Window NFRC ratings and airtightness
I’ve read about NFRC rating. Intuitively, infiltration seems like it should be one of the more important considerations- if the window is leaking air, the U/R value seems secondary.
I’m looking at windows that are NFRC listed. However the NFRC listings on the website do not seem to include infiltration for many windows. You can navigate to the particular window via manufacturer and window type, and from there select the product line, which then brings you to a page with “General Information: and a ratings table.
The General Information includes a cell for Air Leakage, but for many windows, air leakage is blank. In the ratings table, there is a column for U-factor, SHGC, VT and Condensation Resistance. If I click on the CPD# link for the particular window, it brings up a pop up window with a table that includes a box for Air Infiltration, but that box is also blank.
It appears that to meet Energy Star, a window must have infiltration of less than 0.3.cfm/ft2. The manufacturer’s website has a page indicating Air Infiltration is 0.05 (cfm/ft2 of frame @ 1.57 psf wind pressure).
At first glance 0.05 seems good, but since it does not appear on the NFRC site, I’m suspicious of the number. Is there any good reason a window from a national manufacturer would not have it’s air infiltration rating on the NFRC site? Is the psf of 1.57 appropriate? What psf is associated with the Energy Star requirement of 0.3 cfm/ft2 assume?
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Replies
C.L.,
First, window manufacturers who voluntarily decide to test their windows according to NFRC labeling requirements must report U-factor, SHGC, and VT, but they aren't required to report air leakage. Air leakage testing is a voluntary element of this voluntary program. Window manufacturers negotiated for a certain amount of wiggle room -- making the reporting of air leakage optional -- when the NFRC program was established.
The Energy Star program for windows does include a maximum air leakage threshold -- namely ≤0.3 cfm/f2 for most windows. The Energy Star Window Specification does not specifically list the test pressure, but references test standards.
The most common way that window manufacturers comply with the Energy Star requirement for window air leakage is to comply with the NFRC test procedure. The NFRC test procedure for window air leakage references ASTM E283, which requires testing at a pressure difference of 75 pascals (with the exterior of the window pressurized with respect to the interior).
If a window manufacturer is reporting an air leakage result without referencing ASTM E283 or NFRC, there is no way to know the pressure at which the window was tested. If the window was tested at a pressure difference of 5 pascals, it's relatively meaningless.
A test pressure of 1.57 psf (the test pressure reported by the manufacturer you mention) is equivalent to 75 pascals, and is therefore a meaningful test. If you doubt the manufacturer's report, ask a technical help representative to send you a copy of the third-party laboratory result. A reputable window manufacturer should comply with that request.
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Martin - thank you very much for the thorough response.
Based on the information above, is it correct to come to the following conclusions:
1. Although NFRC does not require infiltration testing to be reported, Energy Star does require infiltration testing.
2. If a product is Energy Star certified, the manufacturer had to submit infiltration testing data to prove the infiltration is <0.3 cfm/sf, and one of the acceptable test procedures requires that testing be done at a pressure difference of 75 pascals which is equal to 1.57 psf
So my follow up question is regarding the infiltration numbers, assuming they were done per the Energy Star requirements. How big a difference does it make to use a window that reports it is only Energy Star compliant (assume just met <0.3 cfm/sf, or is 0.299 cfm/sf) vs a window that reports 0.2 cfm/sf. I know that is 0.1 cfm/sf difference, but is that negligible or a big deal? Energy Star only requires doors to meet <0.5 cfm/sf, and there are plenty of doors that still do not carry an Energy Star label...
C.L.,
Your two numbered points are correct statements.
There is no simple way to describe the lower air leakage rate of a high-performance window. Most homeowners with Energy Star windows are happy, and don't complain about air leakage.
However, if your house is on a cliff facing the ocean in northern Maine, or if you are building a Passivhaus, you may want to invest in windows with a lower air leakage rate than a run-of-the-mill Energy Star window.
Calling the mfg for infiltration test data is hopeless. I get bounced around and nobody knows who has the answer, and they all want to know why I need the info...I don't get the impression they don't want to disclose the info, but rather that this is an unusual question, and they have 10's or 100's of people who work there, and I did not happen to get bounced to the 1 or 2 people in their technical department who understand the question, much less know how to provide the answer.
I found the below data buried in a document on their website. This is for a window that, per the NFRC site, meets energy star, so the infiltration must be <0.3/cfm/sf, but beyond that, it would be very helpful if someone could help me understand this data. The air infiltration looks good compared to Energy Star Min, and per Martin's note above, seems to be tested at the proper pascals, but what would these numbers be on a Passive House appropriate window (I'm not building PH, just PGH, but trying to understand order of magnitude difference)? What do the Water Resistance and Design Pressure numbers mean? Any help greatly appreciated.
