Preparing the Floors for Screed
If you’ve been following the series – in which Potton constructs its first Passivhaus showhome and shares words of wisdom which may aid on 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 and an anhydrite (calcium sulphate) cast screed.
We chose to use Optim-R (from our sister company Kingspan Insulation) to insulate under the screed. This is a next-generation product which comprises a microporous core, which is evacuated, encased, and sealed in a thin, gas-tight envelope — giving 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 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.
The Benefits of Anhydrite Screeds
We have used anhydrite screeds previously; indeed, the Wickhambrook barn-style showhome, also at Potton’s Self Build Show Centre, 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.
Traditional sand and cement screeds are slow and labour intensive to install. They are also highly dependent upon the quality of the installer, too. Developed over 20 years ago, anhydrite screed has many obvious benefits compared to traditional sand and cement products. It is substantially quicker to lay, far less labour intensive and 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:
- The mixing lorry turned up and the installation team checked the slab levels and 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 polyethylene sheet with taped joints) to prevent the liquid screed from escaping.
- The Screedflo team filled up the on-board tank with water – in reality this took up almost half the time the team was on site – and began the repeat batch mix process (this allows a pretty much constant stream of mix to be pumped from the lorry into the building).
- The pumped areas were gently agitated to ensure a level surface and to remove air bubbles from the mix.
The screed was installed on a Friday morning and was ‘walkable’ by Monday. We began working again inside the building on the 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.
Preparing for the External Cladding
A two-man team from our factory (James and Joaquín) were co-opted 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 will be fixed vertically, have been supplied by Vincent Timber. We have selected pre-finished western red cedar in a lozenge-shaped profile with 7mm face gaps between them. 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 then 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 showhome 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 provide a contemporary appearance, with a nod to the traditional brickwork elevations of the vernacular.
In common with all new builds, one of our planning conditions related to the appearance of external finishes and so before starting work on site we mocked up several brickwork panels with different coloured mortars and mortar finishes (flush, bucket handle, recessed), and selected the one that appealed most. One thing we do know already is that the very pale cement we have selected is quite tricky to work with and goes sloppy very swiftly 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 meant that 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 are 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 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.
First Fix Begins Inside
Once they’d 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 have constructed a bulkhead unit in the kitchen to support the cooker extractor and framed up the ‘feature’ elements in the bathrooms.
Walls and Plasterboard
They have started fixing plasterboard too. We have used Gyproc Habito from British Gypsum on the walls which will, for the most part, be taped and jointed for direct decoration.
Gyproc Habito is a new product which 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 rather than needing to be 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.
In 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. At some points we have had several trades working alongside each other. It has been a nightmare trying to keep the build clean and tidy; it really is amazing just how much packaging waste and rubbish the construction process of one dwelling generates.
Plumbing and Electrics
The first fix plumbing and electrics were relatively straightforward; the hardest part was confirming the position of everything in the plant room, which at one stage seemed huge but now seems barely adequate. The most nerve-wracking part of the first fix work was cutting the large 260mm diameter ductwork holes in the 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 this series, you’ll know by now that the key features of a Passivhaus include:
- Very 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 the previous article of this series we were on the verge of carrying 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 the impact that opening a window had on 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’ve done a lot of construction work and made several (12 to be precise) holes in the external envelope — and four of them are 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), at short notice 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. I’m pleased to report that we are now more airtight than we were — at 0.41ach@50 Pa. It will be interesting to see how this performance level changes as we work towards completing the build.
Potton Passivhaus Show Home Day
Along with Potton, we would like to invite you to attend a unique preview event to launch Potton’s new Passivhaus Show Home — the first in the UK.
Homebuilding & Renovating’s Michael Holmes will open the Passivhaus to magazine readers on Wednesday 14 September. The Readers’ Day will include presentations and a chance to be the first inside the new house, plus there are also four other stunning Potton homes to explore.
A goody bag, information pack, buffet lunch and a glass of fizz will also be waiting for you!