What difference, if any, does Gap Width make?

A place to ask all your replacement window questions
Post Reply
Message
Author
Mark
Posts: 67
Joined: Thu Oct 07, 2004 9:26 am
Location: Atlanta

What difference, if any, does Gap Width make?

#1 Post by Mark » Wed Oct 13, 2004 10:01 am

In http://www.hfrc.org Looking at one double-hung window by one particular manufacturer, only for double panes, I see 3 different Gap Widths listed - .64, .828, and .75. I can't see where it makes any difference in in U, SHGC, or VT. So why does a manufacturer have 3 different Gap Widths? I looked at another manufacturer and they have a whole bunch more than 3 gap widths.

More importantly, do I, a homeowner looking for replacement windows, have any reason to care what the Gap Width is?

Thank you

Mark
:? [/url]

Oberon
Posts: 205
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

Gap Width

#2 Post by Oberon » Wed Oct 13, 2004 1:15 pm

I am assuming that you are talking about the air gap between the two lites in an IGU.
I will answer accordingly.

You are a window manufacturer. You have several different glass thickness options available in your IGU's...possily even laminated glass.
Now consider having only a singel spacer (gap option). You manufacture two IGU's, one with two pieces of 5/32" glass and one with two pieces of 3/32" glass, and both using a 1/2" spacer.
One completed IGU is 13/16" overall width and the other is 11/16", a full 1/8" difference in the width of the IGU.

What the window manufacturer wants to avoid is having to manufacture different sashes for each possible glass/spacer width.

Using different width spacer (gap width) allows the manufacturer to customize the IGU for the sash which is MUCH easier and less expensive than trying to manufacturer different sashes for each possible IGU width.
So, we end up with a single IGU width applicable to entire product lines and with several available glass options.

Make sense?

Oberon
Posts: 205
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

Gap Width

#3 Post by Oberon » Wed Oct 13, 2004 1:23 pm

I didn't answer the second question.

Gap width can effect the overall energy efficiency of the unit.
But, when considering a dual-pane IGU with argon and a soft-coat Low-E, 7/16" thru about 3/4" gap is not going to display a radical differentation in performance.

A 7/16" gap is about optimum for that particular make-up, and after 3/4" you will start to lose energy performance...but unless the manufacturer varies widely from these numbers you really won't notice the difference.

The old adage of "if a 1/2" is good, then 1" must be twice as good" definitely does not apply in this case!

FenEx
Posts: 553
Joined: Thu Oct 07, 2004 11:18 am
Location: Illinois

reply

#4 Post by FenEx » Wed Oct 13, 2004 1:53 pm

I must disagree. Even with units made by the same manufacturer, with the same spacer and gas fill... the performance of the glass unit increases with the width of the space.. it does not diminish over 3/4" or any other nearby increment. Even the difference of 1/8th of an inch in overall I.G. thickness (all other specs being equal) increases the glass efficiency by 5%. This is easily varified by the NFRC. As any testing is expensive, many manufacturers are allowed to post an approved minimum for multiple units that fall within the required guidelines and specs.

Mark
Posts: 67
Joined: Thu Oct 07, 2004 9:26 am
Location: Atlanta

Re: reply

#5 Post by Mark » Wed Oct 13, 2004 9:37 pm

FenEx wrote: This is easily varified by the NFRC. As any testing is expensive, many manufacturers are allowed to post an approved minimum for multiple units that fall within the required guidelines and specs.
To me, those 2 sentences seem contradictory and I'm asking for clarification to increase my understanding. I can go to NFRC and see that the different gap widths have no reported difference on the specs shown there. There is nothing in NFRC (that I can find) that indicates which units they tested and which they posted the minimum. So help me understand. Since you are saying that the wider the better, I will ask my local dealer for the wider widths. (And I just hope that he won't tell me that I am a nut case for asking about things that no one else has ever asked about!)

Thank you for your help.

ps - in the NFRC ratings, some windows have a 0.00 U-value. Which means an infinite R rating. So it isn't possible. Probably a typo or a human mistake. Just goes to show that not everything on the NFRC website is perfect.

FenEx
Posts: 553
Joined: Thu Oct 07, 2004 11:18 am
Location: Illinois

clarification

#6 Post by FenEx » Thu Oct 14, 2004 9:07 am

First of all, I misread Oberon's post. When he said 3/4"... I was thinking I.G. thickness, not gap of 3/4". Has me curious now. Secondly, you make a valid point by my using "easy" and NFRC in the same sentence. It does get pretty confusing. Awhile back I had come across some testing procedures used by Oak Ridge National Laboratory to calculate SHGC and U-factors on windows. I remember reading that the NFRC offers default values for products that meet or exceed certain criteria as a minimum without re-testing. I'll let you know if I dig up anything that may help clarify.

Oberon
Posts: 205
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

Width - Spacer - performance -

#7 Post by Oberon » Thu Oct 14, 2004 10:10 am

I was researching the internet this morning, hoping to find a graphic presentation of spacer performance. Unfortunately, I couldn't find one anywhere. Oh well.

Thanks for the clarification FenEx about IGU width and airspace width -- I hope that I wasn't confusing when I was discussing gap spacing versus overall IGU spacing.

Anyway, one really interesting fact about using inert gasses and Low-E coatings in an IGU is the performance of the gasses at various "thicknesses" of the airspace(s) within the IGU....which I think was something of the original question.

The current very best, most energy efficient, spacing/gas combination is Xenon in a triple glazed IGU with "airspaces" of about 7/32" each....we are looking at a U value of .09 in that construction.

