Window Research Questions

[MinneGal]

Hi to all the pros and experienced buyers - I'd like some assistance in knowing the importance of various stats/factors and how to find them, among other things.

First - I have a lower end townhome in MN with 28-year old clear double pane wood casement windows (and a wood sgd) on the south and west sides. South side has no shade and those windows have taken a real beating (and need to thus withstand heat/sun in the future application). I want to replace for reasons of maintenance, aesthetics, and of course energy efficiency. I plan to replace most with sliders; one bank of 4 casements will likely become 3-lite casement or possibly 3-lite slider. I want the windows to be in good shape for 25 years, but don't want to overdo it in my home price range.

Questions as I've been researching:

What importance is a difference of .02 in the U factor? i.e. .32 vs. .30, or .30 vs. .28 - are these comparisons essentially equivalent? Is a CR of 59 vs. 56 essentially equivalent?

How important is the DP rating? Where can I find it if it's not part of a company's marketing? One of the windows I was considering apparently has a DP rating of 20 or maybe 30, that sounds low.

Where can I find air infiltration rate (also not on NFRC site)?

On the NFRC site is the gap width representative of the glass package thickness, so I could assume on dual pane smaller gap is less insulative? What is the LowE - Internal Film (surface) ? Haven't found those described there.

How important is reinforcement in the vinyl? Frame and sash, just sash, or doesn't matter too much if the ratings are good?

The sgd is on lower level walkout that is 15 degrees colder than on upper level. Not knowing anything else about my home, could you advise if you think a triple pane sgd would add much more insulative value to the room overall (which also has woodburning fireplace opening) than double pane argon low e?

Thanks in advance!

[Windows on Washington]

Hi to all the pros and experienced buyers - I'd like some assistance in knowing the importance of various stats/factors and how to find them, among other things.

First - I have a lower end townhome in MN with 28-year old clear double pane wood casement windows (and a wood sgd) on the south and west sides. South side has no shade and those windows have taken a real beating (and need to thus withstand heat/sun in the future application). I want to replace for reasons of maintenance, aesthetics, and of course energy efficiency. I plan to replace most with sliders; one bank of 4 casements will likely become 3-lite casement or possibly 3-lite slider. I want the windows to be in good shape for 25 years, but don't want to overdo it in my home price range.

Casements, albeit a bit more expensive than like sized sliders, will net better energy performance than a like sized slider. Keep that in mind.

Any good vinyl replacement should survive quite nicely in direct sunlight and function flawlessly for years to come.

Questions as I've been researching:

What importance is a difference of .02 in the U factor? i.e. .32 vs. .30, or .30 vs. .28 - are these comparisons essentially equivalent? Is a CR of 59 vs. 56 essentially equivalent?

Look at it like a percentage. If you that .32 is a function of percentage, a window with a .30 U-Factor should be about 6% more efficient from a thermal standpoint. Obviously if the window with a .32 U-Factor has a much better air infiltration number, that difference may be negated.

Same thing for the CR. Unless you have existing condensation/humidity issues, I would move that down the list of importance. Most good insulated frame/glass vinyl windows will have sufficient CR numbers.

How important is the DP rating? Where can I find it if it's not part of a company's marketing? One of the windows I was considering apparently has a DP rating of 20 or maybe 30, that sounds low.

DP rating gives a nice representation of overall unit strength and build quality. A window with a low DP will not fair well in high wind situations and typically indicates a unit that was skimped on in the vinyl arena. Usually the result of thin extrusions and a lack of reinforcements. DP 20 is pretty low considering there are well built units that are in the 50's and 60's.

Where can I find air infiltration rate (also not on NFRC site)?

You will usually have to go direct to the manufacturer. Some manufactures post that information on their website and others avoid it like the plague. If a manufacturer is putting it out there, they are probably not ashamed of it.

On the NFRC site is the gap width representative of the glass package thickness, so I could assume on dual pane smaller gap is less insulative? What is the LowE - Internal Film (surface) ? Haven't found those described there.

I don't know this for a fact, but have been told that the optimum air gap is somewhere between 1/2 to 3/4s and going above that range does not increase insulation performance. The internal film surface is a measurement of film thickness if I am not mistaken.

How important is reinforcement in the vinyl? Frame and sash, just sash, or doesn't matter too much if the ratings are good?

I prefer it. Adds strength to the window and it is nice to think of the locks and keepers going into something other than just vinyl. If the frame extrusion is solid, it might be overkill there but I do like them in the sash meeting rails at a minimum.

The sgd is on lower level walkout that is 15 degrees colder than on upper level. Not knowing anything else about my home, could you advise if you think a triple pane sgd would add much more insulative value to the room overall (which also has woodburning fireplace opening) than double pane argon low e?

