Building a house

(Reposted from September, 2009, with pictures added. )

We're now heading fairly surely towards actually building a house. People keep asking me what kind of house it's going to be, and I'm not really sure how to answer. I wonder if they want me to say that I'm building an igloo, or a fourteenth century Venetian palace or a brick Victorian end terrace. Certainly they want some short answer and not a long description of what colour each wall will be and what the doorknobs will be made of.

It certainly seems like I should have a short answer for this question. As building is much more of a philosophical journey than a technical one I'm just going to set out what I want to do and what that means.

I want to build a house that has an energy consumption of less than zero.

An explanation of why I'd like to build a house that produces energy should not be necessary, and in my opinion in a developed country you should not be allowed to build to consume when you can build to conserve.

Following from this basic condition for building are four topics: Energy efficiency, thermal inertia, generation of electricity and collection of heat. They are of course interconnected, but I'll try to explain each one.

When it's going to be ten below zero outside, a big part of energy efficiency is thermal efficiency. First of all, this depends on the size and shape of the building. The bigger it is, the more heat it will need. The bigger the surface area, the more heat it will lose in the winter, also, the more heat it will gain in the summer when it's over thirty outside. As well as the design and layout of the house and the rooms, the materials used are important. It must be well insulated. A lot of the heat is lost through windows, so these are very important.

Thermal inertia will keep the building at a constant temperature. The more heat the building can contain, the better. This can be both active and passive. The air in rooms contains a certain amount of heat, and it will make a difference how this air moves by convection and how it is forced and fanned where it might not otherwise go, and whether heat can be recuperated from air as it leaves the building. Water, or other liquids, can also store heat and can be moved around the house to where heat is needed. In addition, building materials can store heat. While wood is a good insulator, stone can store a lot more heat. Another possibility is phase change materials: for example floor panels containing a liquid that freezes at 19 degrees. Because of the latent heat of freezing, this can absorb a lot of heat as it is melted, and release a lot of heat as if freezes, all at the ideal temperature of 19 degrees.

Photovoltaic solar cells are pretty much the only practical way of generating electricity in a small plot in the middle of an urban area. The amount of solar energy that falls on the earth in one hour is the same as the amount used by the human race in a year, so some kind of harnessing of this power is very feasible. In fact most kinds of energy production come indirectly from the sun, whether you're burning wood, using fossilised wood in the form of coal or prehistoric microbes in the form of oil, or even using wind and waves that have been generated ultimately by temperature changes and the evaporation of the oceans. Only nuclear power and tidal generation are not originally solar. The sun is, of a course, a large nuclear reactor so I suppose you could say that all energy is ultimately nuclear. Solar cells are not highly efficient, perhaps only converting 10 to 20% of the sun's energy that hit them into electricity, but that's worth a whole new blog.

The collection of heat is most simply achieved by facing windows to the south. As the winter sun is low and the summer sun high, eaves can easily be extended to allow heat in in the winter and keep it out in the summer. This, on its own, is not going to be enough to heat the whole house and of course the more windows there are, the more heat is lost. Solar walls, developed to dry grain in Canadian barns, can absorb heat from the winter sun and convert it to hot air inside the house. Roof mounted solar thermal panels can be used to heat air or water. Combined with photovoltaics these can greatly increase efficiency, also worthy of a whole new blog. Heat pumps are another way of getting heat. They use a small amount of power to defy the second law of thermodynamics and get heat from a colder body.

I'll come onto the colour of the walls and the doorknobs later, but these will probably be influenced by the points above.

original post: http://minuszeroeco.blogspot.jp/2009/09/building-house.html

Lovelock and Reynolds

I was listening to James Lovelock talking the other day and something he said struck a chord. Although he's something like Moses to the ecological pantheon, his own views towards the environment and especially global warming are somewhat ambivalent. He lamented that an ecologist used to be someone who enjoyed walking in the country, but has become an angry protester.

Although the reaction of many to the amount of energy we are using is one of horror, he viewed the whole system in terms of the Reynolds number, and the increase in turbulence with more energy. Reynolds studied non-laminar flow, and you can see the effect of his eponymous number as you turn on a tap, first in a steady trickle, then with ripples and curves as the flow increases, and finally with splashes and crashes as it turns to chaos. In the same way, as we use more energy human output becomes much more interesting, and Lovelock suggested a threshold of one kilowatt per square metre for this to happen.

You probably don't need me to tell you this, but that's the amount of energy that comes from the sun. There are two implications, one dreadful, and the other divine.