There was a question posted a few days ago about whether or not triple glazing is an advantage over double glazing. A triple glazed unit with Krypton fill and Low-E on two different surfaces cannot quite match the performance of Xenon fill, but it is available and it is pretty darn hard to beat....U values of .10 are achievable. Dual-pane Argon will theoretically peak at about .13...none of which are particularly bad numbers, but remember that these are also optimum numbers that depend on other factors as well.

At the "thin" space-widths, Argon isn't nearly as effective as the other gasees but by the time you approach a 7/16" width airspace the advantages of Xenon and Krypton are a bit less.

Xenon fill peaks at about 7/32" and begins to decline in perfomance almost immediately, and Krypton peaks at about 5/16" and is pretty consistent up to about 1/2" where its performance begins to decline, but that is also where Argon peaks - but even there Argon does not "quite" match Krypton performance.

Also, when I say perfomance declines, I am not saying that suddenly it doesn't work anymore...in fact, both of those gasses continue to work very well... just that they are past their peak performance area.

Typical Argon performance peaks at about 7/16" to 9/16"...but even at its peak performance, Argon cannot quite match either Xenon or Krypton even when those two gasses are no longer performaing at their optimum.

Hope this makes some sense?

FenEx
Posts: 553
Joined: Thu Oct 07, 2004 11:18 am
Location: Illinois

#8 Post by FenEx » Thu Oct 14, 2004 11:19 am

Now that makes more sense to me. So you are saying that the performance doesn't deteriorate over 3/4", but the rate of improvement is gradually reduced. Kind of like a car that does 0-60 in 5.5 seconds but only 60-90 in the next 5.5.

I would like to add though, that the achievable U-factors (i.e. 0.10) you stated are for glass only... not the entire window unit. Just trying to avoid confusion.

Oberon
Posts: 205
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

Performance

#9 Post by Oberon » Thu Oct 14, 2004 12:20 pm

Great analogy!

But the performance really does go down as the width increases.

Imagine the Xenon gets to a60mph peak in 4 seconds, and then drops down to 54mph over the next minute.
The Krypton gets to 58 in 6 seconds and stays there for the next 10 seconds and then drops down to 53 over the next minute.
And finally, the Argon gets to 53 in 10 seconds and stays there for about 15 seconds and then drops to 50 in the next minute.
Again, this is based on the airspace and not the overall width of the total IGU.
And all of them start to experience a more pronounced drop off in performance after that...that more pronounced drop off is due to convection currents that form within the IGU space as the width increases.
And obviously I did not attempt to make my mph vs time to scale, but the number-ideas are "correct" in the comparison of higher and lower and maximums and minimums - ie: The "minimum" for Xenon is a hair more than maximum for Argon.

And thanks and you are correct that I was speaking entirely of glass-only when speaking of theoretical U values...that was what I meant when I mentioned the "other factors involved", but not clearly stated.

Mark
Posts: 67
Joined: Thu Oct 07, 2004 9:26 am
Location: Atlanta

#10 Post by Mark » Fri Oct 15, 2004 10:11 am

I found this interesting article on the web about the difference in R value that a fill gas makes. While it wasn't talking about windows per se, I don't see why it doesn't apply to windows. Anyway, I thought it fit in well in this topic.
Gas-filled panels contain multiple pockets of sealed polymer film filled with low-conductivity argon, krypton, or xenon gas, which have R-values per inch of 7.2, 12.4, and 20 respectively. Vacuum insulation panels use a vacuum held between two gas-impermeable layers of metal to create R-values of 25 to 40 per inch. Aerogels are low-density solids that resemble wisps of frozen smoke. They are made most commonly from silica and offer R-values of 15 to 35 per inch. All three of these are currently used in appliances such as ovens and refrigerators, but they are too expensive to compete with traditional building insulations. One company, however, is developing a granulated transparent aerogel that will be used to create skylights and windows with R-values of 8 to 20. The first of these should be available in late 2002 or early 2003.

Mark
Posts: 67
Joined: Thu Oct 07, 2004 9:26 am
Location: Atlanta

#11 Post by Mark » Fri Oct 15, 2004 10:27 am

And I found this on http://www.efficientwindows.org/gasfills.cfm
Manufacturers have introduced the use of argon and krypton gas fills, with measurable improvement in thermal performance. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless. The optimal spacing for an argon-filled unit is the same as for air, about 1/2 inch (11-13 mm). Krypton has better thermal performance, but is more expensive to produce. Krypton is particularly useful when the space between glazings must be thinner than normally desired, for example, 1/4 inch (6 mm). The optimum gap width for krypton is 3/8" (9mm). A mixture of krypton and argon gases is also used as a compromise between thermal performance and cost.

Oberon
Posts: 205
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

Aerogels

#12 Post by Oberon » Fri Oct 15, 2004 11:26 am

The aerogels are really something. And when placed in an IGU, they really have very little effect on visible transmission, but an astounding effect on energy performance. In fact, the only thing "better" than performance is price!

Figure an "average size" fixed IG with a glass-alone price tag in the thousands! Seriously, they can be made, but in no way does the perfomance increase even come remotely close to the price increase...
but perhaps in the future?

Mark
Posts: 67
Joined: Thu Oct 07, 2004 9:26 am
Location: Atlanta

#13 Post by Mark » Fri Oct 15, 2004 12:03 pm

Photos of Aerogel are cool!

Image

Post Reply