Passivhaus in the Mediterranean? Strategies for keeping cool in a Passivhaus on the beach

According to the climate modelling presented in the study “Study on Climate Change and Energy in the Mediterranean” carried out by the European Investment Bank, countries in the Mediterranean basin will experience an increase of between 3ºC – 6ºC in average temperatures between the period 2070-2099, based on the 1961-1990 period.

The need to tackle overheating is addressed directly in the European Directive 2010/31/EU for nearly zero energy buildings (nZEB), which states the following: “Priority should be given to strategies that improve the thermal performance of buildings in the summer. To this end, measures should be promoted to prevent overheating, such as shading and sufficient thermal inertia in the construction of buildings, as well as to improve and apply passive cooling techniques (…)” [2]. This article presents the strategies used to improve the thermal performance in summer of a passive house in a Mediterranean climate. 

Caste study: “Esencia Mediterránea”…passive house on the beach 

The home, “Esencia Mediterránea,” has a useful area of 173m2, over two floors, located about 50 meters from the beach, 3 m above sea level (Figure 2, Figure 3, Figure 4) It has an architectural design very much in line with the Mediterranean vernacular tradition, was designed by Guillermo and Iciar Sen, built by House Habitat, and is Passivhaus certified. 

The project team:  

• Architects: Guillermo Sen, Iciar Sen 
• Technical Architect: Javier García Garrido – Garcia & Sala Arquitectes  
• Builder: Pere Linares – House Habitat  
• PHPP & building physics: Oliver Style, Bega Clavero  
• HVAC design: Vicenç Fulcarà – Progetic 
• Passivhaus and Blower Door Certification: Micheel Wassouf, Martín Amado – Energiehaus 

Passive cooling strategies 

Many of the strategies used to improve the summer performance of the home are rooted in traditional Mediterranean vernacular architecture and combined with modern solutions. For the thermal analysis presented here, a PHPP model of the house was prepared to meet the minimum requirements of the Spanish Building Code (CTE). Subsequently, the impact of each strategy on cooling demand is presented, until the limiting cooling demand value required by the Passivhaus standard is achieved. 

Shading analysis 

In order to be able to more accurately check the impact of design strategies for the summer, a shadow study was carried out with the DesignBuilder – EnergyPlus thermo-dynamic tool(Figure 7, Figure 8, Figure 9). The results were used to calculated shading reduction factors for each window, that were later introduced in the PHPP shading tab. 

Thermal inertia and natural night ventilation 

Combining thermal mass with natural cross ventilation is a key Mediterranean passive cooling strategy. Cooling power is logically limited when minimum night-time temperatures are not sufficiently low (< 18 ºC), so the strategy tends to work best in climates that are inland and at a higher altitude than coastal areas. While lightweight Passive Houses in warm climates have shown good summer performance [3, 4], some thermal inertia, in combination with night ventilation, clearly helps to modulate indoor temperatures and dissipate heat, improving comfort conditions and reducing cooling energy consumption. The house in question was built with a lightweight timber system – inherently low inertia. To incorporate some mass and enhance the effect of natural night ventilation, a 5 cm of mortar layer with ceramic floor tiles were installed on the floors of both floors, providing a specific capacity of 85 Wh/K·m² inertia (compared to a very light building of 60 Wh/K·m²). With tilt-and-turn windows partially opened at night, a minimum night ventilation air change rate of 0,8/h was calculated with the PHPP tool, provided by simple, cross and stack effect ventilation. 

Reflective surfaces: walls and roofs 

White painted walls and roofs are another feature of traditional Mediterranean architecture. The house has a white silicate mortar render, with a white roof, both with a solar absorption factor of α = 40 % (black α = 95 %). This helps to reflect more solar radiation and reduces transmission heat gains to the interior of the house. 

Shading devices and solar control 

Solar gains are reduced with balconies set back from the façade, exterior shading devices, and solar control glazing with a solar factor of 36%.

Thermal insulation 

Thermal insulation reduces transmission heat gains, especially through roofs. It is important to find a balance between the insulation thicknesses needed for winter and summer, since an excessive thickness of insulation can in some cases prevent heat dissipation at night in the summer. On the ground floor, 15 cm of wood fibre insulation was used, U = 0.264 W/m²·K. On external walls, 20 cm of wood fibre insulation was installed between the timber structure with 6 cm of external high-density wood fibre insulation, U = 0,158 W/m²·K. The roof has 26 cm of wood fibre insulation, for a U = 0,152 W/m2·K 

Air infiltration 

Reducing air ex/infiltration is a strategy that comes from cold and temperate climates, where the priority is to reduce winter heat losses, when there can often be an indoor-outdoor ΔT of 30 ºC. Outdoor air temperatures would have to be 55 ºC to have the same ΔT in summer. However, reducing air infiltrations in coastal Mediterranean climates with high humidity can help reduce latent cooling loads when active cooling is on and windows are shut. In the case study home presented here, reducing air infiltration from n50 = 5 ach (a typical value for newly built homes) to n50 = 0.4 ach, the latent cooling load is reduced by 39% and the latent cooling demand by 7%. 

