Highest SHGC sliding door in Canada is 0.63. Why not higher?
Melville2
| Posted in Expert Exchange Q&A on
NRCan’s downloadable data for all sliding glass doors has 21400 rows, apparently one row per door option. The highest SHGC is 0.63. Only 4 doors are higher than 0.6.
Why are there no sliding doors with an SHGC higher than 0.63?
Does the frame of each individual door reduce the SHGC?
Does the frame surrounding the complete set of doors reduce the SHGC?
Those two frames mostly overlap so shouldn’t be double-counted.
For a door 36″ x 84″, a 2″ frame all around would reduce the area like so:
(32 x 80) / (36 x 84) = 85%. With a 3″ frame, it’s 81%.
If the frame affects the SHGC, then the theoretical maximum seems to be around 83%, so why is the maximum made 63%?
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EDIT:
For clear glass, no coating, no gas filler:
Single Pane SHGC: ~0.86
Double Pane SHGC: ~0.76–0.79
Triple Pane SHGC: ~0.68–0.72
For a 2-pane plain glass sliding door 3’x7′ with a 2.5″-wide frame:
Glass area = 31″ x 79″ = 2449 sq inches = 17 sq ft. Whole door = 3×7=21.
= 0.615
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Replies
I'm not sure why you would want such high SHGC. The passive solar experiments of the 1970s and 80s were informative but largely failures; building airtight envelopes with appropriate levels of insulation and good quality HVAC equipment and a reasonable amount of modest-SHGC windows is what has proved to work well, consistently.
Clear glass has a SHGC of about 0.87 and each additional layer of glass reduces the SHGC. Most glazing has coatings that reduce it further. SHCG modeling does include the leaf frame and I'm pretty sure the door frame as well. Some manufacturers show both glass-only SHGC and whole-unit SHGC.
Thanks Michael.
Why high SHGC?
Calculations show south-facing glass has a net energy gain, and the gain is greater the higher the shgc value, despite high u-values.
For south-facing sliding doors with large maple trees blocking sunlight when in leaf.
Option 1: SHGC = 0.25, u = 1.36
Option 2: SHGC = 0.63, u = 1.82
Area = 12 ft x 7 ft = 84 sq ft = 7.81 sq m
Insolation = 3 kWh / m² / day (based on PVwatts with tilt = 90, modified because tree leaves will block sun for part of the heating season)
Heating Degree Days = 180 days x 20 degrees avg temp diff in vs out = 3600.
Calculating
Solar Gain = SHGC × Area × Insolation × Heating Days
and
Heat Loss = U-value × Area × HDD × 24 hours/day / 1000 (W / kW)
The calculations below are also in a spreadsheet here: https://docs.google.com/spreadsheets/d/1Dx-t3vKxLH62KqCJay4Dtl2vr7a98Mcrb3L753xN8FE/edit?usp=sharing
Option 1:
Solar Gain = 0.25 × 7.81 × 3 × 180 = 1054 kWh
Heat Loss = 1.36 × 7.81 × 3600 × 24 / 1000 = 918 kWh
Net: = 137 kWh gain per year
Option 2:
Solar Gain = 0.63 × 7.81 × 3 × 180 = 2657 kWh
Heat Loss = 1.82 × 7.81 × 3600 × 24 / 1000 = 1228 kWh
Net = 1429 kWh gain per year
The higher the SHGC the greater the gain, despite higher u-values.
Am I wrong?
Thanks
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(For heating degree days, I could have used 210, but if the outdoor temp is 3 degrees colder than the inside target, and the windows are adding 6 degrees, only 3 of those 6 are of particular benefit, so it seems reasonable to ignore at least a couple weeks at each end of the heating season.)
Given your assumptions, I think your math checks out. The tradeoff is that it won't be as comfortable to sit near those doors on cold evenings, and the room might also become uncomfortably hot on sunny days.
Uncomfortably hot when trees have no leaves? Perhaps that does make sense...
Typical daytime highs in late October here are 9-12 Celcius. Typical daytime highs in early May are 13-16 Celcius.
[EDITED Dec 5 2:30 pm - corrected calculations below]
[EDITED Dec 7 1:30 am - corrected kWh to Wh below, thus finding the windows would only add 1 degree Celcius - so definitely not overheating; in fact disappointingly little heat, but better view without net energy loss I accept..]
On a sunny day in early May (in this large room, 11 m x 7 m x 4 m), I calculate:
Solar Gain = 0.63 × 7.81 metres squared glass × 4 W/m2/day × 1 day = 19.66 kWh
Heat Loss from Glass Doors = 1.82 (u-value) × 7.81 m2 × 7 degrees C × 12* hours = 1194 Wh ÷1000 = 1.19 kWh
Net: 18.47 kWh.
How that affects temperature seems to be highly sensitive to accurate considerations of thermal mass.
18.47 kWh = 66500 kJ
Thermal Mass for the room's air + floor + walls + ceiling = 59000 kJ / degreeC.
66500 kJ absorbed by mass that's 59000 kJ / degree C = 1.13 degrees C.
But there's also:
Heat Loss thru ceiling and the 2 long walls = approx 1 kWh.
(Heat Loss = U-value x Area x ΔT x Hours:
Heat Loss (walls) = 0.09 x 144 x 7 x 9 = 816 Wh = 0.82 kWh.
Heat Loss (ceiling) = 0.05 x 77 x 7 x 9 = 243 Wh = 0.24 kWh.)
Net = 18.47 - 1 = 17.47 kWh.
That's 62,892 kJ, which applied to mass 59000 kj / degree = 1.066 degrees Celcius.
So the temp would only increase by 1 degree. !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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* (The heat loss was calculated for 12 hours not 24 and not 14 (the # of daylight hours) since we're considering the effects during the hours when the sun is significantly coming thru the south-facing glass.)
