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The Passive House in summer

Summer climate in the Passive House – an important issue

The question of low-energy buildings overheating in summer “due to their high level of insulation” is still one that is frequently raised in the public debate.1) In this study we document research on a residential Passive Houses in climates, where a passive temperature control in summer is possible (as was up to the year 2003 the case in most parts of Central Europe). Practical experience with such passive summer performance of Passive Houses has clearly shown that these houses have a pleasant (cool) indoor climate even during excessively hot Central European periods. However, this requires professional planning, made possible by reliable tools like the PHPP. The PHPP implemented a passive cooling sheet. This article deals with the summer characteristics of Passive Houses in climates such as that in Central Europe – where residential buildings typically did not require active cooling so far. This may change with climate change, if the frequency of tropical nights will further increase.

Fig. 1: Indoor temperatures during the hot summer of 1993;
measured on three different floors on the southern side of the
Darmstadt-Kranichstein Passive House (see picture below)

passivhaus_kranichstein_sommer.jpg The four terraced houses of the first Passive House development in Darmstadt-Kranichstein are solid constructions which are oriented exactly towards the south (picture on the left). They are fitted with easy-to-use temporary shading devices (motorised external blinds) and they provide the possibility of free night-time ventilation in summer. In addition, the internal heat sources are very small due to the electricity-saving concept. These are favourable conditions for a cool interior climate in the summer (which will work also in hotter climates, night ventilation of course only, if there are still low temperatures and low humidities during night).

Thermal building simulations are used to determine the summer characteristics of Passive Houses built according to different construction methods and having a different orientation – taking account of the type and level of shading and ventilation. The first systematic investigation was carried out in the “Passive House Summer Climate Study” which was completed in 1998 ([Feist 1998a] ). The study was the result of the joint research project carried out on behalf of G&H Ladenburg, ISORAST GmbH Taunusstein, Nordhessische Kalksandsteinwerke GmbH&Co; Rasch&Partner GmbH Darmstadt; Schwenk Dämmtechnik GmbH Landsberg and VEGLA GmbH Aachen. We should explicitly like to thank the initiators at this point. Since then, the findings have also been confirmed in practice in numerous realised Passive Houses. A metrological concomitant study, in which the focal point was on the summer situation, was published in [Peper/Feist 2002] .

In this paper some parts of the study are summarised and substantiated based on existing measurement results. An earlier version of this article was published in 1999 in the Protocol Volume of the Working Group for Cost-efficient Passive Houses Volume 15 ([Feist 1999] ). A procedure was developed in this working group with which the results for the summer case could be determined more easily. This PHI Summer Case procedure has been documented in the Protocol Volume. Since 2000 the procedure has been included in the form of spreadsheet formulae in the PHPP (Passive House Planning Package) [PHPP 2007] . Each planner of a Passive House can determine the influences, as dealt with below, for his/her own building project by using this and thus achieve a comfortable summer climate by designing the building professionally.

Methodical basis: building simulation, house model

The instationary simulation software DYNBIL was used for the simulation of the thermo-technical characteristics. The program and its procedures were systematically tested in [Feist 1994] . Thus a validated simulation model is available which allows for highly reliable predictions about the temperature processes in the Passive House. (See Dynamic simulation of a building's thermal performance).

For this study, the Frankfurt/M DYNBIL climate year was used as a climate data set; this is based on the test reference years, but uses a corrected data set for the atmospheric counter radiation.

The analyses are based on plans of the inhabited Passive House in Darmstadt-Kranichstein. The plans of a mid-terrace house were reduced to a simpler basic model for the optimisation. The basic model is clear enough while reflecting the zoning of the house and allowing for the simple modification of the essential characteristics of the model. The model with seven zones has been described in detail (in [Feist 1993] ):

zone -I

zone 0

zone I

zone II

zone III

zone IV

zone V

zone VI

zone VII
ground temperature 1m below floor slab

outdoor air temperature


ground floor (front):living area

ground floor (rear):kitchen &entrance area

upper floor(front): children’s bedroom

upper floor(rear): bedroom

attic: guest room/study

centre: bathrooms & staircase
Fig. 2: cross-section of the Darmstadt Kranichstein Passive House including the individual zones.

The model parameters have been documented in detail in the study [Feist 1998a] ; Tab. 1 gives an overview of some of the essential parameters for this base case.

