The following article contains excerpts, details and information from our book Understanding Passivhaus – The Simple Guide to Passivhaus Detailing and Design.
The building fabric has a key role to play in achieving many of the requirements of a Passivhaus. The Passivhaus standard adopts a ‘fabric first’ approach. This essentially means that key priority is placed on ensuring the envelope of the building performs well including high performing insulation and airtightness, so that minimum requirements are placed on ‘bolt on’ technologies such as water heating, space heating and so on. The Passivhaus approach provides high levels of thermal performance and exceptional indoor comfort. The success of the Passivhaus relies heavily on the building envelope.
Many different construction methods have been used to build a Passivhaus, some of the more frequently used ones include:
• Solid masonry walls with exterior insulation composite
• Solid masonry walls with exterior insulation and cladding
• Core insulation construction systems
• Cavity wall systems
• Timber frame construction with infill insulation
• Sustainable construction methods such as straw bales
This high performance continuous envelope includes insulation to achieve low U-values, a good wind tight layer to the outside of the insulation, an airtight barrier on the inside of the insulation and high performance windows and doors.
Passivhaus Roof Design
There are a large variety of solutions available for a Passivhaus roof design. As with any standard roof, the most important issues is waterproofing, with the addition of airtightness / windtightness and additional insulation requirements of a Passivhaus design.
Insulation material and thickness will be closely related to the roof structure chosen. While a timber roof structure is common for a Passivhaus or indeed a standard residential construction, there are a number of other options available including concrete pitched or flat, ICF roof amongst others. We explore a couple of these different options below.
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Passivhaus Roof Insulation
Key to the fabric first approach of the Passivhaus standard, the design will consist of a high performance continuous thermal envelope.
A Passivhaus will require high levels of insulation, sometimes referred to as super insulation, in order to achieve a U-value between 0.08 W/m2K to 0.15 W/m2K. These U-values can be reached with many different methods of construction including masonry, timber, prefabricated components, steel construction and more. The thickness of the walls will vary according to the method chosen.
Insulation materials have thermal conductivity ranging from 0.06 and 0.02 W/mK. Choice of material will have many determining factors including thermal capacity, fire performance, water diffusion resistance, ease of installation and more. Environmental impact and energy required to produce the different insulations must also be considered.
The exact required U-value will be calculated in the PHPP and will take into consideration other design elements such as location, form factor, solar radiation, fenestration, local climate and other variables.
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Passivhaus Roof Airtightness
Air leakage is a common problem in construction, with many buildings suffering significant heat loss due to air leakage. Air leakage can be defined as unplanned movement of air through the building assembly or thermal envelope. Draughts from air leakage can waste a large proportion of heat generated in a building, and also result in decreased occupant comfort.
The Passivhaus standard requires very low levels of leakage, or in other words, high level of airtightness.
With an airtightness barrier on the inside of the insulation the building can keep heat inside, or unwanted heat outside. Essentially, high levels of insulation with good airtightness will reduce the heat flow through the building fabric. As well as insulation to the general building fabric, any penetrations such as pipework and ducting must be well insulated to minimise cold bridging.
Passivhaus Roof Details
Find Passivhaus Projects
Other Passivhaus Resources
The details from our Understanding Passivhaus book are also available on the Detail Library. You can sign up for membership to get access to all the details. Find out more here:
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Photos: Jim Stephenson