My woodstove is heating the forest through my windows
Seriously, with larger windows and low U-values, my question is how much radiant heat is escaping from the warm stuff in my house (warmed surfaces, wood stove, fire in the fireplace) to the outdoors through the giant windows at night? SHGC must account for a certain wavelentgh (IR spectrum energy) and I’m sure the coatings that enhance (lower) the SHGC are placed on a certain side of a certain pane in a 3 pane glazing package and that those are designed to reflect incoming IR energy, which is all well and good for sun trying to get in during the day (whether you want it (high SHGC) or not (low SHGC)), but what about trying to keep that energy in at night (I know, close the shades). What are the dynamics of energy leaving the heating envelope through the windows and how do installed coatings/films address this?
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The radiation leaving the house is mostly in the UV part of the spectrum. Low-E coating are designed to be transparent to visible light but block UV.
Homes primarily radiate in the infrared/far infrared part of the spectrum. UV is short wave/high energy radiation and is only produced by natural sources such as the sun or special light sources (Mercury lamps, high energy LEDs, etc). Unless your house is lit entirely by black lights of course!
You're right, I got my spectra confused.
Assuming a dual pane IG unit with 1/8" glass and 1/2" airspace between the lites, at night (SHGC is zero because there is no sun), and outside temp of 0°F and inside 70°F.
If the IG is clear glass and air between the lites, plan on losing 21 BTU /ft²/hr to radiative loss and 13 BTU /ft²/hr to conductive loss in the IR or infrared spectrum or "heat" spectrum.
Add a single silver LowE coating to glass surface 2 and argon between the lites, you will improve radiative loss to 3 BTU /ft²/hr radiative loss, but conductive loss will actually increase to 15 BTU /ft²/hr because of the metallic coating on the glass. Of course overall loss still favors the coated glass significantly over clear.
With a triple pane with coatings on surface 2 and 5, plus argon fill in both chambers, radiant loss drops to 1 BTU /ft²/hr and conductive to 10 BTU /ft²/hr.
SHGC as a factor in heat gain presupposes that the sun is actually shining on the glass. No sun means no SHGC measurement.
Edit: I meant to include but forgot due to interruption. The numbers I posted are from memory, but the actual test data belongs to Cardinal and these numbers and a lot more are available on their website. My apologies for any confusion.
Heat leaving as radiated energy is mostly in the infrared (IR) range, UV is different. IR is what you get as you go out into the invisible part of the spectrum past the reds that you can see. UV is what you get when you do the same thing but go out past the blue end of the spectrum.
In the case of SHGC (or any of the Low-E coatings, for that matter), the coating is making a sort of filter that blocks (reflects) radiation outside of the visible range. The coatings primarily attempt to block IR, since that's where a lot of the heating comes from, but most also block in the UV portion (as does the glass itself in many cases, but it depends on the composition of the glass). The result is what is known as a "bandpass filter", which passes only the visible portion of the spectrum that you can see, and blocks (reflects) everything else. These filters work in both directions in most cases, so it blocks the sun's heat from getting in just like it blocks the fire's heat from getting out. You'll still lose energy through the window, but not as much as you would with a clear glass window.
The reason the Low-E coatings look a little tinted is because they aren't perfect, so they don't immediately start blocking light at a specific wavelength, they sort of "creep up" on the target. The end result is the filter's bandpass characteristic looks more like a sort of stretched out "U" instead of a more ideal squared off shape "|__|".
Shades work on a different principle, but can also help. Some people even place insulating panels in their windows at night (1/2" polyiso works great for this), but that can have a downside of getting frost or condensation on the interior of the window.
Bill
Thank you Bill.
OK, this is great and really helpful in my understanding. So what I'm understanding is:
- Window layers are numbered from the outside 1 - 4 (double pane) 1-6 (triple-pane) to the inside.
- Coatings (Low-E or all the special named coatings) are all of a metallic nature that reflect targeted wavelengths (Infrared primarily).
- Coatings on the glass reflect essentially evenly regardless of which direction (inside or outside) the IR energy ("light" waves) are coming from.
- Coatings can be adjusted to reflect more or less IR energy which is then tested and given an SHGC number.
- Whether it is the sun's IR radiation coming in or my fireplace's radiation going out, the coating as defined by the SHGC number will determine what percentage of that IR "heat" is passing through the panes of glass.
