What if we told you that implementing a single methodology could go a long way toward combating climate change, slow the spread of COVID-19, and lead to longer, healthier lives? We discuss the Passive House methodology for building sustainable, healthy homes, schools, and offices with Miwa Mori of Passive House Japan, and how anyone can get started.

Editor’s note: This interview has been edited and condensed.

Tokyo Cheapo: Could you tell us about who you are, what you do, and your connection with Passive House?

Miwa Mori: I’m an architect. My educational background is in architecture, in Japan and then later in Germany, so I have two university degrees. I came across the principles of Passive House about 20 years ago. That was when I became interested in the method. I say “method” because when people do the research on Google, they say it’s a strict certification for the energy efficiency of the building, which is true, but from my point of view, Passive House is a method to make your building better. So 20 years ago, I was very impressed with the concept, and since then I started learning about the methodology.



After graduating university in Stuttgart in 2002, I spent the first five years in Germany working in an architectural practice. I spent another five years in Dublin working for another architecture and design firm. My profession has always been designing buildings. At the same time, I was able to develop a deeper understanding of energy-efficient buildings.

After 10 years in Europe, I was looking for a chance to come back to Japan. My last client in Dublin was a client who lived in the USA in a very hot and humid climate,m and I became quite sure that this [Passive House] methodology could also be applied to that kind of climate—not just for a central European climate. I was looking for the chance to come back to Japan and got my first project there in 2009. Since then, my activities have been based in Japan. I established my own architecture practice and at the same time I established the Passive House Japan non-profit organization.

Architect Miwa Mori from Passive House Japan | Photo by Passive House Japan

TC: Could you give us a quick summary of what a Passive House is?

MM: It’s a building standard set by a private organization: the Passive House Institute in Germany. Therefore, it’s a third-party certification process, a quality guarantee of the energy performance of your building. But for me, it’s a methodology for all designers who are interested in making something better than what’s being produced today. What’s being producing today is one level, and Passive House is at another level. Sometimes the gap is so big that people say, “Oh, it’s too much!” They exaggerate and say that Passive Houses costs so much more and that there’s no demand in the market, but if you want to make things a little bit better, you can learn about the Passive House methodology and improve some of your building design.

TC: So it’s not just for people who want to buy a brand new architecturally designed house?

MM: There is also a certification scheme for the refurbishment of existing buildings. It is not only for new buildings. However, the great contribution of the Passive House Institute is that they have invented a spreadsheet tool, which you can buy for less than 200 Euros. With this, any architect on the planet can design buildings with better energy performance. For me, this is the Passive House movement.

Of course, there is a certification process. If you have met the criteria, you can get that certification, your house will be listed in the database, and you can be sure that your energy bill will stay below the expected value from the calculations. It’s a guarantee for the end users and a methodology for the designers to improve the performance with the maximum balance of cost to performance.

TC: Can you tell us about the goals of Passive House Japan?

MM: The idea of Passive House Japan is to spread the ideas and principles of Passive House, to inform the experts and consumers that you can make a better building by using this methodology. That means saving energy while at the same time achieving a high level of living comfort. This is the activity of Passive House Japan, while my architecture firm deals with clients. We are designing one-off buildings for each client using the Passive House methodology.

Passive House Japan now has 250 supporting companies. Most of them are builders, about one-third are architecture and design firms, and about one-fifth are building materials suppliers—supplying things such as insulation, windows, and ventilation systems. After 10 years of activity in Japan, I think we have managed to produce some better-quality building components. For example, we started consulting with window manufacturers to bring a better product to the market.



My goal is to achieve that high standard in the most cost-efficient and affordable way. For that, I had to start by motivating the product suppliers. Now you can buy Japan-made triple-glazed windows, which we didn’t have 10 years ago. So the price of components is going down and it’s getting easier [to build energy efficient houses]. The cost difference between a standard Japanese building and Passive House is getting smaller and smaller. So Passive House building is about 15% [more in comparision], although it should be less than 10% so that this additional cost could be paid back within 25–30 years.

But it’s not just an economic issue, the comfort level of the building is priceless! It’s good for your mental health, it’s good for your family, it’s good for how to spend your time—it has more value than just an economic cost.

Wall assembly for Passive House Kamakura | Photo by Passive House Japan

TC: When did the concept first come to Japan, and how many Passive Houses are there in Japan now?

MM: The first project in Japan was certified when I came back to Japan in 2009. Presently, 40 projects have been certified. So it’s quite slow progress. But as I said, I had to start with consulting with the component suppliers.

TC: Is it growing rapidly? Will there be more Passive Houses coming this year and next year?