Table 1: Parameters for the Darmstadt-Kranichstein Passive House (as built, using data from the simplified model, but all zones summed up) during summer period (mid-terrace house): U-values, ventilation, inernal sources

Evaluation based on operative temperatures

The operative temperature is also a decisive factor for the assessment of summer comfort; moreover, air humidity (sultriness limit!) and air speeds play an important role. As first and foremost the Central European climate is being dealt with here, the study initially concentrates on the operative temperatures [Kirtschig 1998] . It has been shown that with the “standard effective temperature” – (SET) generalisation of this concept is also possible for hot and humid climates [Wang 1996] .

The reference case (Kranichstein Passive House without window ventilation)

Fig. 4 shows the daily mean values of the indoor temperatures during the year for the reference case of the “ Darmstadt-Kranichstein Passive House – without any shading or window ventilation”. For the operation of the ventilation system, it was assumed here that

  • the heat recovery (80%) is in operation only in winter,
  • in the summer (more exactly: from 15th April to 30th September) the ventilation system is operated only as an exhaust system with air changes of 0.475 h-1 .

From around 10th July temperatures of 25 °C or more are prevalent in all rooms, during very hot periods between 30th August and 8th September of the reference year, they even rise to 30 °C. Except for a few days during this hot period, the indoor climate in the Passive House is also comfortable in summer. Later on, other cases will be dealt with which result in much more favourable indoor climates due to increased ventilation.

Fig. 4: The daily mean values of the indoor air temperatures for the base case during the year
(without window ventilation , without temporary shading)

Fig. 5 shows detailed temperature courses during the hot period in the room “South Top Floor”. One can see that all indoor temperatures in this time period gradually increase from a level of barely 25° up to 28 and 30°C. The surface temperatures are still somewhat warmer than the indoor air; however, this temperature increase is less than about ½ degree for the highly insulated solid building components. The interior surface temperature of the window panes is very high: at up to 40°C it is 10 K above the maximum indoor air temperature. It should be noted that in this base case (except for the small overhang of the mini-balcony), neither temporary summertime shading nor window ventilation has been implemented.

Fig. 5: detailed temperature course of surface and indoor air as well as the operative temperature
during the critical time period in the “South Top Floor” room (reference case, without window
ventilation and without temporary shading in summer)

Fig. 6 depicts the indoor air temperatures of the reference case as a regular annual duration curve; it should be understood as follows: in Zone VI (upper floor southern side) of the house the temperature hasn't exceeded 25°C for 7816 h of the year. The temperature is higher only for a time period of 943 h. This is equivalent to a frequency of overheating events of hθ>25°C = 10.77%. This frequency of overheating events is a good indicator of the summertime climate in buildings. Based on previous work, the summer climate in residential buildings is still acceptable if hθ>25°C remains less than or equal to 10% of the year [Kolmetz 1996] . This value has only been exceeded by less than 1 % in the reference case being considered here.

Fig. 6: The indoor air temperatures of the reference case (without window ventilation and without
temporary shading) as a regular annual duration curve.

Base case of a Passive House with "tilted windows when required"

All the information of the reference case dealt with in Section 3 remains unchanged except for intervention by the user:

If temperatures in the house exceed 21 °C and the external temperature is lower than the indoor temperature,
in each room a window is placed in the “tilted” position. This is possible in the Darmstadt Kranichstein Passive House, where there is at least one window with a turn-and-tilt fitting in each habitable room.

The tilted position of the window leads to considerably higher average air changes. Fig. 7 shows that due to this, the temperatures in the house sink perceptibly to constantly comfortable levels during the summer. This is one of several reasons, why we always recommend to have at least one window in each room openable and also have a tool to keep it fixed at a specific level (e.g., a "tilted window").

Fig. 7: Base case of a Passive House with tilted windows when required in summer, without
temporary shading; daily mean values of the indoor air temperatures in six zones; simulation run.

In the Passive House, even during the critical hot period, it can be seen that with this method, an excellent indoor climate prevails, although there is no temporary shading (Fig. 8). The annual duration curve (Fig. 9) shows that the frequency of overheating events becomes extremely small: hθ>25°C = 0.68% (equivalent to 60 h).

The case under consideration corresponds (except for the difference between the end-of-row and mid-terrace houses) to a certain extent with the actual situation that existed in the years 1991 to 1993 in the end-of-row house on the western side of the inhabited Passive House in Darmstadt-Kranichstein - there were no shading possibilities there 2), but ventilation by tilting the windows was possible. Occupants and visitors were often surprised that the house remained pleasantly cool, even in hot periods in the summer. Fig. 1 shows measured temperatures from all three floors, which are even more favourable in comparison with the simulation from Fig. 8. This is mainly due to the fact that the windows could be opened wide for some time in the early morning hours. The summer of 1993 was particularly hot in comparison with the average for many years.

