Following is an article I wrote on the subject several years ago. It's
long, but some listers may find it useful. For others, please forgive me
for boring you with it.
MOISTURE & CONDENSATION IN ATTICS & CRAWLSPACES
By Wilton Strickland
A combination of excessive moisture (high relative humidity in the affected
area), inadequate ventilation and cool surfaces in attics and under-floor
crawlspaces can cause condensation and/or fungal growth, rot, etc., on the
undersides of roof & floor structures and lead to serious damages to the
structures. Generally, if wood has a moisture content of more than about
20%, significant damage can occur, but a moisture meter is not needed to be
able to see that wood and insulation are soaking wet and dripping with
condensation or badly stained and/or covered with significant staining,
surface damage or fungal growth and to know that the wood is wet enough, or
has been wet enough, for serious damages to occur if these conditions
persist. Designers, builders and homeowners should be aware of and
understand this potential problem in order to combat it adequately. Home
inspectors should be able to recognize and understand the problem in order
to properly report it and recommend improvements to alleviate it.
Condensation occurs on building surfaces when the relative humidity is high,
ventilation is poor & the surfaces are cool. Relative humidity is expressed
as a percentage of moisture present in air relative to the amount of
moisture the same air can hold. Generally, warmer air can hold more
moisture than cooler air. When air is cooled below its "dew" point (the
temperature at which the air is saturated or has 100% relative humidity),
some of the moisture condenses into visible water, such as fog, rain, snow,
dew, etc.
This same action causes condensation to form on surfaces of cool objects
that cause the air in immediate contact with the object to cool below the
dew point. For example, condensation on the outside of cold drink glasses,
the insides of cold car and house windows or dew on grass. If enough air
(ventilation) is moving across these surfaces, however, the condensation
does not form - air in contact with the surface does not have time to be
cooled below the dew point.
In confined areas of buildings with inadequate or reduced ventilation,
condensation forms when warm, moist air comes in contact with a surface cold
enough to cool the air in immediate contact with the surface below the dew
point - just like the cold drink glass or car windows. This can occur even
in heated areas with poorly insulated walls and ceilings, inside closets or
any area with poor ventilation, high humidity and cool surfaces.
Condensation often forms on the cold underside of roof sheathing and on
rafters when warm, moist air rising from the living area comes in contact
with the cold roof structure.
It is usually not economically feasible to try to keep moisture from getting
into the attic from the living space. Moisture enters the living space from
several sources: the ground under the house, the air around us, cooking,
laundry, gas logs, bathing, breathing, evaporation from our bodies, from
plants, etc. Moisture from some of these sources can be reasonably reduced
(by reducing water entering the crawlspace from in the yard and off the
roof, by improved vapor barriers and ventilation in crawlspaces and improved
ventilation in bathrooms, kitchens and laundry rooms), but much of this
moisture cannot be reasonably eliminated. Moisture is a natural byproduct
of human life.
Therefore, this moisture must be allowed to escape to the outside without
doing damage to the structure. The most natural way for the moisture to
escape is to rise through the ceiling into the attic. If adequate
ventilation is not maintained in the attic during cold weather, this
moisture can condense on the cold underside of the roof and, in time, cause
serious damage to the structure. Maintaining good attic ventilation,
therefore, is very important year-round - not just in summer. Condensation
does not form on the underside of the roof during hot weather because the
roof is usually much warmer than air in the attic at that time. Attic
ventilation is important in summer to reduce the potential for damage to the
shingles from overheating and to reduce the cooling load in the house, but
it is important in winter to let moisture in the house escape to the outside
without damaging the roof structure. Attic vents should not be blocked or
sealed in winter!
If there are dark, mildew-type stains or wetness on the underside of the
roof or on the rafters not caused by a leak, the homeowner should try to
improve ventilation in the affected area of the attic. This can be done by
installing additional soffit vents & by adding ridge vent(s) or wind
turbines on the roof. The most efficient attic venting system takes
advantage of natural convection - the natural tendency for warm air to rise.
This system has intake vents down low in the attic (such as soffit vents)
and outlet or exhaust vents as high as possible in the same general space
(such as ridge, turbine or gable end vents). Different combinations of
these vents work well in different situations. Electric-powered vent fans
can also be installed, but these are usually meant to aid ventilation during
hot weather and are usually thermostatically controlled. In addition to
costing money for operation, powered fans often fail without the homeowner
knowing it - bearings “freeze” on shafts, blades break off and/or get
jammed, motors burn out, etc.
Often, excessive moisture in an attic and condensation stains on the
underside of the roof structure can be traced to excessive moisture in the
under-floor crawlspace. Excessive moisture in crawlspaces not only can lead
to settlement of footings and foundations, but can also cause serious damage
to the floor structure by fungal growth, rot, etc. During hot, humid
weather, moisture easily condenses on cool, exposed surfaces of floor joists
and the underside of insulation in crawlspace areas with reduced
ventilation, especially when cooling system ducts in the crawlspace cause
the surfaces to become cold enough to cause air in contact with the surfaces
to be cooled below the dew point. In much of N. C. & the southeastern U. S,
the most critical time for condensation problems in under-floor crawlspaces
is roughly April-September.
In too many cases, plastic “vapor barriers” are laid on the ground under
houses in hasty and often, money-grabbing, efforts to solve “moisture
problems” in crawlspaces without attacking the source of the water or
considering the other major factors involved - water intrusion, ventilation
and cool surfaces. In many of these cases, the plastic works as a “swimming
pool liner” to hold water (condensation) dripping off the underside of the
structure or running and seeping in from outside the crawlspace onto the
plastic. In some cases a plastic vapor barrier on the ground can help to
reduce moisture intrusion onto/into the structure, but a “vapor barrier” on
the ground is not a magic solution to all crawlspace moisture problems.
