Friday, 15 September 2017

Too Much Humidity

When we built the house I refused to add an air conditioner for two reasons. First because I didn't think we needed to spend money on cooling when the house was not going to be so hot, and secondly because I'm from Yorkshire where we don't use air conditioners. Actually that's probably just one reason.

It may be global warming, acceptance of reality or weakness to luxury, but I think we need take active measures to remain comfortable in the peak summer heat. I need to take a closer look at passive house and high-temperature high-humidity in a different post.

The temperature is not a huge problem. It rarely goes over 28 degrees, and when it's 35 degrees outside, 28 degrees is a relatively pleasant temperature. The problem is when it is humid, and when it gets over 70% humidity it starts to feel really hot.

A de-humidfier would make the house more comfortable without making it cooler. In terms of thermal efficiency, de-humidification is a good idea since heat gain depends on temperature difference, so taking moisture out of the air makes it feel cooler without encouraging more heat to come in. On the other hand, making the house cooler means a bigger temperature difference, and more heat leaking in from outside.

Actually, we do have an air conditioner in one room, and that air conditioner does have a dehumidifier. But the de-humidify function just seems to work by cooling the air, and that room was not designed for the air to circulate through the rest of the house, so it just gets very cold in there when the dehumidifier is on.

Most dehumidifiers work by running air over a cooling element so that humidity condenses out of it. They differ depending on what happens to the heat that was taken away to cool the air. Either the heat can be put back into the air, or it can be taken out of the building. Our air conditioner does the latter, sending out cold, dry air. If you have a dehumidifer for a basement that gets damp in the winter, you want the former.

So do we want a dehumidifer that transfers the heat out of the house, or one that keeps it inside?

Should we try to dehumidify the air as it comes in through the ventilation system or should we get a standalone dehumdifier?

Would it just be cheaper and easier to get an air conditioner that can de-humidify?

Even if it was more expensive, would we be better off getting an air conditioner that can also cool and heat and do other fancy stuff? Maybe we could even get one that humidifies as well, since we need more moisture in the air in the winter.

Can I fit another air conditioner unit to the compressor that spends over 360 days of the year idle on my roof?

Or will it be cheaper to get another air conditioner with its own compressor?

How much moisture are we talking about?

The last question is easy.

If it's hot and humid outside, the ventilation system is going to be adding saturated air to the house. If it's 28 degrees, 60% humidity inside, with the ventilation system working at 150 cubic metres per hour, that is going to add 1.6 litres per hour. This is how much the dehumidifier needs to remove at peak load.

A closer look at some actual data for temperature and humidity here in July and August shows that the outside air was never actually hot and humid enough to come in saturated. But with a more comfortable 50% humidity at 28 degrees, the peak dehumidification load is 24 litres per day.


One very simple solution would be to switch off the ventilation system, or at least turn down the flow. This is a short term measure, because we do need fresh air in the house, but at night time and in the morning we open the windows and get plenty of fresh air in anyway. In fact the main demand for ventilation is to remove the moisture that we produce when we breath, wash and cook. If there are just a couple of people and a cat in the house, then we should be OK for a few hours. Turning down the ventilation would also be a good solution on cold winter nights when there is a risk of freezing in the drain from the ventilator.

References

​Assume on a hot day ​the air coming in is humid and hotter than the inside air, so humidity will rise to saturation as it passes through the heat exchanger in the ventilation. (Actually this is a pessimistic assumption.)
28 degree air at 100% humidity holds 27 grammes water per cubic metre.
Assume 60% humidity inside. That means an extra 11 g/m3.​
Air flow of 150 cubic metres per hour.
That's 1.6 kg of water per hour to get rid of.

Humans breathing out humid air:
In one hour we breathe in about 450 litres of air.
Assuming exhaled air is 100% humid at 36 degrees C; inhaled air is 60% at 28 degrees C.
1 cubic metre of exhaled air holds 42g of water vapour.
1 cubic metre of inhaled air holds 16g of water vapour.
We each contribute about 12 grammes of water per hour. Is that all?