Fig. 8: Big South facing windows case (19 m²) of a Passive House with tilted windows when
required in summer (no temporary shading); temperature details for the warmest zone during
a pronounced period of hot weather; DYNBIL simulation run.

Fig. 9: Base case with tilted windows when required, without temporary shading; annual duration

Fig. 10 shows the progress of the overheating events when the air exchange is gradually increased by means of a controlled ventilation system. If the heat recovery is also operated in summer, resulting in an energy-equivalent air change rate of about 0.1 h-1, the frequency of extremely high temperatures and overheating events would reach 35%. This type of operation should be excluded in any case:

ventilation units must allow summertime operation without heat recovery

in Climates, in which outside temperatures during Summer are still often less than the 22°C (which is the case almost everywhere in Europe except South of some 45° North) This is also given as a certification criterion for Passive House suitable ventilation systems – and can be achieved by means of a bypass or a summer cassette or by operating the exhaust air fan only.

With exhaust air operation only (corresponds to 0.475 h-1), the reading is 10.77% (see reference case). If the permanent air exchange is further increased, the frequency of overheating events decreases almost proportionally to an n-1 function. Please note, however, that increasing the automatic air exchange to 0.7 h-1 in summer will require additional electricity to run the fans. Nevertheless this is a possibility for buildings in which opening windows is not possible due to reasons of noise protection, for example.

It is better to rely on natural ventilation in summer with tilted windows, as in the case above. It can be seen in Fig. 10 that the case considered above, with windows being opened when required, corresponds to a continuous air exchange using only exhaust air of about 1.4 h-1 . If this exhaust air flow is produced mechanically by means of an efficient system, the electricity consumption in summer for the ventilators would be about 560 kWh; window ventilation is possible without any additional running costs. (Using 560 kWh can also produce some 1000 kWh of active cooling using efficient split-cooling-units. This might in most cases be a better option, espacially in regions with hot or humid night conditions. See the study of Jürgen Schnieders for Mediterranian summer conditions.)

Fig. 10: The influence of the ventilation on the frequency of overheating events in the Passive
House in summer (base case, simulation); adequate cooling can be achieved just by tilting
windows. There is no temporary shading in the case calculated here!

The influence of the window size and quality of glazing

In the Darmstadt-Kranichstein Passive House glazed areas make for 35% of the southern facade. First generation triple glazing with low-e coating was used, which has a U-value of about 0.71 W/(m²K) and a g-value of 49.5%.

Fig. 11 shows how the thermal comfort progresses in summer if the window type and size are changed.

Fig. 11: The influence of the size and quality of glazing in the reference case; the frequency of
overheating events rises sharply with increasing glazing areas of more than 20 %.

The results for the summer climate are given on the basis of the excessive temperature frequencies hθ>25°C :

  • Independently of the glazing, a temperature of 25 °C is not achieved with south-facing glazing areas smaller than 14 %, even in summer. The problem of overheating in summer does not arise in the Passive House with smaller windows; however, with such small windows, the specific heat demand is just below 15 kWh/(m²yr). Larger windows are also recommended for reasons of daylight utilisation (see Ursula Schneider's contribution at the 10th International Passive House Conference [Schneider 2006] ).
  • The frequency of the occurrence of temperatures over 25 °C increases with increasing south-facing windows areas with low-e triple-glazing: with the following glazing proportions in the south-facing facade:
    • about 30% with triple glazing and low-e coating
    • about 25% with “3-Magnetron” (clear glass)

good values can still be achieved without temporary sunshades in the Passive House.

  • In contrast, with a glazing proportion of
    • over 42% with triple glazing and low-e coating
    • over 35% with “3-Magnetron” (clear glass)

there are such high solar gains in the summer in the base case under consideration that additional measures have to be taken. These will be dealt with later on.

It is interesting that, for normal window sizes, the double glazing with low-e coating and the triple Magnetron glazing lead to practically the same summer climate conditions: the g-values of both are almost identical as well.

This analysis shows that in residential buildings possible overheating during the summer is mainly caused by (much too) high solar gains. However, this can be avoided by professional planning and with the help of simple components available on the market (blinds, curtains, overhangs …). This analysis is true in almost all climates. During hot conditions, reducing solar loads through is the most important measure.

The influence of orientation

Fig. 12 shows the effects of different orientations of the main facade. The course of the triple glazing with low-e coating is depicted. A glazing area of 19.127 m² (equivalent to 34%, this is the size in the reference building) is assumed.

