U-value calculation of glazing on custom windows
In the midst of a micro-cabin SIP build that uses very custom sized windows and doors. The frames have been built and are awaiting the custom-ordered glazing. I’ve searched all local suppliers of insulated replacement glass and can’t find anyone that knows what the u-value is on the units which they sell.
Is it possible that u-value is only tested on completed doors and windows (As I’m being told)? If the finished unit has a u-value that is derived from the formulation that considers separate values of frame and glazing, isn’t it reasonable that one would be able to source values for standard glazing with known: glass thickness, number of panes, and type of gas?
I can’t believe these numbers don’t exist except for the Pella and Anderson’s of the fenestration industry when they rate their own finished product.
Any glazing geeks care to help me with this one so that I might have a more accurate shot at modelling with real performance numbers?
Thanks,
AC
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Replies
Glass manufacturers like Cardinal publish u-factor info for their products, but you'll need to pay attention to how they develop those numbers. They are easy to find online. Is your supplier able to provide the specific glass/coating you want?
Al,
Here is a link to U-factor information for some glazing products from Guardian Industries:
https://www.guardian.com/cs/groups/sunguard/documents/native/gi_001675.pdf
Here is a link to U-factor information for some glazing products from Cardinal Glass:
http://www.cardinalcorp.com/wp-content/uploads/pdf/cardinal-architectural-brochure.pdf
As David pointed out, other glazing manufacturers post similar technical manuals online; just do a Google search for the name of the glazing manufacturer and "U-factor."
Thanks David and Martin, But the referenced sources missed the mark. Although Cardinal and Guardian have u-value charts, they are for commercial glazing units. I'm looking for specs on two layers of clear glass 1/8" thick with a 1/2" air-space. No gas and no coatings. This would be the configuration on most residential double pane units. Yet I still can't find any values on that configuration.
Your help is still greatly appreciated.
AC
Al,
Clear double glazing has a U-factor of 0.48. That's the glazing-only U-factor, not a whole-window U-factor.
Hi Al,
Actually, the majority of residential window's manufactured today do contain both a LowE coating and a gas infill between the lites. And while Guardian does produce glass for both commercial and residential applications, Cardinal is almost 100% residential. The links that David and Martin posted for you are applicable to residential windows.
Also, the Pella's and Andersen's of the world don't rate their own windows, their windows are tested and rated by the NFRC just like most (but not all) reputable window manufacturers do (and some less reputable).
If you want to post the applicable information on your windows including glass thickness, width of the airspace, spacer used in the IGU, window style, and sash/frame materials, I can run the calcs for you and give you a reasonable approximation of the numbers that you would get if you were buying from a window company that has their products certified.
Regards,
Oberon
Martin, Thanks for that #. Can I assume that is with no gas? Can I ask where you found that #?
Oberon, Thanks for your help and your offer of calculating the whole window UF. I'll format the drawings and send them over next week. I need to verify the spacer type as the last spec on the drawing. Should be an interesting exercise. I look forward to working with you.
AC
Al,
In the glazing world, the phrase "clear double glazing" refers to an IGU (insulated glazing unit) with two panes of glass, no low-e coating, and no argon or krypton fill.
There are many reference works that provide U-factor information on glazing; ASHRAE Fundamentals is one. If you're looking for an online document, I suggest that you start here: Selecting Windows for Energy Efficiency. I have reproduced a table from that document below. (Click the image to enlarge it.)
For more information on these issues here on the GBA site, I recommend All About Glazing Options.
.
Al,
In the glass/window world there are certain givens. One of those givens is that if you put two pieces of glass together in a window, doesn't matter what spacer you use, doesn't matter if you use wood, vinyl, fiberglass, aluminum, or concrete for the frame, the center-of-glass U-value is .48. If the outside air temp is zero and the inside air temp is 70° (and there is no more than a very light breeze) then the inside center-of-glass temperature will be 43°.
Martin's references are dead on of course, but some things just "are what they are".
Have a great weekend!