Mechanical ventilation with heat and moisture recovery (enthalpy) + automatic bypass in summer 

Mechanical ventilation with heat recovery (MVHR) is another solution that originated in central and northern Europe. Does it work in a Mediterranean summer? When outdoor temperatures rise above the indoor comfort temperature (> 25 ºC), users in air-conditioned Mediterranean homes typically close windows and turn on the cooling system, with negative consequences for indoor air quality. A mechanical ventilation system with heat recovery and automatic summer bypass, ensures constant air change and high air quality. When T_out > T_int the heat recovery unit reduces the temperature of the inlet air, shown in Figure 10, where heat recovery reduces the temperature of incoming air from 35.5 ºC to 29.5 ºC:

When T_out < T_int the automatic bypass opens, providing free cooling and bypassing heat recovery. Additionally, an enthalpy unit reduces the amount of water vapor that enters the house in summer when the absolute humidity of the outdoor air is greater than the extract air (which is often the case in warm humid climates in homes with air conditioning / dehumidification). When the air conditioning is off, users can of course open windows at any point.  

Discussion and conclusions 

Figure 11 shows the PHPP simulation results for each of the strategies described above. The reduction in cooling demands from insulation on the roof is less than on the walls – a result that seems surprising. This is due to shading from the tree canopy, which means the reduction is less than in the walls. The results show that the combination of all strategies is greater than the sum of individual strategies. Cooling demand is reduced from 33 kWh/m2·a with the CTE code-compliant building, to 18 kWh/m2·a, meeting the requirements for the Passivhaus certification in a Barcelona climate.  

Combining traditional Mediterranean passive cooling strategies with solutions included in the Passivhaus standard, the summer performance of buildings can be improved with a reduced need for air conditioning.  “Stress testing” your design with a tool such as PHPP in the early stages of the project is essential, and post-construction monitoring and evaluation is highly recommended to learn from mistakes and improve.

References 

[1] Somot, S. (2005), “Modélisation climatique du bassin Méditerranéen: Variabilité et scénarios de changement climatique.” Thése de Doctorat, Université Toulouse III-Paul Sabatier. UFR Sciences de la Vie et de la Terre. pp 347. Toulouse, Francia, 2005. 

[2] Parlamento Europeo (2010), “Directiva 2010/31/UE del Parlamento Europeo y del Consejo, de 19 de mayo de 2010, relativa a la eficiencia energética de los edificios (refundición)”. Parlamento y Consejo Europeo, Bruselas, 2010. 

[3] Wassouf, M. (2015), “Comfort and Passive House in the Mediterranean summer – monitorization of 2 detached homes in Spain Barcelona”, 19th IPHC, Leipizig, Alemania.  

[4] Oliver Style (2016), “Measured performance of a lightweight straw bale passive house in a Mediterranean heat wave”. 20^th International Passivhaus Conference, Darmstadt, Alemania. 

Hormipresa Arctic Wall: prefabricated construction system achieves Passivhaus component certification

Hormipresa Arctic Wall is a fully industrialised construction system, certified for passive houses in the warm-temperate climate zone, as the regular U-values of the exterior components are below 0,25 W/m²K and the connections meet the criteria of ‘thermal bridge free’.

The system consists of 9cm of PIR thermal insulation, sandwiched between a 15cm reinforced concrete layer internally and a 6cm white concrete layer externally. The two concrete layers are held together with a galvanized steel lattice system with wall ties that minimise heat transmission while providing mechanical strength. Additionally, 4cm of mineral wool insulation is installed internally in the service void. For the purpose of certification, a three-dimensional simulation was carried out to determine the thermal effect of the steel wall ties and lattice system that penetrate the insulation layer. The roof consists of a prestressed concrete hollow core slab with 14cm of XPS insulation. For the ground floor detail, 8cm of XPS insulation is placed on top of the concrete slab.

For the purposes of certification, a standard passive house window (Uw = 1,00 W/m²K with Ug = 0,90 W/m²K) was used. The overall U-value of the installed window of standard size (1,23 m wide by 1,48 m tall) should be no more than 0,05 W/m²K greater than the Uw to ensure occupant comfort – this criteria is met in this instance. 

