Obscured glass entry door and solar heat gain
This is a weird question, but who else would I ask, but all of you. I want to know if there is privacy glass that still offers decent solar heat gain, and if one type is better than another.
The front of my home faces South and also faces the street on a city block. I want a glass entry door for natural light and solar heat gain in the winter (yard is shaded in summer) Provia makes Quad pane doors 2.5″ thick that seem like they might fit the bill with U values as low as 0.13 (the same as my windows, coincidentally): http://www.proviaproducts.com/productdetail/entry-doors/embarq-glass
My issue is that my wife insists on obscured / leaded glass for the door, which seems to kill any chance of solar gain. Is there such thing as a privacy glass that allows a decent amount of solar heat gain? I can’t even find any testing that has been done on privacy glass for SHGC.
Thanks,
Ryan
Minneapolis, MN
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Replies
Solar heat gain isn't worth sacrificing the functionality or aesthetics of the home. There just isn't enough of it to make up for the significant expense of capturing it.
If your front door had a SHGC of 1.0, and zero shading all winter, it would let in roughly 4 MMBtu October through March, by my math (and Minneapolis weather data). A more reasonable SHGC of 0.4 (good triple pane), and we're down to 1.6 MMBtu per year. That's worth $55 with pure electric resistance heat at $0.12/kWh. With a mini-split, it's more like $20.
Those Embarq quad panes have an SHGC of 0.12, with clear glass. That's just .5 MMBtu, under $10/yr for the mini split. You're already practically at zero; cut it in half with privacy glass and you're still at zero.
Tim C:
Thanks so much for taking the time to give an estimate and reality check here. I see your point. I suppose with this reasoning though, do you think one could be justified to pay an extra $1500+ for triple pane to go from U=0.50 (just a guess here, and I will assume the SHGC is much better with a cheaper door) for a non-low-e fiberglass 3/4 light glass door to a U=0.13 Provia quad pane door. Any thoughts on that? Provia seem to run in the $2,500-$3,500 range for the doors. The cheapest JeldWen at home depot is about $1000 with our aesthetic specs. Again, this would be the weakest link since the walls are R-21 spray foam and windows are U=0.13 krypton filled triple panes. If it will pay back in 20 years, I'm OK with that. If it will never payback, I have to question the logic.
Ryan,
The information in this article may influence your decision: Study Shows That Expensive Windows Yield Meager Energy Returns.
According to this
http://9dc159b43a66b1fe0a49-bd2073f7c8dbd16f36eed639782493f0.r48.cf1.rackcdn.com/design-pro-smooth-pro-fiberglass-thermal-ratings.pdf
a Jeld-Wen fiberglass 3/4 lite decorative glass unit has a U-factor of 0.26 and SHGC 0.2
For 8000 HDD, and a 3'x7' door, that's about 1 million BTU/year, or $35/year with resistance heat. The quad-pane cuts that to $16/year, or saves $19 per year.
In other words, all you really need is the title of the link Martin provided.
Also, I like Jeld-Wen because they are the only manufacturer I know of that specifies they use an insulation material that is not made with an extremely high global warming potential blowing agent.
Thanks Charlie. I appreciate the link, as I haven't seen this. Design Pro is JeldWens higher end door. It was over $2,000 with our specs, so close to standard Provia. The Home Depot JeldWen is less than half that amount, so I'm wondering how that one stacks up. Can you tell me where you found this doc? This spec sheet does answer my question that decorative glass is not going to have significant SHGC.
The list of Jeld Wen exterior door thermal data is at
http://www.jeld-wen.com/professional/technical-documents/nfrc-thermal-ratings#category_s=Exterior+Doors&paginate%5Bstart%5D=0&paginate%5Bcount%5D=20&paginate%5Bsort_field%5D=score&paginate%5Bsort_dir%5D=desc
That might not include the home-depot variants.
You can see list prices on Jeld-Wen doors at
http://www.brosco.com/products/price-catalog/default.aspx
I have an entry with several full height panes of frosted glass, 3/16 spacing non lo E glass, and a full light newer lo E glass door, and both seem to create a nice amount of solar gain in the AM. I do not think the frosting makes a difference
Yes, glass made for southern climates with very low solar gain will not work as you wish, but careful selection of high gain glass will give the dog a comfortable place to sleep