The PH15 System timber frame solution provides a Passivhaus suitable construction system that enables the build team to achieve airtight and thermal bridge free houses as simply and cost effectively as possible, supported with appropriate training modules.

The PH15 System is flexible enough to enable bespoke designs and to meet the many and varied planning requirements in the UK. It comprises all the component parts needed to create a weather tight, airtight shell to the Passivhaus standard.

The PH15 System uses natural, low carbon materials. Our system is also ‘vapour permeable’, this means moisture can escape to the external air; we believe this is essential when building with natural materials like timber. 

The components of the PH15 System are outlined in detail below.  The basic shell materials include all the elements that you need to achieve weather-tightness. The shell includes the intermediate floor joists and floor boards, as well as the stud work for the internal walls. The main I-joist frame is pre-cut off-site and arrives packaged ready for erection, with easy to read frame erection drawings. Included is the supply of triple-glazed timber windows and energy-efficient doors, typically clad externally in aluminium for low maintenance. Further options are a folding/sliding door set, triple glazed roof lights and triple-glazed sun tubes.

The final core element is the design and supply of the ventilation system (MVHR) which is Passivhaus Certified. The ventilation system continuously delivers fresh air to every habitable space in your home and extracts continuously from bathrooms and kitchens. The different PH15 elements are delivered to site as you need them, and each new project is supported with an initial training session on site and technical telephone support throughout. On completion of the shell, an air-tightness test is carried out, we typically achieve results of 0.3-0.45 ach@50Pa, which meets full Passivhaus criteria. 

Generally, the final wall & roofing finishes are up to you to supply & install, most finishes are compatible. The external wall finish is always supplied if it is a lime render. If you are wanting a timber rain screen we can also supply. On completion of the shell, the internal fit-out can follow to your own specification.

Our services range from the supply of the shell materials only, through to full house design. For architects and designers there is a PH15 System Design Guide and we can also supply a set of PH15 System Construction Details (scale 1:10) to suit your project. Using PH15, avoids the risks associated with reinventing the wheel – PH15 is a tried and tested construction solution that delivers quality at a fair price. 

FINALISTS - Structural Timber Awards 2016

The PH15 Systems starts with conventional strip foundations familiar to all groundworkers albeit to tight tolerances. The design spec describes all of the necessary components and insulation thickness is calculated in the PHPP (see What is a Passivhaus) although the only components we supply are the Foamglas loadbearing ‘pillars’ which support the slab. The remainder are standard products obtained by the builder/groundworker. The foundation slab and perimeter block work must be laid to a high standard with the insulation fitting tightly and without gaps to ensure a thermal bridge free base for your new home.



Precision cut timber frame

Wooden I-Joists offer more economical use of materials, greater strength and reduced thermal bridging than conventional joists. Solid timber joists or rafters are construction elements that can be inefficient where they act as a thermal bridge. With I-joists (part of the STEICOconstruction Building System) such thermal bridges are reduced to a minimum due to the inherent efficiency of the I-section profile. And by thermally modelling junctions cold bridges can be further reduced. In many instances this gives a ‘free’ boost to performance equivalent to adding more insulation.

STEICO I-Joists meet the requirements of the Building Regulations, NHBC Standards, Zurich Insurance and Robust Details Ltd. precision cut PH15 components

Intermediate floors

Intermediate floors joists are supplied as Ecojoists®.  Ecojoists consist of parallel stress timber flanges joined with engineered V-shaped steel webs with nailplated zones from Gang-Nail®. The result is an easy to install system, and with its open web design, enables fast installation of ventilation, plumbing and electrical systems. All the required hangers, trimmers and strongbacks are also supplied as required.

In addition the floor deck is also supplied as a 22mm t&g Egger Peel Clean Xtra Chipboard.


External wood fibre cladding

On the outside of the building we use woodfibre insulation, adjusting the thickness as approriate which helps keep the building warm in winter and cool in summer. This highly vapour permeable insulation, used in conjunction with a vapour closed internal lining board, ensures that moisture will not build up with-in the structure.

The woodfibre is suitable for use with direct render systems and is part of a full BBA approved system of components or it can be covered with a timber or brick slip rain screen.

A sustainable and FSC® certified product manufactured from forest thinnings and small diameter logs.

external woodfibre cladding

Airtightness tapes and membranes

The PH15 System uses a combination of airtight racking board (Unilin Durelis) and Pro Clima tapes and membranes. This simplifies the process of achieving reliable low ach (air changes per hour) values where a value of 0.6ach or less is required to meet the Passivhaus standard. More often that not it is the failure to achieve this airtightness result which causes Passivhaus schemes to fail.

All the necessary airtightness tapes and membranes are supplied along with external tapes for the window/woodfibre junction ensuring that these junctions will be wind and weathertight prior to applying the external finishes. An airtightness test is included in the System cost and is carried out once the building shell has been completed.


Unilin Durelis airtight vapourblock

Blown in Insulation

High levels of insulation, installed without gaps is an essential part of a Passivhaus. We use either cellulose or recycled glass blown into the frame void fully filling the walls and roof, which for most situations in the UK is 360mm deep. Spaces which are inaccessible after the frame is up are filled with sheepswool as the build progresses.

cellulose-fibre insulation

Passivhaus suitable windows and doors

The PH15 system is supplied with Katzbeck windows and doors from the full range of wood and wood/aluclad styles. The exact type will depend on the overall requirements needed to achieve the Passivhaus standard and your personal taste. In addition to lift and slide doors which allow large, unrestricted openings you can opt to have Lacuna folding sliding doors for a more traditional look.


Katzbeck lift and slide door   Katzbeck-Brochure-enLacuna-brochure

Heat Recovery Ventilation System

The MVHR units and ducting layouts designed and supplied as part of the PH15 System are based on self-balancing, Passivhaus Certified units from Brink or Zehnder, allied to semi-rigid, radial ducting which we have selected as they combine cost effectiveness and ease of installation. The units are self balancing which simplifies set up and ensures ongoing optimum efficiency. The units can also be supplied with CO2 demand control.

The benefits of a whole house ventilation system are very good indoor air quality, improved thermal comfort and continuous removal of smells and unwanted moisture from the environment. An MVHR system does not stop you opening the windows and is designed to compliment natural ventilation.


Training Modules and Support

The PH15 System typically comes with three Training Modules. Firstly, for the groundworks package to ensure a thermally efficient detail and an accurate start for the frame construction is achieved. Secondly, for the erection of the precision cut frame covering window install and airtight sequencing. The third module covers installation of the MVHR.

In addition telephone advice is available from the The Passivhaus Store.

For architects and designers wishing to build using the PH15 system we provide an Architects Support Package which provides technical construction details, MVHR installation and window details.

If you would like to find out more about the PH15 System why not . . .

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