The right “e” for solar gain situation.
severaltypesofnerd
| Posted in General Questions on
Ok, color me confused. I should be up on this, but am not.
What’s the proper glazing type to specify maximizing solar gain, while still having reasonable cold weather performance? Which surface, e-Type, etc?
This would be for double pane plastic/vinyl window glass replacement. Zone 3 Marine, San Francisco Area. Zero to two hot days a year, no A/C. For a room where the emphasis is on bringing in light, and keep warm in winter. Solar gain in summer is no big deal, solar gain in winter is helpful.
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To maximize solar gain, you want to maximize IR transmission. Most low-e coatings are inteded to block IR transmission. You'd want either to specify the highest SHGC number you can find, or no low-E coating at all. i89 on the interior side might help you a bit, but I've never really tried anything like that and I don't know if that coating can be used with a plastic glazing material (I'm guessing it can't).
I think you'd be best off with a double pane assembly with an airspace of about 1/2" with no low-E coating at all if the goal is to maximize solar gain while also keeping the insulating U factor up as much as you can. You can play around with the balance between those two numbers to get the performance mix that's important to you. SHGC is the Solar Heat Gain Coefficient, which you want as high as possible to maximize solar gain through the window. U factor is the insulating value, and you want that as low as possible to trap heat inside. You can see how the different coatings affect those two values here:
https://www.cardinalcorp.com/technology/reference/loe-performance-stats/
If I'm understanding correctly and you're planning to use a plastic/vinyl window that uses a plastic glazing material and not glass, I'd strongly reconsider that. Glass tends to be more durable over time (much less prone to scratching, and much less prone to yellowing and other degradation). If you do want a non-glass glazing, you might try looking into the materials used for greenhouse glazing, such as "triple wall" polycarbonate. These materials are made to let in as much sunlight as possible while still providing good thermal insulation, they're durable, and they may be cheaper than a double glazed window assembly depending on what you're ultaimtely trying to do.
Bill
Is there any such thing as a "one way" IR coating?
Cold glass in winter sucks also. Maybe "solar shutter" on the outside, or an insert on the inside, could accomplish a seasonal switch?
Yes there is. Passive heat coatings limit heat loss (long wave IR) while still allowing most solar gain (short wave IR), in contrast to solar control coatings that limit both. That difference is represented by the SHGC of the glazing.
In CA you have to comply with title 24 requirements, which stipulate low e glazing (shgc 0.24) in conditioned spaces. There are ways to get around this sometimes by doing a whole house T24 model but that is the prescriptive requirement.
thank you for advice!
Not in that area.
No prescriptive shgc.
I think you are confused because you have been convinced that passive solar actually works in the real world.
For more than 50 years smart people have spent tons of money and lots of effort in mostly failed attempts. What passes for success turns out to mostly be super insulated homes that happen to have a lot of windows and succeed in spite of the windows not because of them.
Don’t trust me, trust the numbers!
Build the BEopt computer model for your climate with the different window coating and sizes. My guess is the model with the smallest uncoated double pain windows is the winner.
No one wants to live in a windowless box so limit the windows to 15% or so of the wall area.
If you take up my challenge, be sure to find and watch the YouTube training videos for the BEopt software.
https://www.nrel.gov/buildings/beopt.html
Walta
Walta, I read this thread and thought of you. I assume it will make your eyes roll:
https://forum.heatinghelp.com/discussion/198232/off-grid-hydronic-with-clay-battery
I have made a conscious effort to avoid that forum for fear it would suck up 3 hours out of my life daily. I did go and read the question and 2 replies. Yes, I did roll my eyes as predicted.
All I can say is dreamers are going to dream but I am only one person and I can’t crush every dream with mere facts.
Walta
Yep, it's a bigger job than I realized.
The problem seems to be that when things get past a certain relatively low level of complexity, people's eyes just glaze over and you get the "but, but, but, let's TRY IT" kind of attitude. Then they fail, but AFTER spending gobs of money on something that was never going to work.
