15% is a nice round number that is generally used to cover the wall space taken by windows and doors.... keep in mind, that is wall area, not floor.

This post was edited by HomeSealed_WI on Thu, Jan 15, 15 at 13:41

That's an interesting question with little merit unless the overall home design is considered. Architectural balance, view, orientation, etc should drive % of windows. I like light and a view, so we are over 35% glass. We have zero on our west face walls, around 10% on east facing walls, 30% on north facing walls and over 75 to 80% on south facing (also the main view). The same percentage is reflected in our basement except that north and west facing walls are below grade. But then again I like light and light for the most part is a good thing and worth the decrease in efficiency to me.

lkbum - I realize its a tricky question, with a lot of variables. But still thought it was an interesting one.

For ours, the windows provide views and are in locations that we find beneficial, but I was curious if there were any 'rules of thumb' for perhaps min for aesthetics/natural light, and max for energy efficiency.

...but I also live in an apartment with a window 12' long x 5' high, and its curtained closed 90% of the time. :P

Naween,
I was an engineering student in the 70's. At that time, the world was supposed to be out of oil by the mid 90's and we were going back into an ice age. Consequently, there was a LOT of focus on energy conservation, alternative energy etc. There were several exhaustive studies on the best windows and arrangement for a house that took advantage of solar energy. The conclusion was the best house had no windows (I'm not kidding) and used a mechanical system to positively control ventilation and recover heat from exhausted air (moisture too for that matter). This led to folks championing under ground homes. Sorry to ramble, I think that a balance of windows for architectural style and to take any advantage of the site is what I would strive for so long as it was affordable. It is an interesting question, studies from the 70's showed no windows were best, i.e. a cave :)

Where is the home going to be?

You can over window a home but you can just as easily under window a home as well.

Orientation and glazing design by elevation is critical.

The home is in Northern Canada, where temps flucturate between hot+humid in summer (~90F+humidity), to cold and dry (-40F) in winter.

We have oriented our house so the front faces NNE.

There are no windows on the SSE side of the house, as we hope to put a future greenhouse on that side of the wall.

The dining room, patio, and living room all face roughly SW, to take advantage of the sun when we get home from work, as the sun doesn't set til ~9:30 or 10pm in summer.

I did a quick sun study to ensure that we aren't getting shadows on windows when we are mostly likely to use them, so I think I'm reasonably ok?

No passive solar to really be had in that case. You aren't going to get much in terms of warming rays from the window options you do have.

Windowsonwashington, are you saying its my window LOCATION that limits my passive solar, or their type?

Truthfully, if I have windows for light, and a minimum amount to ensure my ICF walls keep temperatures from fluctuating, I'll probably be happy. My parents had a stick-framed house with a south-facing backyard, and all the rooms on that side were grossly hot in summer.

Naween, you are correct in your goals, minimum windows will be the most efficient. A minor mistake though is ICF walls are insulated on both sides, so the interior insulation actually detracts from the thermal mass effect for stabilizing interior temperatures. This claim by ICF form manufacturers is a little off. The mass does stabilize the wall temperature, but if you have a fluctuating internal load due to something like solar gain, they don't help much. Regarding solar gain, it depends on Latitude and Overhang. In the winter the sun is lower on the horizon in the southern sky so that a south facing window get direct solar energy. In the summer the sun is more directly overhead which reduces the amount of sun on a southern window. This is usually offset by a modest overhang in a solar design. See photo. Note in this photo, the thermal mass is the floor⦠no insulation on top of it which is essentially what the ICF walls are (insulated thermal mass).

This post was edited by lkbum on Fri, Jan 16, 15 at 10:30

Great info from Ikbum and others. I think in any building envelope design, starting with your titled question is a good start. The 15% overall is the default value of Resfen, a useful energy modeling software and probably a good average for homes in N. america. Obviously, this variable varies on style and site but its nice to have for a point of comparison.