MM: Yes, definitely! This year, I have already certified 10 or so. I became an accredited certifier in 2019. There are three certifiers in the Asian market. One here [in Japan], one in China, and one in Korea; so there are three institutes doing the certification for the Asian region, and I am allowed to certify applications from all over the world.

TC: Are there many apartments in Japan that meet the Passive House standard?

M.M.: It’s coming! Maybe 38 out of the 40 Passive Houses in Japan are free-standing, detached, two-storey houses, but we finished an apartment block refurbishment in 2017 that was the first certified apartment block. That project has been visited by about 8,000 people! Apartment units are coming, because Passive House methodology is applicable for any type of building.

TC: What are the current insulation and airtightness standards for newly constructed houses and apartments in Japan?

MM: There is no minimum standard! There is no obligation of doing blower door tests, so you don’t know how airtight your new-build home will be. But it’s said that the air leakage of a standard new-build house is at least five to 10 times the Passive House standard. [Passive House standard is ACH50 0.6, so that would be ACH50 3 to 6], so airtightness is not there! For insulation, you could build houses with 5 cm of insulation in the wall, and you could go for double-pane windows with aluminum frames and you could get maybe 20% to 30% of the thermal performance of a Passive House.

Passive House Kurashiki | Photo by KURASHIKI MOKUZAI

TC: So the average new house has about 20% to 30% of the insulation of a Passive House?

MM: Yes. We would have at least 20 cm to 30 cm of insulation in the wall, depending on the region. In Tohoku, maybe you would need more than 20 cm—25 or 30 cm depending on the type of insulation material. New-build homes by large prefab housing companies have glass wool insulation of maybe 8 cm to 10 cm, and windows are poorly insulated.

TC: Are these standards likely to change in the future?

MM: It’s now being discussed by a group called the Renewable Energy Task Force set up by the government. One of our directors is fighting to make a minimum building code standard there. But a minimum standard won’t be set. Instead, they plan to increase the “encouragement standard” with subsidies as always. They are keen on giving subsidies for zero-energy buildings, which is called ZEH for Zero Energy House. So if you put on lots of solar panels and install at least 8 cm of insulation in the wall, you get subsidies. Now the necessity of a minimum standard is being discussed, but there are so many researchers advising the Ministry of Construction who are actually against the improvement of the standard.

TC: Japan is under a lot of pressure to reduce carbon emissions, and the Japanese government has said they want a 66% reduction in carbon emissions from households. Do you think this is possible with current regulations?

MM: No. We need a really big mind shift to make that happen. It could work if there wasn’t a minimum standard, but if 70% of new build houses were Zero Energy Homes. We have calculated what this means and found that the estimate [they were using] for new-build homes was incorrect. The economy is shrinking and Japan is not able to maintain the volume of new projects—we have to set a much stricter minimum requirement to meet the reduction target.

TC: If all new houses were built to Passive House standards, what would be the implications for Japan?

MM: If the Passive House standard were to be implemented, adding just a small amount of photovoltaic power generation, we could go a long way toward meeting the government’s stated goal of reducing CO2 emissions. It’s been recognized in most countries that reducing energy consumption is the first step before you consider new energy generation.

TC: What is the level of knowledge about Passive House among architects, construction companies and contractors in Japan?

MM: For the housing market, it’s becoming better known what “passive design” means. We’ve completed and certified 40 projects, meanwhile, the same people are building a large number of so-called “nearly Passive House” standard buildings. Maybe they have only 80% of the performance of a Passive House, so we can’t call them Passive Houses. The happiness and the positive feedback from the clients (living in these houses) have been such a strong influence that architects and builders who initially are quite skeptical about this kind of idea accept this quite quickly.

I think we have been able to avoid some of the negative messaging around Passive Houses, such as it being “horrible in summer because it’s not good for our hot, humid climate” and “we need airy, drafty buildings for summer.” It’s always the summer issue or the cost issue, but I think we’ve managed to eliminate these very simple arguments.

The next issue is non-residential projects: public buildings, office buildings, schools, and nursing homes, etc. This is our next target to see how we can change the market.

Passive House Kamakura | Photo by Passive House Japan

TC: Since houses in Japan are so leaky, how is the air quality in most dwellings? Is this a health risk for occupants?

MM: Oh, yes. There are two things.

One is airy, drafty construction, which makes it difficult for mechanical ventilation to work properly. For example, if you vent air from the bathroom, fresh air is supposed to come into the living space, but it will only work if the building is airtight. You have to create negative pressure so you expel air from one side and the fresh air comes in from the other side. This doesn’t work in a Japanese house. Because of the coronavirus, I have to state very clearly that in a standard house, mechanical ventilation doesn’t work. You don’t get the minimum number of required air changes unless you have your window open.