Fig. 12:Dependence of the hθ>25°C and the annual heating demand of a mid-terrace Passive House
on the orientation of the main facade (3-WSK, 19.127 m², reference case, without temporary
shading and without window ventilation)

  • The annual heating demand (between 10 and 12 kWh/(m²yr)) as well as the frequency of summertime overheating events (15 to 18%) change only slightly if there is a deviation from the ideal southern orientation of ± 30° at the most.
  • Then however, overheating events as well as the heating demand increase noticeably. In the area between 60° and 90° towards the south, the frequency of overheating events reaches a maximum of 20%. With an orientation of 90° (west or east), the annual heating demand already reaches a maximum of about 16 kWh/(m²yr).
  • This hardly changes with further deviation from the southern orientation, i.e. in the case of winter, a northern orientation is barely less favourable than an eastern or western orientation. In the case of summer, however: the overheating frequencies fall sharply with further orientation towards the north. The hours with 10% overheating are smallest with ±45° with northern orientation.

Based on these results, it can be understood why buildings with large glazing areas that are mainly oriented towards the east or west have problems with summer comfort. A more detailed analysis shows that with very large glazed areas, the maximum frequency of overheating events is nevertheless reached with a southern orientation because otherwise overheating can also occur during winter and in the transitional periods.

The influence of fixed horizontal shading elements over windows (roof or balcony overhangs)

Here we will assume that there are no fixed shading elements (this is not the reference case!). For this case only the balcony and roof overhangs are increased (measurement is always made from the external surface of the glazing, with a distance of 0.59 m above the upper glazing edge, see Fig. 13).

Fig. 13: Fixed balcony and roof overhangs
above south-oriented glazing can reduce the
energy gains in summer considerably. With
the Passive House level of insulation heating
demand is only slightly increased by these
(see Fig. 14), if the overhang is not too large.
The overhang from the external surface of
the glazing, and the distance from the upper
glazing edge is measured.

Fig. 14 shows the changes for this case study:

  • In a Passive House the annual heating demand does not change with overhangs of up to 1.25m depth (it is different for low-energy houses).
  • In contrast, the frequency of overheating events in summer decreases noticeably with horizontal overhangs with depths between 0.5 m and 1.5 m (from hθ>25°C = 22% to less than 7%).

Even larger overhangs increase the annual heating demand significantly, but there is hardly any improvement of the summer climate.

Fig. 14: Effects of fixed shading elements at a distance of 0.59 m above the glazing edge on the
annual heating demand and the frequency of overheating events (window flush with the outside,
no temporary shading, south-oriented, no window ventilation).

From this, directly practical recommendations can be given: the fixed horizontal shading elements over windows in south-oriented Passive Houses with an overhang of about 1.25 m have a favourable effect on the summer climate in Germany, without increasing the heating demand noticeably.

The influence of temporary shading

Fig. 15 shows that the temperatures in the Passive House (reference case) decrease noticeably with the use of external shading elements during the summer. Temperatures above 25°C do not occur at all (hθ>25°C becomes less than 0.5%).

Roller blinds between the glass panes also have the same effect on the indoor climate as external blinds, and even roller blinds on the inside reduce the temperatures in summer perceptibly: hθ>25°C drops to 6.8% in comparison with 10.8% in the reference case. Hence, although roller blinds on the inside are not as effective as external blinds or roller blinds between panes, together with increased summer ventilation, for example, they can help to mitigate the heat.

Fig. 15: By using temporary shading attachments comfort in the summer can be improved
significantly. However, with the glazing quality with triple panes and higher absorption of the
glass as used in Passive Houses, shading on the inside of the windows is only of limited benefit.

The problem of possible unacceptable warming of the glazing due to back reflection and the heat build-up due to interior blinds will not be discussed at this point. These problems must be resolved before the practical implementation of such systems ([Feist 1998b] ).

Temporary shading is equally effective for all window orientations - but unfortunately this doesn't apply for the fixed overhangs previously dealt with, which only provide an insignificant summer shading benefit with orientation towards the east or west. Temporary shading is therefore appropriate if overheating in summer is expected. This is very much in line with the experiences in Mediterranean countries, for example, where it is customary to equip traditional buildings with window shutters that are kept closed during the day.

The influence of the internal heat sources

For the summer case study, an average internal load of 386.8 watts was calculated in the cases previously considered. This is equaivalent to 2.48 Watt/m² and is slightly higher than the value assumed for internal heat sources in calculations with the Passive house Planning Project (it is safer to use the somewhat higher value for the summer case; however, during the heating period the smaller value should be used in calculations to be on the safe side).

In Fig. 16 shows how, based on the DYNBIL simulation, the annual heating demand is reduced if the useable heat sources increase:

  • At first the heating demand decreases almost in line with the additional usable free heat, with a utilisation factor of about 80 %.
  • However, the curve quickly levels out with higher gains. The “zero-energy house” standard is only achieved when the internal heat sources increase by three and a half times.

