Category: Passive Solar

Eaves

If you go back 39 years most houses had proper eaves but now they are less common.

As I travel around I sometimes see new houses with eaves on the front facade but non elsewhere, which I think looks weird.

If you are thinking about ‘eaves’ on your new house here are some advantages and disadvantages.

Advantages

  • The eaves keep the rain off the walls. As well as improving the weather proofing this helps improve the thermal performance of the walls in winter.
  • They will shade north facing windows in summer while letting the winter sun in. (This effect is negligible for windows facing in other directions and only about 50% effective for full length windows and patio doors)
  • Appearance. I think they give a more finished appearance and the shadow line adds interest.

Disadvantages

  • Cost. A typical cost is around $80/m2 so 600mm eaves all the way round a typical house could add around $4,000 to the cost. 450mm eaves will be a little bit cheaper.
  • You can’t build as close to the boundary which can be important if you have a narrow block.

As for me my first Australian house had eaves, but my next house did not so I have a foot in both camps.

I didn’t mind the look of the design without eaves and I built a pergola on the north side for shading.

Are you for, or against, having eaves on your new house?

 

To find out how big your eaves need to be… see Shading Northern Windows

 

West, or East, Facing Windows

Well the first rule is to avoid West, or East, facing windows . . . or at least keep them as small as possible….and here is why:

Summer

West windows get direct sunlight from mid-afternoon to late in the evening in summer causing overheating.

East facing windows get sun from sunrise to midday.

This isn’t normally as bad because the house is cooler in the morning, but the heat built up during the morning does stay making the house hotter later in the day.

Because of the low angle of morning, afternoon and evening sun the over window shading that works well with North Facing windows will be inadequate for West and East facing windows.

Winter

In the winter there is minimal heating benefit with little or no direct sunlight getting into West or East facing windows.

In the last house we built the master bedroom full length windows all face west.

I would have preferred to decrease the size of the windows but the builder would not alter the size of the windows on the front. (They were quite happy to change all the other windows, just not on the front)

Shading

To shade, our west facing windows, we adopted the following three stage strategy.

  1. A highly reflective tint was applied to the window, by Tint-a-car. This had the immediate effect of lowering the temperature in the room by about five degrees on a sunny afternoon. Even the installer was amazed at the difference it made. Expect to pay around $50-60/m2,
  2. We planted shrubs in front of the windows to shade the bottom third of the windows and also provide a micro-climate to reduce heat loss in winter.
  3. We have fertilised and trimmed a couple of trees on the nature strip, in front of the house, to encourage dense foliage which now shades the window from mid afternoon to sunset.(see photo below)

For similar posts see Passive Solar

Choosing a House . . . A new E-book for only $4 to help plan your new house

 

Shading Northern Windows

In a post about Passive Solar I talked about shading North facing windows.

That doesn’t mean installing roller shutters, or blinds that pull down over the window.

Shutters and blinds tend to make the rooms very gloomy and depressing, and with good design are unnecessary.

The best methods of shading North facing windows are:

• Overhanging eaves
• A well designed veranda
• A pergola

How Much Shade?

For areas North of Brisbane its better to have  enough shade to keep the sun out of the house as much as possible.

For areas from Brisbane and South the keys are:

  • Keep direct sun off the window during the summer months of; December January and February.
  • Allow full sun to shine on all of the window in June July and August.

Dimensions

Keeping the summer sun out for areas South of Brisbane can generally be achieved by making sure the eaves overhang is 45% of the height (H) the shade is above the bottom of the glass.

The eaves, or other method of achieving shade, should also extend horizontally along the wall window 45% of H to either side of the window.

To minimise shading of the window in winter the shading needs to be around 20% of H above the top of the glass.

West and East Facing Windows

West and East facing windows are almost impossible to effectively shade in summer and get minimal benefit from winter sun.

Information on dealing with these windows can be found at: West and East Windows

 

For more Green Ideas see Sustainability

 

Winter Sun

If you are going to get as much winter sun inside your house you want to minimise shading.

That could be from neighbour’s houses or even and boundary fences/hedges.

Here is how to check…………………..

Ideally you should be aiming for around 6 of hours of sunlight into your house on the shortest day (21st June).

To get the exact angle for your location you can get solar calculators on the net that will give you the suns angle for every hour of the day for any day of the year.

The diagram below shows the how to begin working out the effects of shade from a 1.8m fence to the North of a window

 

 

I have provided a TABLE that shows approximate midwinter shade angles ‘A’, and shade slopes ‘H/D’ for various positions of latitude.

