size of the window on a passive house

Hi Guys,

I need some advise! We are about to build a passive solar house ( we live in South Africa). Lounge, study and 1 bedroom facing north ( towards the Sun) and we need to figure out how much the window size needs to be the compared to the wall size. ( Like for instance 30% or 40% of the wall needs to be window.) Anybody having an idea where I could check this out? Or which book gives info about this?

Any help is much appreciated!

I know you can't have more than 60% windows on the sun facing side. You want to put as few windows as possible on the north (Northern Hem). We use honeycomb shades at night to keep the warmth in or down in the summer to keep the cool out.

This is the place that designed our house. You might get some good ideas reading the FAQ.

Something in the house that becomes a heat sink (or the house itself) helps with passive, like the adobe homes of US southwest.

Thank you!

CJ's tip with interior insulated shades is huge. A well built passive solar home can increase its performance with occupants that take the time and effort to do this. It yet another way that passive solar living can increase one's connection with the interplay of nature and home. Insulated shade design and the way it fits to the window to stop air movement is important.

When talking ratios of glass, the amount of glazed area to the home's heated square footage is the main criteria to use when figuring the heating energy of the designs. Generally, 7-8% is considered sun-tempered and 10-12% is the ideal ratio of most passive solar designs. Anyone care to translate to metric? I know people calculate square footage differently throughout the world too but it should be close enough.

Keeping a certain amount of un-glazed area on the South wall can be a smart way of avoiding additional structural components needed to compensate for lack of shear resistance. Sounds like the 60% number would be close to that.

Just left a comment in the "passive solar design and thermal mass" thread that elaborates a bit on my opinion that Airtightness and Insulation are much more important than Thermal Mass. I might also debate that overhang design is more important than Thermal Mass depending on the details.

Thanks Brian!

Brian Knight wrote: I might also debate that overhang design is more important than Thermal Mass depending on the details.

This!!! Overhang (SHADE) is critical. Making it adjustable (through exterior shutters or shade cloth) will help if you have wide temp swings within a season or just are unsure of design needs.

I thought about the overhanging shade myself, too. I just wanted to figure out if anybody knows a more or less accurate window/wall percentage. But thanks for your answer.

With very careful design, you can get away with an absolute maximum of 12% of the floor square footage in window square footage (I'll use feet because I'm more familiar with that unit for house dimensions, but the percentages I'll derive apply equally to any measure of area). That is, 120 square feet of window for a 1000 square foot house. Any more than that and you will end up with over-heating during the day an excessive heat loss at night. If you're not carefully designing the structure to handle the heat load, a maximum of 8% is recommended. The same applies in all climates, with the amount of insulation and thermal mass varied to regulate temperature.

If you are designing for solar illumination, you probably want your interior walls no more than 12 feet (4 m) away from the outside edge of the house. So consider a house of 24 foot (8 m) deep and 40 feet (13 m) across, or 960 square feet. That would give us a maximum of 75 to 115 square feet of windows. Considering you live in a warm climate, I would suggest erring on the smaller side, even with careful design. Heating is cheaper than cooling. If you are building a small home, I would really try hard to find ways to increase the thermal mass inside, and again err on the small side for windows.

In general you'll want the majority (70-85%) of the glazing on the sun-facing wall, a moderate amount (10-15%) on the east wall (to capture morning heating), a small amount (5-10%) on the west (to avoid evening heating), and a minimal amount (0-5%) on the shade wall (to avoid net heat loss). Also pay careful attention to overhang over the sun-facing windows to avoid direct sunlight penetration in the warm months (you are far enough south that the sun is always north of you, even at the solstice). Using the 24x40 square foot example with 8 foot (2.5 m) walls and 8% glass, that would be about 60 sq ft of the 320 square feet on the sun side, or roughly 20% of the sun wall, 10 sq ft of the east (5% glass), 6 sq ft of the west (3% glass), and perhaps 2 sq ft of the shade wall (1% glass). Putting most of the glass on the sun side maximizes winter gain, and minimizes summer gain from sunrise and sunset, an especially important consideration at your near-tropical latitude. Of course, these are guidelines, but the most important part is to resist the temptation of putting in too much glazing.

As humans we are much more comfortable in rooms that have light coming from multiple directions. I would make sure every inhabited room has light entering from at least two directions. If you have to add more glass on the shade wall to accomplish this, I think it's well worth the sacrifice in thermal efficiency for mental sanity.

Also, keep it simple and keep the walls and windows straight vertical. The winter sun is at a lower angle and will penetrate fine, and the higher angle of the summer sun is more likely to bounce off the window than penetrate. Plus due to differences in thermal expansion coefficients, angle mounted windows usually develop leaks over time.

I highly recommend James Kachadorian's book Passive Solar House: The Complete Guide to Heating and Cooling Your Home. I don't know what materials or construction techniques you'll use specifically, but his design concepts are very close to traditional stick build but effective in passive design without much extra cost.