⇒ This would be equivalent to an additional internal energy conversion of 8470 kWh/a; this is neither economically nor ecologically sound.

Fig. 16: The influence of the different levels of internal heat sources on the annual heating demand
and the frequency of overheating events in the Passive House (mid-terrace reference case).

Each additional internal heat source also has an extremely unfavourable effect on the summer comfort, (see Fig. 16).

  • At first, the frequency of overheating events also increases almost in line with the internal heat sources; doubling of the sources equates to about 2.3 times more hours of overheating.
  • The frequency of overheating events increases disproportionately with internal heat sources of more than 5 W/m². In the extreme example mentioned above, with a 3.5-fold increase in heat sources of 8.7 W/m² , the frequency of overheating events hθ>25°C in this building would occur on more than 64% of the year. The summer climate in such a house would be unbearable.

The study demonstrates the importance of paying attention not only to good insulation and heat recovery in the Passive House, so that the annual heating demand is low, but also to lower internal heat sources, in particular heat dissipation from electrical devices and pipes, boilers etc.. Here especially increased energy efficiency is necessary, as this will contribute to summer comfort and in relation to environmental protection and finances it will also have a positive effect. Efficient electricity use is today's requirement!

Reference case “lightweight timber construction”

The daily mean temperatures during the course of the year as shown in Fig. 17 with the same floor plans, ventilation technology and window sizes and glazing qualities as the Passive House built in Darmstadt, but using only lightweight timber building components.

  • The annual heating demand is 12.8 kWh/(m²yr)
  • The frequency of overheating events is 17.7%.

The frequency of overheating events increases significantly in lightweight construction. Due to the lower thermal inertia of the building, the temperatures fluctuate considerably more than they do in the reference case. In particular, indoor temperatures of up to 25 °C in the months from November to February are now also possible. The highest daily mean temperature with 34 °C on 4th September is far above the tolerable level. It can be seen even from this initial rough analysis that for a purely lightweight timber construction, additional measures for providing an acceptable summer climate are necessary in each case (shading and/or additional ventilation).

Fig. 17: The daily mean values for the indoor temperatures in the course of the year in the base
case of the “purely lightweight timber construction”.

Fig. 18 shows in detail the selected surface temperatures and the indoor temperature in the “top floor south side” room during the critical hot period. The increase in temperatures is significantly higher in comparison with Fig. 4 (reference case) – temperatures between 32 and 36°C are reached. Also the daily temperature amplitude is recognisably higher: e.g. for the interior surface of the external wall it is about 2.5 K.

Fig. 18: Details of the temperatures during the hot period at the beginning of September in the
“purely lightweight timber construction” reference case; additional measures are inevitable.

Both are the result of the lower storage mass of the building, due to which the time constant is reduced. For the evaluation of the results it must be ensured that in this reference case

  • no window ventilation and
  • only minimal summer shading

takes place.

⇒ Measures of one kind or another are indispensable in the case of lightweight timber construction considered here.

Fig. 19 shows in an exemplary way that in this case also, the indoor climate can be greatly improved through increased ventilation through tilted windows in summer. The frequency of overheating events will thus be hθ>25°C = 3.9%. That is still considerably more than for a solid construction in a similar case, but even it is an acceptable summer climate. The difference in the instationary characteristics becomes clearer if the hourly temperature values of the hot period from 30th August to 6th September are compared (Fig. 19 in comparison with Fig. 8). The final temperatures for the lightweight construction are higher by 2 K, the temperature amplitudes are also higher.

Fig. 19: Summer ventilation using tilted windows when required: details of temperatures in the
hot period at the beginning of September for the “purely lightweight timber construction”
(compare Fig. 19 with Fig. 18 as well as with Fig. 8 ).

Besides the cases described in the examples here, other parameter variants for the lightweight construction case have also been studied and documented in the final report of the Summer Case Study. Moreover, further construction variants have also been considered in this: e.g. lightweight construction with slightly higher effective mass (due to double planking with plasterboard) and buildings constructed using concrete formwork blocks and in the mixed construction method. The dependence of the heating demand and the frequency of overheating events on the effective heat capacity has also been systematically represented there. In Europe, there is a good summer comfort solution for each construction method - for a specific case the right solution can be found with the PHPP.

Improved thermal insulation: good or bad in the summer?