 

 

Latitude Locations

Noon

9.00am -3.00pm
Slope H/D Angle Slope H/D
38 Melbourne 29.5 0.57 16 0.29

 

  1. The Bottom of the window is 0.15m above ground level
  2. The top of the fence ‘H’ is 1.65m above the bottom of the window(1.8m -0.15m)
  3. To make sure you get 6 hours winter sun in the window, divide the height of the fence above the bottom of the window by the ‘9.00am -3.00pm’ Slope (0.29).
  4. D = 1.65/0.29 = 5.7m. The fence needs to be 5.7m from the window for full sunlight for 6 hours.
  5. If the fence needs to be closer than 5.7m it worth checking if any sun gets to the bottom of the window. To check divide 1.65 by the ‘Noon’ Slope (0.57)
  6. D = 1.65/0.57 = 2.9m. If the fence is closer than 2.9m you may be better reducing the window size by raising the base.

 

This information also applies to Solar Panels for Heating and Power Generation

To provide summer shading see Shading Northern Windows

 

Passive Solar – What Does It Mean?

Do you want a house that is filled with natural light in the summer without overheating, and minimises your heating bills in winter? ……….If you do, then a passive solar house is what you need.

Here are some of the things you consider.

Get The Winter Sun Into The House

From Wikimedia Commons

This means having most of the main living rooms facing North with lots of windows.

It also means making sure the Windows aren’t shaded by fences, trees etc.

Maximising Thermal Mass

This means having dense absorbent materials like concrete  and brick inside the house.

  • In winter the thermal mass heats up during the day and releases the heat during the evening.
  • During summer if you open the house in the evening/night the thermal mass cools and helps keep the house cool the next sunny day.

Ideas for increasing thermal mass include:

  1. Have the house on a concrete slab rather than stumps.
  2. Have tiles or a slate floor especially in front of the north facing windows.
  3. Brick feature walls and or brick fireplaces.

Keeping the Summer Sun Out

  • Minimising West and East facing windows. These windows are the worst for heating up the house with the low morning and evening sun.   (In winter they hardly get any benefit from the sun)
  • Shading North Facing Windows.
    Typically the shading will be in the region of 1 m from the outside wall. It can be either deep eves, a veranda, or a pergola.

Minimise Heat Transfer

That’s transfer of heat from outside to inside in Summer days, and inside to outside in Winter.

To minimise heat transfer:

  • Provide good insulation to walls and ceilings.
  • Have small windows on the South Side.
  • Closing curtains at night in winter.

The first house we built in Australia was built according to these principles. Although we lived in it for 10 years we never felt the need to fit air conditioning. We didn’t require awnings on the windows or wanted to shut the curtains on hot days.

The above advice applies to Australia and other Southern Hemisphere Countries such as New Zealand and South Africa. If you live in the Northern hemisphere you need to have the Large windows on the south side of the house.

What Passive Solar ideas have Worked for you?

For more Green Ideas see Sustainability

 

Which Way Is North?

Don’t rely on a compass!

Or a mobile phone either!

There are different Norths

  • True North
  • Grid North
  • Magnetic North

The difference can be more than 10 degrees which can affect your view and the solar performance of your house and solar panels.

Just to make it more complicated the difference can change depending on where you live and the date!

True North

True North points to the North Pole, the axis of the earth’s rotation.

Grid North

Grid North is the basis of the grid lines on a paper map as your north reference.

Grid north is typically close ( within 2° east or west of true north) and varies with your position within a zone.

It is used when establishing geographic co-ordinates so it will be the direction the North Arrow points on a house plan prepared for construction.

Magnetic North

A compass needle will align with the earths magnetic field and thus points to the ‘magnetic’ poles.

The Earths magnetic field and thus the ‘Magnetic Pole’ moves over time.

The difference between Magnetic and True North (called Declination) is also different depending on your position.

 

Want to find the difference for your site? . . . . Follow this link:  Geoscience

Solar Citizens

Do you want to be a Solar Citizen?

Do you know who the Solar Citizens are?

 

Well I only found out about them a couple of weeks ago.

Basically they are a nationwide group of people that support a more sustainable future.

Amongst other things they are campaigning for are ‘Better Feed In’ rates for people with Soalr PV panels on their roof . . . . they believe a fair rate for Feed In Tariffs is between $0.10 and $0.18 rather than the $0.05 to $0.08 you might be currently getting!

 

To find out more follow the link below

www.solarcitizens.org.au

Energy Through The Windows

Ever wondered how much actual benefit you get from the sun shining in?

How The Sun’s Energy Warms Your Home.

