Size of eaves for passive solar design « Alternative Technology Assn Forums

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Size of eaves for passive solar design

(26 posts) (14 voices)

Started 10 years ago by prettygully
Latest reply from MaxG

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prettygully
Member

Dear Forum,

we are at the planning stage of a new, passive solar house - orientated towards true North, floor length N windows, at about 29 degrees lattitude and 550m altitude. We are aiming at full solar gain in winter & good shading from October to April (incl.). I was under the impression that working out the correct width of the eaves to achieve this was a simple exercise in trigonometry which the building designer could do - but obviously not.

Have looked around and found some conflicting information:

1) http://www.yourhome.gov.au gives a figure of 45% of height for lattitudes up to 27.5 degrees
which for our floor length windows & 9 foot ceilings would translate to about 1200mm!

2) sustainable by design has a nice on-line 'annual overhang analysis' tool
(http://www.susdesign.com/overhang_annual/index.php)
playing with that I get an overhang width of about 900mm for the kind of
shading/solar gain we aim at.

Why the big difference? And how do I pick?
Does anybody know how reliable the on-line calculator is?

would appreciate any advice you may have!

Thanks!!

Posted Monday 14 Nov 2011 @ 11:06:24 pm from IP #
dymonite69
Member

prettygully said:
I was under the impression that working out the correct width of the eaves to achieve this was a simple exercise in trigonometry which the building designer could do - but obviously not.

Have looked around and found some conflicting information:

The answer is both simple and not simple. With either solution you will be within the ball park of creating a comfortable house. They are based on average conditions and work pretty well. There are warm days in early springtime when the sun isn't yet high enough to be completely blocked by the eaves. There are also cooler days in later springtime when no shading would be helpful.

If you want to tweak it slightly further than consider this:

1) I would try to avoid long windows which require longer eaves to shade completely. The general advice is not overglaze. In winter, the top part may constantly be in shade and be a source of heat loss. In summer, the bottom is the last to get shaded until the sun gets overhead and can be a source of heat gain. Summer sun also gets reflected from the ground in front of the window back through the window. To shade the ground would require even longer eaves. Long windows also exaggerate sweeping convection currents off the windows. This can be even be demonstrated with double glazing.

2) Uptilt eaves have the advantage of allowing more solar access to the top portion of the window but may not satisfy the architectural conventions of the house.

3) Try to work out you own indoor temperature comfort levels and compare that with the weather for your locality (see BOM website). Direct sunlight can raise the indoor temperature from 5-10 degrees compared to outside. Great if it is cold. Terrible if it is already warm. You definitely want full shading (even before midday) once it is reaches minimum comfortable temperature. In late spring/early autumn this may occur quite early in the day. If the outside temp is consistently < 8 degrees below comfort levels then you want full sun all day.

Determine what time of the year (and time of day) this occurs then get a sunpath diagram and work out how high the sun is during this time of the year. From that you can use simple trig to work out the correct eave length.

In the end, it is always a compromise. Too much sun when it is already warm inside. Too little sun when it is isn't warm enough.

An active solution is better where you use less glazing but more willing to adjust indoor and outdoor window covering to tailor the solar access for any given day.

Posted Tuesday 15 Nov 2011 @ 12:15:16 am from IP #
Russell Moore
Member

For what it is worth, here are my measurements for my 28 year old solar house.

Location, Green Valley, south western Sydney,34º latitude, orientation 5º east of north. I was told this would help warm the house earlier in winter, we could have easily gone true north. If I had my way over again I would go for true north, as it makes it better for PV and SHW.

The northern wall(the house is close to being a rectangle 20m x 11m)is approximately 2600mm high, it has a 550mm eave overhang (including the guttering), we have 3 large sliding glass doors/windows in that wall. The bottom of the glass starts 230mm up from the floor, and 600mmm down from the ceiling/roof. At your latitude I would think that 500mm of overhang would suffice, if you had glass positioned like mine.

I would highly recommend a glasshouse attached to the northern wall, in our case it covers the centre glass sliding door/windows, and in winter we open up the sliding door, and hot air flows into the house, and cold air flows into the glasshouse, in a constant cycle during the day. In summer we cover the whole north side verandah of the house with shade mesh,this includes the glasshouse, which has two doors on the west and east side, which we leave open over summer.

Hope this helps.

Posted Tuesday 15 Nov 2011 @ 2:49:54 am from IP #
Russell Moore
Member

Removed post.