In general, the best remedy and/or prevention for excessive moisture in a
crawlspace is to maintain good control of water off the roof and in the yard
immediately outside of the house. Many cases of excessive water in
crawlspaces result from improper disposal of water off the roof. Homeowners
should take all reasonable measures to drain water off the roof and in the
yard away from the house - don’t allow water to accumulate under and around
foundation walls and/or seep (or flow) into the crawlspace. Keep areas
around the house immediately outside of foundation walls graded with slight
slope downward away from the house for five to six feet to deter water entry
into crawlspace and/or ponding under and around foundations (a drop of about
two inches in four to five feet horizontally is usually sufficient). Basic
grade (slope) should be established and maintained with topsoil - not sand,
mulch nor gravel. An extremely simple principle of physics is involved -
“WATER RUNS DOWNHILL.” We need to make it run downhill away from the house
as much as reasonably possible.
Low areas immediately inside of foundation walls (usually footing trenches
that have never been properly back-filled) should be filled with soil to
reduce the potential for water entry from outside. In new construction,
crawlspace areas should be properly back-filled and graded before any wood
at all is put on the structure. Ideally, the ground inside the foundation
wall should be at least as high as the ground outside of the wall, but in
this part of the country, this is seldom the case - the ground level in
almost all crawlspaces I have seen is usually several inches below outside
grade.
Water often easily enters crawlspaces via crawlspace access doors and
foundation vents that have their lower edges at or below outside grade.
“Window wells” should be built or installed on the outside of such openings
to deter water entry.
Water also easily enters crawlspaces via openings in foundation walls around
main sewer pipes and heating/cooling Freon pipes or other penetrations.
Such openings should be sealed on the outside of the wall completely around
the pipes (including under the pipe) to prevent water entry.
Landscape edging can also restrict drainage away from a house enough so that
water off the roof runs or seeps into the crawlspace before it can drain
away into the yard. Such edging should be modified or removed as necessary
to improve and maintain positive drainage away from the house.
Roof gutters can also help to better control of water off the roof. Gutters
must be kept cleared of debris to allow proper drainage, and care should be
taken to discharge water from gutter downspouts away from the house. If
gutter downspouts discharge into low areas immediately outside of the house
where the water cannot drain away from the house but easily seeps through or
under the foundation wall into the crawlspace or runs (gushes) through
nearby foundation vents or other openings in the wall into the crawlspace,
the purpose of the gutters is defeated and may even make moisture conditions
worse by concentrating the moisture in a smaller area.
In many cases, inadequate ventilation is a greater contributor to formation
of condensation in crawlspaces than wetness on the ground under the house.
Crawlspace ventilation is often restricted/reduced by vegetation (foundation
plantings), mulch, decks, foundation piers, perimeter sills around sunken
rooms, heating/cooling ducts, chimney bases/foundations and foundation walls
between porch crawlspaces and the house crawlspace. If natural ventilation
can not be reasonably maintained, it may be necessary in some cases to
install humidistatically controlled fan(s). If a fan is to be used, it
should be installed so that it provides air movement in the area needing the
improved ventilation - not far across the house and near other openings so
that it has no effect in the desired area. A fan draws air via the
shortest, least-resistant route, such as an open foundation vent near it.
Adequate ventilation should be maintained in all areas of most crawlspaces
in much of N. C. and the southeastern U. S. during hot, humid weather. If
soil under houses were dry, and if we could isolate the crawlspace from the
ground with a 100% vapor barrier and isolate the crawlspace from the outside
hot, humid air with sealed foundation walls, ventilation would not be
necessary - in fact, it would improve energy efficiency not to ventilate at
all. But few crawlspaces in this area are dry, and perfect seals between
ground and the crawlspace and outside air and the crawlspace are rarely
accomplished. Building codes even contribute to this by requiring
foundation vents. Even when closed, foundation vents leak. If any hot,
humid air gets into the crawlspace, it must not be allowed to be in contact
with cool surfaces long enough for it to be cooled below the dew point and
cause condensation to form on the surfaces. Therefor, in most cases I have
seen, if any ventilation is allowed, full ventilation becomes necessary.
Homeowners should also take reasonable steps to keep the underside of the
structure itself from getting cold enough to cause air in contact with the
surfaces to be cooled below the dew point. If the cooling system is
operated for an extended period with settings below about 76 degrees F on
hot, humid days, ducts in the crawlspace often cause the outsides of the
ducts, exposed areas of floor joists and the underside of under-floor
insulation to get cold enough to cause air in contact with the surfaces to
be cooled below the dew point. Under these conditions, all of these
surfaces often get soaking wet and dripping with condensation. Try not to
cool the interior of the house below about 76? F for extended periods during
hot, humid weather. This not only saves energy, but may also help prevent
serious damage to the underside of the structure.
In review, condensation is caused by a combination of excessive moisture
(high relative humidity in the affected area), inadequate ventilation and
surfaces cold enough to cause air in contact with them to be cooled below
the dew point. Builders and homeowners should take reasonable steps as
necessary to attack the “moisture problem” in all three of these areas -
reduce water and water vapor entry into crawlspaces and attics, keep attics
and crawlspaces adequately vented and try not to cool the inside of the
building so drastically as to make its underside cold enough to cause
condensation to form on the surfaces of insulation and structural
components.
Wilton Strickland 1996
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