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Ask an Architect: Insulation


From December to February, Tokyo apartments are often colder inside than outside. After braving another winter in sub-igloo comfort, we decided to ask someone in the know where exactly the insulation’s at. The following queries were floated to Néojaponisme’s resident architectural adviser Ashizawa Keiji, who has gracefully explained why Japanese residences do not fare so well in Japan’s seasonal extremes.

Why is there so little insulation in Tokyo homes? And why is central heating not used?

First of all, you can’t say that houses in Japan don’t take heating into consideration. The main actor for temperature control in Japanese living spaces is local heating (partial heating), as symbolized by the kotatsu — the heated table that occupies the cha no ma or living area in so many houses even today.

When I think back to my childhood, I remember that everything in the house outside of the kotatsu during winter was almost as cold as outside. So no one wants to ever leave the kotatsu. We would decide by rock-paper-scissors who had to go get the mikan from the entry hall. The entry hall was as cold as a refrigerator, so we used it to store things like mikan. The corridors and bedrooms (if you weren’t under a futon) were so cold that you could see your breath. So, it was really important to warm yourself up in the bath. And it was really hard to get out of bed in the morning.

A greater awareness of insulation began during to the oil shock of 1973. An idea formed that one could heat the entire home through insulation. When people did the math, they realized that the old way of doing things — only heating up an uninsulated six-tatami room full of cracks and openings — was not energy efficient. Therefore you needed to increase the efficacy of insulation and make more airtight construction. But this then led to “shock houses.” The so-called shock house was a house that caused health problems such as allergies or atopy due to the emission of synthetic chemical materials used in construction. This is why specialized alarms are required in residences constructed today. Every room must have an air vent, and the ventilation fan is left on 24 hours a day. Some claim that this makes the house colder, leading many people to shut off the ventilation during the frigid depths of winter, even if they are aware of the shock house problem.

Is there any desire (either by architects, developers, or dwellers) for more insulation or other uses of heating in modern homes?

High levels of thermal insulation or air sealing are part of many house builders’ sales pitch, and paying attention to insulation is gradually becoming commonplace. I should add, however, that some contractors do remain skeptical…

Although the rules are not set as clearly as in Europe or the United States (due to the regulations of the Government Housing Loan Corporation), builders often publicize and reference the volume of insulation in the roof walls and floor. The idea of localized heating is therefore gradually becoming a thing of the past. Now, even cheap rental apartments have air conditioning units [note: Japanese AC units normally include both heaters and coolers] installed, and it has become a standard custom to heat the entire room.

While there are many different kinds of insulation, the most commonly used one is fiberglass, which is extremely inexpensive. Leaving out insulation is therefore not a particularly clever way to save on construction expenses. Of course, there are builders who ignore insulation in their designs. Houses with large openings and houses with extremely simple construction and thin walls — where the delineation between inside and outside is only marked by spraying concrete — are cold in the winter and hot in the summer.

I once heard a story that, when a certain European country was refurbishing its embassy in Japan, the construction fee was over budget, so the European architect requested a Japanese architect to come up with a cost savings plan. When the European architect saw that one of the suggestions was to “leave out insulation,” he was quite surprised and thought it was a joke! This story happened just a few years ago.

How does the situation in Tokyo compare to other parts of Japan?

Yoshida Kenkō wrote in the Tsurezuregusa that when building a house, one should focus on the summer. This has become fundamental to dwellings in many regions of Japan, meaning that the emphasis is on keeping summer as cool as possible. Only in Hokkaido is there such a thing as the Law on Cold Residences, and the Government Housing Loan Corporation gives financial assistance to homes protected against the cold. They say that people from Hokkaido catch colds when they come to Tokyo, because they traditionally live in houses insulated and warmed through central heating.

Do more expensive homes in Tokyo have central heating/cooling?

There are cases in the past where they put central heating into luxury apartments or homes, but I think it’d be very rare now to see that. They use central heating as a general rule in Hokkaido though.

If my current apartment has insulation, why is it so cold in the winter? Is it just because it was built for summer?

It’s probably that the windows are only single-layer glass, which lets cold air pass in and out. Rental apartments rarely use “pair glass” (glass with insulating properties.) The idea is that you should make the apartment tolerable in the summer, and cost-wise, this is a very convenient strategy for the managers.

Which is warmer in winter: wooden structures (アパート) or concrete structures (マンション)?

That’s a tough question. New apartments are clearly better than old apartments. I have lived in both an old concrete apartment and a old wooden apartment, and both were super cold in winter and fiercely hot in summer. The wooden apartment, however, was nicer at night, because the concrete would store the heat, making you need to use an air conditioning unit.



