Ventilation Choices: Three Ways to Keep Indoor Air Fresh

Every House Needs Fresh Air

Exhaust ventilation

Exhaust-only ventilation.
Stale air from bathrooms and the kitchen is pushed out of the house, while fresh makeup air enters the house through random leaks in the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials..

Supply ventilation

Supply-only ventilation.
A fan pulls outside air into the house and distributes it to living rooms and bedrooms, pressurizing the house. Stale air exits the house through random leaks in the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials..

Balanced ventilation

Balanced ventilation.
An exhaust fan pulls stale air from the house while a supply fan furnishes the house with an equal volume of fresh outdoor air. The use of a heat-recovery ventilator(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. or an energy-recovery ventilator(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. reduces the energy penalty associated with the operation of any ventilation system. While balanced ventilation systems are more complex than supply-only or exhaust-only systems, they generally provide more uniform distribution of fresh air.

DIVE DEEPER

OTHER CONSIDERATIONS

ASHRAE minimums

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has established a residential ventilation standard (Standard 62.2) calling for 1 cfm of mechanical ventilation for every 100 square feet of occupiable space, plus an additional 7.5 cfm per occupant. For example, a 2,000-square-foot house with three occupants requires 42.5 cfm of mechanical ventilation (20 cfm + 22.5 cfm).

GREEN POINTS

LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. EQ4 (Environmental Quality) offers up to 3 points for above-code ventilation practices; EQ5 offers up to 2 points for bath and kitchen ventilation; and EQ7 offers up to 2 points for filtering ventilation air.

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 9 — Indoor Environmental Quality: 8 pts. for properly sized and vented kitchen exhaust fan (902.1); up to 9 pts. for timer- or humidistat-controlled bath fan (902.2); 8 pts. for testing and confirmation of fan flow rates (902.3); up to 8 pts. for Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. rated bath exhaust fans (902.4); up to 17 pts. for whole building ventilation per ASHRAE 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. (902.5).

ABOUT VENTILATION

Delivering the right amount of fresh air

All the effort that goes into creating tighter houses results in significant energy savings and greater comfort. But moisture, chemical toxins, and cooking odors can’t dissipate as easily in a house with a tight envelope as in a leaky house. Opening a few windows may provide too much or too little ventilation, with adverse effects on comfort in either case. Even in houses where building materials and furnishings have been carefully chosen to reduce indoor air pollutants, some form of mechanical ventilation is a prerequisite for healthy occupants and a healthy building.

There are two basic strategies to provide mechanical ventilation: spot ventilation, which removes moisture and pollutants at their source, and whole-house ventilation.

Typical spot ventilation strategies include the use of bathroom exhaust fans and an exhaust fan over the kitchen range. Exhaust fans are available in a variety of sizes, styles and price ranges. Control options range from simple wall switches to sophisticated timers, occupancy sensors, or humidistats.

Whole-house mechanical ventilation systems are designed to remove stale air from or supply fresh air to the building as a whole. These systems are more complicated and more expensive than spot ventilation systems, but also more effective.

Use a variable speed furnace fan

Some ventilation systems use the furnace fan to distribute air around the house. In these systems, the fan speed used for heating and cooling is probably too high in a ventilation-only mode. A variable-speed or two-speed fan is more efficient, using less power for ventilation than for heating and cooling.

MORE ABOUT VENTILATION

Air sealing makes a house healthier

In designing any ventilation system, a few rules of thumb apply:

The house should be air-sealed carefully, particularly when it comes to basements, crawl spaces, garages and other areas where toxins and pollutants are common.

Building materials and furnishings that emit the least amounts of pollutants should be a first choice.

The most efficient ducts are smooth, straight, sized correctly, and sealed tightly. Corrugated ducts have greater resistance to airflow, and elbows and long runs of duct also reduce efficiency. Duct joints should be sealed with mastic. Ventilation ducts that pass through unconditioned spaces should be insulated.

Exhaust ducts should always be vented to the outside, not into crawl spaces, basements, or attics.

Don't forget about radon

Radon, a naturally occurring radioactive gas, is the second leading cause of lung cancer in the U.S., accounting for between 15,000 and 22,000 deaths a year, according to the National Cancer Institute. Radon can migrate through the soil and into basements, or enter a building via well water. Some parts of the country are at greater risk than others, but all houses should be tested for it.

Particularly in high-risk areas, a vent system designed to pick up radon from beneath the basement slab should be installed during construction. If tests later show radon levels are too high, it's relatively simple to install a fan and exhaust the gas before it can do any harm. Putting in a radon mitigation system after the fact is that much more difficult.

The EPA has a number of recommendations for radon resistant construction.

FURTHER RESOURCES

Designing a Ventilation System
For an in-depth discussion of this topic, see "Desiging a Good Ventilation System."

BuildingScience.com:
Review of Residential Ventilation Technologies

Green Basics Index

Image Credits:

Fine Homebuilding 178

Tags: ERV, exhaust ventilation, venting, Air-to-Air Energy Recovery Ventilation

Feb 22, 2010
6:58 PM EST

Rule of thumb
by Martin Holladay, GBA Advisor

Christoph,
You may want to read this article on blower door testing:
http://www.greenbuildingadvisor.com/blogs/dept/musings/blower-door-basics

The article includes the following rule of thumb: "If air-sealing work continues until the house is tightened below 1,000 cfm50, it’s advisable to install a whole-house mechanical ventilation system."

Feb 22, 2010
4:31 PM EST

Insulating old house: When is a ventilation system required?
by Christoph W

I'm curious to know at which 'air tightness' level a balanced ventilation system is recommended or required. We bought an old drafty fixer-upper two years ago and on windy days you could feel the air move through the house. Heating bills were $600 and above for some NH winter months. In the meantime we spray-foamed the whole attic, about half the outside walls (we're renovating through the house room by room), and replaced most windows with new, tight HE windows.
A recent blower door test still came back with values of more than 4000 at a 50psi pressure difference (I forget the unit), which from what I know is still a value as if there is a window open. During the test we indeed found a number of so far unknown air leaks in the basement and an addition that we have not touched yet, so right now we still have plenty of air exchanges on their own (even though the highest heating bill this winter was less than $250, I guess previously we had 4 open windows :-)

So long story short, as we continue renovating, insulating and sealing off the house, at some point in time it might be so tight that we require a balanced air exchanger. Is there a criteria that can be determined with a blower door test (e.g. dropping below a certain value)? Any other criteria? Depending on how low the value is (or how high the tightness), would it be acceptable to e.g. only provide air in the central hallway on each floor, or would we have to provide air to each room (particularly the bed rooms, everything else is pretty much an open concept)?

Just FYI, even though we have not really planned for such a system yet, we e.g. replaced the old monster of a boiler from the 70s with a HE boiler that has a sealed combustion. The water heater will at some point in time be replaced by an indirect tank heated by the boiler. So replacement air for combustion inside the house will not be an issue.