Exhaust Fans and Bathroom Ventilation

Ventilation systems for bathrooms are designed to exhaust odors and moist air to the home’s exterior from the bathroom. Typical ventilation systems consist of a ceiling fan unit in the ceiling of the bathroom connected to a duct that terminates at the roof.

Exhaust Fan Ventilation Functions;

The exhaust fan may be controlled in one of several ways:

Most are controlled by a conventional wall switch.
A timer switch may be mounted on the wall.
A wall-mounted humidistat can be pre-set to turn the fan on and off based on different levels of relative humidity
.

Newer exhaust fans may be very quiet but work just fine. Older exhaust fans may be very noisy or very quiet. If an older exhaust fan is quiet, it may not be working well. Inspectors can test for adequate exhaust fan airflow with a chemical smoke pencil or a powder puff bottle.
Bathroom ventilation fans should be inspected for dust buildup that can impede airflow. Particles of moisture-laden animal dander and lint are attracted to the fan because of its static charge. Inspectors should comment on dirty fan covers.

Ventilation systems should be installed in all bathrooms. This includes any bathroom with windows, since windows will not be opened during the winter in cold climates.

Defects in Poor Ventilation

The following conditions indicate insufficient bathroom ventilation:

moisture stains on walls or ceilings.
corrosion of metal.
visible mold on walls or ceilings.
peeling paint or wallpaper.
frost on windows.
high levels of humidity.

The most common defect related to bathroom ventilation systems is improper termination of the duct. Ventilation ducts must terminate at the home exterior. The most common improper ventilation duct terminations locations are:

mid-level in the attic. These are easy to spot.
beneath the insulation. You need to remember to look. The duct may terminate beneath the insulation or there may be no duct installed.
beneath attic vents. The duct must terminate at the home exterior, not just beneath it.

Improperly terminated ventilation systems may appear to work fine from inside the bathroom; you have to look in the attic or on the roof. Sometimes poorly-installed ventilation ducts will loosen or become disconnected at joints or connections.

Ventilation ducts which leak or terminate in attics can cause problems from condensation. Warm, moist air will condense on cold attic framing, insulation or other materials. This condition has the potential to cause health or decay problems from mold, or to damage materials such as drywall. Moisture also reduces the effectiveness of thermal insulation.

Mold and Poor Ventilation

Perhaps the most serious consequence of an improper ventilation setup is the potential accumulation of mold in attics or crawlspaces. Mold may appear as a fuzzy, thread-like, cobwebby fungus, although it can never be identified with certainty without being lab-tested. Health problems caused by mold are related to high concentrations of spores in indoor air. “Spores” are like microscopic seeds, released by mold fungi when they reproduce. Every home has mold. Moisture levels of about 20% in materials will cause mold colonies to grow. Inhaling mold spores can cause health problems in those with asthma or allergies, and can cause serious or fatal fungal infections in those with lung disease or compromised immune systems.

Mold is impossible to identify visually and must be tested by a lab in order to be confidently labeled. Inspectors should refrain from calling anything “mold” but should refer to anything that appears as mold as a material that “appears to be microbial growth.” Inspectors should include in their report, and in the inspection agreement signed by the client, a disclaimer clearly stating that the General Home Inspection is an inspection for safety and system defects, not a mold inspection.

Decay, which is rot, is also caused by fungi. Incipient (early) decay cannot be seen. By the time decay becomes visible, wood may have lost up to 50% of its strength.

In order to grow, mold fungi require that the following conditions are present:

oxygen;
temperatures between approximately 45° F and 85° F;
food. This includes a wider variety of materials found in homes; and
moisture.

If insufficient levels of any of these requirements exist, all mold growth will stop and fungi will go dormant. Most are difficult to actually kill.

Even though mold growth may take place in the attic, mold spores can be sucked into the living areas of a residence by low air pressure. Low air pressure is usually created by the expulsion of household air from exhaust fans in bathrooms, dryers, kitchens and heating equipment.

Improper Ventilation

Ventilation ducts must be made from appropriate materials and oriented effectively in order to ensure that stale air is properly exhausted.

Ventilation ducts must:

terminate outdoors. Ducts should never terminate within the building envelope.
contain a screen or louvered (angled) slats at its termination to prevent bird, rodent and insect entry.
be as short and straight as possible and avoid turns. Longer ducts allow more time for vapor to condense and also force the exhaust fan to work harder.
be insulated, especially in cooler climates. Cold ducts will encourage condensation.
protrude at least several inches from the roof.
be equipped with a roof termination cap that protects the duct from the elements.
be installed to manufacturer’s recommendations.

The following tips are helpful although not required. Ventilation ducts should:

be made from inflexible metal, PVC, or other rigid material. Unlike dryer exhaust vents, they should not droop.
have smooth interiors. Ridges will encourage vapor to condense, allowing water to back-flow into the exhaust fan or leak through joints onto vulnerable surfaces.

