INTRODUCTION
Wood is one of the primary food sources
for mold to thrive |
Mold is not new to mankind. It has been naturally occurring in our
environment since the beginning of time and will continue into the
foreseeable future.
Mold spores are airborne and exist in the air both outside and inside
every structure. Even if it was possible to eliminate mold spores
inside a structure, they would be re-introduced to the interior every
time a
door or window was opened, either transported by air or on the clothing
of the occupants. Because there is no evidence that the level of
mold spores has or is increasing, the question might be, “Are
people becoming less tolerant?”
Investigation indicates that over the last 20 years, changes in
construction materials and regulations have affected moisture levels
in residential
structures. This is due primarily to the reduction in air infiltration
rates in relation to energy conservation. However, structures being
built today are still well within acceptable levels and should not
have a problem.
In general, simple guidelines can be used to determine if moisture
levels in the structure are within acceptable levels. Do any parts
of the home
have a “musty odor”? Are there water stains or signs of water
accumulation on the underside of the roof decking, in the crawl space,
near baseboards, near receptacle and switch plates, doors or windows,
or on sloped ceilings? Do the windows develop condensation (sweat) during
the heating season? If the answer is “NO”, then the moisture
levels inside the structure are moderate and the chance of problems
is slight.
Almost all new homes built today should maintain winter indoor relative
humidity levels below 50%. This is well below the 90%+ surface relative
humidity (RH) required for mold growth. However, because each occupant
adds approximately 4 pints of water each day into the conditioned space
(mostly through breathing and perspiration), the number of people and
their activities in the residence becomes a bearing factor as well. Simply
put, a large number of people in a small space will generate more moisture
and increase the relative humidity.
Energy conservation efforts by the occupants, while well intentioned,
can also raise the indoor relative humidity of a residence above desired
levels. Many homeowners are tempted to shut off heat to areas that are
not in daily use. However, if the average relative humidity inside the
home is 50% at 70 degrees and the temperature in the areas that are shut
off is 55 degrees, then the relative humidity in those areas will be
increase to 85% (very close to mold formation); and cooler wall surfaces
may be at 50 degrees - the condensation temperature.
Cooler indoor temperatures are also directly related to mold formation
in “dead areas” such as exterior corners, closets and behind
furniture. Exterior corners are colder because of “wind washing” from
the outside and thermal bridging through wall framing. Corners frequently
have more wood than insulation in comparison to the rest of the exterior
wall. High local relative humidity in “dead areas” can
be minimized by increasing the temperature, increasing air circulation,
moving furniture away from walls and corners and by providing local
heating.
THE COOLING SEASON
During the cooling season some potential moisture problems inside
the structure disappear. “Dead areas” will now be warmer
which means a lower relative humidity. Areas that are closed off
will also
be warmer with a lower relative humidity.
Interior spaces are now being dehumidified by the operation of the HVAC
equipment and the indoor relative humidity is being maintained at about
50 to 60%. The number of occupants has less effect during the cooling
season, because as they generate moisture, they also generate heat, which
will increase the running cycle of the HVAC system. However, cooler indoor
temperatures make controlling the relative humidity in bathrooms important.
Spot ventilation (bath fans) is an effective method to reduce moisture
loading from bathing. One researcher noted that in his personal home
the run time for his (80 CFM) vent fan controlled by a humidistat was
about one hour after each shower. Complete drying of wet surfaces and
towels in bathrooms through evaporation can be managed by air circulation
from the air conditioning system. During the cooling season, bathroom
windows should not be used for ventilation if the structure is being
air-conditioned.
GOVERNMENTAL REGULATIONS
Increasing moisture levels in residential structures is also directly
related to governmental regulations (the Building Code) and the process
of adopting those rules. In general, the process of adopting governmental
regulations, including building codes, is incremental, commonly known
as “code creep”. The concept is to add regulations in
small increments, thus minimizing cost impact, objections, and scrutiny.