Posted Tuesday 15 Nov 2011 @ 2:52:38 am from IP #
Benny
Member

As usual a comprehensive answer Dymo (-;

Is there a simple expression for the mid-day angle of the sun throughout the year ? Is it a simple sinusoid ? If it is, then at latitude 29deg, the solar azimuth would obviously be 61 deg at the solstices (90 - 29) ie March and Sept, 87deg in summer, 35 in winter and for other months:
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
83.5 74.0 61.0 48.0 38.5 35.0 38.5 48.0 61.0 74.0 83.5 87.0
Is this right ? Can you use the angles for Oct and April prettygully to do the trig - with the wall/window heights you have specified ? You don't specify what you mean by "good" shade - as Dymo points out is that "full" or almost full ?

Posted Tuesday 15 Nov 2011 @ 2:53:11 am from IP #
fireflies
Member

Just to add to the points of view to consider.
An ATA talk I attended by a solar designer a month or so ago here in Perth indicated that the width of eaves should not be too much of a consideration but that the potential for heat gain from the external paving into the slab or rqdiated back through the lower window is actually more of a consideration during high summer months. The reasons he gave was the angle of incidence of light during summer (for Perth) is approx 80 degrees from horizontal (or 10 deg incidence at vertical window pane). Whereas the radiated heat from the paving externally can be almost directly perpendicular to the window and hence penetrate fully through the window. However with the 10 deg incidence high levels of reflection occur so that light/heat does not fully penetrate as much as may be imagined.

So his advise was small eaves but functional insulation space between paving and lower window - for instance low garden with thick loose mulch to reduce absorption at and hence radiation from ground to window.

Posted Tuesday 15 Nov 2011 @ 7:50:33 am from IP #
dymonite69
Member

prettygully said:

Why the big difference? And how do I pick?

It actually isn't a big difference once you consider the solar azimuth at which you get full shading.

900mm eave - full shading at 66 degrees
1200mm eave - full shading at 72 degrees

The sun will rise from 66 deg to 72 deg within 30 min during October.

Alternatively it will be around 66 deg at noontime on the 1st October and 72 deg at noontime on the 15th October i.e. 2 weeks of less shading with the shorter eave.

However, the cost difference to build that is significant once you consider the bigger trusses required.

re ground reflected radiation, some form of external blind arising from ground level is conceivably another way of addressing this.

Posted Tuesday 15 Nov 2011 @ 8:20:40 am from IP #
Ueee
Member

A good example of adjustable eaves is built into the north facing wall of my dads house(Bungendore NSW) as per Russells house it is a rectangle with the long wall facing north, which is all one large room with the kitchen at one end, and it has a full length glasshouse off it. The fixed eaves are just the standard (maybe 500mm) length, and then the rest of the glasshouse has twinwall polycarb roofing. We then fixed several stage sliding covers, with insulation fixed to the top side, between the rafters. These can be slid right back under the fixed eaves or extended out to the full width of the glasshouse.

It works a treat, even to the point where he can grow Frangipanies in the glasshouse, in a climate that regularly see's -10 in winter, and puddles can stay frozen all day if they don't see the sun......

Posted Tuesday 15 Nov 2011 @ 8:38:42 am from IP #
Buzzman
Member

Another alternative is a pergola-like structure, with rails between the extended top chord (rafter) of the roof trusses.
Grow a deciduous vine over it, so it progressively losses shade in autumn and reverse in spring. But in winter it's 'leaf free' and providing full su to the windows.
This is actually a better idea for retro-fit, but no reason why it couldn't work for a new build.
And then it could be 1.5m and thus form a nice shaded arbor/walkway in summer time.
Kinda like a green-roofed 'Roman collonade'.
Those Romans. They were crazy. Not.

Posted Tuesday 15 Nov 2011 @ 12:55:36 pm from IP #
yabbietol
Member

A good tool for looking at energy efficiency and taking into account eve size is the NSW Government BASIX web site.

If you want to you can do a BASIX test on your house. The test is for educational use only and it is not a valid BASIX test. You also need to have a NSW address (If not in NSW Just use a made up address with a valid NSW council area and postcode).

https://www.basix.nsw.gov.au/information/index.jsp

You will need to register, this tool and web site is full of very useful information, not correct for all situations, but pretty good.

