When my partner and I bought our 1960’s home, we knew it was sorely lacking in some key energy efficiency components. Many of the windows, for example, were original to the house and drafts around their edges were immediately apparent. But our house was losing efficiency in other, less obvious ways revealed by our home inspector’s infrared camera. Who knew the recessed toilet paper holder was a source of heat loss?! There were plenty of invisible flaws working against our (admittedly 40-year-old) HVAC system.
While a new heating system and energy-efficient appliances can do a lot, we weren’t going to improve energy efficiency in a significant way until we addressed the big picture: our thermal envelope.
What is a Building Envelope?
Just like the packaging for your mail, a building envelope is the physical barrier between your home’s interior and exterior. It’s a combination of the foundation, walls & siding, roof, and fenestrations (windows & doors). As you can imagine, it protects the interior of the home from wind, rain, snow, dirt, and sunlight. Its protection goes both ways, keeping elements from getting in and preventing heat or AC from getting out. The strength of your building envelope therefore controls temperature, moisture, air pressure, light, and even noise inside your home.
The better your thermal envelope, the less work your system will need to put in to maintain a comfortable temperature—meaning less energy used.
Much of the efforts in the construction industry to improve energy efficiency in both residential and commercial applications focus on enhancing thermal envelopes. For example, every 3 years the International Code Council releases a new set of guidelines, the IECC for energy conservation in building design and construction with standards for insulation, walls, floors, doors, and more.
Passive house design, widely considered the highest standard for home energy efficiency, also focus on perfecting the building envelope to achieve up to 90% less energy use than an average home.
How to Improve Energy Efficiency with a Superb Building Envelope
So, what makes a thermal envelope good? The best envelopes minimize thermal bridging, which occurs when heat passes through conductive materials to enter or escape your home. Studs, joists, metal components, and gaps in the insulation are common causes of thermal bridging. Here’s a breakdown of how to make each component of your building envelope maximally efficient.
Foundation
Your home’s foundation is the literal bedrock for the entire structure and benefits from the earth’s natural insulating properties. Just as it protects tree roots from extreme cold or heat, soil around your foundation can provide a buffer that moderates temperature fluctuations in your home. Typically made from rock or concrete, your foundation also has high thermal mass, helping to stabilize temperatures throughout the day. It’s no wonder my brother and I used to hang out in our family’s basement at the height of each summer.
However, these benefits alone do not always meet modern standards for energy efficiency. Cracks in your foundation, gaps around windows, doors, or the sill where the foundation meets the frame, and rim/band joists can compromise airtightness, allowing heat to escape or cold air (and often moisture) to seep in. Properly sealing and waterproofing your foundation is key to preventing energy loss.
Insulating your foundation also enhances efficiency and reduces the potential for thermal bridging. On new construction, exterior foundation insulation is becoming more common, applied to the outside of the foundation walls before backfilling. Continuous, rigid insulation board, like wood fiber TimberBoard, is most effective for these applications. It’s recommended to add a protective board/panel, 4” of pea gravel, and effective drainage away from the foundation to protect insulation from pests and moisture that could reduce its lifespan.
