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.