That would be fairly difficult to quantify without seeing specific data on the two glass packs. That being said, a SGD is mostly glass and triple pane/krypton is usually a significant improvement over dual pane/argon.

Thanks in advance!

[MinneGal]

Thanks very much for the info and tips, WoW.

Back to the DP rating, is that based on a test "ASTM E330" for Wind Resistance? I have some specs for the ProVia/Sugarcreek window that indicate the DP 20 for a slider and 30 for a casement based on that test. Given the great PR I've heard about that window (it's the highest window recommended by one contractor, above Gorell), such a low DP rating seems odd. The window does have aluminum reinforcement in the sash (and frame for casement) and I was told was 85 mil vinyl (is that good? if average, what would be good?). Air infiltration is .1 cfm/ft2 for slider and .01 for casement (seems good). U is .31 / .28 for these models. Triple weatherstripping including "Q lon".

Gorell is not in the running since its profile is 1" wider than most vinyls and I don't want that loss of glass space. The ProVia is a nice looking window inside and out. Has anyone here had experience with it?

Others in the running are Sunrise (DP 35 for slider, AI of .03) and possibly SL Imperial or Simonton Platinum (5500). The looks of the latter two are less appealing to me, including the welds, the white color, the lack of coving/routing (possibly I saw an old Simonton sample, the welds are not routed and are ugly. This was at a distributor warehouse, I don't yet have a contractor for that option). I know those are highly recommended on this site.

The ProVia salesman said the Sunrise had an interior stop vs. an exterior stop (is that bad?). I don't think Sunrise is reinforced - yet the DP rating is higher (is that due to the thickness of vinyl extrusion, or what?).

I noted that American Craftsman at HD show DP ratings of 35-50 for various styles!

Installation is another matter, am still in process of understanding those components and my feelings of the contractors will play in. But I want to understand the windows.

It would be so much simpler if this information were uniformly available! NFRC is helpful but incomplete. I really appreciate this BB resource.

Thanks in advance (to WoW or anyone else) for insight on these matters!

[Oberon]

On the NFRC site is the gap width representative of the glass package thickness, so I could assume on dual pane smaller gap is less insulative? What is the LowE - Internal Film (surface) ? Haven't found those described there.

The optimum space between the lites in a dual pane IGU is 7/16" to 1/2". Anything wider and you really don't gain anything in themral performance up to about 7/8". Past 7/8" the efficiency of the unit actually begins to decline due to convection currents inside the IGU. This is a very slight and gradual decline, though. Nothing drastic.

I am not sure what you mean by LowE - Internal Film (surface) , but there are a few options available in your environment depending on whether of not you prefer high solar gain windows to help with winter heating of your home - with the downside that the higher the solar heat gain of the unit, the lower the overall thermnal efficiency.

Or, you may prefer improved U-factor performance for increased thermal efficiency in both summer and winter - but with the downside that increased thermal efficiency also means less winter solar gain performance.

High solar heat gain LowE coatings, for optimum performance, should be applied to surface 3 of the IGU. Surface 3 is the inside-the-IGU-airspace surface of the interior lite.

Low solar heat gain LowE coatings should be applied to surface 2 for best performance. Surface 2 is the inside-the-IGU-airspace surface of the exterior lite.

Is that what you might be asking?

How important is the DP rating? Where can I find it if it's not part of a company's marketing? One of the windows I was considering apparently has a DP rating of 20 or maybe 30, that sounds low.

Design Pressure or DP is actually part of the WDMA rating system rather than NFRC. As a rather long follow up of DP, or how DP is tested or how it ultimately relates to window performance...

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. This is independent of the Design Pressure 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 ultimately means is that a window with a DP60, for example, is tested for its water infiltration at 9psf (15% of 60psf) or equivalent of a windspeed of 60mph, while a window with a DP45 is tested at 15% of 45 or 6.75psf or an equivalent 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 winds...so while water infiltration is DP related, air infiltration is not. 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 (or tempering) 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.

And as a final follow-up and a bit off topic. Much has been said recently about increased energy perfomance in windows. Many people point to the advantages of krypton fill in window IG units - it is being discussed extensively in some parts of the window industry.

Well, here is some food for thought - argon is 1/120th ot the earth's atmosphere. Krypton is 1/900,000th of the atmosphere. The question when considering krypton for improved window performance is "how much atmospheric krypton is actually available to use for IG fill?" And is it possible to actually "run out" of krypton for use in window IG systems?

[Windows on Washington]

Oberon,

I was pretty close on the air space. Thanks for clarifying that.

[MinneGal]

Oberon,

Thanks for helping to clarify this ironically opaque subject!

I get the gap width now, thanks, makes sense. I think you solved part of the low e surface issue.... for instance, a rating is .035(2) - meaning applied to surface 2 as I understand now, but what is .035 (or .042 or .027)? Additionally appreciated the U /SHGC thoughts.

