Renovating a 30-year old house into a zero-energy home

If you want a well-insulated house in Japan you usually need to build it yourself. If you are looking at an older house to buy or rent, insulation is usually lacking, unless the original owner decided to go the extra mile to create a home with insulation. For the majority of older homes, however, they tend to be built cheaply and can be cold in winter and hot in summer.

The construction and home building industry in Japan has come a long way in recent years with active efforts to create zero-energy homes. There are a lot of options now for double or triple-glazed glass windows, non-aluminum frames, insulation and thermal breaks.

These insulation options are not just limited to new construction, it is possible to renovate an existing home to make it a zero-energy home.

One of these projects was recently completed in Daizawa near Shimo-kitazawa Station in Tokyo. The 2-storey home was built in 1987. It has a concrete basement, while the above-ground structure is wood-frame. The house was fully renovated by Rebita and YKK AP, resulting in a contemporary home with numerous energy-efficient features, insulation and earthquake-retrofitting.

In addition to 260mm glass wool and 50mm phenolic foam insulation in the walls and ceilings, PVC-frame, Low-E double-glazed windows replaced the original aluminium-framed windows. Compared to aluminium windows, the new windows are estimated to reduce heating and cooling costs by between 18~21% depending on the region and climate.  Aluminum frames tend to be the standard in most homes due to their low cost, but easily transfer heat and cold, causing condensation in winter and bringing in heat during summer. The new windows have 1/1000th of the thermal conductivity of aluminum windows.

Heat transfer coefficient

By 2020 the government is aiming for the standard U-value (heat transfer) in the industry to be 0.87 W/m²K. Well-insulated parts of a house have a low U-value, while poorly insulated areas will have a high U-value.

Before renovations, the Daizawa house had a U-value of 1.53 W/m²K. After the renovations, this dropped to 0.46 W/m²K, well below the planned national standards.

Even in the middle of winter, the house is calculated to be able to keep an average internal room temperature of 16.6C with intermittent heating, an improvement from 14C prior to renovations.

Other energy saving features installed during renovations included low-energy air-conditioners, water-saving toilets, EcoCute hot water system, LED lighting, and a Home Energy Management System (HEMS).

With an optional installation of solar panels on the roof, the heating and cooling costs for the house can be reduced to a quarter of what they were prior to renovations, going from 310,000 Yen per year to an estimate of 73,000 Yen per year.

Earthquake retrofitting

Earthquake resistance is also a concern when buying a much older house, especially wood-framed homes.

One of the easiest ways to provide additional support to a structure is to increase the thickness of the walls. This usually results in the windows being made smaller. For the Daizawa house, extra-strength wood-frame walls were installed around the windows using a method that allowed the windows to stay the same size. Earthquake-retrofitting work to the existing house brought it up to the Level 3 of earthquake resistance. This is the highest level and means that the structure is designed with withstand 1.5 times the predicted force of an earthquake. Prior to renovations, the house had a high chance of collapse in an earthquake producing a seismic intensity (shindo) level of upper 6. After retrofitting, the house should now be able to withstand such an earthquake.


Approximately 35 million Yen (310,000 USD) was spent renovating the Daizawa house. This is more than the typical 20 million Yen that could be spent on a full house renovation. The extra 15 million Yen was due to the additional insulation, earthquake-retrofitting, and other energy-saving features.

Still, renovating the existing house turned out to be around 10 ~ 15 million Yen cheaper than what it would have cost to build a brand new house of the same size and scale.

The renovated house and land is currently listed for sale for 159 million Yen (approx. 1.41 million USD).  The land is 126 sqm and the house has a total floor area of 144 sqm (1,549 sq ft). It is 600 meters from Shimo-kitazawa Station and 3.5km west of Shibuya Station.

Source: Rebita News Release, July 12, 2017.

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