PS In my experience at 35 deg south north facing eve of 1m works pretty well as a compromise in both high summer and mid winter. Made from existing eves (480mm) plus 520mm wide colour bond laid flat on a on a pergola.

Posted Tuesday 15 Nov 2011 @ 8:55:19 pm from IP #
gazmo88
Member

We are going through this at the moment with our design.

The "shading season" in Sydney is allegedly late October to late February.
In Sydney therefore you should aim to prevent any noon sun which falls at an angle above 62 degrees from striking your building.

Says a number of books.

Noon midwinter sun in Sydney falls at 32 degrees. For maximum effect you would like the winter sun to reach as much of the floor as possible.

None of this takes into account shade from trees of course, or the fact that this only accounts for noon sun angles.

There are numerous well documented strategies but in our case this will involve 1.77 metre (ish) eaves:
http://3.bp.blogspot.com/-x4f-2rxwlF4/TrC44mjaIhI/AAAAAAAAADc/kQn_gCa_nD8/s1600/SunAngles.jpg

Such eaves are easily achieved using Bondor, Ritek, or various SIP roofing products.

If you want a feel for your site and have "i" disease then there's an app for that. I have used both LiteTrac and Sun Seeker and found them helpful in visualising sun paths.

Making a small balsa model, orienting it correctly and then watching what the sun does is also easy to do and obviously takes into account all real sun angles provided your neighbours don't shade your models too much.

As well as the above eaves we will be having extensive pergolas to the west which will likely be covered with Wisteria. Alternatively a friend has covered his pergola with an ornamental grape vine (the "grapes" are only 1mm dia) - you could use a regular grape vine if you like having possums and birds as close friends.

A suitably sized pergola also solves the reflected heat (and glare) problem.

Posted Tuesday 15 Nov 2011 @ 10:01:29 pm from IP #
prettygully
Member

Dear All,

Many thanks for all your considered answers - there is a great deal of useful information and good advice!

We'll probably err on the conservative side with the width of eaves, about 900mm (including gutters - hadn't thought of these providing additional shade) which should shade out sun at angles higher than 69 degrees -- noon
sun between mid october and early march -- and put our money towards external roller shutters which should not only help to keep the heat out for out of season hot days but also help with security and bush fires ...
and, of course, we can always add a pergola later if we find we'll need it.

Posted Wednesday 16 Nov 2011 @ 4:45:33 am from IP #
karmacoma
Member

I used Google SketchUp (that's a free CAD program, enormously popular) to design my eaves. That's because you can "site the house" at a particular latitude (actually it has a drop-down list of cities though you can just enter lat long coords if you want) and it throws shadows from the sun any time of day, full year round. So you can make up your own mind when you think the cut off point for shading ought to be, what to do or blocking the low afternoon sun etc. Worked well. Bit techie for some I suppose.

Posted Wednesday 16 Nov 2011 @ 7:03:35 am from IP #
dymonite69
Member

prettygully said:
and put our money towards external roller shutters which should not only help to keep the heat out for out of season hot days but also help with security and bush fires ...
and, of course, we can always add a pergola later if we find we'll need it.

What is important to realise is how effective even modest amounts of solar glazing can keep the house warm on a cold day. It is far more thermally and cost efficient not to overglaze. It will minimise nighttime winter heat loss and there is less window area to shade during summer. A piece of window is always going to be a lot more expensive than a well insulated wall.

Posted Thursday 17 Nov 2011 @ 1:12:42 pm from IP #
Jeffbloggs
Member

I agree with D69

I classify windows as a thermal "hole in the wall". It puts them in a different light. But I can also offer better holes in the wall, made in Germany, uPVC double glazed windows with integrated foamed alumunium electric roller-shutters for around $650sqm. I can also source some aluminium clad timber frame versions for about $850sqm. Triple glazed is about $90sqm more for either. 8 weeks delivery time.

But on the subject of eaves and shade. I think it is important to realise that the sun is not static. It moves around all day long, as well as changes it's position during the seasons. There is no such thing as the "ultimate eave". A house is generally not constructed to deal with this moving around. It's a static "heap of bricks". I always imagine that the ideal house would have to "live", something adaptable like a tree.

We need houses that "react" to the weather, because the weather is always changing. Previously we were nomadic and followed the sun to be keep comfortable. Nowadays we are very settled and stagnant, but our houses are too, and they simply don't adapt enough at all.

