Andrew Wilson

Fred Morgan wrote:As I have accomplished more in my life, I have become less and less willing to comment about what others are doing. I have some very accomplished friends, I haven't done so badly myself. We almost never will offer criticism of each other - but we tend to ask each other for opinion when in doubt, and we are often in doubt.

There is a person who is afraid of being the doer, but still wants to share credit - they will shoot out "help" like a shotgun, perhaps hoping someone will use something they suggest, and then they can share credit.

Ideas are a dime a dozen, and so is advice. Building a plan, following through no matter what - these are the things that separate those who are successful and those who aren't.

I know this is an old post and I am a bit off topic but I just wanted to say this is the most amazing thing Ive read in a long time, I keep going back to read it over.
Thanks Fred

Just to keep it on topic dont change Paul, I look forward to my daily commute to work which we share via podcast, makes an hour feel like 10 minutes.
Thanks Paul

I thought thermal mass was the most important part of passive solar design, obviously i was ill informed.

Does the 10% to 12% work for all climates or is it specific for one with different persentages for warmer/colder areas. I like the eave calculator, it would take all the math out of it. Bill advocates facing the longest wall in the house toward the sun and the house should only be one or two rooms deep with the joining wall between the rooms made of cob or masonry or some other material that has a high thermal mass if the house is two rooms deep, so you shouldnt get the skinny wall problems. The overheating shouldnt be a problem in summer as the windows should be shaded out by the overhanging eaves, if there are especially hot days in winter then it would be a problem but i assume that would also be an issue for the floor area to window area method. Yes the insulation of the walls and roof would also be a large factor, something like a cob house or earth shelter with an earthen roof would do the trick or conventional construction methods with thermal and reflective insulation of appropriate R value. I have no idea what effect the airtightness has on passive solar, i guess that it would help to hold the heat but i wouldnt of thought it would make that much differance, obviously you would keep the doors and windows closed when it was cold but since you are getting radiant heat from your thermal mass i would of thought a little bit of fresh air wouldnt effect it that much and it would help to counter the farty air factor that Paul raises in airtight houses.

"sounded like percentage of south wall thats glass to me"

Yes thats right the percentage is the area of the south facing wall that is windows. So if you lived at a place that was 50 degrees latitude north of the equator, half your south facing wall would be windows if you were following Bill's passive solar design system. The persentage does not relate to floor area only position from the equator. I imagine there are lots of other design systems that can be used with good results, i thought Bill's was cool because it was so simple to calculate the amount of windows needed.

The overall size of the building is not relevant as the areas are in percentages. The percentages are of the south facing wall not how much of the houses walls face south. There are many other principles that go along with passive solar design and i think a house in which only 10% of its walls faced south would not be an efficient passive solar design, the longest dimension of the house should be aligned to the south to take full advantage of the sun to heat the mass of the home, and the shortest dimension of the house should be facing west to minimize over heating from western sun. Also the house should be no more than 2 rooms deep or as you say the north rooms will not be warmed. i am certainly no expert i was just trying to answer the question of what is Bill Mollisons percentage of the window surface system for passive solar design.

"I watched maybe a year ago Bill Mollison's Permaculture Design Course on DVD and I did remember that he was speaking about the % of the window surface, and the house design but I couldn't remember correctly. Since the post I've checked his book, and found some info, although not very much detailed."


I believe that Bills system uses the latitude of your position for the percentage of windows on your sun facing wall, (northern facing in southern hemisphere and southern facing in the northern hemisphere) so that if you are at 27 deg latitude away from the equator like me you would have 27% of your sun facing wall constructed from glass, while if you were in Missoula, which is much cooler with a latitude of about 47 deg from the equator you would have 47% of your sun facing wall constructed from glass.

Hope this helps