The divine implications is that all of the stuff that's happening can be attributed to a concept from physics, and what appears to be chaotic and out of control is just business as usual for the universe. It's tempting to think of the world like a top that has been spinning merrily and steadily for a long time, and is now wobbling erratically before falling over at the end of its turn. Looking at the Reynolds number, rather than about to fall over, we are just starting to taking off.

The dreadful implication is that if we are going to continue to be "interesting" we need to continue using a kilowatt of energy per square metre, and this is not sustainable if we're using primary solar energy in the form of photovoltaics, secondary solar in the form of wind and waves, or historical solar in its various fossil forms.

Humidity makes it hotter ... or colder

When the scientific theory doesn't match your observation of reality, you know you've got one of them wrong. So it's been bothering me for a while that the theory suggests higher humidity makes it feel hotter while the evidence suggests the opposite at colder temperatures.

Finally I've realised the cause of this anomaly.

There are actually two effects of higher humidity on the body losing heat. One reduces the ability of the air to remove heat from the body, and the other increases it.

As we know, the body mainly loses heat through evaporation of the body's perspiration. The ability of this perspiration to evaporate is hindered by high humidity. Humid air just has less carrying capacity for those water droplets and will push up
The other effect is on the air's heat capacity. Water is a very effective carrier of heat, with a kilogramme of the stuff able to hold almost twice as much heat as a kilogramme of air. Adding water to air is going to increase this heat capacity. More heat capacity means a greater ability to take away heat.

Perhaps as it gets hotter, and the difference between body temperature and ambient temperature becomes smaller, the evaporation effect is larger, so humid air makes us feel hotter. Meanwhile, when it gets colder and the difference in temperature is larger, the higher heat capacity effect is larger.

Perhaps, but probably not.

Water certainly does have twice the heat capacity of air, but absolute humidity is measured in grammes of water moisture per kilogramme of air, so the increased heat capacity may only be one percent, comparing dry air with dripping wet air at 10 degrees centigrade. It's difficult to imagine this making the kind of two or three degree differences that humidity makes when it's hot.

Clothes are another matter though. The amount of water they can hold does not depend on temperature and absolute humidity, but on relative humidity and the related vapour pressure. Cotton can hold up to 15% of its weight in water, wool can hold up to 35%. Both of these textiles are hydrophilic and will try to reach an equilibrium with the atmosphere around them steadily releasing or absorbing moisture. That all takes energy, and any water content in the clothes needs to be kept warm.

At last this seems to make sense, although I'm not completely sure it's correct. I know the hiker's adage that cotton kills, but also I've heard that if you are stuck somewhere cold and damp, the best thing you can do is wrap yourself in a woollen blanket, since wool is exothermic.

Notes and references

This article is only tangentially relevant, but has a good explanation of hydrophilic textiles seeking equilibrium with their environment: Iqbal, M., Sohail, M., Ahmed, A., Ahmed, K., Moiz, A. and Ahmed, K. (2012) Textile environmental conditioning: Effect of relative humidity variation on the tensile properties of different fabrics Journal of Analytical Sciences, Methods and Instrumentation 2(2), 92-97

In fact there is much older work on moisture in textiles, for example, Albert C. Walker's Moisture in Textiles (1937, in Bell system technical journal, 16, pp 228-246). This goes into some detail on exactly where the moisture goes within cotton hairs.

And here is some propaganda from New Zealand wool industry: New Zealand Merino Company Limited (no date) Heat and moisture regulation.

You may want to compare this with the moisture content of wood. In textiles terms, of course, wood is just raw rayon.

A Q and A session on physics.stackexchange.com was helpful in debunking my first hypothesis, giving the formula c_s = 1.005 + 1.82H, where 1.005 kJ/kg°C is the heat capacity of dry air, 1.82 kJ/kg°C the heat capacity of water vapour, and H the specific humidity in kg water vapour per kg dry air in the mixture.

The best LED I've ever bought

With its bright light, spidery legs, magnet and sensor, the Ritex Dokodemo portable LED light is probably the best LED I've ever bought. And I've bought quite a few LED lights over the past few years. A lot of them have been completely rubbish. 

I got a battery powered device with three LEDs spaced a few centimetres apart in a row and a magnet on the back that would fit onto the front of the fridge. 

The magnet worked.

Unless you tried to use it to hold something else onto the fridge.

And there was an E17 light that looked pretty promising to replace the incandescent bulb in a table lamp. It had several LEDs in reflective holes that would point light where you wanted it. But it was made of cheap plastic, incapable of conducting the amount of heat away from the LEDs that they would have produced if they'd put out a decent amount of light. 