Because
the changes are small, the overall performance of the structure is
rarely considered, even though the cumulative effect of small changes
over a
period of time can and does impact the entire structure. This is
particularly true with regard to energy conservation efforts over
the past 25 years.
Brick veneer with the required 1" air
space
|
First, the levels of insulation were increased. Later, air infiltration
was reduced. Little consideration was given to moisture levels inside
building envelopes; for good reason. Twenty-five years ago, the typical
new home had an air infiltration rate of 2 or more air changes per hour
(ACH) and moisture levels (RH) during the heating season were very low.
Today, a typical new home has a 0.4 ACH, and some homes are being built
with air infiltration rates as low as 0.1 ACH. During the heating season,
as air infiltration rates come down, relative humidity inside the living
space is going up. This is caused by shorter run times for the HVAC equipment
and reducing the amount of dry outside air entering the structure.
MOISTURE TRANSMISSION
The four methods of moisture transmission inside the structure are:
1) Liquid flow due to gravity and/or air pressure
2) Capillary action
3) Air movement
4) Vapor diffusion.
Transmission by the first three methods is associated primarily with
bulk water and water vapor between the outdoor and indoor environments.
In the fourth method, vapor diffusion, water vapor is transported through
the building envelope by means of a partial-pressure difference of the
water vapor from the warm parts of the structure toward the cooler parts
of the structure.
RH is expressed in percentage and it describes the moisture content
between dry air (zero RH) and moisture-saturated air (100% RH). The
capacity of air to contain water is temperature dependant – it decreases
as the temperature is cooled. For instance, consider air at 70 degrees
F and 100% relative humidity and at 50 degrees F and 100% relative humidity.
The warmer air has twice the amount of water than the cooler air. How
much water does this represent? For a 20’ x 20’ room and
8-foot ceiling, the 70 degree air has ½ gallon of liquid water
and the cooler air has a quart. This liquid water equivalent is actually
water vapor that is mixed with the other gases that comprise air.
NEUTRAL PRESSURE PLANE
The ASTM manual “Moisture Control in Building Envelopes” explains
that when an outdoor to indoor air temperature exists; a pressure difference
is created by the buoyant force of the air densities. A “neutral
plane” occurs where the positive air pressure ends and where
the negative air pressure begins. This means that for the heating
season,
warm indoor air exfiltrates through the upper areas (positive pressure)
of the house while outdoor air infiltrates through the lower (negative
pressure) areas of the house. This means that ventilation fans benefit
from the positive pressure in the upper area of the house, and they
must work against the negative pressure at the lower portions of
the house.
During the cooling season this process will be reversed. Natural
ventilation through operable windows follows the same convention,
where net airflows
are influenced by the local pressure difference.
CRAWL SPACES
It is the intent of the building code and the Indiana Amendments for
crawl spaces to be free of standing water - for good reason. Wet crawl
spaces create mold and musty odor problems inside the crawl space area.
In severe cases, this moisture can condense on insulation and structural
members exposed to crawl space air; or can be transported to other components
of the structure.
The Building Code including Indiana Amendments places the following
requirements on crawl spaces:
- Requires 6 inches of fall in 10 feet for foundation drainage,
which is very important for keeping surface water from entering
the crawl space.
- Requires grading the crawl space to drain to a central collection
point (a sump pit).
- Provides for crawl space vents to be omitted provided the perimeter
walls of the foundation are insulated, the ground is covered
with a vapor retarder and conditioned air is supplied.
The Building Code does NOT require an HVAC return air in crawl spaces.
The ASTM Manual “Moisture Control in Building Envelopes” makes
it clear that crawl space vents contribute to excessive moisture
levels in crawl spaces. Introducing warm humid air (already wet enough
for mold
growth) from outside the structure into the crawl space generates
condensation.