Energy models usually include the amount of time during the year that the space will be either hotter or colder than the ideal range, which is rarely if ever correlates with the least energy used annually. Passive House modeling also includes the effect of surface temperatures on radiant comfort. I have a small house with 100- to 200-year old windows so I experience this radiant cooling all winter.
On the other hand, there is a theory that having conditions be at "ideal" temperatures all of the time isn't healthy, called alliesthesia: https://www.terrapinbrightgreen.com/blog/2018/01/5008/?fbclid=IwAR2thWKCCeQF095Sk6vCsSOwc7HFASj_Td4ZPmq3HlP3tlgNHtHUA0VDtig. If that describes you, then by all means use glass that will technically overheat or provide excessive heat loss.
Since Passive House was mentioned here: in the few projects I've worked on, the projects have been in the same position as the original post-- the SHGC on the commonly available options were too low, so we had to dig around for better options and then make some changes in the rest of the design to account for the fact that we couldn't find our 'ideal' high SHGC glass.
This is just to validate the OP's question and general math.
I personally live in a largely uninsulated house, so while I experience temperature ranges that definitely outside my comfort range (50-90F), I generally enjoy the diversity of temperature around the house. It's like that Solar Cat book
https://www.goodreads.com/book/show/1925463.The_Solar_Cat_Book
"So despite the solar gains, the house would still drop 50 degrees if there was no heat pump?"
I think what's missing from your calculations is that the heat loss is determined by the difference between indoor and outdoor temperature. So as the interior temperature drops the heat loss slows. So what you'd need to solve for is the interior temperature where the heat inflows balance the heat outflows. Then apply the heat capacity of the house to see how much the interior temperature would fluctuate as those inflows and outflows occur.
I attempted a spreadsheet way of making that dynamic.
Feel free to comment or make changes if you wish.
It looks reasonable - but that makes it not match my calculations that showed a 50 degree Celcius net heat loss during this half-day.
https://docs.google.com/spreadsheets/d/1i4wVdlE9xAndvI_g0zzGw_PS-wK8dNsGzQsTL_GFNpU/edit?usp=sharing
Wow, those enclosure U-values are impressive! I did say it might overheat, not knowing anything about your home design. With 8 m2 of south-facing glazing (if that's all of it) in a 77 m2 room, the ratio falls within the 10-12% range that some passive solar designers suggest as reasonable. And, the larger the room, the more surface area to soak up excess heat.
However, I think you might have an error in the decimal placement on your room heat loss calculations. I am getting much smaller heat loss than you.
Can you show me an example where you got a different heat loss calculation?
Heat loss for walls: 0.09 W∕m2·K × 144 m2 × 7 °C × 9 hours = 816 Whr = 0.82 kWhr
Thanks! GBA isn't letting me reply to where you gave your example calculation, but yes, that helped me see my unit conversion error. The result is no longer a bizarre loss of 50 degrees C, but now it's a surprisingly slight gain of 1 degree C.
17.47 kWh = 62900 kJ, which when absorbed by thermal mass of 59000 kJ / degree C means a change of +1 degree Celcius.
Melville2,
Nice work. Good to see someone actually thinking about the free energy the sun provides. Somehow we seem to have forgotten that sunlight is life (think photosynthesis). Get some nice house plants along with those south facing windows and winter will seem almost tropical.
Doug
Doug you are not talking about new ideas. Lots of smart people have thrown time and money trying to make a passive solar house work almost all admitted defeat. I say those that claimed success built a tight well insulated home that succeeded in spite the big windows not because of them.
Check out the single pain doors if the high solar gain is the only factor.
Walta
"Almost all have admitted defeat"
While I believe that's the case, it's just weird that my personal reality is sooo different.
I'm going by a couple of passive solar homes on my commute back tonight, and I know their owners speak the praises of them with their direct gain areas and woodstove backup heating.
15 years ago, I went on a tour of a bunch with a class, and they, by all appearances, seemed to be occupied by happy owners. Maybe they weren't being honest, but I didn't see a lot of evidence that they were complete failures...
I read that clear glass with no coating has a vlt of 80% so if you have two panes the theoretical max possible would be 64% vlt.
I think shgc would also follow the same certifications no? As you add more leaves of glass the shgc has to be reduced
If you are adding glass to gain free solar heat, you are wasting your time
If you are taking advantage of existing or 'necessary' glass to calculate the available energy, this is good design
If you are using plain glass rather than low e for more heat gain, you will be very, very sorry. At night in the winter you will be losing your body heat to outer space, and you will feel it.
IME , the difference between an old R2[U.5] IGU and a triple pane low E argon filled with interior coating [R6 U.17] is mind boggling[I have both in my house, sometimes in the same room]
You cannot feel warm standing next to the old glass. The new glass you literally feel your body heat reflected back at you.
Thanks !
I went through energy modeling on my south facing windows and yes they hard coat low E did reduce yearly energy use. It also made it that some days the house was up to 27C when I got home (this is in the middle of winter). When the sun was not shining though you could feel the cold washing off them.
When the IGUs failed, I replaced with low SHG units with i89 coating. Everything about the new windows is better, temperature is much more even and there isn't the night time chill anymore. I do sometimes miss wearing shorts at home in the winter though.
My point. Don't chase solar gain too much. Very hard to get it right.
Thanks!
Melville, you might consider using this free program (paid for by our tax dollars) to automate your calculations: https://www.nrel.gov/buildings/beopt.html. There is a bit of a learning curve but it's fairly straightforward. (and did I mention, free?)
thanks!
If you decide to try BEopt spend the time to watch the training videos.
https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
When I spun my BEopt model 360° it made almost no difference.
Walta
thanks!