They did not. Somebody in the design department had not considered that. 

The best lights I'd got so far were bedside lights with single high-power LEDs. Nice heavy bases and plenty of luminous power. 

I saw the Ritex docodemo in a hardware shop and decided it would work well under the stairs. 

Instead of wiring in a light there, from the start the plan was to plug a sensor light into a socket high up on the wall in there. Plug-in sensor lights have been around for a while, and recently they have been LEDs. But they don't put out a lot of light. They are probably OK in a corridor or on stairs so you can see where you're going, but if you're in a store room trying to find something they don't really do their job. 

The Ritex has been keeping it light under the stairs when necessary. It is battery powered and the batteries ran out once, but that was when someone had left it switched on, rather than on sensor mode, since the previous day, and it was starting to dim.

I took it camping with us last weekend, and realised there what a great light it was. 

It was in the area of camping that LEDs made their first big commercial success in delivering light rather than just vaguely letting you know that light was there, as they had been doing since the 1970s when they appeared on electronic panels and then in the seminal digital watch that would come on for a few seconds to show you the time in red. LED head-torches have been around for a while, where their reliability, long life, low power and light weight make any extra cost worth paying. 

We've got a decent collection of LED head torches, but have still been adding unideal candidates to our gallery of camp lights. 

Before it got dark, I lined three of them up along the rope above the table that was going to hold up the tarp covering it. One of the lights gets about seven out of ten on my scale of satisfaction. It's a small puck light with a hook on the back, three AAAs inside and not a bad amount of light, although it seems to take a little while to warm up. It works well hanging from the middle of a tent, and is better than nothing above a table, although it doesn't really give out enough. 

Next to it I put our wind-up LED tilley lamp. This gets about six out of ten. The good point with it is the handle on top so you can charge it when it starts getting dim. It can also be charged from the mains or from a car. The big problem is that it's trying to be a tilley lamp, so instead of sending the light down, where you need it, it sends it around in all directions, which may be useful if you're pretending to be a lighthouse, but is not much use otherwise. And if you were going to be a lighthouse you wouldn't be that bright. 

Next to these I hung the Ritex, and when I turned it on, the other two immediately became parodies of themselves. The puck light is smaller and we'll probably bring it with us, but I think the LED tilley lamp will be staying at home for our next trip. One issue with camping lights is that you don't want things to be too bright when you're camping. Being under the stars is great, but you have to be able to see them. Since this is a sensor light, it will go off if nobody moves for a while, so when you're sitting back you can enjoy the night, but when you need to see what you're doing it will help you out. 

So under the stairs it works perfectly. On the campsite it works perfectly. I haven't had cause to try it out in the event of a power cut, or use it for illumination when I'm working outside at night, but I'm confident it will perform wonderfully in these situations too. 

It's so good, in fact, that I'm going to get another one, or perhaps two. The only problem is that they now offer an updated version, on which you can adjust the colours, and I have to decide whether that's worth the extra 500 yen. I think it probably is. 

Ten tips for building a house

The poster presentation on cool homes for a warming planet gave me the opportunity to revisit and update my ten tips for building a house. It also encouraged me towards concision, so the ten tips are now more pithy.

1. Don't do it.

2. Dedicate a couple of years of your life to this project.

3. Have a vision.

4. Don't trust anyone.

5. Work out who is in charge.

6. Don't let them rush you.

7. Sometimes you have to make decisions and then move on.

8. The ideal home, what you want, what you ask for and what you get are four different things.

9. Assume that everyone thinks you are a complete idiot who knows nothing about house building.

10. When it's all finished, if you get more than about half of what you wanted, you've done really well.


11. It will take longer than you originally thought, and they may try to charge you more than the price you thought you had agreed on.

That's all that went on the poster, but here are some notes just to explain a little more:
1. This is not so much a negative give-up-and-don't-bother, but more because there are loads of houses out there and lots of people already making houses, and you have to wonder: do you really want to add to all of it?
2. The way life works out, there's a good chance that building a house will coincide with having small children and working very hard at your career, both of which may seem, and in fact are, more important.

3. If you don't have a clear idea of what kind of home it will be, the other people building the house may use theirs. If you can find someone with a vision you like, that's great! Beware of marketing slogans masquerading as visions. 

4. Ask questions, search the internet, find out the options, get a second opinion. Be particularly wary of free information, like this.

