I'm looking for clarification on required ventilation levels in residential settings.  For instance if I have a home that indicates an ach rate of considerably less than .35 that would suggest the addition of mechanical ventilation. I'm unclear, however on how much is enough, how much is too much?  It would seem that the additional air exchange rate should be based around the actual natural rate difference between the measured amount and .35.

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hey Don, good to see you chime in here. I absolutely agree with you

Eric - you probably do not need to add mechanical ventilation but you are leaving a lot of energy savings behind. 





Mechanical ventilation costs energy - it does not save it. But the energy penalty is the price of good indoor air quality. And both IAQ requirements and most building codes require mechanical ventilation in any new house.

I just downloaded this from DOE website check out 2nd paragraph


Just a heads up, check out the date - that is from 2002 and outdated

Is it possible codes, etc... will change - sure as they always are but the most recent & proven one is listed above 7.5+


Seasons Greetings!!  Yes, that is old data.  The 15 cfm per person or 0.35 ACH numbers were from ASHRAE Standard 62-1989 - 22 years ago!!  When we set about updating 62-89 in 1994, we realized that most of the contaminants were related to the number of people and the size of the house, but not the volume.  So we reduced the ventilation rate per person from 15 cfm/person to 7.5 cfm/person plus an allowance for the size of the house, 0.01 cfm per square foot. BPI has been slow to change their calculation method to update it to the numbers used in the 2003, 2004, 2007, and 2010 versions of ASHRAE 62.2 but they are finally coming around.  DOE now requres that all WX done with federal dollars after January 1st must meet ASHRAE 62.2-2010 so it is well past time to abandon the 15 cfm/person rates that ASHRAE stopped publishing in 2003.  As for code, yes, the 2012 International Energy Conservation Code (IECC) and 2012 International Residential Code (IRC) both require mechanical whole house ventilation at these same rates and both require that all new construction be built to 5 ACH50 for the mild climates and 3 ACH50 for the more severe climates.  Not everyone will adopt the 2012 I-Codes this year, but they are coming.  ASHRAE 62.2 is already the required code in California, Maine, and several other states so it is time to figure out how to do it. 62.2 generally requires only 30 to 80 cfm so it is not a big airflow or energy penalty.  So my recommendation is to mimic Nike and Just Do It!!  Make it tight and ventilate it right at the 62.2 levels.

A short and concise brief on this can be found at



as well as on our own website, www.PureEnergyCoach.com   Have fun!

Tamasin,  Thanks for the pointer.  Good info on the control methods (timers) for fans etc.  

Here is excerpt from the International Residential Code.

Check with your local building department

 Hope this Helps

R303.1 Habitable rooms. All habitable rooms shall have an aggregate glazing area of not less than 8 percent of the floor area of such rooms. Natural ventilation shall be through windows, doors, louvers or other approved openings to the outdoor air. Such openings shall be provided with ready access or shall otherwise be readily controllable by the building occupants.
The minimum openable area to the outdoors shall be 4 percent of the floor area being ventilated.

1. The glazed areas need not be openable where the opening is not required by Section R310 and an approved mechanical ventilation system capable of producing 0.35 air change per hour in the room is installed or a whole-house mechanical ventilation system is installed capable of supplying outdoor ventilation air of 15 cubic feet per minute (cfm) (78 L/s) per occupant computed on the basis of two occupants for the first bedroom and one occupant for each additional bedroom.

In other words if it feels stuffy inside open a window :)

Following ASHRAE 62-2-2010 is relatively simple.  In my view the continued teaching of the .35 and its use by various programs around the country is less then State of the Art!

Amanda Evans pointed me to Rick Karg's app for the iPhone to calculate this.  ResVent62.2 in the app store.  It also works on the iPad.

Worthwhile minor investment for major results.

Could you spell out 62.2 2010's standard in full formula form for me? I am accustomed to calculating a BAS that results in a blower door number at CFM50, and if I have to deal with air changes I get uneasy that I'm not doing it right. If I start with square footage and number of occupants, I understand I multiply the occupants by 7.5 and add .01 for every square foot. What do I do then with that number in order to reach a blower door number at CFM50?

Teresa,   ACH is a function of rate and volume.  Start with the square footage and the volume of the home.  1000 sf home slab on grade. 10 feet ceilings so you have 10,000 cubic feet.

ACH is air change per hour so we are trying to determine the number of times per hour that volume changes.  We may have 1, 3, 5, or any number like that.  Energy Star has a requirement of 5 ACH measured at 50 pa pressure difference inside to outside.

We usually depressurize to -50pa  so we state an air change term of CFM @ 50.  That is an accurate repeatable number that does not require the N-Factor approximations and estimates of the auditor as to shielding.

The formula for taking a Blower Door number (measured at -50 pa depressurizaton) to ACH @ 50 is

(BD Reading * 60)  / Volume

So our home above built in 1950 could easily test at 2500 CFM at 50.

2500 * 60  is 150,000  now divide that by V(plume) and you have 15 ACH @ 50.

Our home built in the 1990 would test closer to 1200 CFM at 50.

1200 * 60 is 72,000 now divide that by V and you have 7.2 ACH at 50.

I tested an Energy Star 4 Bed Room home for Habitat for Humanity last Tuesday.

1176 sf  12,934 Volume  1001 CFM at 50 Blower Door test.

1001 * 60 is 60060 and divide by V and we get 4.64 ACH at 50 Meet Energy Star < 5 ACH @ 50.

The CFM @ 50 number you are looking for, on my Habitat Build, is around 1400

It is now irrelevant.  If the home comes in at 500 or 2000 CFM at 50 Blower Door after air sealing, we still need to add mechanical ventilation. The question is how much and then what strategy; balanced, supply, exhaust.

So 4 Bedrooms (plus 1) at 7.5 is 30 CFM.  Plus you add 1 CFM per 100 sf (1176 / 100) = 11.7 so the BASE requirement for Mechanical Ventilation is 41 CFM

You now figure an allowance for A) Building Tightness  and B) Spot (Intermittent) ventilation.

I use Rick Karg's iPad app (RESVENT 62 also for iPhone) for this.  So the required MV after allowing for the actual BD number was 29. Lower than the Base 41. Adding the 2 Baths and Kitchen with intermittent ventilation did not lower the 29.

If we had a 2500 BD number the app reports 0 CFM needed for MV.

If we had a 1500 BD number the required would be 17 CFM.

If you know the volume of your home and want to predict some ACH numbers for your home with a BD reading you need a couple of magic numbers. You can use the 60 in the formula with 3 and your magic number is 20. Or you use 5 and the 60 in the formula and your magic number is 12.

Divide the volume by 20 and you have the CFM @ 50 reading for 3 ACH at 50   12934 / 20 = 646

Divide the volume by 12  and you have the CFM @ 50 reading for 5 ACH at 50   12934 / 12 = 1077

Actual numbers above.


Does this help?


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