Above all else, a bathroom ventilation fan should be connected to a duct capable of venting water vapor and odors into the outdoors. Mold growth within the bathroom or attic is a clear indication of improper ventilation that must be corrected in order to avoid structural decay and respiratory health issues.

Attic Ventilation Do’s and Dont’s

Ventilation Systems For The Attic

Roof Structure Ventilation

Well ventilated roof structures use air movement to exhaust heat from the attic or roof structure to the outside. Poor roof ventilation can shorten the long-term service life of certain types of roofing materials, especially those which contain asphalt, such as black felt underlayment and asphalt shingles. Ventilation also influences moisture levels in the attic and comfort levels in the home.

Roofing materials absorb sunlight as heat. This heat is then released both upward into the open air and downward into the roof structure.

Cools the Roof

Keeping the attic space or rafter bays cooler helps keep roofing and underlayment cooler and extends their long-term service life. Loss of volatiles, which help keep shingles flexible and waterproof, is accelerated by poor ventilation. Loss of volatiles contributes to the distortion you see in the organic shingles in the photo above.

Ventilation Can Cool the Living Space

1 25 2 Attic Ventilation Dos and Donts

Evacuating heat before it reaches the living space helps keep the home cooler and more comfortable, and reduces cooling costs.

Ventilation Can Help Remove Moisture

The third benefit of roof ventilation is that it can remove excessive moisture vapor. Excessive moisture vapor can cause problems such as mold, decay, corrosion, and roof sheathing expansion.

The roof sheathing in this new home expanded after the shingles were installed, resulting in buckling of the shingles around the panel edges. Oriented strand board, commonly known as OSB, is typically used for roof sheathing, and leaves the mill with a 3% or 4% moisture content, so it’s very dry.

Installing sheathing and roofing it over too soon in a climate with high humidity can cause this problem. Roof sheathing needs time to adjust to local humidity levels. Another way to say this is that roof sheathing needs to reach “equilibrium moisture content” (EMC) with the home site’s environment before roofing materials are installed.

Roof structure ventilation systems can be divided into two basic types: active and passive.

ACTIVE VENTILATION SYSTEMS

Thermostatically Controlled Fan

An active ventilation system requires some kind of power to operate.

Two types of powered ventilation systems are common: attic fans and whole-house fans.

Attic ventilation fans are usually automatically controlled by a thermostat.

These ventilation fans are often installed in a gabled end, as you see here. That’s the adjustable thermostat circled in yellow.

You may also see them installed between rafters, as you see here. The white conductor should be stapled to the framing member nearest to the fan.

1 25 8 Attic Ventilation Dos and Donts

This is what a roof-mounted ventilation fan looks like from the roof.

Whole-House Fan For Ventilation

1 25 9 Attic Ventilation Dos and Donts

Whole-house fans are large fans, usually 24 to 30 inches, installed in the ceiling of the top-most floor in a central part of the home. The hallway ceiling is a common location. The fan may be controlled by a thermostat, a timer, or a manual switch.

1 25 10 Attic Ventilation Dos and Donts

Whole-house fans pull hot air from the living space and exhaust it to the outside through the attic space. They’re typically used with a downstairs window open or with a window-mounted cooling appliance, such as an evaporative cooler, to create a flow of cool air through the home for good ventilation control.

Backdrafting

If they’re used improperly, whole-house fans can cause dangerous backdrafting.

“Backdrafting” is a term used to describe the conditions that result when air is pulled into the home through an exhaust flue.

Instead of flowing up into the exhaust flue and to the outside, exhaust gasses are pushed into the living space by air being pulled down the flue.

This can happen when no fresh air is supplied to replace air pulled out of the living space by the whole-house fan. If no window is left open, replacement air will be pulled into the home through the path of least resistance. If the power of the whole-house fan exceeds the forces propelling the byproducts of combustion to the outside, backdrafting will occur.

1 25 11 Attic Ventilation Dos and Donts

Backdrafting is more likely to occur in homes with atmospheric furnaces, which are furnaces that have no exhaust fans.

It’s also more likely to happen in tightly built homes that have no system for providing makeup air. An example would be a heat-recovery ventilation system, also known as an HRV.

Potential dangers associated with whole-house fans should be addressed with a source of outdoor air, such as an open window or an evaporative cooler that is installed in an open window.

An air conditioner will not provide outdoor air since all it does is cool the air that re-circulates through the living space.

Ventilation FYI

stumbleupon Attic Ventilation Dos and Donts

delicious Attic Ventilation Dos and Donts

Attic Ventilation Systems

Continuous Ridge Vent Attic Ventilation

One of the most efficient attic ventilation methods is the continuous soffit vent combined with the continuous ridge vent.

To form the ridge vent, a slot is cut out of the roof sheathing along the ridge. A fibrous material is installed over the top of the slot and held in place by a cap. The fibrous material will keep out insects while still allowing air to flow. The cap holds the fiber in place and diverts air.

Different manufacturers produce ridge vents in different attic ventilation configurations.

