Climate, siting, and location are critical factors in creating sustainable and comfortable buildings and spaces. A good design negotiates the high and low temperature variations across each season and throughout the entire year while maintaining indoor comfort. Thoughtful façade design, increased energy efficiency and HVAC system performance are some of our best tools to reduce the effects of climate change.
The amount of glass on a façade can impact the HVAC system and the project’s ability to meet sustainability goals. The demand for highly glazed facades – think glass walls, floor-to-ceiling windows and large sliding glass doors – is here to stay for both designers and owners, and needs careful consideration in the design process. This is why it is important to work with an architect. Much of our work as architects, is in NYC and the northeast, a cold climate where heat gain and loss through glass and humidity are big concerns. All things considered; the question then becomes: how do modern facades with large amounts of glass contribute to building performance in cold climates? What is the impact of a glass façade on heating and cooling, and more energy efficient systems such as heat pumps?
Project goals and challenges differ for residential and commercial projects. Often, custom designed homes optimize fantastic, panoramic views where floor-to-ceiling glass facades and/or large picture windows are a desirable design feature. Similarly in office spaces, where people spend much of their day, access to daylight is a top priority. In retail environments, big storefront windows give visibility to the products and space inside and help attract customers. In a cold climate, large windows can throw heating and cooling loads out of balance. We have faced this chicken-and-egg challenge with recent custom residential projects in rural locations.
New Home in Stanford, NY, where deep roof overhangs and existing trees shade the house in summer, and high efficiency windows keep the cold out in winter.
Considerations for residential high performance façade design in cold climates
A well-balanced and high-performance design for a ground-up custom home or extensive retrofit will benefit from robust exterior wall and roof insulation, insulated glass (triple-pane if feasible), and high- efficiency HVAC equipment. The following list goes into more detailed considerations and recommendations for balancing large glass walls and windows with HVAC system demands.
Energy codes
Codes set minimum insulation values for the components of a building – the foundation, above- ground walls, the roof, windows, doors, skylights, etc. Pursuing sustainability programs like Passive House or Energy Star, which set higher targets than code, can bring additional operational energy cost reductions (e.g. lower your electric bill).
High efficiency window and glass specification:
Insulated Units – Glass has a lower insulation value than a solid wall, losing more heat from the inside in the winter and gaining more heat from the outside during summer. Double and triple pane insulated glass units (IGUs) have an air cavity between each layer of glass, resisting heat transfer better than a single pane of glass.
Heat transfer – Use glass with very low transmission values (or U-values) on the north and south facades, a high solar heat gain coefficient (or SHGC) on the south façade, and low SHGCs on the east and west.
Frame – The weakest point of heat transfer in a window is the frame. A higher proportion of frame to glass can help improve window performance, depending on orientation and sun exposure. Specify thermally-broken window frames.
Offsets – Sustainable design requires looking at each part and its relationship to the whole. For example, heat gain and loss from glass in one area of the building can be offset by increasing the insulation at the roof and/or walls elsewhere in the design. This strategy is less applicable with standards like Passive House, where the wall and roof insulation offsets cannot compensate for the lower insulation offered by all-glass exterior walls, especially when they adorn multiple facades.
Solar shading to reduce solar heat gain:
Design with exterior shading devices to make your house cooler, such as roof overhangs, baffles, screens, solar shelves, trees and landscaping, etc. to block sun rays and heat from entering the home through glass. In the design of the Link Farm House, a ground up custom home, roof overhangs shade all sides of the glass-walled living spaces, with deeper overhangs on the southwest and southeast sides where summer sun and solar heat gain are stronger.
Interior roller shades and window treatments, while less effective than exterior shades, can be an easier solution, especially for retrofits.
Heating, ventilation, and air conditioning (HVAC):
Indoor thermal comfort – A lot of glass can drive up the HVAC loads needed to make-up for the heat gained and lost through windows.
Equipment sizing and humidity – Specify HVAC equipment that gets as close to the lowest total energy load required. In summer, an oversized cooling system risks short cycling: cooling spaces too quickly and cycling off before all the humidity is removed. Too much interior humidity can cause moisture related mold and mildew problems and encourage pests. Properly sized equipment is critical for Energy Star, which wants the system to run longer so humidity comes down during the cycle. An oversized system is more expensive to purchase and operate.
Heat pumps (air source) – Heat pumps are more efficient and transfer heat when conditioning indoor spaces. Heat pumps are available in air-source or ground-source models. We often hear the question, “do heat pumps work in cold weather?” The short answer is, yes. Recent advances have made heat pumps a more effective and affordable space heating option in cold weather climates and subfreezing temperatures. Air-source heat pumps are the primary HVAC system for the Egremont House, where (2) outdoor compressors will heat and cool the entire 4,700 SF home.
New Custom Home in Egremont, Massachusetts, designed to Zero Energy Ready Home Program requirements.
Heat pumps (ground-source) – Also called geothermal heat pumps (GHPs), these models exchange heat with the relatively constant earth temperature, instead of the outside air. Our design for the SoKul House uses a geothermal system with seven bores, each of which reaches 300 feet below the house to stable ground temperatures.
The ground-up SoKul House in Dutchess County, NY, is conditioned with geothermal warm air heating and cooling. In construction currently.
Heat pumps (common concerns) –
For a cold climate heat pump design, planning for the lowest temperature can throw the cooling load out of balance. Supplemental heat may be required for air handling units (AHUs) on the coldest code-specified design temperatures. This is critical for spaces with glass facades.
Combining multiple indoor AHUs on a single outdoor compressor is feasible but can result in oversized heating loads, way beyond the demands of the house.
Air-to-water heat pumps are not currently available in the US, meaning separate units are required for space conditioning and hot water.
Heat pumps (incentives)
The federal Inflation Reduction Act and other state-run programs offer rebate and tax credit incentives for qualifying heat pumps.
Rebates are available for low- and middle-income households. Rebate amounts are dependent upon the median income in the project area. Generally, funding is available for home electrification projects and heat pump wiring. Contractors are funded through the state energy office.
Tax credits are not income based and offer up to a 30% reduction on installed product costs, capped at $2k per year. The Inflation Reduction Act allows cap to be applied annually for a 10-year period, incentivizing phased and incremental work.
These incentives are only available for “qualifying” heat pumps. Rebate qualifying heat pumps follow Energy Star 6.1 and tax credit qualifying heat pumps follow the Consortium for Energy Efficiency’s (CEE) highest tier.
For a more complete guideline on energy conservation in design, refer to our write-up on energy efficient home design: Top Tips to Design and Build an Energy Efficient Home.