Ventilation & Filtration

Ventilation is the process of bringing in fresh air, typically by the use of fans, and usually referred to as "mechanical ventilation", but also passive ventilation which generally means opening windows.  This ventilation is brought into the building on purpose, as opposed to infiltration which is air that leaks in on accident (see the infiltration section).  The total amount of fresh air coming into the house then is the sum of the ventilation and infiltration amounts.

Mechanical ventilation is somewhat controversial, although there is no controversy at all among mainstream builders or even among most green builders.  If you don't like mechanical ventilation, then you probably shouldn't build so tight, however as the data below shows, if you live very clean your ventilation requirements could be much lower than the standard.

Fresh Air Requirements

Ventilation air brings in fresh oxygen, and dilutes pollutants by mixing in fresh air with the stale air and can also be used to eliminate excess indoor humidity.  The biological process of life consumes oxygen, and emits carbon dioxide at a generally fixed rate (much more during exercise), while the amount of pollutants entering the building from man made sources depends on what has been brought into the building, hence the amount of oxygenated air depends on the number of occupants, while the amount of dilution air depends on the rate of off-gassing etc from the materials in the house.  There are two different kind of dilution air needs: constant, for things that off gas at a fairly regular rate (furniture, upholstery, or anything that off-gases as its sitting there) and intermittent, for pollutants that are created by a specific activity (cooking, showers, hobbies involving toxics etc).  A home where shoes are left at the door, without pets, without hobbies that involve toxics, without toxics cleaners and pesticides and that avoids VOCs, will need much less fresh air than ones which do have some of these things.

The standard for the total air requirement used to be .35ACH, that is approximately 1/3 of the air in the building changed every hour.  The problem with this was that the number of occupants didn't vary by floor area, so a 3000SF house with two occupants ended up getting twice as much ventilation as a 1500SF one (70CFM versus 140CFM).

The standard1 was changed to 3CFM/100SF of floor space plus 7.5CFM per occupant.  In this case the number of occupants is taken to be the number of bedrooms plus one (eg two bedrooms implies three people), even if turns out that's not how many people will live there.  This is the total fresh air rate, and can be reduced somewhat for building with infiltration rates that are higher than the standard assumes.  For comparison, a person a rest breathes about 1/4CFM, so most of the air requirement is dilution air.

This ventilation requirement is considered the base rate--its a whole house ventilation rate-- and any activity that generates extra pollutants (including water vapor which could lead to excessive humidity) would requires higher rates or spot ventilation.  However, given our low breathing rates, its clear that there is some dilution air already included in the per person requirements, and with the added ventilation requirement based on the floor area, its clear that the assumption is that most houses will need a significant amount of dilution air.  Still you may want additional ventilation during cooking and after showers.

Issues with infiltration

All buildings leak air, and historically this leakage was so high that that no ventilation at all was needed.  As building got a little tighter we started including spot ventilation in kitchens and baths and as they got tighter yet we included whole house ventilation systems.  Given that infiltration rates vary with weather (ie wind, temperature) and house tightness, but ventilation rates are fairly constant, how do we determine how ventilation air we need?  There are four general approaches:

  1. Let the building be very leaky so that even in calm, mind weather there is enough infiltration to meet the fresh air needs.  This is historically what we've done.
  2.  Make the building reasonably tight and provide constant mechanical ventilation. This could be the code required ventilation rate, or it could be reduced somewhat from that based on an assumed likely minimum infiltration rate.  In cold, windy weather the building is over ventilated, but this still uses less energy than the leaving the building be leaky.  This is the standard green building solution.
    Note that if the building isn't too tight (say around 2.5ACH50) the infiltration rate in the winter is generally high enough so as to not need any mechanical ventilation, but in the spring and fall you will need either passive or mechanical ventilation.
  3.  Make the building extremely tight and provide continuous mechanical ventilation.  This is the passive house approach and saves even more energy, particularly if the ventilation is done with an energy efficient HRV.   The infiltration rate here is generally so low that it can be ignored.
  4. Make the house moderately tight, but only provide ventilation when the weather is moderate, ie so that it's never over-ventilated.  While this has been suggested, I know of no buildings that have actually done it.  One obvious solution would be to find the approximate outdoor temperature where the house starts needing mechanical ventilation and shut off the ventilation system when it gets colder than that.2

While this gives solutions to determine ventilation rates, it doesn't address the energy cost of the various solutions (for more on infiltration/ventilation energy see the infiltration section), which suggest that building somewhat tight (tighter in colder climates than warmer ones) is the best solution.  As it turns out there are two other health reasons to do this, all due to side effects of the air movement thru insulation cavities.

