Why the house is so cold, and what fixing it costs
Every foreigner who spends a first winter in an older Japanese house asks the same question. The answer is that there is almost no insulation in it. The more interesting answer is why, because it was not an oversight.
In short
- Traditional Japanese houses were designed against summer humidity, not winter cold. Thin walls, large openings and an open underfloor were the correct answers to the problem being solved.
- Windows lose more heat than anything else, and they can be improved without opening a single wall.
- Insulation is not only a comfort question. Condensation inside an uninsulated wall wets the timber, and that shortens the life of the building.
It was not an oversight
Japan is hot and humid in summer, and traditional housing was built around getting through that. Air movement, ventilation, shade. Thin walls, large openings, and an underfloor left open to the outside.
Winter cold was treated as something to be endured, and heating was applied to a person rather than a building: a heated table, a paraffin stove, later a room air conditioner. The idea of warming a whole house is very recent in ordinary Japanese housing.
The result is a housing stock built, in enormous numbers, to a pattern that looks like this.
- No insulation in the walls, or a very thin layer
- No insulation above the ceiling, or too little
- No insulation under the floor
- Single-glazed aluminium-framed windows
- Low airtightness, with draughts
None of this was cutting corners. It was a normal house for its time. Insulation requirements came to Japanese housing comparatively late and remained a target rather than an obligation for a long period.
Where the heat goes
| Element | Share of the loss |
|---|---|
| Windows and openings | The largest by a clear margin, approaching half in some houses |
| Walls | Next largest |
| Ceiling and roof | Substantial, since warm air rises to meet it |
| Floor | Smaller in share, but this is what cold feet actually are |
| Ventilation and gaps | Not negligible in a house with low airtightness |
Look at the first row. Improving the windows is among the most effective things that can be done, and it is also among the least disruptive, because there is a method that does not involve opening any walls.
What to do, in what order
| Priority | Work | What it involves |
|---|---|---|
| 1 | Windows | A second window fitted inside the existing one, which requires no structural work and is quick, or full replacement, which is more effective and a larger job |
| 2 | Ceiling | Insulation laid in the roof space. Straightforward where there is access, and it stops the loss upward |
| 3 | Floor | Insulation fitted from below where the crawl space allows. Changes the feel of the house at floor level considerably |
| 4 | Walls | Insulation into the wall cavity. The most effective and the most disruptive |
Never insulate walls on their own
Insulating a wall means opening it. If it is open, that is the moment to add seismic connection hardware, renew the wiring behind it and inspect for termite damage.
Opening a wall for insulation alone is the least efficient use of that opportunity there is. This is the same sequencing argument set out in what five, ten and twenty million yen actually buy, and it applies here more sharply than anywhere.
Rough costs
For a detached timber house of roughly 80 to 120 square metres. Orders of magnitude only, and real figures move with structure, region and materials.
| Work | Roughly |
|---|---|
| Secondary glazing throughout | Several hundred thousand yen to somewhat over a million, depending on how many windows and how large |
| Ceiling insulation | Several hundred thousand yen |
| Underfloor insulation | Several hundred thousand yen |
| Wall insulation | High on its own because of the opening-up work. Rational only alongside other work |
| Windows, ceiling and floor together | In the region of two to four million yen |
The useful observation is this: windows and ceiling alone deliver the largest change in how the house feels, for the smallest part of the budget. If everything cannot be done at once, do those two. The difference in winter is not subtle.
There are subsidies, and they have rules
National and municipal subsidy schemes exist for insulation work in Japan, and window improvements have attracted particularly substantial support in recent years.
Four things about them decide whether you actually receive anything.
- Terms change from year to year
- Budgets are capped, and schemes can close early when applications are heavy
- Applications generally have to be made before work begins
- Eligible products and registered installers are often specified, and the forms are in Japanese
Finding out that support existed after the work has started is a common and expensive way to learn this. Check what is available while the work is still a plan.
Insulation is not only about comfort. Water condensing inside an uninsulated wall is wetting the timber every winter.
Condensation is the structural half of this
Heat a poorly insulated house and condensation forms. Not only on the glass, but inside the walls and in the roof space.
Surface condensation can be wiped away. Condensation inside a wall cannot, and it wets structural timber in a place nobody will look for years. That moisture leads to decay and mould, and decay attracts termites, which is the same chain described in by the time it shows on the ceiling, the wood has been wet for a while.
So insulation belongs in the same conversation as the life of the building, not only in the conversation about heating bills.
What to establish before buying
- Whether there is insulation in the walls, ceiling and underfloor, and if so what and how thick
- The window frames and glazing: aluminium and single, or something better
- Any evidence of condensation, at window reveals, at the back of cupboards, behind furniture
- What insulating the house would cost, included in the renovation budget rather than added to it later
- The climate where the property actually is
The last one deserves more weight than it usually gets. Hokkaido and Kyushu are not the same country for this purpose. Winter minimums, snow loading and summer humidity all differ enormously across Japan.
If you are seriously considering a property, visit it in winter once before you buy. Whether you can live with that particular winter is not a question anybody else can answer for you, and it is not answerable from a photograph taken in May.
What these figures are not
Costs given are general orders of magnitude and not estimates for any property or scope of work. Actual costs depend on the structure, specification, location and contractor.
Subsidy terms, eligibility and deadlines change by year and by local authority. Confirm current schemes with the relevant office. En Beau lo does not carry out construction work and introduces contractors where they are needed.
Cheap and liveable are different questions
A house can be an excellent purchase and still be one you cannot comfortably occupy in February. Those are separate assessments, and insulation is where they meet.
What else sits outside the purchase price is covered in what a cheap Japanese house actually costs.
The condition of the building sets the plan
We visit the property and record its condition, including what can be seen of the insulation, the glazing and any signs of condensation, with photographs. Where work is indicated we set out a priority order and introduce contractors.
Ask us to check a property on siteSources and notes
The distribution of heat loss across building elements follows general building physics guidance for Japanese housing and varies by construction. Costs given are general orders of magnitude for a detached timber house of approximately 80 to 120 square metres and are not estimates. National and municipal subsidy schemes for insulation work are revised annually and administered by the relevant authorities.