What Is A Thermal Envelope?

When it comes to building a home that’s eco-efficient and comfortable to live in, building it right is about building it tight. 

What does that entail?

It means that the lived-in areas (spaces that you use energy in to heat or cool them) have to be clearly separated from those that aren’t. In other words, there must be a tightly sealed thermal envelope around the home, or else hot and cold air will quickly enter or exit the house. 

But what is a thermal envelope, exactly? Here’s what you need to know. 

The Thermal Envelope

Like other industries, the world of construction and HVAC have their terminology for concepts that insiders understand, but the general public may not. A thermal envelope is one of these terms. 

Put simply, every home has a thermal envelope. It’s the structure that separates the air inside your home from the air outside. The National Renewable Energy Laboratory describes a thermal envelope as “everything about the house that serves to shield the living space from the outdoors.” 

This includes things like: 

• Windows
• Floors
• Exterior walls
• Outer doors 
• Insulation
• Air/vapor retarders 
• Weatherstripping
• Caulking

In this way, a thermal envelope is more than just one thing. It can be easier to understand as an engineering and design concept. According to EchoTape: 

“At its simplest definition, the building envelope is the exterior or shell of a building that repels the elements. At its most complex definition, it’s an engineering system that meshes elements such as structural integrity, moisture control, temperature control, and air pressure boundaries into a single design strategy.”

Sometimes, the objects that help maintain the home’s seal are referred to as the heat flow control layer. A thermal envelope aims to form an environment that prevents drafts and heat transfer from a building’s interior to the exterior. 

In the winter, a house with a reliable thermal envelope prevents air leaks; in other words, it keeps warm air inside and cold outside. 

In the summer, it’s the opposite. 

To picture this, think of an older home. It tends to be drafty and hot in the summer and cold in the winter. Such houses likely have poor thermal envelopes, resulting in significant energy loss and heat change. Creating a thermal envelope can be done in any home. There are several different insulation types, and Attic Construction is here to help you find the best insulation for your home.

The Purpose of a Thermal Envelope 

A thermal envelope serves three distinct functions:

1. Air control – Managing air movement within your living spaces is important for managing energy consumption, preventing condensation from accruing, ensuring that air quality is optimal, and making the space comfortable. This air control includes both movements through the home’s enclosure and the heat flow control layer. A drafty house, for instance, has poor air control. 
2. Moisture control – Moisture can threaten the structure’s integrity and wreak havoc on the interior thanks to mold and other issues. Left unchecked, it can cause damage to the foundations of the home. As such, an envelope’s ability to control and minimize moisture is one of its most critical tasks, especially in hot-humid climates or those that experience snowy winters.  
3. Thermal control – As the name suggests, a thermal envelope makes it easier to reach and then maintain your ideal temperature no matter the time of the year. If you’re feeling too hot in the summer or too cold in the winter, it may be time to look for ways to increase your building envelope’s efficacy. 

What Characteristics Impact a Home’s Thermal Efficiency? 

Three main elements impact a home’s thermal envelope’s efficiency from a high-level standpoint. They are:

• Heat transfer
• Phase shift
• Water vapor diffusion

Heat Transfer

Heat transfer is the transition of thermal energy from a heated space to a cooler area. According to Low Energy Home, there are three means of heat transfer that need to be considered when planning to make a home energy efficient:

1. Conduction – Transfer of heat through matter from a higher temperature region to a lower temperature region. 
2. Convection – Transfer of heat occurring because of the movement of fluids and gases (e.g., air moving in a wall cavity).
3. Radiation – Electromagnetic radiation emitted from the surface of an object (e.g., radiation emitted from an aluminum foil).

When it comes to building, two types of materials help prevent heat transfer: thermal insulators and radiant barriers. 

Phase Shift 

This determines the amount of time it takes for the highest temperature on your home’s external surface to make the inside of the home reach its highest temperature. Phase shifts are important to consider, particularly in summers to prevent your attic from overheating. 

The phase shift of building materials increases with the thickness of the object, with a target value of 12 hours or more. According to Science Direct, “phase change materials (PCMs) are regarded as a possible solution for reducing the energy consumption of buildings. By storing and releasing heat within a certain temperature range, it raises the building inertia and stabilizes indoor climate.” 