Excessive moisture in crawl spaces can be caused by ventilation when
the outdoor air is warm/humid and when the space is cool from being thermally
coupled to the ground temperature. This is particularly true for crawl
spaces that are partially below grade. The humid outdoor air can reach
its dew point and condensate on cool surfaces, such as on floor insulation,
ducts and plumbing pipes, etc. The Building Code requires exterior sheathing
to be covered by a weather-resistant house wrap, such as Tyvek or 15#
felt, when brick veneer is used. Additional, an air space is required
between the brick veneer and the sheathing.
The Building Code DOES NOT REQUIRE sheathing paper under vinyl or aluminum
siding. However, NAHB Research Center studies demonstrate that in conditions
involving wind-driven rain, these materials suffer leakage. Studies also
show that most self-flashing windows can and do leak (particularly with
multiple units). These studies clearly demonstrate the benefits of protecting
wall cavities from water intrusion by way of placing weather-resistant
sheathing paper over the exterior sheathing under all exterior finish
materials. Further, the weather-resistant sheathing paper can easily
be used as flashing material for doors and windows, self-flashing or
not. Flashing is also necessary to protect the building envelope from
bulk water intrusion. The Building Code requires flashing in the following
locations of the structure:
- At the top of all window and door openings (except self-flashing
windows, which are discussed above)
- At the intersection of chimneys or other masonry construction
with frame or stucco walls
- Under and at the ends of masonry, wood, or metal copings and
sills
- Continuously above all projecting wood trim
- Where exterior porches, decks, or stairs attach to a wall or
floor assembly of wood frame construction
- At wall and roof intersections
- At built-in gutters
HVAC SYSTEM
Air ducts must be wrapped and sealed with
adhesive sealant to maximize efficiency |
HVAC ductwork in unconditioned spaces, both the crawl space and attic,
must be sealed tightly and insulated to prevent the dew point from occurring
inside and on exterior surfaces of the ducts which can add moisture (relative
humidity) to the inside of the structure and the inside of the ductwork.
Insulation installed on duct exteriors must include a vapor barrier
to prevent condensation. Leaky ductwork allows infiltration of outside
air into the system. During the winter, this simply adds dryer air to
the inside of the structure and does not create a moisture problem. It
does, however, decrease the energy efficiency of the system. During the
cooling season, warm humid air introduced into the system will increase
moisture levels (relative humidity) both inside the ductwork and the
structure.
Most attic ducts lying on or near the ceiling of the structure will
not have ambient temperatures that will create a problem. However, this
is not true if the ducts are elevated above the ceiling surface.
Another important factor to consider is that dehumidification by
the air conditioner is affected by run-time and “right-sizing” cooling
capacity to the building load. Grossly over-sizing the cooling capacity
causes short cycling and damp indoor conditions.
EXTERIOR WALL PERMEABILITY
Reducing air infiltration and adding insulation values to the exterior
sheathing are common recommendations to reduce energy bills. However,
combining these materials has an effect on the moisture levels inside
the wall cavity, which is rarely considered.
If a 4-mil vapor retarder (0.08 perm) is used on the warm-in-winter side
of the wall, and a low-perm sheathing (some as low as 0.03 perm) is used
on the outside, then the possibility of water encapsulation in the wall
cavity (caused by leaks and/or dew point) needs to be considered.
If the moisture content of the wood exceeds 19%, the permeability
of a wall (the wall’s ability to dry) becomes a major factor.
Moisture levels in wood components are affected by several factors:
- Vapor Transmission
- Leakage
- Wet Applied Insulation; and/or
- Dew Point
The minimum or maximum amount of permeability needed using common building
materials has not been determined. However, until such studies are done,
NAHB Research confirms what years of experience have taught us, that
7/16-inch structure wood (1.0 perm) protected by a 15# felt does have
a level of permeability that will allow drying of the cavity to the outside.
While conventional wisdom holds fast to the idea that a warm-in-winter
side poly vapor retarder is necessary, most of the studies are based
on cold climate conditions where the vapor transmission is to the outside
of the structure.
