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Lakehouse Juolas is rated an Energy Efficient House as defined by Motiva, the Finnish state-owned Sustainable Development Company.

Motiva describes the qualities for energy efficient detached houses as:

An energy efficient house belongs in energy class A or B.

[Source: What is an energy efficient detached house like? - Motiva / October 2023

Lakehouse Juolas has received a B rating (in a scale from A to G) in the energy effiecincy certification. The enery efficiency certification as specified by Motiva, is described as following:

Energy performance certificates help consumers compare buildings’ energy efficiency. The classification scale familiar from household appliances gives an overall picture of a building’s energy efficiency in a simple way. Energy performance certificates also aim to raise the status of energy efficiency so that it becomes one of the design criteria for buildings.

Energy performance certificates are computed. They are based on the building’s E figure, which is composed of the building’s calculated annual consumption of purchased energy, weighted with the factors of various forms of energy. The factors and definition employed are the same as in the energy efficiency regulations concerning new buildings which took effect on 1 July 2012.

Energy performance certificates are based on buildings’ properties. Computed comparison enables the comparison of the energy efficiency of an old and a new building.

New detached houses currently being built pursuant to building regulations belong in class C. Class D or E for a fairly new, existing detached house is fairly commonplace, while the class for older houses can be E–G. The classification can be influenced by, for instance, improving a house’s thermal insulation, the recovery of ventilation heat or the use of renewable energy.

[Source: Energy performance certificate - Motiva / October 2023]

What makes the Lakehouse Energy Efficient?


The house has very good thermal insulation in the outer walls, the floor and the ceiling. The ceiling itself has 50cm of mineral wool for thermal insulation.


The large windows in the house are extremely energy efficient. They are have 3 layers of glass (thick-thin-thick) with argon noble-gas inbetween to improve thermal isolation. The U-value (measure for the insulation efficiency) for the windows is 0.6.

Energy Recovery

There is a mechanical ventilation in the house which recovers heat from exhaust air to pre-heat the new incoming air. This is very efficient since it will recovery heat energy from any heat sources inside the house: The heat pump, the sauna or the fireplace. With a room temperature of 22 degrees the exhaust air is only 6-7 degrees after the ventilation has transfered the heat the the new incoming ventilation air. This means that about 15 degrees of the heat is reused.

Fireplace Efficiency

The fireplace has an efficiency of 86,4% and a mass of 1300 Kg for storing the heat. This means that less that 14 percent of the heat when burning wood will go out of the chimney. Most of the heat is absorbed into the stone mass from where it is gradually given off into the house in the next 12-24 hours.

Heat Pump

The house has a modern heat pump that is used both for warming and cooling. The house also has floor heating but it is only used to keep the floors at a basic temperature. The heat pump is the primary way warm up the house as it is very energy efficient.

Remote Operation

Everything from the heat pump to floor heating and ventilation can be remotely controlled. This makes it possible to drop the room temperature and the ventilation to a lower setting when the house is not in use for a longer period of time (multiple days).

The energy efficiency is not achieved by any single measure - it is the sum of all of the above issues.

An A-rating would have required to use an even more energy-efficient way of heating - for example geothermal energy. That is however not sensible for a relative small house of this size.