Katrin Klingenberg, PHIUS Executive Director
Passive House Alliance China’s 3rd China Passive Building Summit in Shanghai was followed up by a one-day expert meeting and workshop. The group rode out together to old town Shanghai, a nice area of the city consisting of mostly low-rise buildings. The fall weather had now finally turned a little nippy and drizzlier than the days before, and I was happy to have worn my jacket that day. Shanghai is unique: for two months out of the year (one each in the spring and fall), the temperatures are on the cool side, requiring no cooling and almost no heating, but the humidity is still too high to be comfortable.
The workshop was organized by Passive House Alliance China and took place onsite at a high-performance multifamily retrofit project located in a high-end gated community. Upon our arrival we were welcomed into a beautifully designed lobby where refreshments were being served to the invited stakeholders representing the construction industry and building science field from different cities in China. Following lunch, we had the opportunity to tour several of the completed high-performance apartments and begin to delve deeper into high-performance construction methods in mixed/humid climates with a focus on the cities in the Yangtze Delta. The main focus was on large-scale multifamily buildings, a rather typical and ubiquitous building typology throughout China.
If China and the US are country climate twins, then Shanghai is a close twin to Houston’s southeast Texas climate with the exception that Shanghai gets a little bit more rainfall. The most important challenges for passive design space conditioning solutions in these cities are not the thermal loads – it’s the humidity! Thermal loads are easily reduced to very low peaks by using passive design strategies such as moderate amounts of insulation (4 inches of mineral wool for a larger scale buildings will suffice), balanced ventilation with very good energy recovery efficiencies, excellent windows (double pane with thermally-broken frames), and passive level airtightness. But the high humidity load from ventilation during summer and the shoulder seasons can only be reduced so far. A significant dehumidification load remains, often during seasons when little or no cooling is required, as was the case while we were in China.
Improvements to the building envelope to minimize heating and cooling peaks also effects the ratio of sensible to latent cooling loads – resulting in the latent load becoming equal to or larger than the sensible load. While in less efficient buildings the sensible load far exceeds the latent load and can be taken care of by traditional cooling equipment, in highly efficient passive buildings it is the latent load that is now equal to or even dominant (see examples from Beijing and Hong Kong in the graphs below). This poses a new challenge for low-load comfort systems.
The graphs illustrate how the sensible heat ratio decreases if the building envelope is improved for the climates of Beijing and Hong Kong. (slide credit: Hartwig Kuenzel, Fraunhofer IBP, NAPHC2015 keynote)
Climate specific targets also matter a great deal in this climate. In mixed climates, the right balance between heating and cooling targets becomes critical to avoid over-insulation and overheating risks. Window performance in mixed/humid climates needs to strike the right balance as well in order to not inadvertently increase cooling loads. Windows need to be optimized for both cases, heating and cooling, to perform at their best. In the climate of Shanghai as mentioned previously, good double pane windows with a lower solar heat gain coefficient and thermally broken frames are typically the right choice to meet comfort targets and to avoid contributing to overheating. Accurate assessment of internal gains must take into account culture, lifestyle, occupancy, and other factors as they have a significant impact on the overall energy balance of high-performance passive buildings.
In the case of China for example, cooking plays a major role in the vibrant Chinese lifestyle and culture, as we were lucky enough to experience first hand as our gracious hosts showed us the best and most interesting dining spots around Shanghai. Food is central to the culture and if folks are cooking a lot of flavorful and spiced foods in their homes in a climate with significant cooling loads, they will want directly vented kitchen exhaust hoods! Grease, odors, and heat need to get captured and thrown out right at the source. I was impressed to see a novel solution to this problem as we toured the retrofitted apartments. Each unit had two kitchens: one being the “real kitchen” with the big stove, prep area, and fridges which were separated from the main living space by sliding doors to minimize the negative indoor air impacts on the rest of the apartment, and the other one adjacent was an open kitchen concept with a bar for entertaining! What a brilliant idea (if you can afford it)!
