Passivhaus Roof Details

Loft Conversions

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.

 

See image credits at end of article

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.

See image credits at end of article

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

 
The following details demonstrate a few different options for Passivhaus roof design. All of these Passivhaus roof details are available in the Understanding Passivhaus book which also feature the cad and sketchup for download within the digital book bundles.
 
Passivhaus Flat Roof Detail M06 3D Passivhaus Flat Roof Detail M06
 
M06
External insulation is carried up the wall and to roof level, continuing uninterrupted onto the roof structure. Thermal block acts as thermal break to minimise cold bridging.
 
Passivhaus ICF Pitched Roof Detail M12 3D Passivhaus ICF Pitched Roof Detail M12
M12
ICF roof panels usually consist of a steel structure, encased in moulded panels of EPS. The roof panels interlock, with the joints taped and sealed. The system is often designed by the supplier to ensure correct detailing.
 
Passivhaus pitched roof detail M27 3D Passivhaus pitched roof detail M27
 
M27
A more traditional roof structure consisting of trussed rafters with insulation provided at ceiling level, which allows for a continues insulated layer from wall cavity to roof level.
 
Passivhaus I Beam roof detail T12 3d Passivhaus I Beam roof detail
 
T12
Timber I beams provide structural integrity to the roof with insulation between the beams. The airtightness barrier (VCL) is positioned on the internal side of the structural frame, allowing a continuity between the wall barrier and the roof barrier. The thermal insulation can continue between the wall joists to the roof without reducing the thickness of the insulation at any point through the junction.

Find Passivhaus Projects

 
Check out the Passivhaus buildings database – you can search by location, building type, construction type and more, to see examples of completed Passivhaus buildings and read detailed information about the projects.
 
 

Other Passivhaus Resources

 

Passivhaus Trust

Passive House Accelerator

Passive House Plus Magazine

Detail Library – Passivhaus Details

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:

https://detail-library.co.uk/details/

 

You might also be interested in:

 

Passivhaus Window Details FI
Passivhaus wall details FI

Image Credits

P-House, Girona, Spain

Architects: Tigges Architekt, Energiehaus

Photos: Pol Viladoms

 

Devon Passivhaus

Architects: McLean Quinlan

Photos: Jim Stephenson

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