Friday, 8 September 2017

What is Passive House? Probably not what you think

Here's my short answer:

Passive House is an excel spreadsheet.

There are loads of other definitions and mis-definitions out there. The term is frequently used loosely for any superinsulated building, and often mistakenly for passive solar buildings.

Passive House is not a way of using natural energy. It's true that Passive Houses will take natural energy into consideration, for example considering heat from the sun, but just pointing big windows south will not make a Passive House.

A Passive House is not a building without a heating system. Passive houses invariably have heating systems, but the amount of heating needed is very small. In fact the best definition of a Passive House is one where all heating and cooling needs can be met by heating or cooling the air coming in through the ventilation system.

There are many other things that Passive House is not, and in his excellent blog, Elrond Burrell gives a longer list.

My definition may put you off. I think excel spreadsheets put a lot of people off, including many architects. This is one barrier to the standard's popularity. If Passive House was a simple product you could buy to stick on your house, then lots of people would no doubt buy it. If it was a simple step you could add to the design process, designers would probably take it.

Passive house will help you to reduce your energy bill, and probably help reduce your environmental footprint. But if you want to save the planet, you need to do some sums. Laying a bit of turf on top will not make it green.

And now that we are firmly in the computer age, we can get a spreadsheet to do the sums for us. As with all good spreadsheets, you put various bits of information into the Passive House software, and you get out a simple and accurate picture of what is happening. In this case all the information going in relates to the building size, shape, location, materials and systems.

It is not difficult to find most of the information that you need to put in. But you do need to find it. You need to know the dimensions of the walls and the thicknesses and relative proportions of the various materials going into them.  The spreadsheet needs to know the insulation performance of each material, but most of them are already in there. You can also choose the hot water and ventilation systems, and their efficiencies. You need to know the size of the windows, and also their U values, and the psi values for the thermal bridges. The supplier of the windows should be able to supply these, and if not they may not be the right windows for a low-energy house. You need to know which direction each wall is pointing in. You need to know your local climate, or at least choose your location so that it finds your local climate.

The only piece of information you need to get up from your desk to find is the results of an airtightness test. If you're building an airtight house, then you probably should run an airtightness test anyway, and if you're building a house that is not airtight, start thinking about it.

W​hen​ all the information is in there, you know whether you have met the standard or not. More specifically it will tell you how much energy you need for heating over the year, how much total energy you need, and how often the house will go over 25 degrees centigrade. Even if you are not interested in meeting the standard, the software will give you a very accurate estimate of how much energy you are going to need to run your house.

Thursday, 7 September 2017

Electricity demand in southern Europe to soar with air con

After the hurricane in Texas, there has been a lot of news about how the weather will affect energy use. Of course the big story is how energy use is already affecting weather! I'm sure I heard people twenty years ago warning about global warming making storms bigger and more frequent. 

Another angle is news from the Guardian here about the increase in electricity demand in southern Europe for air conditioning due to increased temperatures. The UK will probably also need more cooling, but will need less heating, so in terms of energy may break even. Obviously the increase in temperature depends partly on whether we do anything about carbon emissions, and of course there will be some feedback if Europe does not de-carbonise the electricity supply.

The article does mention increasing insulation as a way to maintain comfortable temperatures, which is good.

The picture accompanying this shows an array of air conditioners from four different manufacturers, all Japanese.

Here is a report on global demand from the Japan Refrigeration and Air Conditioning Industry Association which shows that demand for air conditioners is already increasing around the world. They estimate 2016 global demand to be around 100 million units, growing 2.9% from the previous year.

In terms of market size, China is the biggest with 40% share, followed by Rest of Asia, North America, Japan, Latin American and then Europe with 6 million unit sales. In terms of market growth there is a very different picture, with Europe growing at over 12%, followed closely by Latin America, then Rest of Asia growing at over 8%. The more mature air conditioner markets of North America and Japan show the lowest growth rates of 1.8% and 2.8% respectively.

