What is the ‘thermal envelope’ of a building?

In our articles about building energy efficiency, we often use the term ‘thermal envelope’: let’s define what we mean by that.

The ‘thermal envelope’ of a building is the union of those structures that separate the conditioned part of the building (subject to being heated and/or cooled) from the outside (including neighboring buildings) or from other parts of the building that are not conditioned.

Within this structure, there are opaque elements such as the roof, external walls, internal walls that separate conditioned rooms from non-conditioned rooms, slabs on ground or external air, and so on.

Exterior windows and doors are also part of the thermal envelope, and play a very important role in the overall energy balance of the building, because they can provide passive solar gains through their glazing. In fact, external openings that are suitable for a Passive House are deemed so because their individual energy balance is positive – in winter, they allow more energy into the building than what they let out.

It’s important to understand that, in a passive building, the mechanical ventilation system is considered to be a fundamental part of the building envelope, instead of being considered part of the building systems.

Other structures of the building, such as interior partitions, or elements that separate non-conditioned rooms from the outside, are not part of the thermal envelope.

In one of our projects, a contractor offered to install “highly insulated garage doors.” Hopefully, after reading the definition of a thermal envelope, you can see why it makes little sense to pay a higher price for insulated garage doors. Garage doors are rarely part of the building envelope in Italian homes.