Energy Design Update in October 1992 published an amusing account
of the origins of the poly vapor retarder requirements by Cliff Shirtliffe,
research scientist at NRC in Ottawa. Shirtliffe explains that years
ago,
some unscrupulous contractors found that they could get double mileage
from batts by peeling off the facers, installing the unfaced batts
in one house and just the kraft facers in another house. To stop
the cheating
the kraft-faced batts were banned and translucent poly was required
so that inspectors could verify the presence of insulation in the
wall cavities.
Thus, the poly vapor barrier was born.”
ASTM states hat in fringe climates a vapor retarder may be unnecessary
and further studies are needed. An estimate of condensation potential
using ASTM C 755 Standard Practices for Selection of Vapor Retarders
for Thermal Insulation demonstrates that drywall with two coats of latex
paint (5 perms) is an adequate vapor retarder for climatic conditions
in Indiana. This is also borne out by experience with cellulose wall
cavity insulation, which is typically installed without a vapor retarder.
Whether or not to use a poly vapor retarder on the warm-in-winter
side of the wall becomes important when attempting to reduce moisture
levels
inside wall cavities in states such as Indiana where we have both
heating and cooling seasons and vapor transmission to the outside
during winter
and to the inside during summer. This also is a factor in the wall’s
ability to dry out from incidental bulk water leakage. Without a
poly vapor retarder, moisture can leave the wall cavity by vapor
diffusion
to the outdoor air and to the indoor air. A poly vapor retarder effectively
reduces the drying area by a factor of one half.
Tyvek house wrap completed and ready for brick |
NAHB Research performed a heating season condensation analysis using
two systems. One was WITHOUT a poly vapor retarder and the other WITH
a poly vapor retarder. Both systems used 7/16-inch structure wood sheathing
and felt on the outside and R13 insulation. Climate conditions of South
Bend, Indiana were used. Both systems performed well. The WITH-poly wall
was estimated to have somewhat less moisture rise during the coldest
winter months, and the evaluation did not identify problematic levels
of moisture gain for the WITHOUT-poly wall. It is important to note that
this analysis was performed for the heating season, when the WITH-poly
wall should perform best.
Use of low-density foam sheathing adds thermal resistance to the
building envelope. In the winter season, this can keep the wall cavity
warmer,
thus minimizing or eliminating the effects of dew point inside the
wall cavity. Some low-density foam sheathing has a very low permeability
(one
manufacturer publishes ranges from 1.0 for one inch unfaced foam
to 0.03 for ½-inch faced foam), but other products have a
very high permeability. Therefore, a word of caution is warranted
for potential condensation
issues that can arise if the foam board is highly vapor permeable
when used with a masonry veneer. Summer time condensation can occur
in air-conditioned
buildings when water vapor is driven by sun-lit rain-soaked masonry
into the wall cavity. An unprotected too permeable sheathing can
allow this
water vapor to migrate into the wall cavity and condense on framing
members, insulation and wallboard.
Given these indicators and the lack of studies to provide guidance, a
conservative approach may be to:
- Maximize the rainwater protection (use a weather barrier – 15#
building felt or an equivalent such as Tyvek under all exterior
finishes).
- Caulk/air seal the sheathing (to minimize air driven moisture
and enhance energy efficiency).
- Use a 7/16-inch structure wood sheathing or equivalent (to provide
a minimum level of permeability to the outside) and
- Eliminate the poly sheathing on interior walls (to allow permeability
to the inside).
Wall cavity condensation potential estimates and experience with cellulose
insulation indicate that the absence of the vapor retarder is not problematic;
and that drywall with two coats of latex paint will manage normal wintertime
indoor humidity.
While it appears that the issue of mold is not going away any time
soon, it does not have to be the catastrophic event some people fear.
Careful
attention to details and an awareness of the homeowner’s responsibilities
can avoid any problems here.