Since they can work as heaters as well as coolers, and since they run off electricity which is the medium of choice for renewable energy, split-unit heat-pump-based air conditioners may increasingly become the unit of choice for domestic heating and cooling needs. I may even get one myself.

Friday, 1 September 2017

These solar panels... are they going to last?

Ugo Bardi writes about the energy return on photovoltaics. Citing an article from Bhandari et al. that looked at 231 studies on ​how much energy comes out of photovoltaics​, and how much energy went into producing them, he comes up with an average return of 11-12 for southern Europe. ​This sounds worthwhile.

(From Dale and Benson)
​This graph paints a slightly different picture. It plots the number of years it takes for panels to generate the energy it took to make them against the growth rate of solar production. The payback got at least three times better in ten years, and the growth also increased three times. This means that, so far, more energy has gone into making solar panels than has come out of them. Hopefully, the growth will stop at some point, and the line will swing into the green as panel production stops growing while the installed panels keep generating. That depends on economics. 

Older estimates were that panels would still generate 80% rated power after 20 years, but according to Engineering. com, panels produced after 2000 will still be producing over 90%, losing only half a percent per year. So technically the panels will still be generating.

Economics is about resources. Somewhere human time is factored into ​it. We consider this resource very precious. ​I remember large scale road building projects in the UK that would decimate forest, destroy habitat and create pollution ​just to take a couple of minutes off people's car journeys. There is an economic pressure to reduce the amount of human time needed for tasks.

Another view is that human time is infinite, and the natural resources are limited. The classical economic view looks at productivity and considers environment assets to be externalities and essentially deems them infinite.  

​Hopefully growth of solar panels will go down, and they will become net energy contributors, but there is a powerful economic mechanism supporting production. If growth increases and we start throwing away the old panels, then that line may stay permanently in the wrong part of the graph, and photovoltaics will have just helped in our longer mission of depleting the world's resources.

The only redeeming feature is that they work very well in space, so we can take them with us when leave the planet! 

​References:​
Bhandari, K. P.,  Collier, J. M., Ellingson, R. J. and Apul, D. S. (2015). Energy Payback Time (EPBT) and Energy Return on Energy Invested (EROI) of Solar Photovoltaic Systems: A Systematic Review and Meta-Analysis. Renewable and Sustainable Energy Reviews​,​ 47(July): 133–41. doi:10.1016/j.rser.2015.02.057.

Friday, 25 August 2017

How to build a house in Japan Part two: Two-by or zairai?


One choice that you probably won't be given by any architect or builder in Japan is whether to build in zairai koho or two-by-four. If you're building a wooden house it's a choice that is made early in the project.

Zairai koho is the traditional wooden building technique in Japan. It consists of a framework of pillars and beams,​ fit together with joints carefully designed to avoid excess stresses, and​ originally held together without any nails or screws, hence the nickname nail-less construction. The standard section size of the pillars is 120 by 120 millimetres, and the beams are multiples of this.

Two-by-four refers to a building technique using beams of two-by-four inches. Rigid panels such as plywood or OSB are added so the structural strength is based on the walls, where zairai traditionally relies on the pillars and beams for the structure. Just to add a little confusion, 2 x 4 beams measure half an inch less by the time they've been cut and dried, measuring 38 x 89 mm rather than around 50 x 100 mm that you might expect.

The two-by-four construction technique developed from balloon framing in the 1830s in the US. It allowed standard sizes of timber to be put together with mass-produced nails by relatively unskilled wood workers.

Compared to zairai, two-by-four constructions is probably cheaper, stronger, easier to build, and easier to insulate. Since the beams are rectangular rather square in ​section, the building more efficiently derives structural strength from the wood. Insulation can be added between pillars and studs, and the wood make​s​ up a smaller proportion of wall, so the insulation performance will be better than a building with square pillars. If the same technique uses 2 by 6, 2 by 8, 2 by 10 or​ even​ 2 by 12 beams, a suitable thickness of insulation can​ easily​ be added between the beams.

It's difficult to find tangible advantages to zairai construction, but I will try.