Now, what about energy modeling? We have often said that more complex climates really should be modeled using dynamic whole building energy balancing tools such as WUFI Plus. What makes the climate more “complex”? Cooling and dehumidification is needed when the exterior temperature gets closer to the interior comfort zone and begins to fluctuate around it. The warm season is dominated by diurnally reversing heat and moisture flows – in during the cooler nighttime, and out during warmer daytime temperatures. Add moisture into this back and forth and it becomes really complex. To be able to accurately predict how components and the whole building will perform from an energy and hygrothermal perspective, the designer really needs to perform a dynamic whole-building energy model based on hourly data to make the right choices. In contrast, in a heating dominated climate, exterior temperatures are swinging far enough away from the interior thermal comfort zone so that heat and moisture flows are mostly flowing out. Static models are accurate enough for simpler climates such as this.
The graphs illustrate conditions for both heating and cooling/mixed climates. The static monthly balance method as employed by WUFI Passive is sufficiently accurate to predict energy use in a heating dominated climate. In cooling/mixed climates such as Shanghai and Houston, dynamic whole-building energy simulation (WUFI Plus) is recommended. (slide credit: Hartwig Kuenzel, Fraunhofer IBP, NAPHC2015 keynote)
Hygrothermal wall performance checks should be best practice for passive designs in mixed/humid climates to avoid any kind of condensation risk. As China ramps up their energy efficiency efforts in varying climates to near passive building levels and experiments with materials it will be critical that these models are created as project teams might not be familiar with just yet or have no long term experience with this risk management in mixed/humid climates, which can lead to critical and significant failures.
Now, what about the high-performance apartment tour, where are the Chinese at with their high-performance solutions today?
I was thoroughly impressed with what they had already in place in terms of execution, performance, details, mechanical solutions, and – to top it all off – a standardized monitoring interface centrally located in the home providing constant feedback on thermal comfort and indoor air quality to the home owner including fine particulate matter (PM 2.5) and inside to outside air quality comparisons. In Shanghai it is often the case that outdoor conditions are worse than indoors due to high pollution levels.
The project we toured was a retrofitted five-story brick building that had been upgraded by adding a 4-inch layer of mineral wool exterior insulation, airtight layer, and a new clay tile façade. The reported tested air-tightness result was 1.5 ACH, which is very respectable for a retrofit! Space conditioning was solved in a very elegant and most comfortable way: a separate energy recovery balanced ventilation system with appropriate filtration and dedicated integrated dehumidification took care of controlling ventilation humidity loads and outdoor pollutants (as evidenced on the screen of the monitoring interface in the living room, see opening photo). Space conditioning was handled by a separate point source solution consisting of hydronic heating and cooling integrated into the room’s ceiling. Radiant heating and cooling is a more costly, yet very comfortable high-end solution. Controlled infiltration and humidity loads are key to this solution to avoid condensation. So is awareness by the homeowner. They need to be put on notice that they can’t cool the home and leave the beautiful lift and slide high-performance balcony door open at the same time!
The developer reported that the passive house approach works financially for them for the high-end market. As you might expect, two bedroom apartments were selling in the millions, as would be the case for similar real estate in any other cosmopolitan global city.
Can passive go mainstream in China?
If I may offer my personal prediction: the Chinese have taken a surprising global lead in fighting climate change and have identified aggressive conservation goals for buildings as a valid strategy. The government has passed mandates to local jurisdictions to find appropriate cost effective solutions. If China addresses the cost optimization of passive building measures based on varying climates, construction paradigms, and energy costs in China similarly to what PHIUS did in the US, then they should certainly be able to generate design guidelines aimed at presenting the most economical path to zero. At the rate that they are going, I believe China will bring passive building to the mainstream before he US does because they have the political will, effective materials and components, knowledge of building science and energy modeling, and cost effectiveness strategies to get there.
What about the state of typical mainstream construction in China?
From what we saw, most apartments in Shanghai already have their own air-to-air heating and cooling heat pump unit sitting on their balcony. Pair that common solution with good airtightness, balanced energy recovery with dedicated dehumidification, moderate amounts of insulation and appropriate hygrothermal wall design, good windows, and you are there.
It would be great to see China taking the lead!