Japanese House Insulation: Standards, Costs and 2026 Grants

Why Japanese houses are so cold

This is not expat folklore, and it is not a climate problem: it is a regulatory legacy. Japan published energy standards in 1980, 1992, 1999 and again in 2016, but none of them was binding for detached houses before April 2025. As late as 2020, a builder could therefore hand over a house insulated to the level of an early-1980s cabin without breaking a single rule.

The result: across a stock of roughly 50 million dwellings, the Ministry of Land, Infrastructure, Transport and Tourism (Kokudo Kōtsū-shō, Kokudo Kōtsū-shō, MLIT) estimated, on the basis of its 2019 data, that only 13 % meet the 2016 energy standard and that around 29 % have no insulation whatsoever. That is precisely the segment you buy into when you target an older house or an akiya to renovate.

A house designed for summer, not for winter

The principle is centuries old. "A house should be built with the summer in mind", wrote Yoshida Kenkō in the Tsurezuregusa (徒然草, Essays in Idleness) back in the fourteenth century. Hence houses that are cross-ventilated, raised off the ground and widely open, built to shed the heat and humidity of July — and therefore perfectly permeable to the cold of January. Paper sliding screens (shōji, shōji) and fusuma (襖, solid sliding partitions) divide rooms without ever sealing them: see our guide to maintaining tatami, shoji and fusuma.

Anyone arriving from northern Europe, Canada or the northern United States should recalibrate here. There is usually no central heating and no whole-house ducting. Heating is done room by room, most often with a reversible air conditioner mounted high on the wall, sometimes with a kerosene stove or a heated table. A British or American buyer instinctively assumes the building envelope does the work; in Japan, for most of the existing stock, it does almost none.

Weak point number one: the windows

According to data from the Japanese building materials and equipment association, relayed by the Agency for Natural Resources and Energy, 58 % of heat escapes through openings in winter and 73 % of heat enters through openings in summer. Yet the dominant frame in Japan remains bare aluminium, an excellent conductor: it transmits the cold, streams with condensation, and a great many pre-2000 dwellings still have nothing but single glazing.

A health issue, not just a comfort issue

Without central heating, you heat one room while the corridor, the toilet and the bathroom sit at close to outdoor temperature. The resulting thermal shock (hīto shokku, ヒートショック) is a documented public health problem: the Ministry of Health's vital statistics record 5,824 deaths by drowning in a domestic bathtub in 2022, around nine in ten of them among people aged 65 and over, with a sharp peak in January. For an investor there is a commercial reality behind the human one: a freezing property collects bad reviews in short-term letting and is hard to re-let.

Dannetsu insulation grades and the UA value, explained

The performance of a Japanese dwelling is read through the dannetsu tō-seinō tōkyū (断熱等性能等級, insulation performance grade), a scale running from 1 to 7 — levels 6 and 7 were created in 2022. It rests on the UA value (the average heat loss coefficient of the envelope, in W/m²·K): the lower the figure, the better insulated the house. A second indicator, ηAC, measures summer solar heat gain.

Since April 2025, the Building Energy Efficiency Act (kenchikubutsu shōene-hō, 建築物省エネ法) has required every new dwelling to reach grade 4 as a minimum. The government has announced that it is aiming for the ZEH level (grade 5) as the baseline by 2030. In other words: the bar is rising, and the entire older stock is falling behind it.

The UA thresholds to know, by climate zone

Japan is divided into 8 climate zones (chiiki kubun, 地域-ku分), from Hokkaidō (zone 1) to Okinawa (zone 8). These are the thresholds worth memorising:

Grade	Zones 1-2 (Hokkaido, northern Tohoku)	Zone 4 (Nagano, upland)	Zones 5-6 (Tokyo, Kyoto, Osaka, Nagoya, Fukuoka)	Zone 7 (southern Kyushu)
Grade 4 — legal minimum for new build since 2025	0.46	0.75	0.87	0.87
Grade 5 — ZEH level	0.40	0.60	0.60	0.60
Grade 6 — genuine year-round comfort	0.28	0.34	0.46	0.46
Grade 7 — very high performance	0.20	0.23	0.26	0.26

Maximum UA values in W/m²·K. Zone 8 (Okinawa) has no UA criterion: only the summer solar heat gain factor is regulated. An uninsulated 1980s house, by contrast, sits below grade 2 — frequently off the scale altogether.

How to find out the grade of an existing property

• The housing performance evaluation certificate (jūtaku seinō hyōkasho, jūtaku性能評価書): rare before 2010, but decisive when it exists. Ask the agency for it by name.
• The BELS label, displayed on some recent or renovated homes, gives the energy class.
• Failing that, you deduce it: year of construction, frame type, presence of insulation in the roof space and under the floor. A pre-1992 property that has never been worked on is almost always uninsulated or very poorly insulated.