Air brings moisture with it, and warm air holds much more moisture than cold air, so any warm air moving thru an insulated cavity can cause condensation if it is slowed down enough on its way out and comes in contact with a cold surface.  If this happens enough, the surface is likely to grow mold or mildew, and if the surface is rot, you get rot.  In winter this is moisture moves from inside to out, which with air conditioning, it moves outside to in.  Not that as long as the air moves thru a cavity fast, most of the condensation happens after the air moves thru the wall: it's only when we slow it down and add barriers that it condenses inside.  Needless to say, in old houses the air went out pretty fast.

For every bit of air moving out a building, some air moves in in an attempt to equalize the pressure.  This air then picks up an fine particulates and and VOCs from materials installed on the outside with the assumption they'd off-gas to the outside, or in the worst case mold spores from any condensation that has happened in the past.  Essentially you don't want any air moving thru your insulated cavities, even if you don't care about energy.  Note that air moving thru dryer vents, bath fan vents, and around windows and doors isn't likely a health problem because the air likely moves thru too fast to condense anywhere.

Sick Building Syndrome

This syndrome started occurring when we started adding greater levels of insulation and making buildings tighter, and while some people still blame the problem on these things, the reality is more complex.  We also started bringing more materials that off-gas around the same time, and in many cases the problem was that we didn't seal the building tight enough and mold accumulated in a wall or ceiling.  The solution for off-gassing is mostly to avoid those materials, but also to make sure there is enough ventilation.  The mold problem can be remediated by ventilation, but preventing it requires some combination of preventing air leaking thru insulated cavities, vapor barriers, and keeping the relative humidity from getting too high.  For a detailed description of the issue and solutions to the problem is in the moisture and condensing potential section.

Natural (Passive) Ventilation

Passive ventilation is ventilation by opening windows, doors and skylights that takes advantage of the naturally occurring pressure differences in a building.  In homes, the operation is almost always manual but windows can be electronically opened and closed. These pressure differences are the same as the ones that drive infiltration, but opening windows generally creates much larger holes than the ones in the envelope (see infiltration for a description of the natural forces).  Passive ventilation is ideal when the outside temperature is in the vicinity of room temperature, ie from somewhere around 60F to around 80F.  Passive nighttime cooling is similar to passive ventilation, but since the windows are generally closed all day, you may need some daytime mechanical ventilation in that situation.

cross venitlation

In designing a building to use natural ventilation, window and skylight opening should be designed to take advantage of both cross ventilation and stack effect ventilation.  To provide cross ventilation, opening windows need to be placed on opposite sides of the house: one should face the prevailing winds the other should face away.  In older houses where windows were placed fairly symmetrically around the house (see diagram at right), no matter what the wind direction, you generally got cross ventilation, even though some rooms might get much less than others, but if there are enough opening windows, you can control the ventilation by how much each window is open.  In general, cross ventilation is only a problem if your taste is for something aesthetically interesting, particularly if entire facades end up with no windows.  Luckily as long as there is at least one opening window in each room, you get plenty of ventilation no matter how they face.

In designing for cross ventilation, keep in mind that air will take the path of least resistance, which in the above diagram is shown as the two places where air comes in one windows and goes out the adjacent one.

Stack effect ventilation is done by having one or more openings somewhat high above the main floor.  If the desired effect is cooling, this kind of ventilation is much more effective on the main floor than it is on upper floors.  In a one floor house, you can get stack effect ventilation thru an opening skylight, but of course you need to think about what effect that skylight will have in all conditions to know whether putting it in is a good idea.

The downside of passive ventilation is that every so often the air is so still and the temperature so close to indoor temperature that air just doesn't want to move.  You'll still get enough fresh air, but if the desire is also cooling, you won't get much.

Exhaust Fans & Depressurization & Back drafting

In a tight house, the use of exhaust fans (especially downdraft cooktops) can result in depressurization of the house, which can make combustion devices back-draft (vent to indoors instead of outside).  Typical shower fan use (90CFM for 30 minutes), in a moderately tight house probably won't cause much depressurization, but in a very tight house (especially a small one), or with a more powerful fan (like a 300CFM range hood), depressurization is likely.  Downdraft cooktops often run at more than 500CFM and are a problem in even vaguely tight homes.  Clothes dryers are also essentially exhaust fans running in the 100 to 200CFM range, so they can also depressurize a building.  Back drafting can be a problem even in areas you wouldn't expect, like a furnace or hot water tank in a basement.  This can happen when the house gets depressurized and that causes air to be sucked out of the basement, when then depressurizes it also.