The most important influencing parameter for the annual heating demand which decides whether the Passive House standard is achieved or not (i.e. not exceeding 15 kWh/(m²yr)) is the thermal protection of the opaque external building components, particularly that of the roof and external walls. It is often assumed that increasing the level of thermal protection would lead to increased problems with overheating in summer. In order to study the influence of the insulation on the indoor summer climate, the thermal transmittance coefficients of the roof and external wall were varied according to Table 2.

of the level
of insulation
Thickness of
the insulation
panels EW
Thickness of
the insulation
panels RO
EW and RO
Variant 1 300 0,126 400 0,093 0,116 11,3
Variant 2 175 0,209 300 0,122 0,182 14,6
Variant 3 100 0,342 200 0,175 0,289 20,4
Variant 4 50 0,598 100 0,312 0,508 32,5

Tab. 2: Variation of the level of insulation and the resulting thermal transmittance coefficient of the opaque
building component (external wall EW and roof RO); the same external components were used as for the
Passive House built in Darmstadt-Kranichstein.

Fig. 20 shows that the frequency of overheating events actually does increase with improved insulation for the summer reference case “exhaust air operation only”: the internal heat load and the solar gains in summer are so high that additional heat losses due to poorly insulated building components lead to increased removal of surplus heat. According to this result, Passive Houses should actually have greater problems with indoor climate in summer than ordinary houses with the same layout and same solar apertures.

Fig. 20: dependence of the hθ>25°C on the average U-value of the opaque building component for
the reference case of the mid-terrace house for different insulation thicknesses; summer operation:
purely exhaust air without window ventilation.

At first, this result is apparently in contrast with the excellent indoor climate in summer in the Passive House in Kranichstein.

  • The contradiction is resolved if the excessive temperature frequencies are considered using a different and practically-oriented summer ventilation strategy (Fig. 21): if the windows are tilted in summer when required, the excessive temperature frequencies decrease quite considerably for this solid construction.
  • Not only that: also the thermal protection level of the roof and wall has the reverse effect. With poor insulation, at first there are higher excessive temperature frequencies (about 0.5 %), which fall to a minimum within the range of the Passive House Standard.

Fig. 21: Dependence of hθ>25°C on the average U-value of the opaque building components in the
Passive House with modified insulation thicknesses; but this time with windows tilted open in
summer when required. It now becomes clear that better thermal protection is advantageous for
a good indoor climate in summer.

The difference in the summer comfort with different levels of insulation is not very great. However, from this study, it is clear that, if it is possible to tilt open the windows in summer in Passive Houses, the indoor climate is not any worse than it is in low-energy houses or with a poorer standard of insulation.

The results can be easily explained:

  • If it is possible to ventilate through windows in the summer, surplus heat can be “disposed of” effectively, if the outdoor temperatures are low enough. The house can thus be cooled to a comfortable level in summer.
  • In contrast, if it is very hot outside, the windows are kept closed: the improved ventilation even helps to restrict the entry of heat through the opaque building components. Such a house can be kept “cool” more easily than houses with poorer insulation.
Good insulation helps in summer as well as in winter

The result of the study about the level of thermal protection leads to another guideline for planning: improved thermal protection reduces the heat losses in winter quite considerably for one, and secondly it helps to keep the indoor climate cool – but only if adequate ventilation is possible in the summer.

Does the temperature amplitude ratio still have any influence?

In many older publications emphasis has been placed on the importance of the temperature amplitude ratio (TAR) and the phase shift of external building components for comfort, particularly in summer. Even today some authors hold the view that these parameters have a great influence.

In the Summer Case Study, various external walls were selected so that instationary heat transfer characteristics varied as much as possible, in order to fully analyse these effects (Fig. 21).

Fig. 22: Dependence of hθ>25°C on the temperature amplitude ratio of exterior Passive House walls
having the same level of insulation and the same heat capacity.
The glazing ratio serves as an independent variable.

1. “LS17&30”: “solid construction”; exterior insulation: 300 mm expanded polystyrene (EPS); interior: 175 mm lime-sandstone
wall (LS wall); U=0.126 W/(m²K); area-specific heat capacity: 106.5 Wh/(m²K).
2. “LS IntInsul”: The “inverse“ of the previous component: EPS insulation on the inside; lime-sandstone on the outside;
same heat capacity, same U-value.
3. “2*Sandwich“: insulation and LS wall devided in two parts: 87.5 mm LS; 150 mm EPS; 87.5 mm LS; 150 mm EPS;
same heat capacity, same U-value
4. “ HomWa“: “homogenised” average thermal resistance and heat capacity for the component. UW= 0.126 W/(m²K),
same area specific heat capacity 106.5 Wh/(m²K).