Thanks for explaining the WDMA rating system and DP. What is the factor to convert to mph - i.e., DP 60 is related to 60 mph, but DP 45 is related to 52 mph?

I have since discovered that AAMA gold certification requires DP of 15. Not sure what that converts to in driving rain with windspeeds, but in MN environment with exposed windows to a 40 yard field wouldn't I want something rated at least 30 mph or more? I should add I've not had any problems with the 28 year old wood casements (except peeling paint).

And is there a measure to judge the quality of vinyl - thickness, perhaps? 80 mil or greater? or...?

Thanks

[Oberon]

WoW - you were right on at 1/2" to 3/4" since there is no difference in thermal (after rereading my original post I noticed that apparently I have issues spelling "thermal"!) performance of the unit in that range. Without a LowE coating, then figure 1/2" for peak thermal performance and with a LowE coating figure 7/16" max. But really, except to the engineers who design these things, anything from 7/16" to 7/8" is basically the same. Unless one includes krypton into the argument - then we have a new factor involved.

Minnegal, what part of the cities do you call home? I grew up in St Paul.

The numbers that you are referencing for the LowE coating - .027, .035, .042, etc - are probably the emissivity ratings of the specific version of the LowE coating. I say probably since I don't know your source of the number...but I am about 99% sure that that is what you are referring to.

My first suggestion is to forget about specific emissivity numbers. Again, like the specific airspace width, the emissivity numbers are of real use only to the PhD-types who design the coatings or to the engineers who design the IG / window. And I will also suggest that very few engineers really pay any attention to the specific emissivity of the coating. The coating manufacturers have folks making big bucks to do the explanations for them.

Now if you wanted the basic definition of the number I would tell you that it is the the ratio of the radiation intensity of a nonblack body to the radiation intensity of a blackbody at the same temperature. And if that particular bit of scientific trivia actually makes sense to you then we can discuss the specifics - but, otherwise, I wouldn't worry about it at all - except to say that, like U-factor, the lower the number the better the coatings thermal "blocking" ability (which isn't technically what it indicates, but the ultimate "result" is close enough and it is the factor that matters to the window buyer - pun 100% intended - sorry).

The simplest way to relate a DP rating to a windspeed is to take the squareroot of the DP rating and multiply it by 20 (well, 20.02 is closer, but again that engineer thing...).

Say a window has a DP of 65. The squareroot of 64 is 8 and 8 * 20 = 160. So, DP 65 equates (roughly) to 160mph wind. And while I know a few folks who would cringe at the approximation, in the real world that estimate is within a couple of mph and is absolutely close enough for any real world comparison.

Okay, a few more examples...

DP9 = sqrt of 3 * 20 = 60mph
DP16 = 4 * 20 = 80
DP25 = 5 * 20 = 100
DP36 = 6 * 20 = 120
DP49 = 7 * 20 = 140
DP64 = 8 * 20 = 160
DP81 = 9 * 20 = 180

Are you seeing the mathematical relationship between DP and windspeed? You may have already noticed that the that the ratios of the design pressures are the square of the ratios of the wind-speeds.

If not clear, then consider only DP 9, 36, and 81.

DP 9 = 60mph
DP 36 = 120mph
DP 81 = 180mph

So, while DP36 is 4 times DP 9, 120mph is only double 60mph
and while DP81 is 9 times DP 6, 180mph is only 3 times 60mph

Okay, enough of that stuff for now...I got carried away a bit. It happens often!

Have a great day!!

[Oberon]

and on my other question...anyone care to venture a thought about krypton availability in the future???

[Window4U (IL)]

and on my other question...anyone care to venture a thought about krypton availability in the future???

It seems like something drastic is happening in the krypton market with the huge increases in price happening lately.
Is it an increase in demand that current producers can't keep up with like I was thinking, or is it that the atmosphere is getting scarce of it? Or is that scenario just a thought to ponder for the future?

[Windows on Washington]

I have seen the price increase bulletins the last several weeks and they are significant in some cases.

Are there any other inert gases that work in that same capacity that do well in small spaces like krypton?

Xenon seems to be next on the list after Kryton but I don't know about the atmospheric availability.

[FenEx]

Hey Oberon, or anyone else...

Do you have any data on STC performance of cellulose vs open cell spray foam when used in wall cavities? Which is better?

[Window4U (IL)]

I found this a while back while researching.....

http://guardianpolymers.com/sound%20absorption.htm

[FenEx]

So the winner is a combination of both. Thanks Dave!

[Window4U (IL)]

Yeah, but the cellulose machine's drum gets real sticky when mixing em

[Window4U (IL)]

It looks like the cellulose does an outstanding job all by itself at a 49. That will be my choice for the work I'm doing on my home this year.