Not only will it keep temperatures down, but it will also keep costs down too.

Water Vapor Diffusion 

This is the process by which water vapor can move through permeable materials due to a variance in water vapor pressure. By applying the proper materials within your building—such as vapor barriers or vapor diffusion retarders—one side prevents water vapor from entering, and the other side ensures that water vapor is dried out.     

Why Does a Home’s Thermal Envelope Fail? 

Although it depends on the structure itself, there are a few primary reasons why thermal envelopes end up being leaky. These include: 

• Heat bridges – These are places where the house’s support structure meets a part of the home’s heat flow control layer, such as a wall meeting a window or a door. When this happens, it creates the opportunity for gaps (i.e., heat bridges) to occur. These areas are weak spots in the home’s thermal envelope that are more susceptible to cold or hot air moving in or out. Even a well-sealed home will have to pay special attention to potential thermal bridges. 
• Poor home design – Sometimes, the home’s architect uses materials or designs that aren’t ideal for living. They might use materials that are incompatible with other materials they interact with or aren’t resistant to water penetration or thermal movement. Generally speaking, older homes tend to have more flawed thermal envelope design.
• Low-quality installation – The design is only as good as the people who build it. You may have a perfectly designed house, but faulty craftsmanship led to improper installation of the structure and its heat flow control layer. When this occurs, there’s a significant likelihood of heat bridges popping up throughout the home.   
• Materials fail – Sometimes, the materials you use in the home simply fail to live up to the expected performance levels—whether through the fault of manufacturing, storage, handling, or installation.
• Acts of Nature – Nature can mess up even the most perfectly finished thermal envelope. Winds, rain, snow, and extreme temperature fluctuations can wear the envelope away over time, slowly chipping away at the thermal envelope. This gradual deterioration can be prevented via routine inspections and then fixing small problems before they can evolve into larger ones.

Benefits of a Thermal Envelope 

Making sure that your home has a tightly sealed thermal envelope is important for several reasons, including: 

• Heat and cold transfer – The most apparent reason it’s better to have a properly installed thermal envelope in your home is to ensure that the home’s temperature stays where you want it to be. If you run the A/C, the house will cool quickly and stay cool. If you run the heat, the house will heat up and remain warm.
• Eco-efficiency – Heating and cooling processes can consume a significant amount of energy. When there are breaches in the envelope or heat bridges, it’s far too easy for air to escape or enter. If you want to do your part in helping waste less energy to keep the environment green, tightening your envelope could go a long way. 
• Money saved – The more energy-efficient your home is, the less money you’ll need to spend on heating or cooling it. You can learn some ways to lower your power bill for a little upfront investment and can significantly decrease your lifetime utility bill. 
• Indoor air quality – When a home is airtight, it becomes much harder for allergens and pollution to get into your home without first being filtered through your HVAC system. 

Attic Construction—Sealing Your Home’s Thermal Envelope 

If you want to save money and stay comfortable, your home’s thermal envelope must be in tip-top shape. Fortunately, there are easy ways to fortify your home and prevent unwanted thermal escape or entry. 

One of the easiest ways to do this is by isolating the attic from the exterior walls and conditioned spaces and ensuring that seals, ventilation, and insulation are properly installed.  

That’s something the experts at Attic Construction do regularly. As the #1 rated San Diego & Orange County attic company, we’re the professionals you can trust to make sure that your home’s thermal envelope is secure. 

Check out our attic insulation installation services and contact us today! 

Sources:

1. NREL. Elements of an Energy-Efficient House. https://www.nrel.gov/docs/fy00osti/27835.pdf
2. Echo Tape. Contractor’s Field Guide to The Building Envelope. https://www.echotape.com/blog/contractors-field-guide-building-envelope/
3. Low Energy Home. Heat Transfer. https://sites.google.com/site/lowenergyhome/thermal-envelope/heat-transfer
4. Science Direct. Phase change materials for building applications: A state-of-the-art review. https://www.sciencedirect.com/science/article/abs/pii/S0378778810001180