Question:
We are installing a new wood and glass door — with the style of 15 panes of double glaze glass (sort of a traditional style door) — and, on top of that a new single-glaze storm door. The question is, which door is best to put Low E glass in? The outermost storm door, or the inner double-glaze wood door? This will be on the south side of the house. (and would it make any difference if the inner wood-glass door is 15 actual panes or simulated divided lites?)
The architect suggested we use Low E glass, as that side of the house is sunny, but he doesn’t know which door is best to put the Low E into. He thought the inner wood door might be the best place, as it has a tighter seal and is two layers of glass, but he doesn’t know for sure. I thought maybe the outermost door (the storm door) is the best place for Low E, so it deflects heat before it enters the few inches inward into the building envelope and heats up the space between the storm door and the multi-pane glass/wood door. But, I’m just guessing. Also, in summer, the top 1/2 of the storm door won’t have glass, but will have a screen, so Low E on the storm door would be only on the lower half of the door for about 1/2 of the year. (there will be a small overhanging roof, but only about 3 ft of overhang)
So, with that info, what might the science really suggest is the right place to put the Low E glass?
Answer: With a south facing door with a 3’ overhang you have about the optimal set up for gaining the maximum heat from the sun in winter (when the sun is low in the sky) and still preventing overheating in summer because the sun is high the the sky and the overhang shades the glass. In our house in these circumstances we used no lowE coating on the windows and patio doors. This allows a lot of sun into the house on sunny winter days which makes lovely places to warm up in the sun. We don’t have pets, but I have heard that pets love spaces like this to curl up in the warmth! So I suggest just clear glass, no lowE coating. This is for the outside. You still want the lowE i89 coating on the inside because this reflects heat (infra red light) back into the house keeping it much warmer in winter. The storm door will not add much insulation because air moves behind it. Storm doors protect the real door from wind and rain, they do not add much insulation. If it is possible to get the door with triple-pane panels that would be better. The insulation value of a glass door with simulated divided lites will be higher than that of door with real wood strips dividing 15 smaller panes of glass because even a plain double glazed window is about R2 whereas wood is about R1 per inch and the depth of the wood is about an inch, hence the wood is the most thermally leaky part of the door.
I hope this helps!
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Zero Carbon Home Website Home Page: https://greenzerocarbonhome.com
Database of State Incentive for Renewable Energy (DSIRE) https://www.dsireusa.org
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Department of Energy, EnergySage, “Zero Carbon Home”https://www.energysage.com/project/7230/zero-carbon-home/
Zero Energy Project, “My Zero Energy Retrofit beats my 401k”https://zeroenergyproject.org/2018/09/23/my-zero-energy-retrofit-beats-my-401k/
Cool Effect, “Mr. Green’s Zero Carbon Home” https://www.cooleffect.org/content/news/green-testimonial?utm_source=Cool_Effect_CRM&utm_medium=EMAIL&utm_campaign=20180814Engage_Testimonial&utm_content=green_CTA
Green Energy Times, “My Zero Energy Pool is a Great Investment!” http://www.greenenergytimes.org/wp-content/uploads/2019/05/GET_May-2019.pdf
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