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Daylight cellar as a solar collector?

WMzJQvhd8a | Posted in General Questions on

I have a 28×36 daylight foundation with a 2ft knee wall on the south facing 28′ gable end. It is in the lakes region of NH. I am using floor trusses so I can leave the south gable unsupported on the basement level. I am considering banking that entire wall with recycled slider glazings and painting my cellar floor black. Then cutting registers in my 2×6 spruce floor above to let all that free heat flow on up to the living space.

I may bust up the cellar floor first to insulate it and make sure there is good radon abatement as well. It is 15 years old.

Please advise.

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  1. Riversong | | #1

    Yes, you'll have to insulate the slab, bottom and edges, very well if you want to store solar heat. The floor (collector) doesn't have to be black if you want to use it as livable space - anything from red to brown to green makes a very good collector surface. And, if you want the basement to be livable and not just a solar oven (and a night-time cooler), then you need maintain the correct glass-to-mass ratio.

    Generally, a passive solar space should have no more than 12%-15% of the floor area in solar glazing, though you could increase the downstairs ratio if you decrease the upstairs ratio.

    Typically, up to 7% of floor area in solar glazing requires no additional thermal mass beyond the normal structure and finish. Some thumb rules are:

    An additional 1 sq. ft. of south glass may be added for every:
    • 5.5 sq. ft. of sunlit thermal mass floor (up to 1.5 times total south glazing area)
    • 40 sq. ft. of floor not in direct sunshine
    • 8.3 sq. ft. of thermal mass wall

    And, remember, warm air can't flow upstairs unless there is a cool air return path of the same size. A strategically-located stairwell can be the return path.

  2. WMzJQvhd8a | | #2

    Thanks Robert. I think I'll steer clear of my solar oven idea for resale reasons and the complexities you've mentioned. I'll use your equasions to size my windows. You've been a big help.

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