Glass Choice Double Check
Hello everyone, this is my first post so I apologize if I screw up any etiquette.
I found this site while searching for an answer to my window question and it looks like many of the people here are a wealth of knowledge, more-so than the building professionals and salespeople that I’ve come into contact with so far.
I will be ordering windows soon (within a week) for my house in the Boston area that I’m renovating. We are pretty much set on Marvin double hung*. It seems like the default glass that most people order is double pane Low E2. Most windows (in my quote) with this glass end up with a U-value around 0.30 and SHGC around 0.27.
The HERS rater suggested going to E3 glass. which drops U-value to 0.28 and SHGC to 0.19. From what I have read, it seems like in my location** that SHGC is on the low side and I would benefit from a higher number. Cardinal has a ton of glass but the standard options don’t appear to have the right combination of low u-factor and high(er) SHGC that I think (I could be wrong) would be the right choice for this house. I could request something else (which the Marvin rep on the phone said they may or may not accept).
I’m thinking of requesting a double pane Low E1 with ERS (in Cardinal terms, I believe that is Low E180 with i89). Maybe spending a little more for triple pane on a handful of select windows in the living and dining areas where people would be close enough to that the comfort factor would be worth the extra expense. Any suggestions on something different from anyone here?
Thanks!
*need double hung for historic reasons, builder and architect both like and recommend Marvin, pricing was much better than Pella and close to Anderson, 1st and 2nd floors will be Signature Ultimate series, basement and 3rd floor will be Elevate series (to save some money)
**Boston area, in a neighborhood laid out in a grid with houses on quarter acre lots, a decent amount of shade from trees and neighboring houses, front door faces northeast (living space mostly faces southeast and southwest). House is a very typical looking 2.5 story 100-year old stucco house (no overhang above windows except for roof on upper levels)
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Replies
The first number used for Cardinal coatings designations indicates the number of layers of silver in the coating, the next two numbers indicate the VT of the glass in dual pane configuration.
For example, LoE-180 means that there is one layer of silver and the VT is 80%. Standard LoE²-272 has two layers of silver and 72% VT, while LoE³-366 has three silver layers and 66% VT. Although LoE³-366 has the lowest U factor, as you noted it also has the lowest VT and SHGC. Conversely, LoE-180 has the highest U factor as well as highest VT and SHGC.
Ideally you are looking for the U factor of the 366 with the VT and SHGC of the 180, but current state-of-the-art coating technology isn't there yet, so there needs to be compromises.
Adding i89 to surface 4 is an option because it lowers U factor with little effect to either VT or SHGC. Surface 4 coatings work by reflecting room heat back into the room rather than allowing it to radiate to the outdoors. The upside is a warmer room, particularly in the area directly in front of the window, the downside is that some of the heat that is being reflected back into the room by the coating that would normally be absorbed by the window glass isn't there resulting in colder glass. Sounds counter-intuitive, but that's how it works. Since the glass is colder when using a surface 4 coating, it can be more prone to condensation formation on the glass given the right (or wrong) environmental conditions. With the caveat that while the glass with surface 4 coating is colder than a comparable LowE coated IGU, it's still warmer than a non-coated dual pane glass unit. If you currently have LowE coated dual pane windows in your home that have condensation issues in the winter then there is a good chance that adding a surface 4 coating to the new windows will continue that trend, if they don't have current condensation issues then you might be fine adding the surface 4 coating. If you have single pane or non-coated dual pane and don't have current condensation problems, then you should have no problems with adding the surface 4 coating....and so on.
The difference in U factor between the various coatings is generally not much more than a couple of hundredths, while as you have seen the difference in SHGC and VT can be very significant. But keep in mind that the U factor, SHGC, and VT that you get from the window company are (and if not, should be) whole window performance values, not just glass. All of these factors can be significantly altered when compared with glass values depending on window frame style and materials used.
Unless you have some serious shading on the western side of your home, I would not suggest using a high solar heat gain coating facing that direction because it's likely to result in some serious heat gain in any rooms facing that direction in the summer months. It's even entirely possible to overheat west-facing rooms in the winter when using a HSGC coating facing west.
I do like the idea of triples. If you are going to go that route, and not going to do all your windows with triples, then I would at least suggest anything facing north or west would be good candidates for that upgrade. For a triple facing north or west I would recommend using LoE²-272 on IG surface 2 and 180 on glass surface 5, this is a very common triple pane IGU configuration and it works well. I think it has the best combination of U, SHGC, and VT for those conditions.
For windows facing east or south, assuming these are dual pane, I would look at using 272 on IG surface 2 with i89 on surface 4. I think that this is also the best combination for your environment to balance summer and winter requirements.
Thanks you for the detailed reply. I will read it a couple more times to make sure I pick up on everything.
Quick followup question: do you think there would be any noticeable difference in tint/color from the inside if I did the combo of double and triple you recommend at the end? Also, how would you recommend treating southeast, southwest, northeast, northwest windows rather than due south, north, etc?
Unfortunately, I am not living in the house so I don't have any past experience with the existing windows. Bought it to renovate just recently and the seller was an estate so I can't ask them anything about it either. My only reference point for condensation are the windows in my current and previous homes which I assume are the default dual pane option any spec builder would use (so probably not i89). I've never had any condensation issues outside of a small amount of frost on the outside edges on a handful of the very coldest winter days.
Thank you
You're welcome...
Although there is a slight VT difference between dual pane with 272 on surface 2 and triple with 272 on surface 2 with 180 on surface 5, you really shouldn't notice a color or VT difference between them looking out in most cases. If they are right next to one another in the same room it might be noticeable in the right lighting conditions, but that's as much due to the extra lite in the triple as much as the extra coating. Also there is no appreciable change to glass VT or color or hue with i89 added.
Ultimately if a window gets full or significant direct sun exposure in summer I would be concerned about blocking solar heat gain. Conversely if a window is not getting much sun in winter, or it's facing prevailing winter winds, I would be focused on a tight unit with a low u factor.
I also think that your environment is probably a good fit for the surface 4 coating, with the caveat that I would still prefer triples.
Thanks again for the advice! I'm going with tripane 272 and 180.
FYI for anyone that comes across this thread later, get all the pricing and options from Marvin (or whoever you are getting windows from) upfront and they don't have all the options and if they can get alternatives they can be priced differently (for example, e272 w/ i89 was $347 more than e272 in a dual pane window; the tripane option I went with [272 and 180] was only $135 more than dual pane 272).