5. It's probably not you and if you want to be in charge you may have a battle on your hands unless you are really going to build the house yourself. An architect will try to keep himself as your sole contact and prevent you from talking to anyone else on the project. He may then leave everything up to the site foreman or the carpenter, and they may in fact be in charge. Make sure they are on your side!
6. Spend another few days making that decision. Don't worry about changing your mind. You will probably be living in the house and possibly paying back the loan for the rest of your life, and it should be just what you want. It's like shoes: if they don't feel comfortable when you try them on in the shop, then don't buy them. They won't ever feel comfortable on your feet outside, you won't grow into them, and they won't be just right when you've worn them in.
There are two main reasons they want you to make a decision quickly. First, they want to get your project out of the way, and start working on the next one. Second, they have promised one of their friends in the building trade that they are going to be contracting some work, and the timing suits their friends.

7. Sometimes you just have put the decision behind you and start treating it as an unchangeable fact that you will have to live with, and which there is no point in agonising over any more. If buts and ands were pots and pans you'd need to build an extension onto your kitchen.

9. For the most part, they are probably right. The building trade, like many others, seems to work on the principle: a fool and his money are easily separated. People will only be interested in what you think or what you want at the beginning. Very soon they will just ignore you and work on what they think you want.

And you can see even more in the original post published here in June 2012.

Further over-complications for what is, essentially, a big bucket

Or, instead of a leaky syphon, it could just be a very thin syphon.
Going back to first principles, what I want is to divert the rain water from the roof into the garden, and slow it down as much as possible so that it arrives days, or even weeks after falling from the sky. If I can get a syphon working between each water tank, and make it thin enough, it may be able to do this.
The speed of the fluid in a syphon depends only on the difference in height between the top of the water and the bottom of the outlet pipe. According to wikipedia, it's the square root of the height times 2g. So water in a syphon coming out of a 50cm tank will be moving at around 3 metres per second.

Converting that to volume, if you had a pipe of 1cm diameter, it would drain at 14 litres per minute. A hundred-litre tank would be empty in seven minutes. If the pipe was a millimetre in diameter, it would drain at 8 litres per hour and be empty in 12 hours. Five of these tanks in series and the water is going to get from top to bottom in 2 and a half days.

Of course the rainwater is going to have to be really clean with a pipe this thin as it would be really easy to block it. Also this would have to be in addition to a regular overflow since a pipe this thin is not going to be much help in the event of heavy rainfall.

This syphon is essential going to be the same as having a pipe coming out of the bottom of the tank, leading to the tank below. The differences are that a pipe coming out of the bottom of the tank needs a hole in the bottom of the tank, and the syphon will only start working after the tank has overflowed, and will may not start sending water down the system until it has.

Wikipedia also mentions self-priming syphons with a cotton-filled hose that will suck water up by capillary action, then send it down the other side. These are slower than an open hose, which is even better. The system would then start slowly syphon water down as soon as the bottom of the tank is wet.

Of course it may be a good idea to have a hole in the bottom of every tank since it will make them much easier to drain, unless the tanks are going to be fairly small. The drain could lead to a tap into a lower level of tank, and it would be possible to control the trickle down the system.

A rainwater system with syphons and leaks

I've been trying to work out a system of cascading water barrels. Ideally each barrel would contain enough water for one watering. When it emptied it would fill from the next one up, and then when it was full it would empty and water the garden. This would get regular irrigation for the garden. Better still it would only work in the early morning or in the evening, and would not work when it was raining, but those are separate issues.

Right now I just want to think of a system that will run only on gravity and rainwater.

Each water tank needs up to three pipes connecting to it: water in, water out via a tap at the bottom, and an overflow. Generally I want to get water out of the tank at the bottom, so the tanks further up the system don't necessarily need a water-out tap, or if they do it will normally be closed.

The water-in can go on the lid of the tank, which should be straightforward. The overflow needs to go high up on the side, and usually will be dry, so the seal around the hole is not critical. The water-out needs to be near the bottom, and will be wet and under pressure, so the seal is critical. It would be a good thing if I could get out of having to make these. 

If I use a syphon overflow, so a pipe goes into the higher tank high up on the side, and the pipe reaches all the way to the bottom of the higher tank, then feeds into a lower tank so the other end of the pipe is at or below this end, then it will work as a syphon. When the water goes above the overflow hole and the high point of the overflow pipe, the pipe will fill with water, then water will head down the other side of the pipe, and suck water from the bottom of the upper tank. And because it will work as a syphon, it will continue to suck water out either until all the water has gone into the lower tank, or if the lower tank is only a little lower, then it will empty until it reaches an equilibrium and the water will go to the same level in the two tanks. 