There are a number of solutions to this problems.  First, avoid high CFM fans. If you do use them, you'll like need to open a window in order to get enough air supply.  If you use sealed combustion (power vented) hot water heaters and furnaces, you avoid the problem and obviously is you avoid combustion appliances completely you avoid back-drafting.  Fireplaces and wood stoves and be a real problem here also, but since they usually create such a large draft themselves, if they're back-drafting you have a big problem.  Many wood stoves now come with fresh air kits that allow them to burn with outside air and that is clearly the way to go in a tight house. Fireplaces often produce many hundred CFM drafts themselves, so they are not a good idea in tight houses.

Ventilation and indoor humidity

During winter, ventilation tends to lower indoor humidity levels because cold air has very little water in it.  Buildings with high infiltration rates already experience this, so a tight building with controlled ventilation rates is less likely to have a problem with dry indoor air.  How much of a problem this is depends on the ventilation control system and how frequently the combination of ventilation and infiltration results in more air than necessary.

During summer, particularly in humid climates, the ventilation air is cooled when it enters the building, and hence increases the indoor humidity.  This is obviously more of a problem with air conditioned buildings where the indoor temperature is much lower than outside.


Many people assume that inside air is generally cleaner than outside, but in fact almost always the opposite is true, often to a very large degree.  Intuitively this is because unless you filter, the inside air tends to have all the pollutants from outside plus the ones introduced indoors.  Unless you live in an especially dirty location, there is usually no reason to filter incoming air.  Indoor air filters are usually part of an air recirculation system, which may also be fed with outside air.

Filtration systems for indoor air come in two varities: the standard filter is there to protect your furnace fan and filters only large particles.  If you want filtration beyond that you will need a better filter.  If you keep your house fairly free of of allergens and dust, and don't have any significant allergies or other health problems, you probably don't need additional filtration.  If you do want filtration, you will need a whole house ventilation system that comes with a fan powerful enough to overcome the resistance of the filter. Building details are in the construction section.

Up until recently, there were a number of different rating systems used for filters, and should you run into one of these older rating systems, you need to know when you buy a filter that is "90% efficient" what it is 90% efficient at removing, as they are rated by one of three methods: the arrestance method, the dustspot method and the D.O.P method.    A filter which is 97% efficient in the arrestance method, is only 50% efficient in the dust spot method, and only 10% efficient by the D.O.P. method.  Medium efficiency filters are rated by the dustspot method, and a filter of 30-40% should be the minimum standard, with a 60-70% filter being a better compromise.  HEPA filters are the cream of the crop, typically achieving 95% efficiency in the D.O.P. method.

Most filters are now rated using the merv rating system, which is on a scale of 1 to 16, where the number tell what size particles the filter removes.  A Merv-1 filter removes only the biggest particles, while a Merv-16 removes very tiny particles.  Merv-5 to Merv-8 are low  to medium efficiency filters (particles > 3-10 microns, dustspot 20-35%), while Merv 9-12 are medium to high efficiency (particles > 1-3 microns, dustspot 40-75%) , and Merv-13+ are very high efficiency.  Merv 8 is good enough to eliminate most dust, but if you want to remove the problem particles, you need higher level of filtration.

The challenge for filtering fine particles is that people create them every time you move (rubbing skin, cloth, furniture etc), and that the ventilation system must filter ALL the air in the house, which means the ducts must be very carefully laid out.  As the filter efficiency goes up, so does the energy needed by the fan to push air thru the less porous filter, although this can be (and often is) alleviated somewhat by making a wider filter.

As the efficiency of a filter goes up, so does it resistance to air flow, and so the efficiency of the filter is usually limited by how much it affects the furnace or ventilation fan.  One way to get higher efficiency without affecting the blower motor is to increase the diameter of the filter, which increases its surface area.



Understanding Ventilation, John Bower, Healthy House Institute, 1997

Builders Guide Series, Joe Lstiburek, EEBA, 2000.

The Health House Workbook, American Lung Association, 1995

Healthy by Design, David Rousseau & James Wasley, Hartley & Marks, 1997


1. the standard is ASHRE 62.2, but beware there are not only many versions of this, but various addendums.  This standard referenced here is 62.2-2010 addendum R.  Previous to addendum R, the rate based on floor area was 1CFM/100SF.

2: this is pure conjecture on my part.