PH15 is a Passivhaus suitable construction solution. PH15 delivers airtight and thermal bridge free houses using simple, UK friendly, construction details. The PH15 approach ensures an appropriate level of specification, at a fair price, with robust details. PH15 was developed jointly by an experienced Passivhaus architect and contractor, recognizing the UK’s need to build much better houses within a longer term mindset. 

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 (walls and roofs), alongside the glazing package and ventilation system.

The PH15 System uses natural, low carbon materials, including an off-site, pre-cut timber frame. The PH15 timber frame is ‘vapour permeable’, this means moisture within walls and roofs can escape to the external air; maintaining vapour permeability is critical when working with natural materials. PH15 is supported with training modules for the erection team.

PH15 system logo

Main components of the PH15 System are:

A. THE WEATHER-TIGHT SHELL: The timber frame is designed in 3-D, which includes structural engineering. The shell includes the intermediate ‘open webbed’ floor joists and the floor boards, as well as the stud work for the internal walls (no stairs). The main I-joist frame is all pre-cut off-site and arrives flat packed ready for erection, with easy to read frame erection drawings, and each element with its own unique code. Other elements included are the high quality air-tightness tapes, membranes and grommets, the guaranteed air-tightness/racking boards, all external wood fibre boards and the wall/roof insulation’s (the blown-in insulation is also installed on site by us). The PH15 wall/roof construction comprises over 85% insulation, giving PH15 a very low timber fraction. PH15 also covers two air-tightness tests.

B. THE WINDOWS & DOORS: Supply of high performance, Passivhaus suitable, triple-glazed timber windows and energy-efficient doors, typically clad externally in aluminium for low maintenance. Further options are a folding/sliding door set, sliding doors, patio doors, triple glazed roof lights and triple-glazed sun tubes. The windows and doors can be selected from a range of design options, including colours and finishes.

C. THE VENTILATION SYSTEM:  Design and supply of the ventilation system (MVHR) which is Passivhaus Certified, and typically 85-90% efficient. The ventilation system continuously delivers fresh air to every habitable space in your home and extracts continuously from bathrooms and kitchens.


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.

PROJECT PLAN: Each project has a bespoke PH15 Plan and the different PH15 elements are delivered to site to fit in with your build programme. Each new project is supported with an initial training session and technical telephone support throughout. On completion of the shell, an air-tightness test is carried out, we typically achieve results of 0.3-0.4 ach@50Pa, which meets full Passivhaus criteria.

FINISHING: 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 or silicate render and comes with a guarantee If you are wanting a timber rain screen we can also supply. The internal fit-out can also follow to your own specification and there is an internal services void for easy install of electrics and plumbing.

Our services range from the supply only of the PH15 System, through to full house design. For architects and designers there is a PH15 System Design Guide and we 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. You can have cost certainty on your Passivhaus components within a 5% range at planning stage, and confidence that exceptional air tightness can be achieved that relies only on standard UK carpentry skill.


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

project snippets
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Passivhaus Trust Member
15 40 architecture collective
certified passivhaus consultant