Hi Mark,

Thank you very much for your detailed answer, it's much appreciated!

Hi Mark,

Sorry this morning I was very tired, couldn't answer longer. So, we'll build a sandbag house with ecobeams, and according to the builder it has got a very good thermal mass. But I still wanted to check out the passive design, because we are committed to use as little energy as possible. One of our rooms where we currently stay happens to have a wonderful design( I'm sure accidentally ) where we get sunlight through the window during winter, and the room warms up very nicely, during summer it does not happen, so it's quite cool. 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.
So, thanks again for the answer!

Then why do earthships have 90-98% glazing on their south facing wall? They experience 0' temps in northern New Mexico.

I find it hard to believe that 10% of my south wall should be glass, and 90% wall.

The ratio that passive solar designers typically use are the south window's square footage to the home's floor area square footage. A percentage of windows for the south facing wall area tells us nothing about the shape/size of the house and the heating potential of the windows to the homes total area.

Earthships do some really cool stuff but I would be careful about basing a design on an earthship. Many of their design details are highly questionable.

"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

That method sounds flawed. It still speaks nothing of the size, shape or heating potential to the overall size of the dwelling. If only 10% your home's wall area faces south you may not have enough window area and would probably experience more temperature difference between south rooms and north rooms. If 30% of your home's walls faced south you would be overheated during swing seasons and lose too much heat at night without very high performance windows or insulative window coverings.

Good Passive solar designs walk a fine line between too much and not enough. Going over 12% south window area to floor area will increase overheating risks during swing seasons and unseasonably warm weather with excessive heat loss at night and during cold weather. Good windows are also pretty expensive.

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.

That I can agree with. "27/47% of sun facing wall constructed from glass" sounded like percentage of south wall thats glass to me. "% of south wall in glass" is not as descriptive as "% of south window to floor area".

Your exactly right when you talk about the shape and depth. So you think Bill was referring to south wall area? What about glass?

"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.

Interestingly, that little rule of thumb does seem to work out. It wont prevent the problems you mentioned of the skinny side of the house facing the winter sun. A small, sun facing wall may not heat a deep room or house no matter what its % of windows. One would need another factor saying how big the sun facing wall is in relationship to the other walls.

The easiest and most telling ratio or % for heating performance of passive solar design fenestration involves only 2 variables: 1. Sun-facing glass area 2. Floor area of intended heated space.

Sun tempered: 7%, 70 sqft of glass for 1000sqft home.
Passive solar 10-12%, 100-120 sqft for a typical passive solar 1000sqft home. Higher % means more risk of overheating during hot times and too much thermal loss in cold times.

This seems to work out in all climates and latitudes for most homes and buildings. Very large or tiny homes are more likely to be thrown off by other variables. Latitude is usually adjusted for with overhang design. http://www.susdesign.com/overhang/ The SHGC value of the glass in important as is the Ufactor/Rvalue.

Other envelope performance variables more important than passive solar fenestration: 1. Airtightness as verified with a blower door test. 2. R-value of the walls and roof.
A passive solar home that ignores those more important things could be outperformed by a non-passive solar design that pays attention to them.

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.

I think it works for all climates although cooling load dominated might be wise to pick lower % or extra Thermal Mass TM to compensate. Still even in warm, cooling dominated climates those ratios seem to work good with the right overhang design.

Yes, the longest wall faces winter sun and having those 7-12% ratios will usually force you into it on a flexible site. Overheating is always a concern with Passive Solar. Even good designs will experience it during certain weather and times of the year which is typically late summer and early fall in our climate.

While this thread has focused on the ratios of fenestration for passive solar, the subject is not the most important thing for a passive solar home. Airtightness and insulation is.

Insulation is obviously very important. When you create heat (or want to keep it out in the summer) insulation is a main part of the equation. A passive solar home that loses its heat as soon as the sun goes down is little better than a non-passive solar home.

Airtightness is the not so obvious part of the picture. Would you leave windows open if you were trying to keep your heat in overnight? That's essentially what you are doing when you dont build airtight. People are surprised to learn that all the cracks and holes in a leaky house can add up to the size of windows. If you are not achieving a certain level of airtightness, insulation is a waste and not doing its job.

The subject of Indoor Air Quality is a contentious one. Most experts say the key for indoor air quality in thermally efficient homes is to build tight and ventilate right. How much ventilation seems to be the main question up for debate. If you are not providing a means of ventilation then there is nothing to argue about. Either smell your farts or be wasteful with your energy use. I argue a home that is not airtight has the worst of both worlds.

What about Thermal Mass? Its arguably not that important. Building envelope performance is measured in Airtightness, Insulation and Fenestration. Thermal mass can improve performance by blunting interior temperature swings but it has to be completely inside the insulated envelope.

Mass walls like cob, earth and solid logs typically do not have high R-value. They will work against you in passive solar design.

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