Once it's in this situation, adding more water to the top tank will send some water down to the lower tank, so that it will maintain this equilibrium, even if the level of water in the top tank is below the overflow level. 

I was thinking that I could use the same kind of syphon overflow between each tank, from the top to the bottom. However, because the syphons are going to keep tanks in equilibrium after they have filled up, they are going to try to get to the same level, and all the water will syphon itself out. 

So a syphon overflow may work for two tanks that are on more or less the same level, but will not work so well for the whole array. 

Maybe what I need instead is a leaky system. Rather than trying to make leak-free taps coming out of the tanks, perhaps I should be letting water gradually leak down the cascade.

Or perhaps I need to think about leaking air. The syphon won't carry on trying to keep the two tanks level if air is leaking into it. If I can get a little air hole at the top of the syphon tube, when the water goes above that level, the syphon tube will fill with water and start syphoning out. It will carry on syphoning out while there is still water at the top of the syphon tube, but as more and more air gets in, the flow will decrease and then stop. The bigger the air hole, the faster air will leak in and the less the water will syphon out. This could limit the amount of water going to the tank below each time.

Of course another priority is a low-maintenance, self-cleaning system, and the chances are that if the system relied on leaks to work properly, then rather than leaks getting worse as they do when you don't want them, they would fill themselves in. 

Recycle or reduce

In keeping with the ecological theme of this blog, I've decided to start recycling posts. Not the wooden kind.

To help identify these recycled posts, I've added a new tag "old" by which to label them. This seems to me much better than labelling any posts that are new "new". If I do that, they are not going to be new for long, so I'd have to change the label later. Posts that are old when they are posted will always be old.

Probably the recycled posts will have some new comments added. In fact I've been putting this off for a while because I know that by the time I've read through the post, checked the spelling and grammar for the inevitable typos, revised some of my more naive and abrasive comments, mended broken links to any photos in the post, or added images if there were none, it would have taken less time to write a completely new post. So maybe I should be labelling them something like "20% extra new".

Or perhaps rather than recycling posts, I should just be reducing them, and keeping true to the ecological mantra.

Built to last or built to lose

There have been articles about this on treehugger.com and in the Guardian recently, so here is a re-post from August 2011, embellished by a graph from a paper they cite.

It is sometimes hard for me to come to terms with the disposable nature of house building in Japan. Apparently the average life time of a house in Japan is 17 years. In the UK, it would take 1700 years to replace the entire building stock. Although those two numbers are not equivalent, it gives some idea of the difference. I come from a country where houses are built to last. I grew up in a house that was a couple of hundred years old, which was not particularly unusual. The house we rent here now is about a hundred years old, and it's a constant surprise that it is still here.

(This shows the value of buildings dropping to zero after 15 years, from this paper by Richard Koo and Masaya Sasaki.)

It is easy to write this off as bad workmanship or see it in terms of a nation that loves new things and is obsessed with the disposal of the old. It has been suggested that Japan must have a large construction industry as there are periodic needs for mass rebuilding after natural disasters. It seems that the construction industry is a powerful lobby and they can veto any suggestions to improve building standards. There is also, no doubt, something left over from the post-war rebuilding of Japan where fast, cheap building was the only option. I think there is no simple reason.

But there is a vicious circle, as I found when I was asking the bank about loans. As far as they are concerned, and as far as the taxman is concerned too, a house is worth nothing after twenty-five years. The biggest drop in value is the moment you move in. In most cases, the house is worth less than you paid for it as soon as you turn the key and walk over the threshold.

The people at the bank weren't particularly interested in the building specs when they were valuing the property, instead they look at the houses in the neighbourhood and take an average per floor area. In fact as far as collateral, they don't really take the house into consideration but just look at the value of the land. So unless you're building with cash, and have lots of it, you're at the mercy of a bank that is going to encourage you to reduce the spec. There is little incentive to build something that will last more than 25 years, although one glimmer of hope is a recent standard for a hundred-year house that can open the door to lower mortgage rates.

Houses in the UK, and probably the rest of Europe, the US and Australia, steadily increase in value. From when they are built, they start to get more valuable. After a while, when they hit an unfashionable or unserviceable age they stop getting more valuable, but even then they will hold their value. A little later they start to go up again. There are certainly stories of people with negative equity and people who lose out, but that's usually short term and a combination of local conditions and some measure of extra bad luck for the house owners, forcing them to buy and sell at the wrong times.