Secondly, in a standard house, the health risk for occupants is caused by poor insulation levels in the walls, which causes mould and condensation. Occupants can get asthma or sick building syndrome. It’s when your window has condensation in winter and it goes into the timber construction. Eighty percent of allergies are caused by mould, and the build-up of moisture in the wall can make people sick. Things like Volatile Organic Compounds (VOCs) such as formaldehyde from furniture and fittings are not the main factor in allergies; the majority stems from mould.

TC: COVID-19 has raised the awareness of good indoor ventilation. Has this had much impact on new home builds and retrofits? Are more builders and architects utilizing mechanical ventilation?

MM: There is a mindset that we have to keep the window open to get the maximum number of air changes, but no one has defined the minimum air change rate needed for when one of your family members has COVID-19. It depends on the season. If you can keep the window open as much as possible without using the air conditioner or heating, that’s good. But if you have a cooling period and a heating period, then you need to maintain an acceptable temperature to keep your immune system as strong as possible, so you have to use mechanical ventilation. Mechanical ventilation can work only if the building is airtight.

The image of a Passive House or an airtight building is that you can’t breathe or that you can’t get any fresh air. It’s complete nonsense. Of course you can leave the window open, and there is always mechanical ventilation required by law. I think Passive House is the best solution for the COVID-19 pandemic situation because you don’t need any air circulating system to cool down or heat up the house. If you have an air conditioner, it blows into the space and recirculates the air, which increases the risk of spreading viruses. In a standard house, you have a large volume of air recirculating in your house and a very inefficient ventilation system, which cannot create negative pressure because the building envelope is not airtight.

The same situation happens in housing and also in offices, shops, restaurants, karaoke, izakaya and everywhere. So many COVID-19 clusters are from locations where many people are inside a building that doesn’t have sufficient ventilation.

TC: R-values are commonly used to measure the insulation level of materials and wall assemblies. Does Japan use the metric RSI value or the American R-value?

MM: We use the U-value.

TC: That’s 1 divided by the R-value, right?

MM: Exactly. So the lower the U-value, the higher the insulation performance.

TC: So it’s the metric system?

MM: Yes.

TC: A U-value of 0.2 would be something like an R-value of 50 in the US?

M.M.: I am not familiar with R-50 actually. If you use a converter, you should be able to work it out. (Note: Metric RSI is W/m2K, while imperial R is usually degree Fahrenheit square-foot hour per British Thermal Unit.)

You can also use a Lambda value which is Watts per meter Kelvin, which is a value that is free from the thickness of the material. It converts the material into a one-meter thickness.

TC: What are the typical U-values for floors, walls and ceilings in Japan if you were building a Passive House?

MM: Somewhere around U0.2W/m2K or so.

TC: Would that be for each element, or would you generally put more in the roof?

MM: Yeah, the roof would be U0.15W/m2K. It’s obviously less than in Germany because we have a warmer climate, so we need maybe 70% of the insulation thickness compared to a German Passive House.

TC: In places like the US, they have specific climate zones that help architects and builders make decisions about insulation, air barriers, vapor barriers and HVAC systems. Is there a similar climate classification system for Japan?

MM: Yes, we have eight climate zones, and each zone has an average U-value for the building envelope defined by the ministry. But it isn’t a minimum standard, it’s only for the Zero Energy Home. If you go for a ZEH, you have to fulfill that level, but it’s U0.4, and for Tokyo only U0.48 or so. For a Passive House, it’s U0.25, so the (ZEH) standard is not high enough.

TC: Do you use the climate zones as a guide when planning a Passive House?

MM: No. We have more detailed climate data sets. We have 840 data points from the Japan Meteorological Agency and have converted that data for use with the Passive House tool for more precise data to start with.

TC: With the COVID-19 pandemic, a lot of people are shifting out of the city and buying older houses in the countryside. Are there any examples of Passive House retrofits for older houses? How about for traditional homes like kominka or machiya?

MM: That is the most difficult version of refurbishment because you have to keep the appearance of the building from the outside. That means you have to refurbish from the inside. Unfortunately, we don’t have an example yet for that very traditional construction. However, there is a large potential for thermal improvement in those traditional buildings as there is almost no insulation at all!

Irori in a kominka
Interior of a traditional Japanese farmhouse | Photo by iStock.com/jpskenn

TC: Would retrofitting a house to a high energy efficiency standard add to the resale value? Do buyers put a premium on energy efficient houses?

