What does the the N factor stand for

I have a theory for what the N stands for in the N factor but really no clue. I understand what the n factor intends to do and how to use it but fail to understand how it applies in the real world and in particular to San Diego.

The N factor is a correction number that smooths the data for blower door numbers in multiple climate zones of homes of different size and height and brings the homes into a level playing field. We then can use this number to determine ventilation rates and put everyone on the same page.

Using the N factor is rather simple. We take the number and plug it into a formula. See my MVG page*.

It’s simple its slick and it is easy to use. It is even been updated in a new ASHRAE formula that I learned about in a training session that I was fortunate to attend taught by ventilation guru Paul Raymer, great class and very informative. At one point the class went to using the new N formula. We did the math for our climate zone plugged the numbers in and Voila, our 2500 sq ft 2 story home here in San Diego needs ventilation at 2778 cfm50. Paul asks for what number should we shoot for a tight home? 2500 the answer comes from the back.

I sat and said nothing but inside I was screaming “2500 cfm50 is not a tight home! It’s a %$#^&* colander with a roof and a door!” Paul had allot to teach and the class was moving fast so I kept it to myself. This conversation is not for this class.

So as we use the N factor and on face value it says if your home meets a certain tightness limit you need mechanical ventilation. Now I have no issue with the mechanical ventilation part but have some major issues with the factor N tightness thresholds. As we explain this we contend if your house reaches a certain tightness level you need Mechanical Ventilation. Right or wrong explanation it is one I have often heard. Why is my tightness level so out of whack and not tight at all?

I have a basic MVG calculator* that I use from the BPI standard that uses the LBL N factor and the one that most of us have been trained in. I will use it for this discussion. First there are four climate zones for the United States. Really? Only four? Heck we have 16 climate zones according to California standards and four right here in San Diego County. Fine let’s accept four and move on.

So we will figure out a 1000 sq ft shielded one story home and the MVG for each home in the following

Zone 1 Fargo ND                  MVG=  871

Zone 2 Ellenville NY             MVG= 1039  Fargo plus 19 percent

Zone 3 Miami Fl                   MVG= 1207  Fargo plus 39 percent

Zone 4 San Diego CA           MVG=1376 Fargo plus 59 percent

Ok so lets take this data and bring into basic blower door formulas***and estimate the size hole we will have for each home at this MVG I am using a basic formulas for comparison. I got these formulas from a post on JLC by Martin Holiday** of Green Building Advisor but I plugged it into a excel spreadsheet a long time ago. I think I might have gotten a couple of others from other locations as well. I did not develop any of these on my own. While these numbers might not be precise they should serve for comparison.


                                       Natural Infiltration      ACH        EqLA        ELA

Fargo  ND                        43.5                                 6.5           87.1         48

Ellenville  NY                   52                                    7.79         104           58

Miami       FL                  60.35                               9.05          121          67

San Diego   CA               68.8                                 10.32         137          76 

So let’s look at our houses Lets say they are 25 wide by 40 feet long we then have a surface area of 3040

Fargo ND    We have a total leakage of 2.875 percent or a window open at all times of about 7x7 open

Ellenville NY we have a total leakage of 3.425 percent or a window open at all times of 7.5 x7.5

Miami FL we have a total leakage of 4 percent or a window open at all times of 8x8

San Diego CA we have a total leakage of 4.5 percent or a window open at all times of  8.5x8.5 and that this is where I am considered a tight house

I would argue that none of these homes are tight. But why does the home in San Diego require ventilation at almost 60 percent higher rate of leakage than Fargo ND? This is rather counter intuitive. I am certainly more inclined to have a window open in the winter, spring, fall and summer than my neighbor in Fargo due to weather conditions. The chance of me getting fresh air from an open window for natural ventilation year round are greater in my area than any other in the US. We have little to worry about with humidity. I heard a comedian once refer to San Diego as a huge unfurnished apartment. I am simply lost when looking for reason for this standard. Why are we required mechanical ventilation before any other climate zone but more importantly why are we considered tight at such a loose state?

Quite honestly it is not the mechanical ventilation I have issue with. it is simply a good idea. Seal it tight and Ventilate it right.

 What makes this tricky in my minds eye is that we use this formula as a delta of which homes are loose and which homes are tight. My main concern of course is in my territory. How do I convey to my customer base that the national recognized standard is pretty much laughable and that almost any home can reach the tightness level by simply closing their doors and using a can a spray foam in a couple of key areas here in Americas Finest City. That when we reach our “tightness” level we are looser than a nut without a bolt. Our infiltration rate in a “tight” home is unacceptable and should be thrown out into the trash and moved on down the road buried and forgotten.

I don’t know what the answer is but I do know I don’t like it when I have to label a home with somewhere around 10 ach and more than a 75 sq ft hole in the structure as tight. I know I want my customer’s homes far below that level before I would entertain the thought they are tightened up on their envelope.

And while I respect the folks at LBL and recognize that they probably forgot yesterday more than I will ever know about blower door data I am not in love with the N factor. 

I think it stands Necromancy.