Passive solar design's most descriptive variable is % of high SHGC , sun-facing(south) window area to floor area. Ikbum's example suggests over 15% which is getting too high for most designs. This can lead to overheating even in the winter. A better performing variable from Ikbum's example is 0% west facing glass. This is seldom seen as a factor in traditional passive solar design but has everything to do with its essence as good solar design should exclude unwanted heat as much as gain it during times of need. Very cold climates often have more to gain from the west than lose but it depends..

A better range for most passive solar designs with building envelopes in the 2012 IECC performance range is perhaps closer to 10%. For those building even better, like net-zero optimized or passive haus levels should probably be closer to 5-6% to avoid overheating.

As for thermal mass TM, we like to use exposed concrete slabs as the most cost effective approach. Adding anymore beyond what's needed for construction anyway has debatable benefits. TM delays thermostat to temperature response which can be a double edged sword in comfort performance. I agree with the ICF comments and would add that very few of them have the effective R values called for by IECC minimums in cold climates.

Wow's response is due to the fact that your home seems to be oriented in the opposite direction of ideal. North facing windows are performance killers in your climate. I would be looking into high shgc, triple panes to slow the bleeding on those orientations. Sounds like you would benefit from hiring a third party energy rater thats familiar with your site and climate to advise you on selecting your window package.

Here is a link that might be useful: Cost-effective passive solar design

Duplicate,post

This post was edited by lkbum on Fri, Jan 16, 15 at 17:10

Brianknight, Not to hijack this thread too badly, but you are spot on with regards to overheating. In my early days doing solar consulting, almost every job I did was for someone who loaded up with south facing windows with either too little thermal storage or no real mechanism to get the energy into the storage without a large temperature difference. The result was inside temperatures over 90F on a clear winter day. I think the ICF guys oversell the stability added to internal temperature as they insulate the mass from inside and outside temps. Our home is two years old with a LOT of glass. But we used the ZIP system for sheathing, a radiant barrier in the attic (part of the zip system), blown in cellulose for insulation with 6" exterior walls and casement windows with no sliders on the first floor. Very comfortable and extremely efficient result. The link below is old, but will give you an idea of the amount of glass used, the main wall faces south with large overhang from the porches. If you scroll down in the thread, there is a pic that shows the ZIP sheathing. This stuff is incredible.

Here is a link that might be useful: Link to lkbum house with large glass areas

Great points and Iam a fan of ZIP, especially the +R version and integrated with sips. Great looking home! For other interested readers, in designs like that, the large overhangs are helping control overheating effects from the larger than 10-15% South window to floor area. Its not as optimized performance as other arrangements but has worthy aesthetics for the supposed compromise in performance and upfront costs.

Gosh... in my house it's all over the place. I live in an odd house though. And we live in the boonies, where security isn't that much of an issue.
One part of the house is single story and was built in the 50's, another part two story in the 70's, and a joining part between the two in the 90's.

Not counting interior walls, and only counting exterior walls... Some walls have about 75-80%, some are more 40-60%, and some are 10-15%. So in the dining room, it's mostly south facing windows, like 80%, but in one of the bedrooms it's one standard window, around 15%.
And I do have a couple of interior walls that have windows as well, like the master bathroom, living room, and kitchen that open to light areas, plus some interior glass doors to light areas.

So... our house is around 4,100 sqft. We have 57 exterior windows, 6 interior windows, 3 interior glass doors that act as windows, 6 exterior doors including a pair of full windowed doors and two half window doors, and 4 exterior patio doors. And a 10x10 skylight on the solarium.

Energy efficient? Depends on the time of year. When it's nice, hell yes- open up the windows and the blow through is perfect, even when it gets really hot. We don't have AC, and if it gets stupid hot, we just shut the windows and suck air from the chilly basement. In the winter- it's absolutely horrid. If I could afford to replace all the windows with more efficient ones I would in a heartbeat.

Beautiful 4100 sq ft home with 5 dozen windows. Can't afford to replace them: #firstworldproblems

LOL. Sounds like an AMAZING home on all accounts.

Bunch of good information in here.

Thanks for bringing up the data and information presence lkbum and Brian.

Oberon was getting tired of carrying the full load.

Great info and great thread for future readers.