Air Infiltration

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Joined: Thu Oct 07, 2004 11:18 am
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#16 Post by FenEx »


I was going to leave this thread alone as it was becoming very complicated to most readers, but I'll try to briefly clarify some earlier posts. This thread was originally about air infiltration and was taken over by DP ratings. They are not directly connected but yet they are.

DP ratings incorporate wind loads and include water into those loads... not air infiltration. Even a residential unit rated DP-65 at 195 mph winds, is still tested for air infiltration at only 25 mph... that of a light storm or windy coastal region. Only heavy commercial or architectural rated units that are tested include deflection... not the residential units. Any sash or frame deflection WILL allow tremendous amounts of air infiltration but the water penetration won't necessarily follow the same paths allowing it to achieve a higher DP rating but yet performing poorly for many nationwide applications regarding energy efficiency. DP ratings do not include air-infiltration... but they can affect it drastically. A more rigid and stronger, reinforced frame and sash often used to achieve a higher DP rating, will have less deflection allowing the seals to be more effective against air infiltration.

Oberon, perhaps a clarification of the following statement?

"My point being that while DP ratings can be very important, even crucial, in some environments, the idea that a person in an area that rarely or never experiences 100mph wind gusts is worried about the difference between a DP45 versus a DP60 is worrying over something that really isn’t a significant factor in their situation…A window or door with a DP15 or DP20 can be every bit as good – in the correct environment – as a window or door with a DP80."

It's not a false statement with regards to structural damage given specific perimeters... but within the context of this post including air infiltration and overall performance, it can be misleading.

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air infiltration

#17 Post by Oberon »

Good post FenEx, you are correct that I did lose the initial point of the thread. I was thinking structural and didn't even consider air infiltration in my answer.

As FenEx pointed out, air, water, structural is a three part test that determines much about a windows overall performance. Air infiltration is the first phase, water penetration is next, and structural is the third part of the test.

As he also pointed out, windows are tested for air infiltration simulating a 25mph wind or a 1.56PSF pressure load. This does seem rather low, especially for windows that are destined to be installed in a higher wind zones area. Unfortunately, this is the current reality.

Also as FenEx noted, air infiltration is treated separately from both water infiltration and structural.

Both water penetration and structural testing are based on the window’s DP rating (in psf). Water infiltration is tested at 15% of the design pressure and structural is tested at 150% of DP rating.

What this ultimately means is that a window with a DP60 is tested for its water infiltration at 9psf (15% of 60psf) or a windspeed of 60mph, while a window with a DP45 is tested at 15% of 45 or 6.75psf or a windspeed of approximately 52mph.

A window with a DP45 rating should be able to keep out rain when its driven by 52mph winds and a window with a DP60 should be able to keep out rain when driven by 60mph while water infiltration is DP related, air infiltration is not…and like FenEx said, the nature of air and water infiltration is different and even if a window is water tight, it can still have a great deal of air infiltration.

Structural rating 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 of the glass as well as the use of higher-end hardware and good quality sealants in the frame and sash. But there is nothing in the structural rating that 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 units that can do both.

Ultimately, there is no direct correlation between DP rating and air infiltration numbers. Just because a unit has a great DP does not necessarily mean that it will have correspondingly great air infiltration figures. Conversely, just because a window has a lower DP does not necessarily indicate that it will be leaky.

Certainly a window built with top quality components and care can have both a great DP rating and a great air infiltration number; but air infiltration and DP rating have to be considered separately when purchasing a window system because one rating may not indicate the other and a window or door with a lower DP might have excellent energy numbers despite its inappropriateness in a particular environment.

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Re: air infiltration

#18 Post by tru_blue »

Oberon wrote:Ultimately, there is no direct correlation between DP rating and air infiltration numbers. Just because a unit has a great DP does not necessarily mean that it will have correspondingly great air infiltration figures.
Great explanations Fen and Oberon. I have noticed at times when comparing two different windows that the DP of window A might be higher than window B, but the air infiltration of window B was tighter than window A. Now I understand why. :idea:

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