Thermal Envelope Design in Home Building (Explained)

Weathertight, thermal insulation, and airtight are the three continuous functional layers in thermal envelope design in home building to provide adequate protection from the elements.

These layers’ performance must be tested on paper and in the field. Once these layers are installed, gaining access to correct any remaining flaws is prohibitively expensive.

If weaknesses persist, they will negatively impact the well-being of the homeowners. So, before it’s too late, the layers must be well designed and tested! 

A healthy home requires light and sunshine, but windows in the thermal envelope need careful design to ensure that the amount of sunlight that enters the house can delight rather than cause discomfort. 

For the construction of energy-efficient homes, windows are an essential component of the thermal envelope. 

The thermal envelope is the most important component for a healthy and comfortable home, and its performance is locked in for a very long time. Of course, you can always add solar panels to your roof later, but changing the insulation or air-tightness in an existing home is difficult.

What Is a Home’s Thermal Envelope?

A homes thermal envelope consists of three main layers; an air barrier, insulation, and weather-resistant layer. More details below:

Airtightness layer An air barrier must be installed. Which in heating climates you’ll want to include it on the inside of the insulation. This will help prevent condensation from escaping air on cold surfaces. Along with draughts, noise pollution, and excess heat loss.
Insulation layer The insulation layer slows energy loss through the envelope and creates thermal comfort. It also helps noise pollution as the mass dissipates the sound waves.
Wind-/weathertightness layer (WRB)A weather resistant barrier (WRB) should be on the outside of the insulation. It should repel water, be wind-proof, and breathable to allow building materials to dry.

The airtightness and wind-/weathertightness layer is susceptible to perforations (electrical pipes, venting, etc). But, all three control layers must be continuous and uninterrupted to function optimally.

Think of a balloon. Even a small leak will eventually deflate it. This becomes problematic when the home building design doesn’t take the control layers into account. As achieving continuity between connecting joints could become challenging.

Joe Lstiburek states, “before you can control air, you must first enclose air.” You should be able to take a pen and draw a continuous line around the entire building design.

Airtightness Layer

An airtightness layer eliminates uncontrolled air passage and moisture and sound transfer. This requires airtight materials, sealing, and blower-door testing.

Before we continue, let’s answer a question: “Do thermal envelope leaks help provide fresh air?” Are airtight houses bad for indoor air quality?

No, unequivocally. First, to provide the required flow rate of fresh air for good indoor air quality (—30 m3 per person/h) on a calm day, the thermal envelope would need to be so leaky that living in the house on a windy day would be extremely uncomfortable, with double or more of the required air thrusting in.

Is the air that enters through building cracks fresh? Most of the time the answer is no.

Leaks don’t improve indoor air quality or meet fresh air needs. The airtightness layer is essential for high-quality air and helps keep the building comfortable. Avoiding:

  • Uncontrolled air movement between the inside and outside of the building can cause heat loss or gain, discomfort, and mold formation on surfaces and in cavities.
  • Air movement through or around insulation materials reduces their effectiveness as insulators. 
  • A malfunctioning ventilation system. These systems are designed and balanced to deliver the right amount of air (and/or heat/cool) throughout the building. Air leaks prevent proper performance.

Pro Tip: All building trades must understand the control layers and know where they can run their materials. Electrical pipes are often public enemy number one on the airtightness layer. Creating a wall design that allows for a service cavity for trades to run pipes and vents will help prevent poor air-sealing.

Test the Air Control Layer With a Blower Door

blower door testing a home for air-tightness

After careful joint design and construction, a blower-door test can identify remaining envelope leaks. Blower doors are frames with fans and pressure gauges that fit in standard openings.

Most leakage rates are evaluated at 50 Pa. (Pa). The result at 50 Pa is calculated from measurements at a range of pressure differentials to cover all reasonable possibilities.

The volume exchanged at 50 Pa pressure differential can be spread over the thermal envelope (q50) to calculate a leakage rate per m2 or surface area, the volume in the house (n50) to calculate air changes per hour, or the treated floor area (vv50).

The blower-door test verifies construction, but its greatest value is identifying leaks. To maximize the test’s value, have rolls of tape or wet plaster, ladders or scaffolds to access hard-to-reach areas, and contractors on site to fix leaks. Leak checking and repair can often improve the initial result.

Insulation Control Layer

insulating an attic with cellulose

Insulation protects us from the elements. It’s a house blanket. Only your budget limits insulation thickness. If done well, adding insulation is always beneficial. Insulation prevents:

  • Outdoors conditions affecting indoor climate
  • Excess heat gain or loss
  • Noise pollution from entering the home

Not all thermal insulation materials insulate sound. Rigid materials don’t help keep noise out, but flexible insulation can improve thermal envelope acoustics. In addition, mineral wool and other inorganic insulation materials can resist fire spread.

Insulating materials are made from a variety of raw materials. To be effective, they must have low thermal conductivity, be continuous, and be airtight, windproof, and weatherproof. Loose-fill insulation forms a gapless blanket in a cavity because it conforms to structural elements and liners (often used in attics).

Rigid materials are best for exterior wall insulation or under slabs, where they don’t have to fit between structural elements.

Insulation must stop conduction, radiation, and convection.

Wind & Weather-Tight Control Layer (WRB)

zip sheathing for a home air barrier

The final thermal envelope control layer provides wind and weather tightness.

Air-tightness vs. Wind-tightness

Airtightness prevents indoor air from penetrating the thermal envelope; wind tightness prevents outdoor air from entering.

The wind tightness function is usually combined with keeping out water, but for keeping out wind, joints must be taped.

This layer prevents:

  • Water from entering wall assemblies.
  • Wind-washing insulation which decreases performance.
  • Creating pressure zones in building cavities.

Encapsulate insulation on both sides with air-impermeable layers, eliminate voids and communicating cavities, and leave no gaps behind or between insulation materials.

Small & Compact is Key

The thermal envelope must tightly encompass the spaces requiring thermal control, which is impossible with a loose-fitting thermal envelope. 

The thermal envelope should also be as small as possible because the amount of heat transmission loss is directly proportional to its surface area.

The only surfaces that must be kept as small as possible (cube shaped homes are best) are those that enclose conditioned spaces. Sunrooms, sheds, attics, balconies, and garages that are unconditioned and adjacent spaces can be used to break up otherwise monotonous facades without expanding the thermal envelope.

Small and simple envelope shapes have several benefits. Smaller structures lose less heat. They’re more straightforward and cheaper to build.

The decreased complexity makes them more adaptable to shifting demands and trends while lasting longer—size matters. Smaller homes cost less, use less energy, and make sense.