Half a croissant, on a plate, with a sign in front of it saying '50c'
h a l f b a k e r y
Apply directly to forehead.

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Variable-albedo roof covering

 Appropriately cool or heat a dwelling with active roof materials.
  (+7)
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A roof covering which changes reflectivity with temperature: dark when cold, reflective when hot. Thin transparent blisters contain a mixture of opaque white fluid and tiny dark plastic grains. The grains have a nearly-neutral buoyancy, and work as a Galilean thermometer. When the liquid is cold, the grains float, thus making the surface dark and heat-absorbent. When the liquid heats up, its density goes down, and the grains sink underneath the reflective white liquid. Thus the albedo changes with temperature


For the sqeamish, perhaps plastic tubes are better than blisters. I reserve the right to delete revolting analogies.

(NB: this is a more workable version of helloc's "Heat-sensible tar").

—rmutt, Apr 19 2001

Heat-sensible tar http://www.halfbake...Heat-sensible_20tar
Credit where due. [rmutt, Apr 19 2001, last modified Oct 17 2004]

Galilean Thermometer http://www.hewgill....lilean-thermometer/
Why this works. [rmutt, Apr 19 2001, last modified Oct 21 2004]

"Mood Home" http://www.slashdot...sid=01/04/20/144241
Pretty much baked. [egnor, Apr 19 2001, last modified Oct 17 2004]

Student Lenticular Shingle Idea http://engineering....oof%20Materials.pdf
An idea developed for students that comes very close. [talldave, Jul 10 2012]

[link]






       I hope these blisters are made of a very strechable material, otherwise they'll pop in the hottest and coldest times of year. Also, wouldn't this give one an all but irresistable urge to pop-pop-pop it like bubble wrap? I might have to buy a few tiles for just such a purpose, although opaque white fluid running out of blisters brings back unpleasant memories of early puberty.
—nick_n_uit, Apr 19 2001
   

       Are you sure this idea would not be improved by adding custard?
—Aristotle, Apr 19 2001
   

       Halfbakery imitates life.

I'd enjoy telling people, "The roof of my house does what your face does when its not cool, all the white quits circulating!!"
—reensure, Apr 19 2001
   

       For bored teenagers, the comedy value of setting light to people's roofs would be almost irresistable.
—DrBob, Apr 20 2001
   

       DrBob, tar burns pretty well, as do cedar shingles. Why are bored teenagers not already on an arson spree?
—wiml, Apr 20 2001
   

       Hey! You could lay out some cut-outs and shade a portion of the material, and send your neighbor a very large message about which team won on Sunday!
—feet1st, Oct 12 2004
   

       Lenticular roof tiles could reflect sunlight in the summer and absorb in the winter, by virtue of the different angles of the sun at different times of year.   

       There was a student team that came up with a similar idea.   

       But they just wanted the white roofs to look pretty from the street, not take advantage of the sun's angles being different at different times of the year.
—talldave, Jul 10 2012
   
      
[annotate]
   


 

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