12 – Envelopes – What Does High Performance Mean?

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LMN Architects

When we think of a building, we usually picture the outside: classical columns, brutalist façades, or contemporary compositions of windows and opaque areas. Prior to modern heating, cooling and electric lighting, architects designed façades to ensure adequate daylight and thermal comfort though massing, window layout, and design of heating sources that included proper solar orientation. We quickly forgot these lessons as energy for heating, cooling, and electric lighting seemed limitless and without downsides; we relied on engineers to make highly glazed sealed boxes habitable.

We now know better, rediscovering that controlling heat gain and loss through windows and facades are a critical part of architectural design intent. Façade design also now includes a conscious embodied carbon investment in materials and has an influence on the size, cost and embodied carbon of mechanical systems and energy generation systems as well. As we invest more embodied carbon in facades to improve comfort, resilience, and energy performance, how do we assess whether we are making a carbon smart choice?


Understanding Façade Materials’ Embodied Carbon

The free Kaleidoscope Tool compares several opaque façade types, including options to look only at upfront embodied carbon or to include a 60-year timeframe (this includes less durable cladding materials that require periodic replacement). These emissions match LMN’s Whose Wall Wins study, showing roughly 5 to 25 kgCO2e/sf of new façade area, and this façade embodied carbon is around 5-15% of total embodied carbon (with many assumptions and caveats).

Some examples exist of carbon sequestering facades.

Opaque facades usually include a structural material (often studs), insulation, sheathing and waterproofing, and a cladding material, along with windows and doors. Heavier façade materials require additional structure at the floor level or within the stud area that should be included when comparing systems. We have embodied carbon EPD data on many of these products.

Two opaque materials stand out: insulation and heavy cladding materials. Insulation embodied carbon can vary significantly from XPS (averaging around 2 kgCO2e/sf/unit R, although some state’s laws significantly lower this) to mineral wool (0.5 kgCO2e/sf/unit R) to carbon-sequestering biogenic materials like cellulose, cork, or mushroom board. Many buildings are also designed to be fossil fuel free despite using fossil fuel-based insulations. Unlike many other materials that see the majority of their impacts in stages A1-A3, insulation has significant impacts across product lifespans. Blowing agents used in foamed insulation are responsible for much of the embodied carbon and are released as the product off-gas during its life span. For this reason, CLF Baselines, which typically consider only A1-A3, include modules A1-A5, B1, C2, and C4 for insulation materials. Advanced blowing agents developed in the last decade significantly reduce embodied carbon, and increasing blowing agent regulation is standardizing lower-GWP insulations.

Heavy cladding materials like precast concrete, brick, and stone often have higher upfront embodied carbon due to their components as well as requiring additional structure for support. However, with good design and maintenance these materials can last for over a hundred years; however, impacts and risk of being torn down early should be considered if products cannot be disassembled and continue as part of the circular economy.

Openings: Not many EPDs exist for windows or glazing systems, but for A1-A3 Arcadia’s Curtain Wall shows around 24 kgCO2e/ft2, Kawneer’s Storefront shows around 14 kg CO2e/sf, Inline’s fiberglass window is around 7 kgCO2e/sf, and a study of European fenestration provides a range of 15-48 kgCO2e/sf for commercial systems. Commercial buildings are increasingly built with premanufactured window systems, often with integrated spandrel panels. To make up for poor thermal performance, spandrel curtain wall often includes an insulated back-pan with an additional, stick-built insulated wall behind it – this often has a higher embodied carbon than opaque wall, along with poorer overall performance due to thermal bridging. Wood and fiberglass window frames have relatively low embodied carbon.

