Part 7: Getting the Details of Your Passivhaus Design Right

1 July 2016 Brent Ackerman, Delta Project Management
Image of a new bath fitted into the SIPs show house
As Potton’s Passivhaus show home takes shape, our attention now turns to fix the plumbing and electrics, as well as lay the screed floors. Dr Paul Newman shares the lessons learned from this stage of the build.

Preparing the Floors for Screed

If you’ve been following our step-by-step guide in which Potton constructs its first Passivhaus show home and shares words of wisdom to help your own self-build project, you’ll be aware that the ground floor construction consists of a solid reinforced concrete raft. The raft is 225mm thick and sits on 250mm of Kingspan Styrozone floor insulation. The whole construction is raised up out of the ground by 600mm because we are on the edge of the flood-plain.

Our building superstructure is constructed on a blockwork upstand, inside of which we have laid additional insulation as well as anhydrite (calcium sulphate) cast screed. We chose to use Optim-R (from our sister company Kingspan Insulation) to insulate under the screed.

This next-generation product has a microporous core, which is evacuated, encased and sealed in a thin gas-tight envelope to give outstanding thermal conductivity while also providing the thinnest possible solution for any situation where insulation is required. With a design (aged and edge effect) thermal conductivity of 0.007W/m.K, it provides an insulating performance that is up to five times better than commonly used insulation materials.

Unlike other insulation products, the installation layout for Optim-R is pre-designed to ensure that the panels delivered to the site fit in the intended position. Where necessary, small infill panels are used to complete the insulation jigsaw, and the whole layer is installed on a 3mm layer of rubber crumb sheet. Once complete, the entire insulated area is overlaid with a thin ether foam sheet and then the screeding can begin. We found that the Optim-R product was surprisingly easy to install, and thankfully, we managed to complete the jigsaw without mishap.

What Are the Benefits of Anhydrite Screed?

We have used anhydrite screeds previously, notably for the Wickhambrook barn-style show home at the Potton’s Self-Build Show Centre, which features this type of screed. The quality, speed and simplicity of installation was something that we were keen to repeat and demonstrate again on this project.

Not only are traditional sand and cement screeds slow and labour-intensive to install, but they’re also highly dependent on the quality of the installer.

Anhydrite screed, on the other hand, has many obvious benefits in comparison. Developed over 20 years ago, it is substantially quicker to lay, far less labour-intensive and is ideal for use with underfloor heating.

Our screed was supplied and installed by a two-man team from Screedflo in less than three hours. The process was as follows:

1. The mixing lorry turned up and the installation team checked the slab levels, as well as positioned a couple of dozen “spiders” to ensure the screed was pumped to the correct depth. Our site team had already installed a tanking membrane (500 gauge polyethene sheet with taped joints) to prevent the liquid screed from escaping.

2. The Screedflo team filled up the onboard tank with water — in reality, this took up almost half the time that the team was on-site — and began the repeated batch mix process (this allows a pretty much constant stream of mix to be pumped from the lorry into the building).

3. The pumped areas were gently agitated to ensure a level surface and remove air bubbles from the mix.

The screed was installed on a Friday morning and was “walkable” by Monday. We began working inside the building again on Tuesday. At depths of 40mm, the screed can take foot traffic after 48 hours and will take full working loads after seven days. It dries at a rate of 1.4mm a day, depending on ambient conditions.

Building a Passivhaus Show Home

A complete step by step guide, we take you through the story of building our Passivhaus show home. From the design process, constructing the superstructure, achieving airtightness right through to the finishing touches.

Download the guide here.

Building a Passivhaus Download

Preparing for External Cladding

A two-man team from our factory was appointed to install the additional layer of 100mm Kingspan Kooltherm insulation we needed to meet the performance levels given in our Passive House Planning Package (PHPP) analysis.

They also fitted the timber battens to support the external cladding. The timber cladding boards, which were fixed vertically, were supplied by Vincent Timber. We selected prefinished western red cedar in a lozenge-shaped profile with 7mm face gaps between them. Additionally, we mocked up several different gap widths to determine which looked best and, after much deliberation, selected 7mm as the most visually appealing option.

It’s important to note that if you are opting for a “gapped” cladding effect, you must use a membrane behind it that is capable of withstanding permanent exposure to UV light — most breathable membranes cannot do this. We used Protect 5000 Façade Wall Membrane.

One of the slightly unusual things about our Passivhaus show home is that it features masonry cladding too, in the form of Terca Megaline Grey Multi bricks from Wienerberger.