As we were looking around Matsumoto for houses and land, we often saw old houses for sale that were very reasonable. If they weren't sold after a year or two, they were knocked down, and the price of the land, without a house, would go up. There is a common wisdom here that renovating old houses is more expensive than building new ones, and I think it may be true if you're comparing a low-cost new-build with restoring a ruin to its ancient form. I think it's more likely to be propaganda by the building trade, a symptom of few people or businesses that renovate, and the prevailing trend of not looking after houses, but letting them wear out until they are knocked down, which is all part of the vicious cycle.

Having said that, if I look at the house we are in now and if we were to bring it up to a comfortable level to live in, we'd have to replace the roof, replace the windows that make up the north and south walls, and pull up the floors and do some work on what's underneath. By the time we'd taken all the bits off that need changing, we'd be left with a wooden frame, and that probably would have to be made earthquake proof as their are no diagonal supports and the whole thing is a mechanism. Also, I'd want to raise all the horizontal beams so the doorways are at least twenty or thirty centimetres above my head rather than two or three centimetres below, just where there is a permanent bruise on my forehead.

One way of looking at this difference is in terms of agriculture. The UK traditionally has pastoral farming, so buildings have been essential to provide shelter for people and animals, so that animals can feed off the surrounding land. Buildings have intrinsic value in this sense. Japanese agriculture is arable, so that land itself is valuable for intensive planting of crops. Any building is going to reduce this value by stopping the production of crops.

Another consequence is in the notion of "home". For the British, a home is a solid thing. An Englishman's home is his castle. For Japanese people, any building seems arbitrary and the sense of belonging is to a community of people.

So while I see what I am doing as an investment, and put myself on a mission to make a small change to the way houses are built and treated here, I'm probably just pouring cash into a hole in the ground, and the main interest of most of the people involved is to catch some of that cash as it falls. I'm sure the house could be built to a similar specification for less cost, and hopefully everyone involved will learn something on the way, so if another idiot comes along asking for a house that doesn't consume, it'll be easier for everyone concerned.

Original post: Built to last or built to lose
Thank you PJ for sending the treehugger article.

Rain water tanks - buy or build

An early decision as I plant the seeds that will eventually lead to a rain water harvest is whether to buy or build. Either way needs careful consideration of design features of the system.

Panasonic do a few rainwater tanks. They have a 150 litre tank for 55,000 yen, and a 200 litre tank for 70,000 yen. They also have a 340 litre wall tank for 65,000 yen. It actually looks like a wall, although I don't think any walls actually look like this. You'd have to get your house sidings redone to match it! Not sure if this was a translation error.

They are not the most expensive, but certainly not the cheapest. Maruichi have a 140 litre tank for 29,000 yen

There's a whole range of tanks here from Direct Tank, with and without lids. They look very simple, with a basic design, no frills, no included connecting parts. And expensive! For example a 300 litre tank with a lid for 52,000 yen.

Then there is Takiron, with a 120 litre tank for 29,700 yen. They also have a 150 litre wall type for 41,000 yen. You can pay more for something that is less attractive!

The best looking tanks may be either the German Groban Slimline 300 litre at 44,000 yen. Except that it's beige. Only 99 euros on ebay, so somebody is making a profit. It's got the ridges around the edge, which I'm not completely convinced about aesthetically, but I guess they have a pretty good functional rationale. 

Or the Aquatower, 150-200 litres depending where you put the tap for 39,700 yen. Or 100 litres depending which website you look at. They have a very clever looking drainpipe fitting which will stop dirty drizzle from getting into the tank, and also act as an overflow if it is aligned with the top of the tank. 

The best I can get is something like 150 yen per litre of storage. This is very close to the price I pay for each cubic metre. A cubic metre is 1,000 litres, so I would need to fill and empty the tank 1,000 times for it to pay off in purely financial terms. If I tried really hard I could probably fill and empty the tank 20 times in a year, so it would take 50 years to cover the initial cost

Of course everything is not purely financial. If I'd been interested in making money, I would probably have invested in shares of the oil industry or a nuclear power station rather than putting solar panels on my roof. Using the water off the roof will not only have benefits in reducing the demand for local water, and providing my garden with cleaner water than the tap does, it will provide a store of relatively clean water in the case of some natural disaster that destroys the municipal water supply. At least it would if such a disaster did not destroy my water system too. Anyway, I can probably build a system for much less than this cost, provided I spend several hours thinking about it, and not putting any financial value to that time.