Black steel roof as a solar air furnace?
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I’m building a 28×36 cape in the lakes region of NH. The gable end is perfectly south facing, so I will be employing several passive solar features. The last of which may be an attempt at a solar roof air furnace. The attic is unconditioned with continuous ridge and soffett vents. The pitch is 10/12. I was planning on using decking on top of the trusses, but now I’m thinking of screwing the steel right to the trusses and building collector boxes in-between them which would also stop the roof from racking.
Anyone have experience with this type of design?
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Replies
Stephen,
If the gable end of your house faces south, that means that your steel roofing faces east and west -- not ideal for solar collection.
Remember, when you really want the heat, your roofing will be covered with snow.
Good points on the east - west Martin. I have another building on the property facing the same direction with a 5/12 pitch steel roof that sheds snow big time.
Stephen,
That's odd. I have a 12:12 steel roof that doesn't shed snow. Maybe that's because I live in a colder climate.
The property is a solar goldmine. Surrounded by small hills to the north and east. It just bakes the roofs.
Basically, you're talking about turning your roof into a Thermosiphon Air Panel (TAP).
A TAP is most efficient facing due south and oriented vertically (to receive the low winter sun and ground reflected sun), and has to be glazed over the black collector surface to contain the heated air.
But no TAP panel is as effective as a south-facing lowE high SHGC window for passive solar gain.
That's a good reason to use a high albedo (highly solar reflective) roofing to minimize summertime radiant gains (and consequently reduce global warming) and concentrate your passive solar design on south windows with proper summer shading and, perhaps, additional south-side TAPs.
Robert, I will already be using several pella proline natural sun windows thanks to your previous advice. Can't pull the trigger on size. I need the glazing to wall ratio if there is one. I'll keep digging.
I will abort my mission on this type of roof for the reasons you've all pointed out. Thank you all.
Is this a question? If so, could you be more specific?
I will make another thread for my window question for good forum etiquette.