Airtightness of the system is achieved in the following way: windows and doors are taped with Iso-Connect Inside Blue Line airtight tapes. The airtight layer of the wall and floor slab is the reinforced concrete layer. In the roof, the airtight layer is the hollow-core slab. Joints between panels and connections with other building elements are sealed with Sikaflex 11-FC elastomeric sealant and painted with Soudal Soudatight SP airtight paint. 

The Passive House Institute has defined international component criteria for seven climate zones based on hygiene, comfort and affordability criteria. In principle, components which have been certified for climate zones with higher requirements may also be used in climates with less stringent requirements. Their use might make economic sense in certain circumstances. 

Check out the Arctic Wall system on the Passivhaus Component database

Many thanks to Soraya Lopez from the Passive House Institute for her considerable efforts to complete the certification process on time.

Lessons learned during 10 years of Passivhaus projects

The Heat Loss Form Factor: Keep it boxy but beautiful!

The compacity of a building has a large influence on energy performance, measured by the form factor, which is the ratio between the thermal envelope and Treated Floor Area (Athermal envelope [m2] ÷ TFA [m2]). The greater the form factor, the greater the exposed heat loss area of the envelope per m2 of TFA, requiring higher levels of thermal protection to achieve the same level of energy efficiency. To illustrate the fact, PHPP modelling results from a range of Passivhaus Classic buildings have been analysed, for varying typologies (single-family, multi-residential, offices and nursing homes) across different Spanish climates. The Form Factor has been compared with two parameters: insulation thickness, using an equivalent thermal conductivity of 0.040 W/m·K (Figure 1) and average Uwindow thermal transmittance (Figure 2).

As expected, the results show a clear correlation: the higher the form factor, the greater the insulation thickness required, and the lower the U_window. The results also indicate that other factors also play a part, such as orientation, solar gain, thermal bridges and- to a lesser extent- thermal inertia.

Blower door test: the moment of truth

In timber construction, OSB boards are often used as the airtight layer and vapour retarder. Research by the Passivhaus Institut [Peper 2014] shows a wide range of air permeability between different manufacturers of OSB boards, with 22mm OSB 4 generally showing best airtightness. Table 1 shows Blower Door results in which different grades and thicknesses of OSB were used:

The sample is very small, but the results suggest that 18 mm OSB 3 is sufficient to achieve an airtightness of n50 ≤ 0.6/h, as long as all joints and connections are well sealed. Taping a plastic sheet to a 1m2 area of the OSB board (not across joints), prior to the depressurization test, can provide a simple and visual means of assessing the air permeability of an OSB board.

Airtightness in sliding windows

Sliding windows are a standard feature in Mediterranean Spanish homes, which can present challenges in terms of airtightness. Lift and slide windows can now achieve Class 4 air permeability, making them suitable for Passivhaus buildings. However, Blower Door test results with three sliding panes have shown to be problematic (Table 2), with the best results achieved at least one fixed pane. If there are several sliding panes, best results were found with the fixed pane in the centre position.

Table 2: Results of the Blower Door test of 3 buildings with sliding window frames.

Heating, cooling, and ventilation, better not together

The experience of using the ventilation system for heating and cooling, whereby a water-fed heating/cooling coil heats or cools ventilation supply air, has been shown to be problematic, above all during summer heat waves, in humid Mediterranean climates in urban settings. Conditioning outdoor air, rather than recirculating indoor air, means that the increase in heating and cooling power is far from linear as flow rates are increased. Also, flow rates are generally small, further limiting heating/cooling power. Despite thermal insulation sleeves on supply air ventilation ducts, long duct runs have been shown to incur large heat losses in heating mode, and large heat gains in cooling mode, meaning that valve supply temperatures at the end of the duct run are often much lower than those used in the calculations during the design phase.

References

[Burrell 2015] Burrell, E.: What is the Heat Loss Form Factor? Searched on September 9th 2019, accessible at https://elrondburrell.com/blog/passivhaus-heatloss-formfactor/

[Peper 2014] Peper, S.; Bangert, A.; Bastian Z.: Integrating wood beams into the airtight layer. Passive House Institut. 2014

Keep cool and carry on: Passivhaus cooling experiences in the warm climates

With rising global temperatures and increasingly frequent summer heat waves, keeping cool in Passive House buildings has become a hot topic.

Bad jokes aside, overheating can be deadly: in France the peak mortality rate during the 2003 heat wave was higher than during the first wave of COVID in 2020, shown in Figure 1 [Parienté et al 2021].

Active cooling in all climates warmer than warm-temperate looks like it will become standard. The question is: for cool-temperature climates, is passive cooling sufficient, or is active cooling unavoidable? What are the pros and cons of the different systems? Will my client ring me up during the next heat wave and give me an ear full?

