The Passivhaus Building Envelope

Elrond Burrell

The Passivhaus Standard delivers exceptional indoor comfort and building energy efficiency. To do this, a high-performance thermal envelope is required Elrond Burrell, architect and author of the popular blog 'Passivhaus in Plain English & More' examines ways it can be achieved.

In passivhaus design and construction, there are frequent references to the “building envelope” and the “thermal envelope.” Neither are exclusive to the Passivhaus Standard, but both are important aspects of the standard.

A building envelope is the physical separators between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer. The three basic elements of a building envelope area weather barrier, air barrier, and thermal barrier. [Wikipedia]

In simple terms, this means that the building envelope is made up of the walls, floors, roofs (or ceilings), windows and doors that separate the inside from the outside. The passivhaus building envelope is also made up of these elements, but there are some key aspects that make the passivhaus building envelope distinct.

The passivhaus building envelope requires a high-performance thermal envelope, it must be continuous and it is key to the fabric first approach. 

 

The Passivhaus Building Envelope: A High-Performance Thermal Envelope
 

The Passivhaus Standard delivers exceptional indoor comfort and building energy efficiency. To do this, a high-performance thermal envelope is required. In basic terms, this means super-insulation, an airtight barrier on the inside of the insulation and a windtight barrier on the outside of the insulation. High-performance windows and doors are also needed.

‘Super-insulation’ is a relative term and does not always mean extremely thick insulation though. The greater the amount of external surface area a building has (essentially the ground floor, external walls and the roof) relative to the floor area of the building, the greater the thickness of insulation that will be required. The less surface area relative to the floor area, the less thick the insulation can be.

Insulation keeps the heat inside the building. And equally important, it keeps unwanted heat out. Insulation reduces the flow of heat energy in both directions through the building envelope.

An airtight barrier on the inside of the insulation does several things:

  • It stops air (and therefore heat energy) that is inside the building from escaping through uncontrolled gaps in the building envelope. This ensures air only goes in and out of the building where intended to through the designed ventilation.
  • It prevents moisture from inside the building getting into the building envelope where it might come into contact with a colder surface and condense. Where moisture does condense within the building envelope (‘Interstitial condensation’) it increases the risk of mould and associated dangers, as well as the risk of damage to the building envelope.
  • It preserves the performance of the insulation. Air movement across the surface, or even into the insulation, reduces the effectiveness of the insulation.

A windtight barrier on the outside of the insulation is also required to preserve the performance of the insulation. Wind blowing across, or even worse, into the insulation, can reduced the performance of the insulation by as much as 40%.

For more detailed information on this aspect of the passivhaus thermal envelope, I suggest you start with this article on ‘Thermal Bypass’ by Mark Siddall. There are also several related papers available for free download on Mark Siddall’s website.

A high-performance thermal envelope is well insulated, airtight and windtight.

 

The Passivhaus Building Envelope Must be Continuous
 

All building envelopes should be continuous – to keep the weather out and to keep the inside comfortable and dry. The Passivhaus Standard is particularly rigorous when it comes to the continuity of the thermal envelope though.

One way to check if the building envelope is continuous is to use the 'Pen Test.' This is where you take a plan or section drawing (or any drawing actually) and trace around particular elements (eg the weather protection) in a continuous line. If you have to lift the pen off the paper at any point, there is a weak spot or break in the continuity. Three examples are shown in this Energy Vanguard blog post: The Pen Test — A Control Layers Tool for Architects and Contractors.

For passivhaus, this test should be applied to the insulation layer, the airtight layer, and the windtight layer – all the elements of the high-performance thermal envelope. An example is indicated on passipedia.

In theory, there should be no breaks at all in the passivhaus thermal envelope. This means that there are no “thermal bridges”, or elements that allow heat to pass through the insulation. Any breaks in the insulation would allow unwanted heat loss, reduce the insulation effectiveness, create cold spots that compromise indoor comfort, and risk moisture issues within the building envelope.

In practice, there often are areas of minor thermal bridging, or reduced insulation performance. The rigor of the Passivhaus Standard requires that all such thermal bridges must be identified. This ensures that any risk of moisture issues can be identified and resolved. It also means the additional heat loss can be calculated and included in the Passive House Planning Package (PHPP). If it is too much, the standard won’t be met.

The passivhaus building envelope includes a continuous thermal envelope.

 

The Passivhaus Building Envelope: Fabric First
 

The design of the building envelope determines if a building can meet the Passivhaus Standard or not. This is because passivhaus takes a ‘Fabric First’ approach. It’s no good designing a poorly performing building and hoping to add something to it to achieve the Passivhaus Standard. It doesn’t work that way.

One reason it doesn’t work that way is that the Passivhaus Standard has a radically low heating demand benchmark to meet. It isn’t possible to reach the benchmark with a poor building envelope and a mediocre thermal envelope that isn’t airtight. Not even with the most efficient heating systems available. A fabric first approach that prioritises the building envelope, including a high-performance thermal envelope, is required.

Another reason it doesn’t work that way is that the Passivhaus Standard has very stringent comfort benchmarks to meet. Taking a fabric first approach ensures that indoor surfaces are always warm without cold spots and there are no draughts or radiant cold from windows and doors. The passivhaus building envelope provides the exceptional indoor comfort that the standard promises.

To read more about the fabric first approach of the Passivhaus Standard follow this link.

The passivhaus building envelope delivers energy efficiency and comfort through a fabric first approach.

 

Is the Passivhaus Building Envelope just the Thermal Envelope?
 

It’s easy to think that the passivhaus building envelope is simply the thermal envelope.p

It’s easy to think that because the Passivhaus Standard delivers such radical energy efficiency, passivhaus architecture needs to be optimised just for an efficient thermal envelope.

This is not the case though.

It is true that the most cost-effective approach to passivhaus is to optimise the thermal envelope. A complex and difficult to construct thermal envelope is likely to be expensive to build well.

But optimising the thermal envelope can go hand in hand with a creative approach to the building envelope. Passivhaus architecture is much more than just the thermal envelope. For example, just take a look at the rich variety of passivhaus architecture emerging from the Brussels region illustrated in this article on Treehugger.

To read more about the design of the passivhaus building envelope follow this link.

The passivhaus building envelope: a high-performance thermal envelope and a creative opportunity.

 

 

About the author:

Elrond Burrell is an Architect, Writer & Speaker with over 15 years experience in designing and delivering excellent buildings. His areas of expertise are Sustainable Design, Passivhaus, Timber construction, and Building Information Modeling (BIM). He is a passionate advocate of these subjects and loves writing and speaking about them, as he often does in the UK and around the world. You can connect with Elrond via his website.

Elrond Burrell is an Associate at Architype Ltd, however, this article is written in a personal capacity and represents his personal opinions, not those of his employer.

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