I think roller-shutters are essential, there when you need them, and gone when you don't. I have them in my house and use them all year around for climate control. Even though I went to great lengths to "calculate" my eaves, they do not compare in performance to the shutters. Plus the shutters offer night time thermal transfer reductions as well, eaves can't help there at all.

Being close to the ocean the shutters are also great for reducing wind noise and possible damage. Security is also increased with shutters.

The best thing about my shutters is that they are electric. This allows them to be set dynamically with ease, and centrally, with different heights, and different windows, and they can be used and programmed according to the time of day or season. At night they also automatically close, unless you want to leave them open for ventilation purposes. I always say that if it is not convenient to use, then it generally won't be used much. No-one wants to "waste" 15 minutes walking around opening and closing shutters, so when they are automated, it really makes them an "active" contributor to saving energy and maintaining comfort. Rather than being a feature one might have, but couldn't be bothered using.

They also offer security and better resale. Ever heard of anyone trying to sell the correct eaves width? Shutters are sort after. I'd rather have a bigger insulated wall area, with a small insulated window that has a roller shutter, than a big window with the "correct" eaves.

Posted Thursday 17 Nov 2011 @ 4:56:12 pm from IP #
gazmo88
Member

JB,

Yup no ultimate eave, not only does that tricky sun bob about in the sky but things sometimes get in its way.

Equally one decent hail storm will leave your pergola in a sub-optimal state .... for the rest of the season.

But there are shutters:
http://www.moderngroup.com.au/images/content/shutters-55_large.jpg
(ooooh ugly but so cheap to install)

and there are the nice "built as part of the structure look like they belong" type that you see all over Europe .... probably those Romans again. I especially like the ones that are also hinged at the top so that you can also open them like an awning window for a touch of early morning ventilation.

And people DO buy eaves e.g.:
http://www.richardcolearchitecture.com.au/#/RESIDENTIAL/2011/?catID=474&imageID=4770
or
http://www.richardcolearchitecture.com.au/#/RESIDENTIAL/--2010/?catID=365&imageID=3685

In some architectural quarters eaves are the new black.

In fact you can pay a lot of money for eaves ..... or not.

Eaves require no electricity or controllers or mechanical repair..... but are unable to respond to a cloudy day

Eaves of course add nothing to the R values of your windows ..... but I don't think shutters add a whole lot either.
The roller shutter vendors seem to avoid the issue.
What are the R value of your roller shutters ?

Eaves can be relatively inexpensive and shutters can add up to a lot of money if you need a lot of shading.

So I don't think its as black and white as you paint it.

I do like the idea of an adaptive house on all sorts of levels (pergolas, shutters etc.) though, and you could easily construct a very responsive climate control system using shutters alone.

Roller shutters are not the only option of course, its pretty easy to construct sliding shutters using centor hardware and its not too tricky to automate these either.

Do you have any links to the windows you mention above ? I am about to buy a bunch of windows and I am looking at everything I can. Never been a big fan of plastic windows though despite their practicality. Do they come in colours other than white ?

Posted Thursday 17 Nov 2011 @ 11:39:58 pm from IP #
dymonite69
Member

I would ideally want my adaptive house to have simple mechanics, low maintenance but reasonably easy and quick to control. Electric motors breakdown, gears seize up, electronics burn out.

My parents noticed their bedroom was a lot warmer in an ACT winter with external shutters. Although shutters don't offer much R value, they reduce convective wind movement off the windows (i.e. wind chill). I reckon heat loss can be substantive if there is a moderate breeze and will significantly degrade the quote R values of most windows (even DG ones). In one sense they act as another convective barrier like DG and TG.

Allegedly in medieval times, just a primitive construction involving two layers of curtain hanging in front of an opening in a stone wall would greatly improve comfort conditions. Science today now specifies the exact spacing distance and manufacturing offers airtight solutions but the principles remain the same.

Posted Friday 18 Nov 2011 @ 12:06:17 am from IP #
gazmo88
Member

"I would ideally want my adaptive house to have simple mechanics, low maintenance but reasonably easy and quick to control. Electric motors breakdown, gears seize up, electronics burn out."

interestingly having spoken to a few shutter people recently they all claim that the electrically driven ones (using Somfy motors) are actually more reliable than the manually activated ones. This could be rubbish of course as it comes from sales people, in general I am a huge fan of "agricultural" when it comes to systems but perhaps a motor direct coupled to a shaft really is simpler than all those cogs and gears and straps and stuff.