The article provides technical analysis and lessons learnt from 10 years of experience applying passive and active cooling strategies in residential Passive House buildings in Catalonia, Spain. 6 different kinds of active cooling systems that have been designed and installed in single-family Passivhaus homes are compared, assessing the simplicity or complexity of design, installation, and commissioning; upfront costs; user-friendliness, robustness, and comfort; environmental impact of refrigerants for active cooling systems; and the bottom line: measured performance and monitoring data.

Passive cooling

Careful design, moderately sized and well shaded openings, close attention to local climate, and active users have been found to be some of the key drivers of successful passive cooling. Reduction of internal heat gains is key, with efficient appliances and compact Domestic Hot Water (DHW) systems that avoid the need for recirculation circuits. If recirculation circuits are unavoidable, they need high levels of insulation and flow rate control, with reduced DHW water supply temperatures- so long as this is compatible with Legionella prevention. Ultrafiltration and chemical shock disinfection are promising alternatives to the conventional solution of high DHW supply temperatures and thermal shock prevention.

External shading devices are essential, ideally with motorised external blinds, either user controlled or automated. Even north facing windows in Passivhaus buildings need shading: the very long time constant of Passivhaus buildings mean diffuse solar radiation can cause overheating. If only fixed shading can be used, then appropriate solutions for each orientation are key, with particular attention to east and west facing glazing, which receive more solar radiation in the summer than south-facing glazing (shown in Figure 2). Deep horizontal overhangs on southern glazing work best (or northern glazing in the southern hemisphere), with slanted vertical fins for east and west-facing glazing.

Natural night ventilation combined with thermal inertia works well when night temperatures are sufficiently low. Cool external colours, high levels of insulation on roofs, ground coupling, and ceiling fans, are also effective strategies. Figure 3 shows an example of measured data for a single-family home employing many of those strategies (but with little thermal inertia).

Passive cooling is in general simple, low-cost, easy to install, maintain and commission, and can deliver good performance. However, passive cooling strategies are highly climate dependent and rely on careful user behaviour. Where average air temperatures and levels of solar radiation are high (hot ground), minimum night temperatures do not drop below ~ 20 ºC (limited cooling power from natural night ventilation), and night sky temperatures and outdoor air humidity are high, passive cooling won’t work. In this case, active cooling is unavoidable to maintain comfort. Figure 4 and Figure 5 show an example of climate conditions in which passive cooling strategies won’t work, during the 2015 heat wave in Barcelona, Spain.

Active cooling

6 different cooling systems that have been designed and installed in residential Passive Houses are compared, assessing 6 criteria through a simple 1-to-5 point scoring system, shown in Table 1. Monitored data is provided for systems System 1, 2, 5 and 6.

Radiant cooling systems offer a high level of comfort and efficiency, but are more complex to design, install and commission, with higher capital cost. Humidity control and cooling power has been found to be problematic in warm and humid climates, where users go in and out of the house (garden/balcony etc.). Monitored data for an underfloor cooling system is shown in Figure 6. Figure 7 shows monitored data for a radiant ceiling system.

Low-temperature radiators (set in the floor, or wall-mounted) offer a reasonable balance between simplicity, cost, efficiency, and comfort, albeit with less cooling power than fan-coils and splits, requiring a greater number of individual units to cover the same given area than a ducted fan-coil/split system (where 1 indoor unit can cool several rooms). Figure 8 shows monitored data for this kind of system.

Supply air cooling has been found to be problematic, due to limited cooling power (a consequence of conditioning outdoor air rather than recirculating indoor air, low flow rates, and heat gain along long duct runs). Power can be increased using partial recirculation but monitored data in Figure 9 shows that temperature and relative humidity often move outside the extended comfort range. The advantages of this kind of system are simplicity and low capital cost, but the limited cooling power means passive cooling measures must be robust: once overheating has set in, the system will struggle to eliminate heat.

Conventional convective solutions through the recirculation of indoor air (i.e., ducted or wall-mounted fan-coil or split systems) have been found to be the most robust. They are generally simple to design, install and commission, have a lower capital cost than radiant systems, and power can be modulated to deal with peak loads. They are less comfortable than radiant systems. Split systems using refrigerant distribution offer greater dehumidification power (due to lower refrigerant sink temperatures than water) with a faster response than fan-coils and water distribution. However, the Glower Warming Potential (GWP) of the refrigerants and risk of leaks from on-site manipulation is an important consideration. Figure 10 shows monitored data for ducted split system. Temperatures outside the extended comfort range are during hours when the home was not occupied, and the clients report a high level of thermal comfort.