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In low-energy buildings in climates with a heating demand the entire building envelope has to be well insulated. The building envelope consists of all the building elements which separate the inside from the outside. Its main purpose is to provide for a comfortable indoor climate – irrespective of the outdoor climate which is determined by the weather.
During cold periods (typically from the middle of October to the end of April in the winter cold climates) the temperature inside the building envelope is usually higher than it is outside. As a result, heat is lost through the envelope and, unless this heat is replaced, the inside of the building cools down adjusting to the outdoor temperature. The inverse applies for hot climates (or during hot periods) with excessive heat entering the building through its envelope. Therefore, it makes sense to restrict the heat flow in any building irrespective of the climate – and this is where thermal protection comes in.
→ Good thermal protection can be achieved for all construction methods and has already been successfully implemented in solid construction, timber construction, prefabricated building elements, formwork element technology, steel construction and all types of mixed constructions.
→ An appropriate level of insulation can also be applied to existing buildings at any given point of time.
An important principle has been derived from experience with new low-energy constructions:
“If you do it, do it right!” - Don’t save on the insulation when it comes to thermal protection measures. This principle is taken very seriously in Passive Houses – because quality insulation is a very affordable way of saving energy.
As a matter of fact, it is thermal insulation and not heat storage which is important (see insulation or storage). A high level of insulation has always proven to be effective; to learn more please go to the following page: Thermal protection works.
Heat losses through external walls and roofs account for more than 70% of the total heat losses in existing buildings. Therefore, improving thermal insulation is the most effective way to save energy. At the same time it will help improve thermal comfort and prevent structural damage (see further information about thermal insulation). Financial support such as low-interest loans, as currently available in a number of countries, reduces the initial investment for improved thermal insulation; yet, even without such incentives, the investment will pay off in the long term, as shown by a careful analysis.
U-values (thermal transmittance) of external walls, floor slabs and roof areas of Passive Houses range from 0.10 to 0.15 W/(m²K) (for Central European climate; these values may be slightly higher or lower depending on the climate). These values are not only benchmarks for all construction methods but also the most cost-effective values at today's energy prices.
The heat losses during cold periods are thus negligibly small, and the temperature of the interior surfaces is nearly the same as the air temperature, irrespective of the type of heating used. This leads to a very high level of comfort and reliable prevention of building damage due to moisture build up.
In warmer climates or during summer months, good insulation also provides protection against heat. Effective sunshades for the windows and sufficient ventilation are also essential to ensure a maximum level of comfort during hot periods.
Good insulation and airtight construction have proved to be extremely effective in Passive Houses. Another essential principle is “thermal bridge free design”: the insulation is applied without any “weak spots” around the whole building so as to eliminate cold corners as well as excessive heat losses. This method is another essential principle assuring a high level of quality and comfort in Passive Houses while preventing damages due to moisture build up.
The heat losses though a standard building component, i.e. external wall, floor, top floor ceiling or roof, are defined by the U-value or overall heat transfer coefficient (formerly k-value) 1). This value indicates the rate of heat transfer through a specific component over a given area if the temperature difference is one degree (1 Kelvin). The measurement unit of the U-value is therefore “W/(m²K)”. The smaller the U-value the better the level of insulation.
To calculate the heat loss through a wall, one must multiply the U-value with the area and temperature difference 2). In Central Europe, the average temperatures measured during severe winter periods are –12 °C outside and 21 °C inside.
To calculate the annual heating losses, one must multiply the U-value with the average temperature difference in the heating period with the duration of the heating period, or in other terms, multiply the U-value by the heating degree hours – which is 78,000 degree hours for an average Central European climate.
Using the example of a small single-family house with an external wall surface of 100 m², the following values were calculated for various U-values:
| heat loss rate|
| annual heating losses |
| annual costs 3) external wall only
The heat loss is a significant factor in the energy balance of a building. Any heat loss must be compensated for by a corresponding heat gain, otherwise the temperature inside the house will drop.
A typical Passive House compact heating system can provide a heating power of about 1,000 W (that's the typical output of a hair dryer). The U-value of a Passive House wall needs to be quite low; otherwise a considerable portion of this power would be used up by the external wall: For typical Central European buildings, U-values of Passive House walls should range between 0.10 and 0.15 W/(m²K); depending on the climate, these figures may be somewhat higher or lower.
What does this imply for the insulating building envelope?
