For do it yourselfers, the classic site is BuildItSolar, passive heating designs, trobe walls,
solar water heater designs, weather stripping, insulation too (as it relates to passively keeping warm)
https://www.builditsolar.com/index.htm there are many options under projects here is the section for passive heating
https://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
The guy who started the site also has a section called the "half project" where he documents his efforts to cut his
carbon footprint in half, gatting to half was so easy he went further. He has some charts that show the biggest bang for the buck etc...
https://www.builditsolar.com/References/Half/ProjectCharts.htm