MM: At the moment, there is no evaluation system for existing buildings, so you cannot sell the good points of the refurbished house, but the government is preparing a rating system for existing buildings so you can show that your house is properly refurbished with improved energy efficiency. At the moment you cannot prove that you should have a higher price if you have done something like an EnerPHit refurbishment.

TC: Japan is a very seismically active country with regular earthquakes. What are the structural implications because of all the extra weight of the insulation? How about airtightness being affected by earthquakes?

MM: There are a few ways to achieve air tightness. We go for a long-lasting method instead of urethane foam, which doesn’t last for a long time. There needs to be flexibility for the structure to move, so we use flexible tape to seal the sheet joints. There are so many products that allow the movement of the building and also make it airtight. The extra weight for the insulation is minimal—it’s much lighter than having a brick wall or a brick facade. As long as you have light material for the facade finish, the insulation behind doesn’t add much.

TC: In places like North America and Europe, there is a big ecosystem around Passive House construction. For example, there are integrated external insulation systems, lots of eco-window options, insulated doors, blower door testers, open and closed foam sprayers, etc. How does this compare to Japan? Are there as many choices?

MM: Unfortunately, the material selection is rather small in Japan. The majority of Japanese people are very keen on reducing the embodied energy (the energy required to produce a material) rather than reducing the energy consumed for heating and cooling—that’s why they hate insulation materials! They don’t want to have plastic in the wall construction, so they’re really willing to use blown cellulose fibre insulation or blown wood fiber. They love to have timber-framed windows if they can afford them.

There’s kind of an allergic reaction to petroleum-based materials. So people reduce the thickness of the insulation in order to choose a more ecologically friendly material. Urethane foam is much cheaper than sheep’s wool insulation for example, but Japanese people are likely to go for the sheep’s wool insulation and they just put half the thickness that they need. It’s not a bad thing, but I just have to emphasize the importance of the thickness of the insulation! I don’t need to convince Japanese people to go for the ecologically friendly material.

TC: Which brings me to the second part of my question: Is there a wide choice of materials available?

MM: The choice of materials isn’t wide, but we have a few choices.

TC: Is this growing?

MM: Yeah, I think so, but the choices are not more than about five manufacturers.

TC: Does that mean that people end up importing materials?

MM: Foreigners living in Japan have no fear of importing construction materials from abroad, but it’s not something that Japanese people do. But we have importing companies, so we can get plenty of high-performance materials from North America or Europe.

TC: A lot of our readers are renting or don’t have a huge budget for a new build or a full retrofit. What are some steps that people can take to improve the energy efficiency of their houses or apartments? For example, would shade sails over exposed windows help?

MM: It depends on the orientation of the windows, so the best thing is to analyze what the weakness is of the building envelope. For that, there’s an app and infrared camera for smartphones. The cheapest one is about 40,000 yen. With these gadgets you can find the weakest point in the house and you can make some improvements in winter and summer.

And before renting a house, you should check this with the infrared camera before you sign the lease. That way, you can find the best property. For office space, you need to check that before you rent; otherwise you’ll end up paying a lot in energy bills.

Controlling the heat radiation temperature is the key to Passive House design—shading for the windows, improving insulation to eliminate the weakest point. It’s not just for the surface temperature issue, but also for condensation issues, so eliminating weak points reduces the risk of mould and condensation. It not only saves energy, but it also improves the comfort level. It’s worth investing in energy generation such as solar panels, but it won’t make your life more comfortable!

So that’s how to decide where to invest your limited budget for the improvement of your life.

TC: Where should readers go to find out more about Passive House?

MM: There is plenty of information available in English! If they just google “Passive House example” or “Passive House Japan” they’ll find something about us. Also, there is Passipedia to explain the basics. There’s so much information on the internet and on YouTube.

TC: Is there a database of architects, construction companies and contractors that are qualified in aspects of Passive House in Japan?

MM: We have a supporting member’s list on our website. That doesn’t mean that they all have experience in real projects, some of them are still in the learning process. We have a mark that indicates companies with a built example. So you can find out who is experienced and who isn’t. We don’t make any recommendations directly to consumers, so people who visit our website should search and contact each company directly.

TC: I think a lot of people may be intimidated to talk with an architect before they have a firm plan. Are there any forums or perhaps consultants that people can go to for advice?

MM: I’m an architect myself and we usually do free consultations. It’s standard, unless you go to a world-famous architect like Kengo Kuma! Usually you will be invited to a free talking session, so don’t hesitate to visit an architect! For a Passive House, you need a designer. You need good builders too, but you need an architect who knows about Passive House from the very beginning of your house project.

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