Basically when the suns energy (which includes both visible and invisible energy such as Infra Red and Ultra Violet [UV] radiation) hits your window three things happen to the energy:

  • Reflection – Plain glass will reflect around 8% of the total energy.
  • Absorption – 5mm plain glass (the minimum thickness for most domestic windows) will absorb around 12% of the total energy. (Typically around 1/3rd of the energy that is initially absorbed is re-radiated to the inside of your house)
  • Transmission –  The remaining energy, 82% of total energy for 5mm glass will be transmitted into your home.This is known as the RAT Equation

    Solar Heat Gain Co-Efficient (SHGC)

    Rather than have to keep working through the RAT Equation the glass industry has come up with a Solar Heat Gain Co-Efficient which makes calculations easier.

    Values of the SHGC range from 0.82 (5mm float glass) to around 0.15 (double glazing with reflective coatings.

    There is more information including SHGC values for a wide range of glazing types at www.nationalglass.com.au

    To find the amount of energy coming in through a North facing window you multiply the radiation from the sun by the SHGC.

    Winter Example Calculation

    For a sunny winters day in Melbourne that the average angle of the sun on the window will be around 20 degrees, so around 800 watts of energy will be  falling on each square metre of glass.

    The Solar Heat Gain = 8oo watts/m2  x  SHGC 0.82  =  652 watts/m2

    Assuming that on that sunny winters day the outside temperature is 10 degrees and the internal temperature is 22

    Heat loss through the window = 12 degrees x ‘u’ value of 5.8 = 70 watts/m2

    Net heating benefit of the window =  652 – 70 =  582 watts / m2.

    Summer Evening Example

    For a summer evening in Melbourne  the average angle of the sun on a window facing the sun will be around 10 degrees, so around 900 watts of energy will be  falling on each square metre of glass.

    The Solar Heat Gain = 900 watts/m2  x  SHGC 0.82  =  738 watts/m2

    Assuming that on that summer evening the outside temperature is 35 degrees and the internal temperature, with air conditioning, is 22

    Heat gain through the window = 13 degrees x ‘u’ value of 5.8 = 75 watts/m2

    Net heating gain of the window =  738 + 75 =  813 watts / m2. (no wonder the air conditioning is working hard!)

For more see Sustainability

 

Energy From Sun.

Have you ever wondered why it gets so HOT in the roof space on a sunny day?

Well there is a lot of Energy coming from the Sun . . . .approximately 1.37 kw /m2 shines on the Earth

By the time the solar energy gets through the atmosphere to the ground it’s about 1kw/m2  for a flat surface square on to the Sun.

Even in Melbourne on an average day the typical 200m2 house will get around 8ookw hours of heat radiation onto the roof.  In January it can be almost twice as much!

Some interesting Solar Exposure Maps, like this one, are available on the Bureau of Meteorology website at this link: Solar Exposure, if you want to check the solar radiation for your location.

This shows the solar energy received in Mega Joules (MJ) on a horizontal surface. (to convert MJ to kwh multiply by 0.28).

The maps are based on measured values so they includes things like times of cloud cover.

If you follow the above link you will be able to find a map of values for each month.

 

See Sustainability for more posts.

 

Sustainability Modifications to a Standard Design

Lots of people say you can’t get a sustainable house from a standard builder . . . . but I disagree!

As an example I have started with this standard floor plan from the ‘MYPAD’ design of HomeStart, Perth first home buyer grant specialist.

Here is how I would look to get most sustainable performance from this layout.

Orientation

The best Orientation will be with the house facing East.

That means the Family Room, Dining Room, and Kitchen will all face North and be able to look out onto a garden.

The Bedrooms, Bathrooms, and Laundry will all face South and act as a ‘Buffer Zone‘.

If your block faces East you will be able to get similar performance by asking the builder to ‘Hand’ the floor plan (build it as a mirror image)

Shading

In order to keep the Summer sun out of the house, while letting the winter sun in, I would look to provide approximately 1m of Shading to the north facing windows.

This could be either by having the house built with eaves or building a pergola after you move in.

Window Locations

Windows that face East, and especially those that face West, can be a real problem as they get too much sun in summer and hardly any in Winter.

The window in Bedroom 3 would be best relocated to the other external wall.

The Family Room window at the end of the house would be better deleted altogether. There will still be plenty of window area and with one less window the layout will actually be more flexible!

Although the Master Bedroom windows will face East the application of a reflective film, after you move in, will prevent the room overheating in summer.

Window Sizes

Both bedroom windows would be best with a sill at least 1m from the floor level. This will give adequate light but the Smaller Window, and more solid wall will result in better thermal performance.

Laundry Door

The Laundry door in this design is a patio door. By changing to a solid wall with window and conventional door you will significantly reduce the heat transfer.

Overall Effect

Making alterations like these to a builder’s standard design allowed me to increase the energy rating of my last home by the equivalent of 1.5 Stars!

 

See Passive Solar for more Posts

 

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