Performance
Meets or Exceeds AAMA/WDMA: Ratings R30-R50; Hallmark Certified
- Air Infiltration (cfm/ft2 of frame @ 1.57 psf wind pressure): 0.05
- Water Resistance: 7.5 psf
- Design Pressure: 30-50 psf
C L
I fell into the same chasm as you recently. That info you are looking for is non-existent on same company's websites, or proudly displayed on others. You will find that some windows with great U values can have horrid air infiltration numbers, or unacceptable water resistance numbers. If you start looking at sliding doors and these values, god help you. Some doors fail so badly I would be ashamed to be the seller. Some of those doors are incredibly expensive. Anyway, for the best explanation, I refer to this quote by the mighty window guru Oberon476:
"The DP rating of a window or door is based on laboratory pressure testing in pounds per square foot or psf.
Air, water, structural is a three part test that determines much about a window's overall performance. Air infiltration is the first phase, water penetration is next, and structural is the third part of the test.
Windows are tested for air infiltration simulating a 25mph wind or a 1.56PSF pressure load - air infiltration is treated separately from both water infiltration and structural and it is independent of the design pressure of the unit. Said again - the air infiltration rate in a window is not based on the design pressure rating of the unit.
Both water penetration and structural testing, on the other hand, are based on the window DP rating. Water infiltration is tested at 15% of the design pressure and structural is tested at 150% of DP rating.
What this means is that a window with a DP30 is tested for water infiltration at 4.5psf (15% of 30psf) while a window with a DP40 is tested at 6psf (15% of 40).
A window with a DP30 rating should be able to keep out rain when its driven by 42mph winds and a window with a DP40 should be able to keep out rain when driven by 49mph winds...so while water infiltration is DP related - and air infiltration is not - the nature of air and water infiltration is different.
The structural rating of a window is as much about the glass as it is about the frame and sash system. In order to get a higher DP rating the window manufacturer has to consider the thickness and possible heat-strengthening (or tempering) of the glass as well as the use of higher-end hardware and good quality sealants in the frame and sash system. But, interestingly, there is nothing in the structural rating that specifically requires that the unit be air-tight.
A window can leak air like a sieve and still achieve an excellent DP rating. Likewise a window that is sealed tightly can have a lower DP rating but excellent air infiltration numbers. Obviously there are also many units that have both excellent air infiltration numbers and a satisfactory DP rating (relating to both structural strength and water infiltration).
Simply stated, the relationship between DP and windspeed is -- "the ratios of the design pressures in psf are the square of the ratios of the wind-speeds in mph".
A window with a DP30 is rated to a pressure level equivalent to a 110mph windspeed, but it is tested (for structural) at a pressure equivalent to 164mph.
A window with a DP40 is rated to a pressure level equivalent to a 127mph windspeed, but it is tested (for structural) at a pressure equivalent to 190mph.
If you are curious about calculating the numbers yourself, a while back (at the request of a specific window company as a matter of fact) I wrote a couple of simple formulas that will allow you to do so...
If wind-speed is known, then:
W/25 * 0.0624 * W = psf
Where W = wind-speed
Or, if Design Pressure is known, then:
SQRT(psf) * 20.01 = wind-speed"
Take note of what he says about wind speed and DP ratings--i.e. The DP30 window should be able to keep out wind driven rain at 42mph. I don't know about you, but where I live, that is nothing. We can easily get 60mph+ rains. A DP 50 rated window would withstand a 55mph wind driven rain. The specs you show at the end of you post are actually pretty good from what I have found. A DP50, with water infiltration tested to 7.5psf and that super low air infiltration would be an above average window. I compared some slider doors recently:
Sierra Pacific sliding door, 4 panels, with the middle two being the sliders, outside 2 fixed. DP15! Water tested to 3.13psf, so expect leaks with a 35mph wind-driven rain. It barely passes on the air infiltration at .26 cfm/sf.
Jump up to the Cascadia Windows compression slider and we see air infiltration of .002 cfm/sf, and water penetration tested to 15psf(a 78mph rain). They rock!
Ok, so I am designing a PGH, a simple cottage, one floor, 32 feet x 30 feet simple rectangle with 9-foot ceilings. I'm thinking about Air Changes per Hour and blower door tests.
Enclosed volume is 32x30x9=8640 cubic feet
There will be 4 operating windows of 10 square feet each = 40 sq ft fenestration area. Assume they meet the minimum 0.3 cu ft/ min /sq ft standard, then expected air infiltration on a blower door test would be not more than 0.3 x 40 sq ft = 12 cu ft. / min = 720 cu ft / hour.
So an otherwise hermetically sealed building would expect 720/8640 = 0.083 air changes per hour (ACH) due to the window leakage alone.
That's not much ACH due to the operating windows. It's only about 14% of the PassiveHaus standard of 0.6 ACH. Admittedly i's a small number of windows, but also a small structure. Other fixed windows would add nothing to ACH, and would be OK to use liberally as far as ACH is concerned.
So is the above analysis correct? Can you use window air infiltration performance specs directly to calculate their contribution to blower door ACH results? Or, like just about everything else, is there a 'gotcha' lurking somewhere?
This is germane to budgeting because going from a 0.3 ordinary performance window to a 0.03 high performance window (10 times better) might not make enough difference to be worthwhile in an otherwise well-sealed building.