None of this ever appears in the statutory disclosure document (jūyō jikō setsumei): nobody will tell you if you do not ask.

Assessing insulation before you buy: the viewing checklist

Twenty well-run minutes on site beat any quotation. This is what we look at systematically during a naiken (naiken, property viewing), including a remote viewing done over video:

1. Year of construction. Before 1981: check the seismic side as well (the shin-taishin standard). Before 1992: insulation is almost certainly absent or token.
2. The window frames. Bare aluminium (cold to the touch, condensation), aluminium-resin hybrid, or resin (jushi sasshi, 樹脂サッシ, uPVC frames)? Is the glazing single or double? A doubled flame reflection in the pane gives double glazing away.
3. Condensation. Black marks in north-facing corners, behind furniture, at the base of walls, on the tatami: mould is the visible symptom of a cold wall.
4. The roof space. Open the attic hatch (tenjō uraguchi, 天井裏口): is there any mineral wool? How thick? Is it compressed, fallen out of place, damp?
5. The crawl space (yukashita, 床下). Underfloor insulation, damp, bare earth, and above all signs of termites (shiroari, シロアリ).
6. The bathroom. An old-style tiled wet room is a radiator in reverse; a recent prefabricated bathroom unit (yunitto basu, ユニットバス) is insulated.
7. Draughts. Run your hand along the base of partitions, around the frames and at the attic hatch: in an older house, air moves freely between the crawl space and the roof.
8. Orientation and obstructions. An unobstructed south elevation changes everything in winter; so does a blind north party wall in zone 4 or 5.

Expert tip — if you can, view between December and February, or ask for a filmed viewing with an infrared thermometer pointed at the walls, the glazing and the floor. The gap between air temperature and surface temperature tells you everything a paper file never will. Our full buying checklist incorporates these points.

What an insulation retrofit costs, item by item

Here are the orders of magnitude on the Japanese market in 2026, before subsidies. Ranges vary with the region, how accessible the site is and the level of finish; they are there to frame a budget, not to replace a quotation.

Item	Range in yen	≈ €	Comfort impact
Glazing replaced alone (Low-E double glazing in the existing frame)	40,000 – 150,000 / pane	270 – 1,000 €	Moderate
Added inner window (uchi-mado, 内窓, secondary inner window)	60,000 – 200,000 / opening	400 – 1,330 €	Very high
Full window replacement, so-called cover method	150,000 – 500,000 / opening	1,000 – 3,330 €	Very high
Insulated front door	250,000 – 600,000	1,670 – 4,000 €	Moderate
Roof space or roof insulation	200,000 – 600,000	1,330 – 4,000 €	High
Ground floor insulation from the crawl space	200,000 – 600,000	1,330 – 4,000 €	High
Walls, cavity fill with partitions stripped out	1,000,000 – 3,000,000	6,700 – 20,000 €	High
Insulated prefabricated bathroom unit (full replacement)	800,000 – 1,500,000	5,300 – 10,000 €	High
Whole-envelope retrofit (external wall insulation included)	2,000,000 – 7,000,000	13,300 – 46,700 €	Maximum

The running order that returns the most per yen spent

Priority	Work	Why
1	Windows (inner window first)	The number one source of heat loss, the most heavily subsidised, no structural work and no need to move out
2	Roof space / roof	Heat rises; a simple job and cheap per square metre
3	Ground floor plus airflow stops	Removes the freezing-floor sensation, done from the crawl space without touching the rooms
4	Walls	Effective but heavy: pool it with an interior refit or with seismic strengthening

Who does the work? A local building contractor (kōmuten, kōmuten) is often cheaper and more available than a large renovation chain, but not all of them are registered with the grant schemes — check before you sign. See our guide to finding an architect and tradespeople in Japan and the detailed budgets in the cost of renovating a machiya or kominka.

Financing. An important reminder: a Japanese mortgage is in practice reserved for salaried residents of Japan. A non-resident buyer funds the purchase and the works from their own capital, so the insulation budget has to be set out at the offer stage rather than discovered afterwards.

The 2026 grants: up to 1 million yen on windows

This is the blind spot of most foreign buyers: Japan subsidises energy retrofits heavily, and landlords, including companies, are eligible. The schemes are grouped under a single annual campaign, the Jūtaku Shōene 2026 Campaign (jūtaku省エネ2026キャンペーン).