Your point re your parents bedroom being warmer is interesting - it underlines something I have been thinking for a while which is that we may well not be using the right metric. Perhaps we need to measure heat transfer under various conditions not just static ones.

I'd still be interested in knowing some actual real R values though.

Posted Friday 18 Nov 2011 @ 12:23:50 am from IP #
prettygully
Member

JB,

the windows with integrated shutters you described sound very interesting -
if it is not too much bother, could you provide some more details or links, please?

Posted Friday 18 Nov 2011 @ 3:26:02 am from IP #
Jeffbloggs
Member

Prettygully/Gazmo

Sadly all the information I have on them is in German (being German made). However I am putting together some English brochures that I could forward soon. In the meantime I will PM you with my email details and we can take it from there.

Here some further details:
Overall the product is excellent value for money, from what I have seen and compared on the Australian DG market. They can be ordered to size, to the nearest mm, and can be configured up to 700-3000mm by 500-2500mm for a single and double window frame, and up to 3600 by 2500mm for a three window frame, and 4900mm by 2500 for a four window frame. they can also have an additional overhead frame above the doors for heights up to 2800mm, but these cost a bit extra for freight.

They can be configured as a fix, tilt and hinge opening window or door, or as a lift-n-seal sliding door. They all open inwards, but can be configured to open outwards for an extra. Each window frame can be configured individually, so in a 3window frame you could have one tilt, and two fix windows etc.

uPVC Colours range from Mahagoni, Oak etc veneer (+20%) to green, blue brown etc.
You can also specify the inside only to be white.

The windows are delivered to site in one piece with the glazing installed, and are generally only installed after all the brickwork and plastering is finished for lock-up. The brickies do not install the frames, and only have to leave openings under the lintels. Plus the plaster's don't get them dirty as well.

The roller-shutters are aluminium for rigidity, and are foam filled slats with 10mm light "seams" when partially raised. Louvre or awning type shutters are also possible, but are not integrated into the window frame. They also cost a bit more. The louvere version is actually quite good as it allows light ingress without the direct heat, even if used without an eave. As previously stated they are electric driven, with electronic end stops. The shutters can be a different colour to the windows as well.

Window panes can be configured for thickness and e-type glass, in double or three panes, with or without safety glass, with or without further sound insulation features.

Doors and windows can be fitted with exterior and interior cylinder deadlocks. Each window or door has a minimum of three locking points around the frame (one per side). The larger the frame the more locking points.

Aluminium exterior window sills and slide opening fly screens are included on both windows and doors. Handles can be specified or retrofitted as desired. Flashings can also be provided. 5 year manufacturers warranty with local support. Delivery and install can be arranged.

I have left you both a PM with my contact details. Let me know if you have any further questions, and I can then send you some pictures as well.
Regards
JB

Posted Friday 18 Nov 2011 @ 5:13:02 am from IP #
dave lawrence
Member

900-1200 eaves are excellent in the subtropics . remember eaves have other functions than shading windows from unwanted sun . they protect walls and openings from driving rain . equally as important for long term maintenance of house .

Posted Friday 18 Nov 2011 @ 7:49:48 pm from IP #
MaxG
Member

@JeffBloggs: do you mind revealing the brand / company, as I can easily read German

Posted Tuesday 4 Dec 2012 @ 12:15:59 am from IP #
Johnnojack
Member

I have shutters on my one Eastern and three Western windows. (Foam filled aluminium slotted when part open type). I didn't like shutters and resisted them for years but finally realised that nothing else would do the job. Now have to admit they are one of the best investments I have ever made. Keep heat in in winter and keep it out in summer and do it very well indeed. Went for the rechargeable battery electric type.
Jeffs sound very interesting. A photo?

Posted Tuesday 4 Dec 2012 @ 1:34:28 am from IP #
MaxG
Member

What brand are they?

Posted Tuesday 4 Dec 2012 @ 3:13:59 am from IP #
Johnnojack
Member

Streamline Max.

Posted Wednesday 5 Dec 2012 @ 12:32:41 pm from IP #
MaxG
Member

Thanks... and sorry, should have been more specific: do they have a website?
I was looking for "streamline" and came across this: http://www.productreview.com.au/p/modern-streamline-roller-shutters.html which I hope is not the company you referred to :o)

Posted Thursday 6 Dec 2012 @ 3:31:48 am from IP #

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