Fig. 22 shows the development of hθ>25°C with varying south-facing window areas for building variants for which the external walls with the different building components given above have extremely different TAR values; the values differ only slightly for the whole range. We can conclude that:

The temperature amplitude ratio of opaque external building components does not play a role for the Passive House insulation standard any more – neither for the annual heating demand nor for summer comfort. The reason is that highly insulated components already cause such a reduction of the amplitudes, irrespective of the time constant, such that the additional dynamic attenuating effects are not relevant any longer. With (more) poorly insulated components the influence of the becomes recognisable.

Summary and conclusions regarding indoor comfort in Passive Houses in summer

The Passive House Summer Case Study dealt with the influence of various structural parameters on the level of comfort in Central European summertime. The frequency of overheating events with “overheating referring to temperatures exceeding 25°C was used as the most important parameter, i.e. the ratio of the number of hours in which the temperature exceeds 25 °C to the number of hours in the year (8760 h). The instationary simulation program DYNBIL was used for the study, for which a building model of a mid-terrace house that had been validated using the Darmstadt-Kranichstein Passive House was applied.

The variation of the structural parameters enables the following conclusions to be drawn:

  • With regard to the insulation level: with sensible user behaviour, better thermal protection can help to improve comfort in summer. Contrary to the fears often expressed about this, the Passive House does not have a specific “summer climate problem” in Central Europe.
  • With regard to ventilation: of course it is advised that the balanced ventilation with a heat exchanger existing in the Passive House be operated without the heat recovery system periodically in summer. Very good results can be obtained if summer ventilation through tilted windows is consciously carried out when required (especially at night).
  • With regard to glazing: apart from the ventilation, the most important influencing parameter for summer comfort is the effective solar aperture. Overheating does not occur with small windows in any case. A vertical southern orientation is much more favourable than, for example, an eastern or western orientation. Guidelines are provided detailing the additional measures (shading) required recommended for a certain type of aperture.
  • With regard to shading: In contrast with an ordinary low-energy house, for south-oriented windows, horizontal fixed shading (balcony overhangs) with a depth that is not too large(1.2 to 1.6 m for room-height windows) is very effective for sun protection in summer, without increasing the annual heating demand too much. Temporary shading equipment on the outside and also shading in the outer space between the glass panes of the triple-glazing is very effective.
  • With regard to the building mass: In Central Europe, it is easier to keep buildings with a larger effective internal mass at cool temperatures than purely lightweight buildings. With excellent thermal protection of a Passive House a good indoor climate in summer can be achieved for the latter. Solutions for a good indoor climate in the summer are available for all construction methods. The PHPP Summer Sheet can be used for planning this.
  • With regard to the temperature amplitude ratio TAR: With the insulation quality of the Passive House the stationary attenuation is already so large that the dynamic attenuation and thus the TAR no longer play a role.

With the help of the PHPP Summer Sheet, the architect can design a building with an acceptable level of thermal comfort in the summer using this information. Simplified algorithms were developed and tested within the framework of the scientific programme of the Working Group for cost-efficient Passive Houses, which allow a sufficiently accurate representation of the dependence of the sum- mertime indoor climate on the structural parameters as described in this article. The resulting procedure was published in [Feist 1999] .

The procedure was later converted to spreadsheet formulae and incorporated into the PHPP as the Summer Sheet. This Sheet uses the same data input that has already been entered for the PHPP monthly balance method - only the specific values needed for the summer case are additionally required; these are:

  • Type and amount of additional summer ventilation,
  • Type and cap factor of the temporary summer shading equipment for each window,
  • Applicable maximum temperature limit for summertime.

Based on these values, the Summer Sheet calculates the frequency of hours with excessive temperatures hθ>25/26°C.

Using this tool, it is very easy to plan a Passive House with superior thermal comfort in summer in Central Europe.

See also


[DIN 1946] „Raumluftqualität, Gesundheitstechnische Anforderungen (VDI-Lüftungsregeln)“; Januar 1994 (“Indoor Air Quality, Hygiene Requirements (Ventilation Regulations of the German Engineers’ Association)”, January 1994)

[Feist 1993] Feist, Wolfgang: „Passivhäuser in Mitteleuropa“; Dissertation, Universität Kassel GhK, Kassel 1993 (“Passive Houses in Central Europe”; Dissertation, Comprehensive University of Kassel, 1993)

[Feist 1994] Feist, Wolfgang: „Thermische Gebäudesimulation“; 1.Auflage Karlsruhe 1994 (“Thermal Simulation of Buildings”, 1st Edition Karlsruhe 1994)

[Feist 1997] Feist, Wolfgang (Hrsg.): „Energiebilanz und Temperaturverhalten“; Protokollband Nr. 5 des Arbeitskreises kosten¬günstige Passivhäuser; Darmstadt 1997 (“Energy Balance and Temperature Characteristics”, Protocol Volume No. 5 of the Working Group for Cost-Efficient Passive Houses; Darmstadt 1997)

