Passive houses are working examples of dwellings with very good thermal insulation standards.

An exemplar of correct detailing: a highly insulated eaves joint (CEPHEUS-Project Horn/Austria, Architect Treberspurg/Vienna).

Examples of constructions suitable for passive houses with an excellent thermal insulation: Masonry construction with thermal insulation compound system, timber construction, lost forms of insulation.

Infrared picture of the joint between the basement floor an the external wall in a passive home (taken from the inside). There are no "cold spots" (photo: PHI).

Infrared pictures of an old building and a passive house (at the bottom) for comparison (photos: PHI).

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.

Lots of the insulation technologis will be demonstrated at the exhibition taking place during the 11th international conference on passive houses.

Experience

The experience during construction of passive houses has shown, that the high insulation thickness can be realised with conventional insulation materials without any problem:

• The place for the insulation is available in almost all construction tasks. In cases, where the additional place is missing or where it is expensive, one can have a workaround using materials with lower thermal conductivity.
• The high insulation thickness is easy to deel with at the building site. Done the right way, the effort for construction ist only slightly higher compared to lower thickness. What remains are the costs of the pile of insulation material – but those are comparably cost efficient materials. How to design a reasonable construction suitable for passive houses using different materials will be demonstrated at the exhibition taking place during the 10th conference on passive houses and at the study trip on Mai 1st.
• It tourned out that all construction used in building envelopes in Germany can be improved in a way to be suitable for passive houses. That has been demonstrated in a manifold of already completed passive houses: there are masonry constructions (cavity walls as well as constructions with external thermal insulation compound systems or with curtain walls), prefabricated elements from porous concrete, prefabricated concrete slabs, timber constructions (classical stud walls or using light weight studs), lost forms of rigid insulation materials filled with concrete on site, metal frame construction and semitranslucent wall elements.
• The monitoring of built passive house has shown, that the insulation effect of the thick insulation layers meets exactly the expectation. The heat loss is indeed that very low, it has been calculated and the buildings can be kept comfortable using the calculated low heat production capacity. That can be seen directly by the high temperatures of the internal surfaces - made visible by IR-pictures (see the thermographic photos on the left hand side).

Super insulating components, like those beeing used in passive houses, have important advantages compared to poor or mediocre insulated building envelopes.

• High surface temperatures of the interior surface during winter are an automatic consequence of the low heat loss. It is not needed to heat these surfaces directly. On this reson the radiation temperature differences from various directions are small, this is a good condition for high thermal comfort. In addition the high surface temperatures reduce the relative humidity at this place. Therefor demages by humidity coming from the indoor air can be excluded in passive houses.
• During Summer the internal surface temperatures are near to the indoor are temperatures, again - i.d. beeing lower than with poorly insulated construction, that will transport more heat from outside to inside. Looking at the instationary behaivior. Highly insulated constructions do provide a high damping of the temperature amplitude even with low heat capacities (a double layer of gipsum board will be sufficient). More important is a high time constant of the whole building. This emerges from a good insulation, which will contribute to a good approach to the interior heat capacities. On this reason night cooling is a very good strategy in passive houses - the structure can be kept cool during the day – provided that the solar load is limited to a certain level.
• Super insulated constructions in a certain extend forgive still existent thermal bridges compared to mediocre insulations - this can be important for the refurbishment of existing buildings. This is contradictionary to prejudices, but has been proven in numerous demonstration buildings and is easy to understand: If the constructive structure is on the inside of a thick insulation, this will have a high temperature through and through. Some thermal bridges up to a certain extend will not change that – on the other hand, if a major part of the construction is cold anyhow, an additional thermal bridge can rapidly lead to a temperature below the dew point. Not to be misunderstood - of course thermal bridges may lead to increased heat losses, even in a passive house. Therefore the strategy of a design avoiding thermal bridges is strictly recommended. But during refurbisment of old buildings it might not be totally successfull - and it is in this case, there one could profit from the forgiveness of super insulation.

Latest research results and experiences with highly insulating constructions will be presented in workshop 4 of the 11th International Conference an Passive Houses.

Complete examples of building elements for passive houses will be presented at the exhibition in Bregenz. One will find:

• Masonary constructions with external thermal insulation compound systems and well designed details for the slab / wall-joint, the joint from the external wall to the window and the eave (see also figure 2 from top).
• Lost form from rigid insulation with an exhaustive catalogue of joints avoiding thermal bridges and know-how for airtightness.
• Timber panel construction including all important joint details with various different constructions.

(updated: 2006-09-23 Author: Wolfgang Feist; thanks to Gavin Hodgson (BRE Ltd UK) for proof reading the translation of the 1st edition 
© Passive House Institute; unchanged copy is permitted, please give reference to this site)