そもそも日本には、暖房を意識した家はなかったと言われています。 いまだに多くの家庭のリビング、あるいは茶の間を占領するこたつに象徴されるように、局所暖房(部分的な暖房)が日本の住居において主役でした。

僕の幼少時代を思い出してみても、こたつの中以外は、ほとんど外のように寒かった記憶があります。だから、みなこたつから出れなってしまう。玄関に置かれたみかんをとりにいくのをジャンケンできめていました。玄関はまるで冷蔵庫のように寒かったので、みかん程度のものであれば貯蔵庫として使っていました。廊下はもちろん、寝室も布団の中以外は吐く息が白くなる程でした。だから、お風呂で体をしっかり暖めることが重要だったともいえます。 さらに、朝は布団の中からなかなか出れなくなるわけです。

断熱について意識が向かうようになったのは、1973年のオイルショックが引きがねになったといわれています。断熱することによって、家全体を暖めるという発想がでてきたわけですが、いままでの隙間だらけで、断熱されていない6畳の部屋を暖めていたときよりも、結局のところ省エネではなかったという統計がでています。そこでさらに断熱材の性能を上げ、気密を上げることが 必要となり、こんどはシックハウスの原因となってしまいました。シックハウスとは、建材にふくまれた化学物質が家の中に放出されることによってアトピーやアレルギーなどの体の不調をおこしてしまう家のことです。よって現在つくられている住宅は24時間喚起が義務付けられています。すべての部屋に換気口がもうけられ、24時間換気扇をまわしっぱなしにします。これが寒いというクレームがあるのですが、シックハウスの問題を知りながらも真冬の時期は、切ってしまう人もおおいようです。


上の文章でもかきましたが、高断熱、高気密というのは多くのハウスメーカーの売り文句であり、断熱について気をつかうということは、常識になりつつあります。 建築家は懐疑的な人もいますが・・・。

ヨーロッパや、アメリカ合衆国のようにルールが明確にきまっているわけではありませんが、 住宅金融公庫基準という形で、屋根、壁、床の部位における断熱量が公表され、参考にするケースが多いです。そして局所暖房という考え方は過去のものとなりつつあります。いまや、安い賃貸アパートでもエアコンが常備され、部屋全体をあたためるのが常識となりつつあります。

断熱材にはいろいろな種類がありますが、一般的によく使われる断熱材はグラスウールと呼ばれるもので、非常に安価なものです。よって、建築コストを抑えるために断熱材を抜くということはさほど賢い方法ではありません。もちろん建築家の中には、デザインのためにあえて、断熱については無視をしている人もいなくはありません。開口部が大きな家、 非常にシンプルな構造、薄い壁、コンクリート打ち放しだけで内外を仕切っている家は、冬寒くまた夏、暑いです。








どちらが暖かいですか: 木造建築 (アパート) または コンクリート (マンション)?


Related Articles:
As an Architect: Concrete Façades
Interview with Ashizawa Keiji


Translation by

May 7, 2008

Jean Snow lives and breathes design and pop culture in Tokyo — sustained by an unhealthy addiction to magazines and frequent visits to his favorites cafes. His personal website is located at

Roy Berman lives in Kyoto and is one of the three writers at excellent Japan/Asia blog Mutant Frog Travelogue.

Posted in Architecture, Development and Construction, The Present 20 Comments »

20 Responses

  1. Patrick Says:

    > It’s probably that the windows are only single-layer glass

    I think that’s exactly it. As far as I know you need paired glass, with some air in between, to have any insulation happen.

    My apartment even has a window that opens and closes in a blind-style in the bathroom. It’s never ever 100% closed, needless to say the bathroom gets very cold in winter.

    In my home country it’s custom to replace windows to “pair glass” when winter comes (or keep openable ones all year long), but apartments in Japan don’t provide such.

  2. Matt TREYVAUD Says:

    Did someone say Tsurezuregusa?


    I have to say that I agree with this. In winter, you can always put on more clothes or get into the kotatsu. In summer, there’s a limit to how naked a person can get (without shaving), and of course AC is a relatively recent development.

  3. alin Says:

    > It’s probably that the windows are only single-layer glass

    try curtains.
    thick, plastickey ones are good; i’m personally saving up for some custom-made gore-tex curtains for next winter.

  4. mcvmcv Says:

    This reminds me that I really like the Tsurezuregusa. If anyone is interested, you can look through a copy on google book search.

    “The pleasantest of all diversions is to sit alone under a lamp, a book spread out before you, and to make friends with people of a distant past you have never known.”

  5. W. David MARX Says:

    I guess the next question is: does double glass really cost all that much for developers?