[Feist 1998a] Feist, Wolfgang: „Passivhaus Sommerklima-Studie“; Passivhaus Institut, Darmstadt 1998 (see PHI's list of publications) (“Passive House Summer Climate Study”; Passive House Institute, Darmstadt 1998)

[Feist 1998b] Feist, W. und Holtmann, K.: „Erhöhter Glaseinstand kann Gefahr von thermisch induzierten Scheibensprüngen reduzieren“; Gff (Glas Fenster Fassade), Heft 5/1998 (“Increasing the mounting depth of the glass pane can reduce the risk of thermally induced cracking of glass”; Gff Magazine (Glass, Windows, Facades), Issue 5/1998)

[Feist 1999] Feist, Wolfgang (Hrsg.): „Passivhaus Sommerfall“; Protokollband Nr. 15 des Arbeitskreises kostengünstige Passivhäuser, Passivhaus Institut, Darmstadt 1999 (”Passive House Summer Case”; Protocol Volume No. 15 of the Working Group for Cost-Efficient Passive Houses; Passive House Institute, Darmstadt 1997) (see PHI's list of publications)

[Peper/Feist 2002] Peper, Sören; Feist, Wolfgang: “Klimaneutrale Passivhaussiedlung Hannover-Kronsberg Analyse im dritten Betriebsjahr”; 1. Auflage, Proklima, Hannover 2002; (“Climate-neutral Passive House Development in Hannover-Kronsberg - Analysis in the third year of operation”; 1st Edition, Proklima Hannover 2002; please click here to download a free PDF version of this report (in German).

[Kirtschig 1998] Kirtschig, Thomas; Werner, Johannes; Feist, Wolfgang: „Thermische Behaglichkeit im Passivhaus Kranichstein - eine Wohneinheit als Nullheizenergiehaus: Winter 1994/95“; Passivhaus-Bericht Nr. 16, Institut Wohnen und Umwelt GmbH, Februar 1998 (“Thermal Comfort in the Passive House in Kranichstein – An Accommodation Unit as a Zero-energy House: Winter 1994/5”; Passive House Report No. 16, Institute for Housing and Environment, February 1998)

[Knissel 1998] Knissel, Jens: „Validierung des Simulationsprogramms TAS; Vergleich mit Messergebnissen aus dem Passivhaus Damstadt-Kranichstein“; Institut Wohnen und Umwelt, 1998 (“Validation of the TAS Simulation Program; Comparison with Measured Results from the Passive House in Darmstadt-Kranichstein”; Institute for Housing and Environment, 1998)

[Kolmetz 1996] Kolmetz, S.; „Thermische Bewertung von Gebäuden unter sommerlichen Randbedingungen – Ein vereinfachtes Verfahren zur Ermittlung von Raumtemperaturen in Gebäuden im Sommer und deren Häufigkeit“; Dissertation Universität Gesamthochschule Kassel 1996. (“Thermal Assessment of Buildings under Summer Conditions – A simplified method for determining indoor temperatures in buildings and their frequency in summer”; Dissertation, Comprehensive University of Kassel 1996)

[PHPP 2007] Feist, W.; Pfluger, R.; Kaufmann, B.; Schnieders, J.; Kah, O.: Passivhaus Projektierungs Paket 2007, Passivhaus Institut Darmstadt, 2007 (Passive House Planning Package 2007, Passive House Institute, Darmstadt, 2007)

[Schneider 2006] Schneider, U.: Grünes Licht, Lichtstandards für Passivhäuser; im Tagungsband der 10. Passivhaustagung, Hannover, Passivhaus Institut Darmstadt, 2006 (see PHI's list of publications) (Natural Light Standards for Passive Houses; in the Conference Proceedings of the 10th International Passive House Conference, Passive House Institute Darmstadt, 2006)

[Wang 1996] Wang, Zhiwu: “Controlling Indoor Climate”; Dissertation, Lund University, Department of Building Science, 1996

First of all, some general remarks concerning the laws of physics: insulation does not “create” any additional heat; it only reduces the heat exchange between systems with different temperatures. Therefore, it also protects a cool system from gaining heat from the surroundings. For this reason, cooling devices are thermally protected – a popular example is that of keeping chilled water cool in a (well-insulating) thermos flask.
The other three accommodation units had and still have external blinds for temporary summer shading. No blinds were installed for this end-of-row house because it was fitted with insulated sliding panels for the zero-energy house experiment in the autumn of 1993.
basics/summer.txt · Last modified: 2021/10/13 10:44 by