The long, narrow format brick with 8mm mortar joints provides a contemporary appearance, with a nod to the traditional brickwork elevations of the vernacular. A common aspect of all new builds is making sure a planning condition relates to the appearance of the external finish, so before starting work on-site, we mocked up several brickwork panels with different coloured mortars and mortar finishes (flush, bucket handle and recessed), and selected the one that appealed the most. One thing we did know already is that the very pale cement we selected is quite tricky to work with as it can swiftly go sloppy if overworked.

Most SIPs (structural insulated panels) and timber frame Passivhaus builds exclusively use lightweight cladding systems, which are carried by or fixed back to the building structure. To use the masonry cladding and not introduce any thermal bridging, we needed to construct a separate foundation just to carry the brickwork façade. This was a simple strip foundation — it is completely isolated from the main insulated raft.

The brickwork support channels, which provide restraint, were screwed back through the external insulation into the 15mm OSB (oriented strand board) face of the Kingspan TEK® Building System of SIPs-based panels. The impact of the repeating screw fixings was added into the U value calculations for the external wall and, in this instance, it had little impact because of the large amount of insulation in the wall. This would not be the case in constructions with lower levels of insulation.

The First Fix Begins Inside

Once our external cladding team completed their work externally, our intrepid team from the factory moved indoors and helped out with the first-fix carpentry work, fitting service battens inside the Kingspan TEK® Building System and plasterboard support timbers where required. A separate team constructed a bulkhead unit in the kitchen to support the cooker extractor and framed up the “feature” elements in the bathrooms.

Walls and Plasterboard

The intrepid team started fixing plasterboard too. We used Gyproc Habito from British Gypsum on the walls which have, for the most part, been taped and jointed for direct decoration.

Gyproc Habito is a product that allows you to fix up to 15kg directly to a wall using only a single-standard wood screw. Shelves, curtain poles and TVs can be fixed without the need for drills or specialist fixings — you simply screw directly into the wall surface. It makes DIY easier and gives you more freedom to have your home just the way you want it.

The product is also much more durable and damage-resistant compared to standard plasterboard products. The Habito boards are the same size as standard plasterboard and are installed in pretty much the same way; they can often be scored and snapped as opposed to being cut with a saw. We have finished a few walls with Thistle Magnetic Plaster and I think we’ll have fun seeing how creative we can be using the walls.


Among all of this work, we fitted the ductwork elements required for the MVHR (mechanical ventilation with heat recovery) system and carried out the first-fix plumbing and electrics tasks. At some moments, we had several trades working alongside each other. It had been a nightmare trying to keep the build clean and tidy — it really is amazing just how much packaging waste and rubbish is generated during the construction process of one dwelling.


Plumbing and Electrics

The first fix plumbing and electrics were relatively straightforward and the hardest part was confirming the position of everything in the plant room, which at one stage seemed huge but later seemed barely adequate.

The most nerve-wracking part of the first fix work was cutting the large 260mm diameter ductwork holes in the Kingspan TEK® Building System for the MVHR system and micro air-source heat pump.

The Impact of First Fix on Airtightness in a Passivhaus

If you’ve been following our step-by-step guide, you’ll know by now that the key features of a Passivhaus include:

●    High levels of insulation
●    Extremely high-performance windows with insulated frames
●    Thermal bridge-free construction
●    An airtight building fabric
●    A mechanical ventilation system with highly efficient heat recovery
●    Accurate design using the Passive House Planning Package (PHPP)

In our previous article on how to build an airtight house, we had carried out an airtightness test in public as part of our Self-Build Live events. The result was 0.50ach@50 Pa — a pleasing figure given the somewhat complex geometry of our new showhome (the more complex the design, the more challenging achieving airtightness is).

We were also able to demonstrate how opening a window can impact the overall performance. Opening a large downstairs window by about 150mm reduced our airtightness from 0.50ach to 3.00ach@50Pa — a figure still comfortably in excess of most new build properties in the UK.

Since this test was carried out, we did a lot of construction work and made several (twelve, to be precise) holes in the external envelope — and four of them were very large! To check that we weren’t setting ourselves up for failure in the future (in other words, to make sure we were still at the required level of airtightness), we got our friendly airtightness testers to carry out an extra test.

On reflection, this is something we should have planned to do from the start. This work resulted in the build being more airtight than previously at 0.41ach@50 Pa.

Read Part 8: Part 8: The Final Stages of Building a Passive House (Passivhaus)
Potton Elsworth Passivhaus Show House

The Elsworth is open to the public

We have five self-build homes for you to browse, but the Elsworth is the UK's first permanent show home built to Passivhaus standard, created using our Kingspan TEK Building System.

Come and experience the Elsworth show house for yourself.

Editor’s note: This blog post was updated in June 2019 for accuracy.

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