The Passive House
The most important principle of a Passive House: insulation (yellow), applied continuously around the building envelope without thermal bridging - this reduces the heat losses like a warm coat. Most insulation materials are not airtight, however. Therefore the envelope has to be airtight, too. The airtight envelope can be seen in the section, too - it is given by the red line. The reduction of thermal bridges is very important; A special design method, the "Construction Avoiding Thermal Bridging" has been developed to simplify this design detail.
In a Passive Home the whole building envelope has an excellent thermal insulation. The envelope consists of all parts of the construction, which separate the indoor climate from the outdoor climate. All construction methods can be used for Passive Houses and have been tested successfully: masonry construction, lightweight construction, prefabricated elements, insulating concrete formwork construction, steel construction, and all combinations of the methods above.
The thermal heat loss coefficients (R-Values) of external walls, slabs to the ground, and roofs are within 0.1 to 0.15 W/(m²K) (for Central European climate; other climates will be discussed at the conference). These are upper level values for all contemporary constructions. As a consequence the transmission heat losses during the cold season are negligible. Another consequence is that the temperatures of the internal surfaces are almost the same as the indoor air temperature. This leads to a very comfortable indoor climate and avoids damages caused by the humidity of indoor air.
During hot periods in summer, a high thermal insulation is a protection against heat, too. To ensure high thermal comfort during summer, well designed shading and sufficient ventilation are important, too.
Good thermal insulation and an airtight construction are well proved in Passive Houses. Another basic principle used is "Construction Avoiding Thermal Bridging": The insulation is applied continuously around the envelope of the building without serious thermal bridging. By this method there will be no cold spots and no increased heat losses. This is a contribution to high quality, comfortable, and long lasting construction, too.
A key feature of a passive house is that they incorporate very high standards of insulation. This reduces the amount of heat lost through the building fabric to a very low level. When achieving these very high standards of insulation the purpose provided heating requirement, even on the coldest days, is reduced to a minimum and hence it is possible to adequately heat the dwelling by just preheating the fresh air entering the rooms.
The heat loss through a regular construction (an external wall, a floor to the basement or a slab on ground, a ceiling or a roof) is characterised by the thermal heat loss coefficient or U-value. This value shows, how much heat (in Watts) is lost per m2 at a standard temperature difference of 1 degree Kelvin. The international unit of the U-value therefore is “W/(m²K)”. To calculate the heat loss of a wall you multiply the U-value by the area and the temperature difference.
The following table presents the typical heat losses for different external walls based on a typical European single family house with an external wall area of 100 m². Winter temperatures of -12 °C outside and 21 °C at the inside are used as they are typical of Central Europe.
U-value heat loss annual annual costs (2005)
(load) heat loss only of the heat loss
W/m²K W kWh/(m²a) of external walls €/a
1,00 3300 78 429.-
0,80 2640 62 343.-
0,60 1980 47 257.-
0,40 1320 31 172.-
0,20 660 16 86.-
0,15 495 12 64.-
0,10 330 8 43.-
A typical compact services unit for a passive house will typically deliver ~1000 W without a problem. For the compact service unit to meet the total heat loss (floor, windows, doors, roof in addition to the external walls) the U-value of the wall has to be really low, suitable are values in the range between 0.1 to 0.15 W/(m²K).This means that the heating requirement matches the output of the compact services unit.
Ho does this translate into the construction of the building envelope? First it is obvious that U-values that low only can be achieved using really good insulating materials. The following table shows the thickness needed of an exterior construction, if that is solely built from the material given, to meet a typical passive house U-value of 0,13 W/(m²K):
Material heat- thickness needed
conductivity to meet U=0.13 W/(m²K)
W/mK _ m
concrete B50 2.100 15.80
solid brick 0.800 6.02
hollow brick 0.400 3.01
wood 0.130 0.98
porous bricks,
porous concr. 0.110 0.83
==========================
straw 0.055 0.410
typical insu-
lation material 0.040 0.300
highly insulation
material 0.025 0.188
nanoporous
"super insulation"
(normal
pressure) 0.015 0.113
vacuum-
insulation
(silica) 0.008 0.060
vacuum-
insulation
(high
vacuum) 0.002 0.015
From this table it can be seen, that a reasonable thickness of components is available only if a quite good insulating material is used. All materials beyond the double underline "===" are suitable. Of course, constructions with combinations of different materials are suitable as well and may be needed in many cases: e.g. a concrete wall with an external insulation or a monolithic wall from porous concrete and a mineral foam insulation panel.