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Comment by John Brooks on October 12, 2012 at 2:44pm

I think the correct thing to do is to build tight and provide mechanical ventilation.

and Not to rely on construction flaws for ventilation....

and Not to build or upgrade "just tight enough" to avoid mechanical ventilation.

I am not surprised that some people get "funny" ideas about the purpose of BTL.

here is a quote from {Residential Energy}

"Blower-door testing is the most practical way to predict energy savings from air-sealing methods.

Measuring air leakage allows technicians to apply a Building Tightness Limit (BTL) and various

economic limits to air sealing.

The BTL prevents over-tightening buildings that have no ventilation system.

Economic air-sealing limits prevent technicians from spending too much time and material

sealing small, elusive air leaks."

Comment by Glen Gallo on October 12, 2012 at 11:25am


Thanks for you input. I am off to put food on the table and will make corrections over the weekend

While both of us  agree that it is not a definition of a tight house. The fact that tightness is in the label misleads some to believe otherwise. It is a misconception to be sure however one I have dealt with more than once that is quite frankly frustrating

Comment by Robert Riversong on October 12, 2012 at 11:15am


Your spreadsheets are just as uninformative as the OP above, as you don't label your input units. Martin Holladay's ELA and EqLA formulas are rough approximations, and they both are expressed in square inches not square feet (that's that happens when you don't specify your units).

And I don't know where you got the idea that BTL is the definition of a tight house, when it was never meant to be anything other than the point at which mechanical ventilation is required to supplement natural infiltration.

Comment by Glen Gallo on October 12, 2012 at 10:16am


I was being tongue and cheek with my first statement,  N = Nat 

The formulas are linked below the article. They are in an excel spreadsheet and easy to examine. As far as their accuracy I address that in the blog as not being accurate but being used for comparison. If you show me my errors Robert I would be grateful as I am always willing to learn and correct my mistakes.

The point of my article was that the BTL can be misinterpreted as being a tight home and I think often is which is the point I have issue with. I understand that their has to be standard somewhere. I simply think the rate of a "tight" home is way off base.


As you know there are different programs and codes. It would certainly be simpler if ASHRAE 62.2 2010 were accepted globally. Here in California for new homes we work off the 2007 version. For rebate programs on existing homes it is still 1989 as of today

As you know the 2010 still has the N formula but it is used much differently as an infiltration credit than it was as a ventilation guideline 1989 version but the N factor and the  BTL is still there. My target market is existing homes so ASHRAE 2010 or 1989 do not apply in most cases as codes are not retroactive. In existing homes I contend that our measure are sold and not bought. As we put figures of the BTL in a proposal it would be easy to interpret this minimum guideline as being tight.

Furthermore with the new N factor as I reviewed it it shows China Lake in the Mojave as the same N factor as San Diego.  Weather wise China Lake and San Diego are so far apart it is odd to see them on the same number. When I visit my friend there there are certain rules. In Fall only open one car door at a time or the entire contents of your is flushed out of the vehicle because of the high winds. In Summer get out of the sun by about 10 am or suffer severely from the brutal heat.In Winter it gets pretty cold when the sun goes down. Spring is very pleasant

I would have to conclude there is more than Stack Effect in this formula as the stack in the severe weather conditions of China Lake is far different than in mild San Diego and the stack would not even close to the same

Comment by Robert Riversong on October 11, 2012 at 1:28pm

N stands for natural, but it's a very rough approximation of the expected natural air exchange averaged over the year. It tells us nothing about how much air change is occurring on any given day, and will mean that on mild still days there will be no air exchange while on cold windy days there will be too much.

Lacking any better guideline, many states used the BTL formula to determine at what level mechanical ventilation would be required to insure a reasonable likelihood of sufficient air exchange to maintain IAQ.

As for your numbers, you don't show your formulas or use units, so it's impossible to validate them, but your conclusion that a house in San Diego requires a 75 SF hole is way off base. That it requires more leakage area than a house in Fargo is because there is far less delta-T stack effect to drive natural infiltration.

Comment by George Kopf on October 11, 2012 at 1:04pm

Correct me if I am wrong, but, if you're using ASHRAE 62.2.2010, all of this is a moot point.  That being said, as someone who has used the N Factor for the last 4 years, I recognize (along with a lot of other folks) it has limitations.  I still think it gives a fairly accurate and, more importantly, "field friendly" tool to make sure we have a healthy flow of air through the home.  For example, in my 1,330 sq.ft. (9' ceilings) 3 story townhome, N factor tells me I need right around 70 CFMnat.  A recent blower door test showed I actually have around 80 CFMnat - so pretty close.  Having lived there 3 years, I'd say it's about right:  not too tight, not too loose.

Not very scientific I will admit but, at the end of the day, aren't we concerned about comfort (along with health, safety, durability and efficiency)?  I am comfortable in my home.  Bills are pretty low (~$75 for a family of 4), air quality is great, no moisture issues, etc.  That's a win in my column.  And if it's good enough for me, I am happy to use this standard for our customers. 

N factor isn't perfect, but it works.  IMHO...

Comment by Evan Mills on October 7, 2012 at 4:12am

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