Such low U-values can only be achieved with very well-insulating materials. The following table shows how thick an external building element, consisting only of the material specified, should be in order to achieve a typical Passive House U-value of 0.13 W/(m²K).
|material|| thermal conductivity|
| thickness required for U=0.13 W/(m²K)
|porous brick, porous concrete||0.11||0.83|
|typical insulation material||0.040||0.30|
|high-quality conventional insulation material||0.025||0.19|
|nanoporous super-insulating material normal pressure||0.015||0.11|
|vacuum insulation material (silica)||0.008||0.06|
|vacuum insulation material (high vacuum)||0.002||0.015|
The table graphically demonstrates that:
→ Even a straw-bale wall of 50 cm or more would be suitable for the Passive House.
→ Typical conventional insulation materials (mineral wool, polystyrene, cellulose) require layers of about 30 cm.
→ The thickness can even be reduced to 20 cm with common polyurethane foam insulation materials.
→ State of the art vacuum insulation materials allow for very slender, yet highly insulated, building elements.
→ “Semi-translucent envelopes“ are another, somewhat different approach which has also proven to provide efficient insulation for buildings.
It directs a certain share of the global radiation inside the insulated construction thereby
reducing the temperature differences and achieving a lower equivalent U-value.
It is a widely held view that the level of insulation required in Passive Houses is not affordable. Let's figure it out!!
Have another look at the table right at the top of this article. The fourth column lists the total annual costs for covering the heat losses through the external wall. (Please note that the following calculation is based on figures typical for Germany. The results should therefore serve as an example and may vary depending on the specific climatic conditions and energy prices of the region in question.)
Natural gas, heating oil, district heat, or electricity is used for heating – in Germany, the current or future costs for heating is rather unlikely to drop below 6.6 Eurocents per kWh 4). In fact, average energy prices were even higher over the last few years. The annual heating costs just for offsetting the heat losses through the external wall (100 m²) can be calculated as shown in the last column. Here is a section of the table again:
| heat loss rate|
| annual heating losses|
| annual costs external wall only
The values for a typical wall of an old building, which is not even poorly insulated, are given in the first row. The occupants will spend about € 644 each year just to compensate for the heat losses through 100 m² of this wall. Applying insulation according to the Passive House standard, heat losses will decrease by a factor of 10; the annual costs for the energy loss through the external wall are reduced to less than 64 €/year. This means:
|€ 580 savings in heating costs every year!|
What should be done in order to achieve these savings?
Do you think this is just a zero-sum situation? Will you end up spending all the money you saved on energy costs on trade services instead? No, you won’t, because:
Existing Passive Houses illustrate that thicker layers of insulation required for conventional insulation materials can easily be realised:
Due to the low heat losses, interior surface stay at the same pleasant temperature year round – even without heating surfaces in the components. As a result, the difference between the radiation temperatures from various directions in the room is small, which is a prerequisite for excellent comfort. The high interior surface temperatures also help prevent condensation on the surface of the components. With normal usage, damage due to moisture build up in external building components can be practically excluded in the Passive House. This has also been proven in practice.
|Thermography (infrared image) of the base point of a
Passive House taken at the inside of an external wall
Average surface temperature approx. 20°C
Minimum temperature at the edge 19°C
In warmer climates or during summer months the interior surface temperature is also close to the indoor air temperature which means that it is lower than that of poorly insulated components which allow heat to be transported from the outside towards the inside. Highly insulated constructions have a high temperature amplitude attenuation reducing the temperature fluctuation of external building components, even with very small masses (e.g. double plaster board). This effect is so great that it provides for optimal “summer behaviour” of the component. What is even more important though is the long time constant of the building due to the good insulation, which allows for the full utilisation of the thermally connected inner building mass. As a result, a Passive House in Central Europe can be cooled by night-ventilation and will stay pleasantly cool throughout the day, provided that solar radiation is limited to a reasonable extent. The “summer case” should be just as well-planned as the winter situation: the Passive House Planning Package (PHPP) is an excellent tool for this purpose.
To a certain extent, highly insulated components mitigate any remaining thermal bridges compared with moderately insulated components – this is particularly important in refurbishments. Although people tend to believe it must be the other way round, this has been proven to be true in numerous cases and can be explained quite simply: In highly insulated buildings the supporting structures and the inner component layer are protected by thick layers of insulation and stay evenly warm in continuous areas. As a result, they aren’t even affected by minor thermal bridges. In poorly insulated constructions on the other hand, great parts of the structure are already cold. Additional thermal bridges quickly cause temperatures to fall below the dew point. Nevertheless, thermal bridges do cause additional heat losses in Passive Houses too. That is why, in spite of the large error margin, we recommend that thermal bridges be reduced to a minimum when designing a Passive House.