The construction thickness will be less, the lower the heat conductivity of the insulating material is. A straw bale construction of typical thickness (50 cm and more) does already meet the requirements for a passive house. Using typical conventional insulating materials (mineral wool, polystyrene, cellulose) the thickness needed is some 300 mm. This can be reduced to 200 mm by using polyurethane foam, which is more expensive, however. In Germany vacuum insulation materials have been used in the building industry in some cases.
Another approach already used with success is a construction with a "semi-translucent envelope": In doing so the global radiation is allowed to be absorbed somewhere deeper in the construction, this leads to a reduction of the temperature difference and therefore in a lower equivalent U-value. Be careful with this approach in hot climates - while the classical insulation approach is working quite well against heat loads in hot periods, the semi-translucent insulation will heat up even more.
What about economy?
It is a wide spread believe, that super insulation, like it is used passive houses, does not pay back. Let us check that again! Please glance at the table provided. In the third column the total heat losses of one year per m² of the construction area are given. It is quite simple to calculate those: you multiply the U-value by the average temperature difference and the time interval of the heating period; or, even simpler, just U-value times heating degree hours - in a Central European climate 78000 degree hours. For producing the heat natural gas, heating oil, district heating or electricity is used - it will not be possible to buy the heat for less than 5,5 €Cent per kWh nowadays and the future energy price wont be lower on average. Therefore the annual costs for heating just to substitute the losses of the external wall (100 m²) will be as high as given in the last column. See a section of the table here:
U-value heat loss- annual annual heating
(load) heat costs (2005) only for
W/m²K W loss kWh/(m²a) external wall losses €/a
1.25 4125 98 536,-
0.125 412 10 54,-
In the first row (red) the values for a typical external wall of an old building are given, not the less insulated one. Some 536 € have to be payed every year just to compensate the heat losses through 100 m² of this wall. With an additional insulation of a quality used in passive houses (green) the heat loss is reduced by a factor 10. The annual costs of the energy loss now are lower than 54 €/a. That means:
482 €/a cost reduction for heating.
What has to be done to achieve at this energy saving? This is, what we recommend: To wait, until the external wall need a new plaster or a new painting anyhow - that can not last too long, unless it has just been done. Then the costs for the scaffold and the new facade painting have to be payed anyhow, that ammounts to some 2500 €. Now ask your credit institute for the volume of a hypothecary credit to be payed back by a annual rate of 480 €/a for interest and redemption over 20 years. This credit will ammount to some 6300 € taking contempory conditions into account. Together with the 2500 € anyhow-retrofit costs it is 8800 € now for the measures to be taking at the external surface of this wall. There is no question that one can get a top super insulation by spending this money. And, for a new construction, it will be even more attractive.
Do you think that just sounds like a zero-sum game? To spend all the money saved just for handycrafts work? Well, it is not the whole truth:
1. It is very probable that energy costs in the near future will be even higher than calculated here.
2. The insulation will last at least 40 years, even if the facade has to be repainted again in 15-25 years - like a not insulated wall as well. But the insulation will do its work, the saving of energy costs, also after the 20 years of the credit duration. And there will be no costs for that at all. This is called the "golden end" of investments in the case of power stations and similar things.
3. The additional advantages of the insulation are free of cost: No cold edges, no mould behind the cabinet, a very comfortable indoor climate without cold raditation an without cold air flow at the floor.
4. ...and, if it is a new construction or a comprehensive upgrading, it will be a step towards a passive house, with an asured anduring high thermal comfort.
5. Last not least: These measures are in Germany and Austria supported by governmental money. That has not been taken into account in the calculation given above.
Conclusion: It is attractive. The right desicion is "whensoever, take the best insulation available" . This holds for new construction and for refurbishments.
http://www.passivhaustagung.de/Passive_House_E/insulation_passive_House.html
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