The window scheme, by far the most generous

Run by the Ministry of the Environment, the advanced window renovation programme (senshinteki mado renobe, 先進的窓リノベ2026jigyō) is capped at 1,000,000 yen per dwelling (≈ 6,700 €), down from 2,000,000 yen in 2025: the envelope has been halved, which is all the more reason not to wait. Amounts per opening, for grade S products (the highest performing tier):

Work (grade S)	Small	Medium	Large	Extra large
Added inner window	22,000 yen	34,000 yen	52,000 yen	76,000 yen
Window replacement, cover method	60,000 yen	92,000 yen	124,000 yen	156,000 yen
Window replacement with the frame removed	46,000 yen	70,000 yen	95,000 yen	119,000 yen
Glazing only, per pane	7,000 yen	23,000 yen	35,000 yen	53,000 yen

New for 2026: for inner windows, only grade S is still subsidised, grade A having been dropped. Work must have started on or after 28 November 2025, applications are filed between 31 March and 16 November 2026 (or until the budget runs out) and the job must be finished by 31 December 2026 at the latest. The minimum claim is 50,000 yen per application. As of 24 July 2026, the official counter showed 13 % of the envelope committed.

The envelope scheme: walls, floors, ceilings

The Mirai Eco Jūtaku 2026 programme (みらいエコjūtaku2026jigyō), run by MLIT and the successor to the "green housing" scheme that closed in February 2026, subsidises insulation work on the envelope — floors, walls, ceilings — in addition to openings, for dwellings built before the end of 2016. The order of magnitude for the retrofit cap is 1,000,000 yen per dwelling, with applications filed in windows running to 31 December 2026 or until the budget is exhausted.

Three rules to know before you sign a quotation

• You never file the application yourself. Only a company registered with the scheme can do it, and it passes the grant back to you, usually as a deduction from the final invoice. Choosing an unregistered tradesperson means giving up the money.
• Work must start after the opening date of the current year's scheme. A job started too early cannot be retrofitted into a claim.
• Budgets close on a first-come, first-served basis. In previous years, some strands closed before the official deadline.

On top of this come municipal grants: many local authorities stack their own renovation subsidy, sometimes uprated for taking on a vacant akiya property or for settling permanently in the municipality. As a non-resident owner, plan for a local contact (a management agency or a representative) and a Japanese bank account to receive the payments.

Case study: insulating a 1988 house one hour from Tokyo

A 95 m² timber house built in 1988, therefore post-dating the 1981 seismic standard, 55 minutes from Tokyo by train, bought for 9,000,000 yen (60,000 €). Acquisition costs kept to a maximum of 6 %, so roughly 540,000 yen (3,600 €). Condition: single glazing in aluminium frames throughout, compressed mineral wool in the roof space, nothing under the floor.

Item	Amount in yen	≈ €
12 Low-E inner windows (grade S)	1,150,000	7,700 €
Roof space insulation, 40 m²	380,000	2,500 €
Ground floor plus airflow stops from the crawl space	420,000	2,800 €
Insulated front door	350,000	2,300 €
Total works	2,300,000	15,300 €
Estimated window grant (4 large, 6 medium, 2 small)	− 456,000	− 3,000 €
Net cost after grant	1,844,000	12,300 €

Total cost of the operation: 9,000,000 + 540,000 + 1,844,000 ≈ 11,384,000 yen (≈ 76,000 €), that is 20 % more than the purchase price — an order of magnitude to build into any project involving uninsulated older stock from the very start.

What it changes

• The windows stop being radiators in reverse. Morning condensation disappears, and with it the mould on frames and tatami — one maintenance line item gone.
• The energy bill falls markedly, the order of magnitude commonly cited by professionals being 20 to 40 % on heating and cooling depending on the scope of the work and how the house is used.
• The property becomes lettable all year round. On an assumed rent of 80,000 yen a month (to be validated city by city), that is 960,000 yen a year, and a gross yield of around 8.4 % on the 11.4 million yen invested. Test your own scenario with our yield simulator.
• In short-term letting, winter stops being the two-star review season: with 42.7 million visitors in 2025, the demand is there all year, provided the property is bearable in January. See Airbnb profitability city by city.

One reminder that avoids misunderstandings: this work, like the purchase itself, grants no right of residence in Japan. You buy and renovate freely as a non-resident foreigner; the visa is an entirely separate matter.

Kominka and machiya: insulating without ruining the house

On a traditional house the logic is reversed: the building is designed to breathe. Earthen walls (tsuchikabe, 土壁) absorb and release moisture; posts and beams are left exposed (shinkabe, 真壁). Fitting a vapour barrier in the wrong place, or stuffing an earthen wall with mineral wool, means trapping water inside the wall: interstitial condensation, rotting timber, an open invitation to termites. Many degraded Kyoto machiya were ruined by badly conceived "modern" renovations.

