How do you build a green doghouse? The same way you would build a regular house, only smaller! Use as many recycled or salvaged materials as possible and use passive solar techniques for heating.
The floor could be leftover bricks from a home-improvement job?ask your friends and you might get lucky. Bricks will give the floor thermal mass, keeping it cool in the summer and warm in the winter if the sun shines on the floor. Use wood scraps from your workshop or a neighbor?s, or take apart pallets found in a construction yard (though be sure to ask the foreman before helping yourself). There?s perfectly good wood there that would otherwise end up in a landfill.
If you can find an old single-pane window, use it on the south side of the doghouse?but make it removable so that the breeze can blow through in the summer.
Since a doghouse is so small, you don't need much in the way of roofing?you might even be able to find asphalt shingles left over from a roofing job in your neighborhood. Leave enough of an overhang so that it will shade the opening from the high summer sun, but not too much. (The lower winter sun has to shine into the windows to heat the space.) Go to the Sustainable by Design website for a great tool to calculate how much of an overhang will work. While you're at it, you could even build a nice wraparound porch so that the dog can rest in the shade!
Normally, I would insulate a house to the fullest extent possible to keep the heat in. Since this is just a doghouse, I wouldn't bother unless I had some insulation lying around. If you do insulate, don?t use fiberglass batting. Fiberglass decays and becomes airborne, and it?s a possible carcinogen. Rigid insulation would be better for your purposes. If you?re concerned with keeping the doghouse cool in the summer, add the insulation under the roof. If you want it warm in the winter, you have some challenges: You?ll want to insulate the walls, but all your heat will go out through the window. Also, you?ll need to add a door that a dog can open and close. I suggest that you let the dog into your house in the winter. The dog can sleep with you and you can turn the thermostat down a couple of degrees!
Use any leftover paint you have to paint your doghouse?or leave it natural and funky. Good luck!