Blower door tests and Equivalent Leakage Area
In reading Dr. Lstiburek’s book Series “Builders Guide to… ‘name your climate'”, I concluded (though it was never specified quite clearly) that for 1000 CFM at 50 Pa the EqLA was 1 square foot. That would be about 7 CFM50 for each square inch of EqLA.
More recently, in an article by an editor of Journal of Light Construction, a duct blaster was used to conduct a blower door test in a very tight house. The conclusion was that the 50-65 CFM50 equated to an EqLA of about 2 ½ sq. in.. That would be about 23 CFM50 for each square inch of EqLA.
It doesn’t seem that both can be correct – 7 vs. 23. Is there a consensus of what CFM50 equates to a square inch of EqLA?
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Ted,
I'm waiting to hear back from the technical help hotline at the Energy Conservatory for an answer to your question.
While we're waiting, here's info from an NREL document -- and the info leads me to suspect that the answer is, "It's complicated."
The reference in this document to 4 Pascals (instead of 10 Pascals) leads me to suspect that the author is confusing "equivalent leakage area" with "effective leakage area."
"The equivalent leakage area (ELA) is defined as the area of a calibrated orifice that would have the same air flow rate the house does at a pressure of 4 Pa. The ELA, therefore, is an estimate of the aggregate size of all the leaks in the building. An ELA can be calculated from the results of a multipoint blower door test. This is generally done using a laptop computer with TECTITE software to automatically control the blower door for multipoint tests. Typically, 100 data points at each of 8 different pressures between 15 and 50 Pa are used to determine the relationship between pressure and leakage rate for the test home. The ELA is based on the leakage rate at a 4 Pa pressure difference, which is determined by a curve fit to the blower door test data at various pressures. The ELA can then be used in conjunction with weather conditions at the home site to model the natural ACH at particular times and to estimate long-term or annual infiltration rates."
Ted,
Here's the answer, to the best of my knowledge, from research I did for an Energy Design Update article many years ago.
Equivalent leak area (EqLA) is the area of a theoretical sharp-edged hole in the building envelope that would leak as much as all of the building’s actual holes at a pressure difference of 10 Pa. EqLA (in square inches) approximately equals cfm50 divided by 10.
Effective leak area (ELA) is the area of a theoretical hole (with rounded edges) in the building envelope that would leak as much as all of the building’s actual holes at a pressure difference of 4 Pa. ELA (in square inches) approximately equals cfm50 divided by 18.
A short answer is that it is not possible to determine that actual area of leaks from just CFM50, because leakage paths of different shapes but the same area let different amounts of air through. With more data (CFM4, CFM10, CFM25 and CFM50 would be a good data set to work with) and more effort or software it is possible to get a more accurate estimate. But there's rarely a reason to try to estimate leakage area accurately. The only reason I know of to calculate ELA is to get an idea of how big a hole or how many holes you are looking for when you go to do the air sealing, or to communicate to a homeowner how bad it it. For either purpose a rough estimate is fine.
That helps a lot; and I'm so glad you didn't stop after answer #1.
I suspect there's confusion out there about what someone means when "ELA" is used. Does it mean Equivalent or Effective Leakage Area. Even when the terms (abbreviations) Equivalent (EqLA) or Effective (ELA) are used, one wonders whether the author is using them appropriately; knows the difference; or even that there is a difference.
I suppose, in the end, the difference is of not much practical importance.
Thanks .
More on equivalent leakage area: Paul Morin of The Energy Conservatory just sent me this information by e-mail:
"An older blower door manual shows that the formula for Equivalent Leakage Area = 0.2939 x CFM10. Also attached is Appendix D of the old manual that shows some test results and those results graphed on log log graph paper.
"Here are some ways to calculate CFM10:
• The most obvious way would be to perform a single point test in the PR / FL mode at 10 Pascals. However; on a windy day it is difficult to get an accurate CFM10.
• Do a single point CFM50 test and convert the CFM50 to CFM10 using a default flow exponent of 0.65. CFM10 = CFM50 x (10/50 )^0.65. This would be more accurate, but the 0.65 flow exponent is a default number and is likely not the correct flow exponent for the house you are testing.
• Record a multi-point blower door test in the PR / FL Mode and plot the points on log log graph paper. Draw a line through the points to calculate the CFM10. This is the old school way (BC, Before Computers).
• Do a multipoint test using the TECTITE software and let the software calculate the actual flow exponent and the Equivalent Leakage Area. This will give you the most accurate results."
Before anyone gets bent out of shape due to apparent contradictions in the formulas provided, make sure that you note that some of these formulas refer to cfm @ 50 pascals, and others refer to cfm @ 10 pascals.
Actually the reason that I want to know is related to a new construction home that I will begin soon. My builder (with whom I enjoy a good mutual constructive relationship) typically builds HERS 29-31 houses, without PV, and is always pursuing the next lower number. I visit his projects regularly and tease him about the size of the hole (EqLA) he left in the last home, by wearing a patch depicting the size of the hole. This is by way of encouraging him to push the envelope especially when he gets to my home, where the goal is to achieve 0.1ACH50 (the decimal is correct) though I realize that will be very difficult.
Unfortunately there appears to be a lower limit in the HERS system below which lowering the leakage no longer earns any additional lowering of the HERS score. For my builder, pursuing a lower HERS score means affecting other elements of construction and insulation where the economics of triple paned windows or higher levels of insulation don't seem to make much sense. Without PV, a HERS score of around 28 or 29 seems to be the economic threshold around here, upper CZ-4, in southern Indiana.
My impulse is to say that the PH target of 0.6 ACH50 is an excellent goal, but 1.0 ACH50 is good and achievable with reasonable design and construction practices, and will result in an efficient, comfortable, durable house if I haven’t done something stupid. [“(Stupidity avoidance continues to be an important aspect of building science.)” – R. Baldwin] Clearly air-sealing is a major key to any PGH or better building, but eventually other design aspects are more important to making a pleasant home than chasing the last theoretical square-edged hole in the envelope. And it’s probably reasonable to limit HERS scoring at some point for theoretical leakage rates, especially since holes may not leak much in real life, depending on where they are in the envelope.
But it sounds like you have a good relationship with your builder, so I won’t say anything.