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
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.
annual costs (2005)
only of the heat loss
of external walls €/a
3300 78 429.-
0,60 1980 47 257.-
0,40 1320 31 172.-
0,15 495 12
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):
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
porous concr. 0.110 0.83
lation material 0.040 0.300
material 0.025 0.188
pressure) 0.015 0.113
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.
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:
heat loss- annual annual
costs (2005) only for
loss kWh/(m²a) external
wall losses €/a
0.125 412 10
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
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:
- It is very probable that energy costs in the
near future will be even higher than calculated here.
- 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.
- 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.
- ...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.
- 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.
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
The experience during construction
of passive houses has shown, that the high insulation thickness
can be realised with conventional insulation materials without any
- 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
- 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
- Timber panel construction including all important
joint details with various different constructions.
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)