air.
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If you have windows with good storm sash
or double-pane insulating glass (both keep the inside glass surface
warmer), there isn't very much more you can do to the windows
except possibly add triple-glazing (see below). But again, troublesome
condensation indicates excessive humidity, and that problem must
be eliminated at the source.
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Yes. Double-pane insulating glass permits
about 37% relative indoor humidity (at 70º F inside, 0º
F outside) without condensation. Compare that to single-pane windows
that permit just 12% relative indoor humidity (at 70º F inside,
0º F outside).
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There are several ways you can plan your
home to prevent excessive condensation:
Choose pvc or thermally broken aluminium
rather than metal for windows and doors. This will reduce the
likelihood of condensation on the frame and sash. Metal frames
without thermal breaks, on the other hand, conduct heat quite
readily. So in the winter, the inside surface of a metal window
frame is cold. When humid inside air comes into contact with cold
metal, condensation, and often freezing, occurs.
Make sure the attic and crawl space are
cross-ventilated. The crawl space should be covered with a vapour
barrier. This will prevent water vapour from rising from the soil
into your home.
If you have a hot air furnace, install
an outside fresh air intake.
Make sure your clothes dryer and all gas
appliances have outside vents. Water vapour is one of the products
of gas combustion.
If you have a basement, take the necessary
steps to prevent leakage of soil moisture into the basement. These
steps will vary with soil and drainage conditions on your plot.
Make sure heat outlets are placed below
glass areas of windows and sliding glass doors.
Use exhaust fans in the kitchen, bathrooms,
and laundry. They'll not only help reduce excessive moisture,
but will clear away stale air as well.
Select windows and sliding glass doors
with double-pane insulating glass, Low-E glass or triple-glazing
systems.
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Recognize that the best way to stop condensation
is to reduce the moisture in the inside air.
Be sure that attic or crawl spaces are
properly ventilated. Add a vapour barrier to cover the earth in
the crawl space.
If you have single-pane windows, install
storm panels. If you have double-pane-insulating glass, install
triple- glazing panels.
Be willing to try living in lower humidities.
Eliminate any sources of moisture in your
home that you can control.
In winter, provide more controlled ways
for moist inside air to get out. Run kitchen or other ventilating
fans longer and more often than you normally do.
If troublesome condensation persists,
see your heating contractor about an outside air intake for your
furnace, about ventilation of gas-burning heaters and appliances,
or about installation of ventilating fans. Heating and ventilating
contractors are listed in the Yellow Pages.
Whey you're planning a home, take the
necessary steps outlined in this circular to prevent condensation
problems. Again, work with your heating and ventilating engineer.
Your gas or electric company may have a residential heating engineer
you can consult.
Remember that the best way to avoid condensation
is to reduce the humidity in the inside air.
Window condensation is certainly a problem.
But in the vast majority of cases, it's a problem that can be
controlled or eliminated. If you follow the steps and suggestions
outlined in this circular, you shouldn't have a serious condensation
problem.
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"The more I want to get something
done, the less I call it work."
Richard Bach |