- With zero SHGC glass, the best way to prohibit the IR passing into the house is outdoor shading (an awning in my case) and to keep the heat in at night, the best way would be to install a mylar-lined (reflective, fully light occlusive), loose fitting window shade inside the glass in the room with the fireplace and the wood stove, leaving enough room for air circulation to prevent condensation. I'm steering away from the foam aspect of the insulation (1/2" polyiso) suggestion, as this question is mostly theoretical on IR passing through coated glass (to help me make the right purchase) and assuming that the U-factor of the glazing package can be adjusted (for a price) to keep the other modes of heat transfer tamed.
And thanks Oberon476 for the math on this question!
I hope I interpreted what you both said correctly.
In terms of *radiated* light energy, which would include heat radiated as IR, a reflective (radiant) barrier is like a super Low-E coating that blocks 100% of the light, including all of the visible spectrum. Some aluminized mylar sheet would work just fine for this, or even some of the Reflectix stuff that is basically aluminized bubble wrap. All you are doing is making a mirror, basically.
Conducted heat is where regular insulation like foam comes into play, although those materials usually stop radiated energy too, just in a different way (absorbing and warming up instead of reflecting the energy).
Bill
You might find this presentation on Low-E useful:
https://www.bdcuniversity.com/sites/sgc-university/files/Vitro%20course%20-%20Understanding%20Low-E%20Coatings%20REVISED%2010.11.18.pdf
It also gets into the differences between short wave IR and long wave IR.
Is the real question, will replacing the window save enough fuel to pay for a new window?
The short answer is NO.
Will expensive new windows make the home more comfortable? Yes
Only you can decide what your comfort is worth.
Walta
- Window layers are numbered from the outside 1 - 4 (double pane) 1-6 (triple-pane) to the inside.
*yes*
- Coatings (Low-E or all the special named coatings) are all of a metallic nature that reflect targeted wavelengths (Infrared primarily).
* Yes, that is true. LowE coatings come in two varieties, hard coat (pyrolytic), soft coat (sputter).
Hard coats are primarily tin oxide while soft coats have multiple layers of various metals and metal oxides with silver being the operative metal in most cases.
The vast majority of residential coatings are soft coat. Soft coats come in three versions depending on the number of layers of silver in the coating stack. One layer of silver = high solar heat gain, two layers of silver = moderate solar heat gain, three or more layers of silver = low solar heat gain. Many major window companies offer all three versions of soft coatings while smaller companies might be hit or miss, especially if you want a high solar heat gain product.
As a side benefit, the more layers of silver the more the coating blocks UV as well as IR.
- Coatings on the glass reflect essentially evenly regardless of which direction (inside or outside) the IR energy ("light" waves) are coming from.
*Not exactly. Coatings are directional per SHGC performance but not for U factor performance. Coatings applied to surface 2 allow less solar heat gain than the same coating applied to surface 3. High solar heat gain coatings are designed for surface 3 application, while moderate and low solar heat gain coatings are designed to be applied to surface 2.
Coatings have a "front" and a "back" and the same coating applied to surface 2 of one IG and surface 3 or a second IG will have different SHGC performance, but will also be a different color, they will look different.
- Coatings can be adjusted to reflect more or less IR energy which is then tested and given an SHGC number.
* Sort of to not really....SHGC (solar heat gain coefficient) is the percentage of the available level of direct solar heat that passes through the window compared with the total level of direct solar heat available. SHGC is a rating from 0 to 1 with 1 being 100%, so a SHGC of .25 means that 25% of the available solar heat energy passes through the glass and coating. A SHGC of .5 means 50% of available heat makes it through.
Direct solar heat in this case is specific to NEAR Infrared in the electromagnetic spectrum.
Visible light spectrum is 380-780nm, UV is below 380 and IR is above 780. As I recall (don't quote me) NEAR IR is 780-2500nm. This is the spectrum that SHGC is based on. This is direct solar heat gain. Your fireplace, your furnace, are not NEAR IR and are not referenced by SHGC.
Coatings are designed to meet a certain level of performance rather than given a performance level after testing.
- Whether it is the sun's IR radiation coming in or my fireplace's radiation going out, the coating as defined by the SHGC number will determine what percentage of that IR "heat" is passing through the panes of glass.