1 27 7 Attic Ventilation Systems

Occasionally, you’ll find a ridge vent with nothing installed to keep insects out. This is a defective attic ventilation installation.

1 27 8 Attic Ventilation Systems

This grey fabric used in this attic ventilation ridge vent is one type of barrier, but you will see other types.

This is the vent for the bathroom exhaust fan improperly terminated inside the attic. Bathroom and laundry vents should terminate at the outside — not just beneath the ridge vent…

1 27 9 Attic Ventilation Systems

…and not under the insulation.

1 27 10 Attic Ventilation Systems

Although thermal buoyancy — which is the tendency for hot air to rise – will evacuate hot air from an attic space or rafter bay, attic ventilation is much more effective if the air is actively pulled out of the upper vents by other forces, such as air-pressure differentials.

1 27 11 Attic Ventilation Systems

When ridge vents have baffles, the baffles divert wind blowing across the roof upward. This creates an area of low pressure just above the opening, producing an air-pressure differential between the attic space and the exterior, which helps pull air out of the attic.

This effect is lost if the prevailing winds blow parallel to the ridge.

You may not know the direction from which prevailing winds blow. A home may have a well-designed attic ventilation system which is poorly oriented to the wind. In this case, it may be out-performed by a more poorly designed system which is better oriented to the wind.

You don’t have to know exactly how well the attic ventilation system performs, but knowing how systems are designed to perform may help if you find a problem that may be related to poor roof or attic ventilation.

You’ll find different types of continuous ridge vents installed. Whatever the type, you should check to make sure that the cap is securely fastened, since longer nails are required because of the additional thickness.

1 27 12 Attic Ventilation Systems

This ridge vent was improvised using sections of soffit vent. Obviously, the fasteners did not hold well enough.

Adequate Attic Ventilation

Most home inspection Standards of Practice require comments on the adequacy of attic ventilation.

The most commonly accepted formula for determining adequate attic ventilation is a minimum of 12 square inches of net free area for every 150 square feet of attic space. Twelve square inches might be an opening of 3 inches by 4 inches.

“Net free” means that louvers or other devices that reduce the actual size of the opening cannot encroach upon the opening.

This minimum can be reduced if a low-perm vapor barrier is installed on the warm side of the attic floor, which would be beneath the insulation. If this is the case, the formula changes to 12 square inches of net free area for every 300 square feet of attic space. So, with no air barrier, it’s 150, and with an air barrier, it’s 300.

An example of a commonly installed, low-perm vapor barrier is plastic attached to the underside of the ceiling joists before the ceiling was drywalled.

Attic Ventilation Radiant Barriers

Radiant barriers are materials that are thermally reflective. They reflect heat. They often look like foil bonded to cardboard. Here’s an example. You may see sheets stapled to roof framing in the attic.

Stapling sheets to the bottoms of rafters will trap hot air in the rafter bays.

Installing them beneath solid roof sheathing is also unacceptable. This roof originally had wood shakes installed. When they were replaced with asphalt composition shingles, a radiant barrier was installed on top of the spaced sheathing before a solid roof deck was installed as a substrate for the shingles.

Heat radiated downward by the roofing materials will be trapped and increase the temperature on the roof. This condition can contribute to premature failure of roofing materials, especially asphalt underlayment and asphalt shingles. It will also void the shingle manufacturer’s warranty. If the radiant barrier installation instructions conflict with the shingle manufacturer’s instructions, the shingle manufacturer’s instructions should take precedence.

A better installation and attic ventilation would be to fasten radiant sheets to the attic floor. The amount of heat radiated to the living space will be reduced, but the attic ventilation system will still be able to evacuate hot attic air to the outside.

Generally, asphalt shingle manufacturers require that the underside of the roof sheathing be ventilated for proper attic ventilation for the warranty to be effective.

Home Inspection FYI

Providing Help, Knowledge and Other Useful Information To Home Owners Everywhere

Tag Archives: ventilation systems

Exhaust Fans and Bathroom Ventilation

Ventilation systems for bathrooms are designed to exhaust odors and moist air to the home’s exterior from the bathroom. Typical ventilation systems consist of a ceiling fan unit in the ceiling of the bathroom connected to a duct that terminates at the roof.

Exhaust Fan Ventilation Functions;

Defects in Poor Ventilation

Mold and Poor Ventilation

In order to grow, mold fungi require that the following conditions are present:

Improper Ventilation

Ventilation ducts must:

The following tips are helpful although not required. Ventilation ducts should:

Attic Ventilation Do’s and Dont’s

Ventilation Systems For The Attic

Roof Structure Ventilation

Ventilation Can Cool the Living Space

Ventilation Can Help Remove Moisture

ACTIVE VENTILATION SYSTEMS

Whole-House Fan For Ventilation

Backdrafting

Attic Ventilation Systems

Attic Ventilation Systems

Adequate Attic Ventilation

Attic Ventilation Radiant Barriers

Generally, asphalt shingle manufacturers require that the underside of the roof sheathing be ventilated for proper attic ventilation for the warranty to be effective.