Existing façade work can take a more surgical approach – see Post 10 for COTE Top Ten winners that reused existing buildings. In general, cost-effective envelope energy upgrades (two examples) are likely to pencil only when capital is already allocated for improvement or replacement of some envelope systems, or during major renovations or repurposing. Well-designed opaque facades should last for hundreds of years, but modern double or triple glazed openings last only as long as the seals and the desiccant strips within them, perhaps 25-40 years, meaning replacement cycles occur more frequently. When glazing replacement occurs, advanced glazing, airtightness, and additional insulation can pay back. From a circular economy perspective, most modern windows use laminated glass or low-e coatings that make them difficult to recycle, but aluminum is a great circular economy material; from our perspective including the end-of-life recycling scenario in aluminum is reasonable.


How Do Facades Improve Building Performance?

Facades mediate between outdoor conditions and desired indoor comfort conditions: temperature, humidity, water, wind, solar energy, light, views, and more. While mechanical systems provide the actual heating and cooling, their size, cost, and energy use is a result of massing and façade design.

Comfort. Our experience of thermal comfort is deeply impacted by facades, especially near them. Windows and uninsulated walls are often cold to the touch in winter and warm in summer; since thermal comfort is more impacted by nearby surface temperatures than air temperature (can be called asymmetrical discomfort) this means otherwise pleasant air temperatures are uncomfortable when we are near cold or warm surfaces such as under-insulated or overglazed facades.

To avoid perimeter discomfort in cold climates, many engineers suggest a second heating system at the perimeter that requires space and increases cost and embodied carbon; in many cases a better envelope may provide equal comfort and cost less. You can see some results by adjusting the operative temperature within the Berkeley Comfort Tool. The Glazing and Winter Comfort Tool explores being near a cold window in more detail.

Resilience can mean many things, from social resilience in the face of threats to continued physical operations like water treatment after disasters. Within the buildings sector, high performance facades add resilience: during heatwaves or cold snaps, mechanical systems don’t provide enough heating or cooling, but a great envelope can maintain more consistent temperatures, called passive survivability. The TEDI metric used in Canada, considers this. Since more people die in heatwaves than in any other type of disaster, proper orientation, correct glazing, and operable windows can be part of a critical life-saving resilience strategy.

Loads. During normal operation, the required heating and cooling to maintain comfort are commonly called ‘loads,’ and there are two primary kinds of loads influenced by the envelope design:

Annual Heating and Cooling Loads: The amount of hour-by-hour heating or cooling required by a building to maintain comfort over the course of a year. Heating and cooling loads are a sizeable part of the total annual Energy Use Intensity (EUI, measured in kBtu/sf/year in the US), a common measure of building performance.

  • A ‘free run’ energy model looks at the heating and cooling loads imposed by the façade regardless of mechanical systems. The TEDI metric uses this method to set the minimum envelope performance.
  • Heating Loads are estimated by calculating the thermal conductance (UA value) of an envelope. The U value describes the amount of heat that flows through a material or assembly, and A refers to the area of the material. By multiplying all areas of façade by its U value and summing them, you can quickly compare design options for the envelope contribution to heating load. Thermal bridging must be part of any U value consideration for all assemblies. Note also that the ‘center of glass’ U value is given for most windows; The ‘assembly U value,’ including thermal bridging through the frame, is the only realistic way to assess window thermal performance.

Peak Heating and Cooling Loads: the amount of heating or cooling required at the most demanding hour of the year. A cold winter morning with no solar gain, lights, equipment or people to provide heating usually determines the peak heating load; a hot, sunny afternoon with full occupancy, lighting, and equipment often determines the peak cooling load. Mechanical systems are designed to handle peak loads, so increasing peak loads generally increases the size and cost of the system. Peak loads often correspond to the most carbon-intensive and highest priced electricity as well (Post 06). Solar loads through windows are often the biggest design driver of peak cooling loads, so reducing these through window to wall ratio, low SHGC, and exterior shading has a significant impact on total carbon, well beyond just the energy savings from these measures. Light-colored roofs and low-SHGC windows in sunny areas are often a no-cost peak cooling load reduction.

Note that peak heating and cooling loads can be calculated and used at a building scale to understand equipment sizing, as well as at a room scale to determine air supply temperature and other aspects of mechanical design. Sometimes the peak cooling in one space determines the air supply temperature for a large portion of a building.

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