*Not true, see previous comment....
Soft coat LowE coatings are applied in a process called Magnetron Sputter Vacuum Deposition. Basically the coating is applied in plasma filled vacuum chambers layer by layer in layers that are measured by how many atoms thick they are.
It's a crazy precise and very expensive process. A complete new coating line for a typical three or four layer silver coating is going to cost upwards of 20 million dollars not including all the peripheral construction and support facilities and equipment required.
I coordinate an undergraduate program in Canada that teaches optical technology and trains folks how to design and manufacture optical coatings, so I love this conversation and it's taking a massive amount of effort on my part not to go into a long detailed lecture on the differences between the various IR regions in the spectrum, how that is related to thermal radiation, and optical coatings --- however ---
Walta's comment #9 is the real takeaway.
We once designed a coating that would allow greenhouse glass to transmit the spectrum of light needed by plants, while redirecting the 'waste' for energy generation. But the coatings are complex and extremely expensive. You can do the same thing with windows, but the cost just isn't worth it. If you are really concerned about it, as Bill suggested, a cheap reflective/silvered mylar curtain (like an emergency blanket) that you can pull down at night will do the trick nicely and will work far better than a custom (insanely expensive) coating on your window.
Thank you all again. Alex, I wish I was in Canada to take your class and Oberon476, I really appreciate your digging in to this and explaining so thoroughly.
The fact that this is a new build and not a replacement situation allows me to think a little beyond the cost factor - I must buy windows for the house, after all - because what I'm really learning from this is what questions to ask that will help separate out the window companies that are doing good work from those that aren't (and it's hard to tell when you can't see what's inside the glass panes).
Any thoughts on triple-pane tilt-turn window companies that are more trustworthy or capable of producing quality results than others? I don't think Consumer Reports has yet to dive into this one.
If you go with triple pane windows, be sure to get full depth 1-3/8" IGUs for maximum benefit. The narrower 7/8" IGUs really aren't thick enough to allow triple pane to offer much benefit in most cases.
Personally, I don't like the super aggressive Low-E coatings because of the tint. I prefer a triple pane window with Low-E 180 (Cardinal's number), not the 272 or 366 coatings. I think you'd also like the i89 coating that goes on the innermost surface of the IGU, which is intended to help minimize heat LOSS from the house, instead of heat GAIN from the sun.
Bill
Thank you. Great!
Being new construction and so far north from a comfort point of view spend the money and get the best U value you budget will allow.
Consider building a BEopt model of your home and letting it guide you coating selection. When I modeled my home, the coating did not seem to change the dollar numbers enough to justify the cost.
https://www.nrel.gov/buildings/beopt.html
training video
https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
Walta
Thank you. I'll have a look.
I have triple pane tilt/turn windows from Innotech in Vancouver BC that are 17 years old that look and operate and perform like the day they were installed, so I would highly recommend going that route if it's something that you are really interested in pursuing.
What coating you choose really comes down to personal preference on how you want it to perform; do you want to allow solar heat in or keep it out is the primary consideration when determining the coating you want. Also keep in mind that you need to know what coatings the window company actually offers as well.
Throwing out SHGC for the moment, LowE coatings also affect the U factor. At it's simplest, single pane glass is R1, dual pane R2, triple pane R3. Dual pane with LowE coating is R3, triple pane with two LowE coatings is R5. Argon in the mix adds about R.5 per space, so dual pane with LowE and argon is R3.5 and triple with LowE on two lites plus two chambers with argon is R6...again those are very basic numbers for simple comparison and actual performance values will be affected by what coating, airspace width, glass thickness, so on, but for quick and dirty reference they work. Anything being claimed that is drastically different from those numbers needs close scrutiny to understand why. And I used R value rather than U factor because it's easier to get a feel for differences using R than U, which is why the pink fluffy insulation guys thought up R value in the first place.
All soft coat LowE coatings are going to be relatively similar in how much they improve U factor performance in a window, with roughly about .2 U factor improvement between single silver and triple silver coatings. But the real difference is in SHGC numbers as discussed in earlier post about high solar heat gain coatings versus low solar heat gain coatings.
Thanks again - this is a really great set of "rule of thumb" I never knew about.