Zairai construction is traditional.

That ​is probably enough to illicit approving nods from fans of tradition, and disparaging scowls from anyone who has been paying attention since the Age of Reason.

Zairai construction is based around standard sizes and scales that suit the human body. Measurements are in the traditional units of shaku and sun. One shaku is within a hair of an imperial foot, and was standardised in 1891 to 10/33 of a metre. Traditional Japanese units are decimal, so a sun is one tenth of a shaku. Traditional zairai beams are 6 shaku, or 180 cm, from the floor. That's around my height, and​ after a few years living ​in a traditional Japanese house I was beginning to develop calluses on my forehead and a stoop in my back. The average height in Japan increased 8 cm in the second half of the twentieth century, so I suspect for most of the history of Japanese architecture, the lintels were at an appropriate height. ​In modern houses they are higher. Of course there is nothing to stop you from using human-scaled dimensions in a two-by-four construction. Also, you may have noticed that the shaku is remarkably close to the imperial foot, and the standard lengths of two-by-four (inch) beams are all in feet.

Zairai construction is based on a woodworking tradition at least a thousand years old, which can be seen in the oldest and the largest wooden structures in the world. By building a house in zairai you are helping to keep this tradition alive. But what exactly is being preserved? Why do home builders have to pay more to preserve it? They still make temples and shrines, so couldn't the fantastically wealthy priests preserve their tradition?

The joints of zairai are all supposed to fit together without any nails, except now they do use nails.
bolts​ or other connectors for the joints. And since the traditional pillar-and-beam structures do not meet modern earthquake regulations, to get planning approval for zairai buildings, you need to add structural walls, just like they do in two-by-four construction.

The square beams were traditionally prepared locally from​ round​ trees. Now timber is usually cut in saw mills, often using state-of-the art CNC machinery.

​So is your modern zairai building just a two-by-four construction with more wood in it, and more complicated joints?

I guess you could see an advantage in it being more difficult to build, since that means you have more highly-skilled carpenters. You have to squint a bit to see this, since you are also making the job more difficult, but there are some places where more highly skilled wood workers will make a tangible difference to your house.

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Our house uses zairai koho, and it is on the list of things I would probably have done differently. Luckily ​that​'s a short list! The point when I realised that there had been a different option to this vast array of square-section wooden pillars was at the stage in the process where it was not possible to change the building technique.

You're always at some ​stage in a process.

There had earlier seemed to be a great rush to get the structure all sorted out, coinciding with a busy time in my day job. I was a bit disappointed as I was quite interested in structures, and would have liked to have had some input into it. It seemed like a lower priority than the insulation work and the systems we were considering, so that was a battle I chose not to fight. Qualified architects in Japan, or anywhere else, can be trusted to make structures that will not fall down.

After this urgent decision had been made​ to finalise the structure​, there seemed to be a couple of months when absolutely nothing happened. Ben talks about a similar artificial deadline in his retire Japan Blog and I think this is a common technique in the building trade.

When we were looking at ways to fit at least 250 mm of insulation into walls with 120 mm pillars, I had an idea of using two-by-tens as studs between the ​load-bearing ​pillars, which would have allowed one insulation layer rather than the three we have ended up with. This seemed like a bad idea as it was mixing two different techniques, and would leave a few awkward sized gaps. So I wondered about getting rid of the square pillars ​altogether ​and just using two by tens throughout.

As you will remember from lesson 4, the calculation of the thermal performance depends on how much wood there is in the insulation layer. This information needs to be added into the Passive House software, and I was checking the figure of 18.1% that we had. A more conservative estimation put it more like 25%, so a whole quarter of the wall was made up of wood, much of it by square pillars. This could have been halved by switching to rectangular sections. That would also have meant less wood​ to pay ​for​.

I suggested this to the architect who said it would take a month or two to change the structure, and he'd need to get someone else to calculate the stresses.

So we have a house beautifully built in wood, but ​we can't see any of it it, since it had to be covered to meet fire regulations.