The strategy that works

• Treat the openings first: a discreet inner window fitted behind the shōji preserves the look of the room while eliminating the draughts.
• Insulate from above and from below: ceiling or roof underside, then the floor from the crawl space. These are the two least invasive jobs on an old structure.
• Let the earthen walls breathe, or use only capillary, vapour-open materials (wood fibre, cellulose, wood wool) fitted by someone who genuinely understands the subject.
• Zone the house: insulate two or three living rooms properly — the "house within a house" principle — rather than chasing a new-build standard across 150 m² of old fabric.

In Kyoto, add the townscape constraint: elevations visible from the street are regulated, which naturally steers you towards internal solutions. Neighbourhood-by-neighbourhood detail in our guide Kyoto district by district.

The mistakes that cost the most

• Insulating walls without fitting airflow stops (kiryū-dome, 気流止め) at the head and foot of the partitions. In an older house, cold air from the crawl space travels up inside the partitions to the roof space: without stops, it simply bypasses the insulation you have just paid for.
• Sealing without ventilating. New build has required continuous ventilation since 2003, and for good reason: a house made airtight without organised air change becomes a house with mould.
• Insulating before sorting out the water. Roof, gutters, rising damp and termites all come before insulation. Wet insulation stops insulating and rots what it touches.
• Separating insulation from seismic strengthening. Both jobs open up the same wall build-ups: running them together saves stripping out twice.
• Relying on "insulating" films and curtains. They limit radiation, not structural heat loss. That is a tenant's workaround, not an owner's strategy.
• Viewing in August. A country house in August is charming; the same house in January, with no insulation, is uninhabitable. Have its winter behaviour checked.
• Using a tradesperson not registered with the grant scheme, or worse, doing it yourself: that is several hundred thousand yen of subsidy given away.
• Over-investing in a property you plan to flip. In Japan, the building depreciates fast against the land: see why a Japanese house loses value. Insulation pays for itself through use and rental yield, rarely through capital gain alone.

Conclusion: the highest-return lever in Japanese renovation

Japan's housing stock is behind on insulation, and that is exactly what creates the opportunity: the buyer who builds an insulation budget into the offer ends up with a comfortable, healthy property that can be let all year round, in a country where the rules have finally been tightened for new build. Windows remain the first place to spend, the state refunds a meaningful share of them until the end of 2026, and the whole exercise stays in a completely different league from the cost of touching the structure.

To go further: our complete guide to buying property in Japan sets out the steps and the costs, our akiya selection shows real properties with their year of construction, and our supported projects illustrate renovation budgets that were actually spent. If you would like a property examined from this angle before you make an offer, tell us about your plans through immoJapon's buyer support.

Frequently asked questions

Are Japanese houses really that badly insulated?

Yes, statistically. Out of roughly 50 million dwellings, the Ministry of Land, Infrastructure, Transport and Tourism estimates that only 13 % meet the 2016 energy standard and that around 29 % have no insulation at all. The obligation to insulate has applied to new homes only since April 2025: everything built before that was left to the builder's discretion.

Can a foreign owner claim the energy retrofit grants?

The schemes target the owner of the dwelling, including landlords and companies, with no nationality condition. The constraint is procedural: the application is filed by a company registered with the programme, never by the individual, and the payment passes through that company. Plan for a local contact and a Japanese bank account.

How much does it cost to insulate a 100 sqm Japanese house?

A targeted, effective package (inner windows, roof space, ground floor, front door) commonly falls between 2 and 3 million yen (13,000 to 20,000 €) before subsidy. A full envelope retrofit, walls and external insulation included, frequently exceeds 5 million yen (33,000 €).

Should insulation come before or after seismic strengthening?

Both at once, whenever that is possible. Strengthening a structure and insulating walls both mean opening up the same build-ups: combining the two jobs avoids paying twice for stripping out and reinstating the interior finishes.

Are secondary inner windows enough on their own?

They will not turn a house into a low-energy building, but they are the best result-to-price ratio on the Japanese market: openings account for most of the winter heat loss, fitting takes a few hours per room with no structural work, and they are the most heavily subsidised item in 2026.

Does insulating increase resale value in Japan?

Only moderately. The Japanese market prices the land above all and writes the building down quickly. Insulation pays back first through use, through lower bills and through letting: a comfortable property rents faster, for more, and earns better reviews in short-term letting.

Can a kominka be insulated without spoiling it?

Yes, provided you respect the physics of an old building: work on the openings, the ceiling and the floor, let the earthen walls breathe or use only vapour-open materials, and concentrate performance on two or three living rooms rather than the whole house.

Which insulation grade should you target for short-term letting?

Reaching the new-build regulatory grade (grade 4) in the occupied rooms is a realistic target on older stock, and the ZEH level (grade 5) is an excellent standard if the budget allows. What matters most is that no room a guest can enter, bathroom included, is freezing in January.