  6. nate Says:

    ….twice as much? ;)

  7. masaki Says:

    An old professor of mine who specialized in Japanese classic literature, like Tsurezuregusa, always said that you need to live in an old “hot as hell, cold as hell” Japanese house to understand Japanese seasons and classic lit as well. After having done so for many years my wonder is what are the retrofitting possibilities of these old houses. The windows of course, but I’m more curious about an injectable insulation. Most of these old houses have two pane stucco or plaster walls with open air in between. This acts as a natural insulator but it seems it that this space could easily be upgraded. Picture it, double pane windows and modern insulation in the walls and ceiling, your old Japanese house would be ready for many more years to come.

  8. sdr Says:

    “断熱性能をもつガラス” – I’m no glazier, but is Ashizawa talking about double glazing here? If so, nitty-gritty pricing questions could start here:

  9. W. David MARX Says:

    Picture it, double pane windows and modern insulation in the walls and ceiling, your old Japanese house would be ready for many more years to come.

    Why preserve a beautiful traditional Japanese house when you can raze it and build two instantly-unattractive stucco three-stories on the same spot?

  10. masaki Says:

    But haven’t you been to those fabulous model house malls speckling the western suburbs. More quickly gratifying to yearn for a bunch of lined-up boxes you can’ afford rather than form a judgment about some old shack based on your own aesthetic.

    Where are you DIY injecto-spray insulation?

  11. W. David MARX Says:

    Am I wrong to think that the Japanese residential ideal is based on suffering through the seasonal extremes as a way of “experiencing” their aesthetic?

    Is Hokkaido exempt because it’s not in the sacred 四季 (four season) zone? Should other people at 35˚40′ N (like in South Carolina) also feel bad about being warm in winter?

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  13. James Says:

    “Am I wrong to think that the Japanese residential ideal is based on suffering through the seasonal extremes as a way of “experiencing” their aesthetic?”

    I’m glad someone made this point.
    I suspect this has a lot to do with the lack of efficient insulation more than the “shock house” theory.

    I feel this piece did not really get to the bottom of the insulation issue (to no fault of the interviewer)

    Is this another mystery that can be assigned to the Japanese aesthetic or culture or am I traversing into “Nihonjinron” territory?

    James T.K

  14. W. David MARX Says:

    I feel this piece did not really get to the bottom of the insulation issue

    I kind of agree, but was confused on what follow-up questions would really lead to a definitive answer. The mystery deepens…

  15. Matt TREYVAUD Says:

    Some people think that sleeping on a futon on the floor is suffering. It all depends on your perspective. I don’t have AC in my home, and I like it that way.

  16. Hlem Says:

    Someone I worked with in Japan used to frequently assert that the lack of double-pane windows was due to pressure from power companies eager to sell more juice.

    He also frequently asserted rather crazy things about Jews and 9/11 though, so I wasn’t sure if I should take his insulation dirt seriously.

  17. Keiji Ashizawa Says:

    I would like to write a reply in Japanese and hope that someone trancelate them in English.
    日本人論“Nihonjinron”は、わたしに答えられるような代物ではありませんが、長年 木と紙の家にすむこと、住み続けることによって、生まれてきた日本人の作法というものはあるのだろうと思っています。いやおなく四季を感じるということはそのうちの1つでしょう。虫の声を、美しいと感じるのも日本人の家があまりにも外部と一体化していたために培われた感性かもしれません。

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  19. Matt TREYVAUD Says:

    Quick and dirty translation of Keiji’s post:

    “I don’t know about ‘Nihonjinron’, but I do think that that living in wood and paper houses for so many years has resulted in a certain ‘Japanese manners/way of doing things’ (日本人の作法). Part of this would be feeling the seasons whether you like it or not. Perhaps finding beauty in the voices of insects is also a sensibility fostered by the Japanese home and its close union with the outside.
    I think that the reason that the average Japanese child is (was?) so well-behaved is not only that they lived in such delicate houses but also because their parents taught them not to damage the shoji or fusama. It’s also often said that [Japanese homes] have no privacy (i.e., rooms have no locks and are separated only by thin doors), but in general, before you open the fusama you call out to the person inside to let them know. If you go to a ryokan or ryotei [upscale restaurant], these ‘ways’ (作法) can still be observed. I believe that this style of interaction is connected to how people live. Perhaps the rareness of crime (of course, the ‘myth of [Japanese] safety’ is gradually crumbling) is also the ‘soil’ which allows paper and wood houses to exist. Until about the time when I was in junior high school, most people didn’t even carry a key around. Even today I often forget to lock my door.”

  20. Keiji Ashizawa Says:

    Thank you Matt-san.
    It’s not Dirty at all.