I run into this stuff all the time explaining energy transfer and storage of all kinds to people. Physics is a lot of math, and math is hard, apparently. Oh well.
BTW, any time I see "thermal storage" that doesn't include phase change I know it's a problem. Even in real phase change systems, like ice chillers, the economics are often so poor that there is no payoff for the initial installation cost within a reasonable period of time, if ever. Unfortunate maybe, but usually true.
Bill
figured this thread is as good as any to ask and I suck at finding anything on google; maybe you or someone else can answer.
does acid etched/sandblasted frosted glass change shgc much at all?
i have a small north wall bathroom 24x36 window that only has 2x low e 180 + frosted glass. compared to my old windows with no low e, theres definitely less light in the room but i figured thats more from the frosted glass than anything.
more of just a random curiosity question 😅
Frosted glass scatters light coming through it so that VT is lower than in clear glass, but I don't recall ever discussing whether frosted glass (sand blasted or acid etched) affects the SHGC. My immediate gut reaction is that it doesn't, and I was initially reply to your question that it doesn't, but on reflection... maybe?
That is a really good question and I am going to follow up tomorrow after a quick phone call to someone who absolutely knows the answer.
Also, does 2x low e 180 + mean that you have triple pane with 180+ on two panes?
yea, its a triple pane with e180 on two panes. https://i.imgur.com/OGpPNDF.png
decided on that since its north facing, has very very little direct light, is the only natural light in the bathroom, and is 24x36 on top having a chunky tilt and turn vinyl frame/sash. i trusted that https://www.cardinalcorp.com/technology/reference/loe-performance-stats/ was somewhat accurate.
i'm guessing the shgc is unchanged since theres next to nothing about it online.
ty for response.
I don't have any numbers to back this up, but my gut feel is that the frosting doesn't only scatter light, it also reflects a decent chunk of that light, which would result in a lower SHGC due to that reflectance. All the low-E coatings do is selectively pass or reflect various wavelengths, so it's the same basic principle but by a different means in this case.
Bill
(response to #4) I think the passive house folks have some pretty good data on the success of their approach. And it's very much a passive solar program. Perhaps you believe different, and I can steel man that too
I'd love to hear it. Might be better in a new thread though.
Passive House definitely includes passive solar heat gain in its calculations, but the result is always far less glass than you would expect when you hear "passive solar."
"Always?"
I'm always careful with categorical generalizations. But if I read that as "usually" , or almost always, then I take your point. My formal training in passive solar taught what I believe was modest amounts of glazing (~8% of CFA).
In my experience in passive house projects, I've had issues in all of my projects with too much shading on glazing and advised either more solar gain through glass, or more glass that's fully exposed. Of course, less glazing was also an option, but also not really an option for the designers.
This article should cover all of your questions, as well as important questions you didn't ask: https://www.greenbuildingadvisor.com/article/choosing-window-glazing. It's behind the paywall but if you're doing work on or building a home, the subscription fee is a bargain.
In this conversation with a well-known Passive House expert, which I co-host, we discuss high-performance windows: https://www.greenbuildingadvisor.com/article/choosing-and-using-high-performance-windows
If you're interested in modeling to get the best results, this Q+A is relevant: https://www.greenbuildingadvisor.com/question/simple-modeling-for-choosing-windows
This article discusses one of the other important aspect of window selection: https://www.greenbuildingadvisor.com/article/choosing-windows-condensation-resistance
And more articles here: https://www.greenbuildingadvisor.com/collection/windows
I'm sure you've noted the increase in hot days in the Bay in recent years (i.e. just 3 weeks ago) Maybe you're on Pacifica or Bolinas so it's not really hitting you yet. As high of shgc as possible and low uvalue as possible. It won't be easy. Most window reps in the area just aren't used to thinking that as energy efficient and are biased by title 24 standards which have been shoving low shgc for 20 years due to peak grid issues (which are still very much an issue in the summer, even on the coast).