Things have been moving at a pace on site. Wet weather at the beginning of the week slowed down the progress somewhat, but come mid week, Mike’s team (Mike, Henry, Bisto, Petras, Liam and part time me) got on to taping all the airtightness membranes (particularly around tricky corners), installed timber wall plates and started with the roof installation. It was critical to do all the taping for airtightness before wall plates were in place, otherwise access would be impossible. The strategy is for airtightness layer to switch from outside (walls) to inside (roof), and this swap happens on top of straw timber panels, below wallplates. Unlike walls, which were pre-fabricated as insulated panels, roof is installed as individual timber I-beams, spanning between glulam wallplates and central glulam ridge beam. These 400mm deep I-beams were pre-cut and reinforced in the Ecococon factory, hugely speeding up the installation process on site. Later on they will be fully insulated with blown cellulose insulation. The south canopy overhang is formed with a combination of Douglas Fir rafters cantilevered over wall plates and smaller, 200mm deep I-beams. A team of four of us had all the beams tacked into place in about two hours and Mike’s apprentice Liam topped the house just after lunch on Thursday. This made a huge transformation to the shell – we can really start to appreciate the feel of internal spaces now.
Wednesday 21 September was a big day as we finally started assembling Ecococon straw/ timber panels. One of the main benefits is the speed of construction: it took us only three days to assemble all the external walls, two internal load bearing walls and the main glulam ridge beam. It was helpful that all panels were referenced to colour coded Sketchup renders – this was really easy to follow on site, just like a giant LEGO set. Each panel has a couple of holes at top & bottom, and wooden dowels are used to locate the adjacent panels accurately (like IKEA shelves) before they are screwed together. Unlike ‘traditional’ timber frame, Ecococon system has the airtightness layer on the outside face of the wall, covered with further layer of woodfibre insulation and, in our case, timber rainscreen cladding. Airtightness layer is a vapour open membrane with Sd value of less than 0.2m – this is important in terms of moisture control in the wall assembly. Airtightness membrane is taped to DPM at low level, which in turn is sealed to concrete slab under the sole plates, ensuring no unwanted air leakage. It is critical to keep the straw panels dry – we had a few ‘moments’ during Day 1 when sudden showers forced us to stop assembly and quickly cover the panels up with plastic sheeting.
It is the first time this system has been used in UK and we didn’t quite know what to expect in terms of accuracy of panels. We needn’t have worried – the walls fitted perfectly onto the timber sole plates, even over the longest 17m walls the tolerances were measured in milimeters rather than inches. Very impressive. It was great to have Bjørn Kierulf from the Slovakian architectural studio Createrra available to answer some technical questions during the first day – Bjørn was originally the one who introduced me to the Ecococon system several years ago (thanks!) and I have wanted to find use for it ever since.
Hopefully we will be able to fully wrap the walls up and install the roof over the next week or so, fingers crossed for some dry weather!
With a small time window before we start the assembly of the frame and a sunny weekend to bag, it’s been the perfect opportunity to do some ‘Shou sugi ban’ aka timber charring. We have air dried locally grown Western Red Cedar cladding in one of the sheds for the past four months, with the moisture levels dropped nicely from the original 30-40 odd to a more suitable 20-25%.
Timber charring is a way of preserving cedar, it helps protect the timber cladding against fungi and insects, it enhances fire-resisting properties and last but not least, makes the cladding beautifully textural.
The traditional Japanese technique involves creating triangular ‘chimneys’ of cedar planks and burning the inside face over a small fire. The stack effect draws the fire in and flames rapidly spread upward. I have tried this technique, but had variable results in terms of consistency of char. It is rather difficult to control the degree of burn with 3.6m long planks.
I have also tried another technique, this time using a propane blowtorch, where a rectangular box is created and torch is fired inside. This produced some spectacular burn (ses photo below), but again the control of char was quite limited and some of the edges had to be finished off separately.
Finally, I resorted to a fairly standard blowtorch technique, laying several planks at any one time to make the most of the flame. Low moisture content of the cladding meant a really clean burn, with sweet smell of cedar filing the air.
I’ve tried couple of options finishing the charred cladding – soft brushing & staining with OSMO black oil woodstain, and just oiling with OSMO UV oil with no brushing. Both options produce a beautiful finish, but the brushed version is a lot more time consuming (and dirty!). Having looked at the design of the house, I’ve decided to limit the brushed finish cladding to the covered verandah, where the cladding will be in most contact with us. ‘The Gator’ effect cladding (without the brushing) will cover the rest of the house.
Ecococon is a company based in Lithuania, manufacturing prefabricated straw/ timber panels to exacting standards. Combined with internal clay plasters (applied directly to straw), the panels provide a healthy, toxin free internal environment that is comfortable in all seasons. With extra layer of woodfibre insulation fitted to the outside, it is relatively easy to achieve passivhaus levels of insulation. Unlike the traditional straw bale building, Ecococon system is very accurate, predictable and consistent – drawings are worked out in milimeters rather than bale sizes 🙂 Further details about the company can be found here.
After months of detailed design coordination, we have finally taken delivery of the wall panels and materials for roof construction, all fitted neatly on two lorries. The straw fairies were present and we’ve been blessed with the warmest day of September. Six of us and a JCB made a relatively quick job of unloading, stacking and covering up all the materials. Starting with assembling this giant house lego next week – cannot wait!
Windows and doors play a key role in how the building feels and performs. They have to balance visual appeal with daylighting, thermal & acoustic performance and overheating. Many architects’ dream is to have slimline profiles, but this usually comes at a cost and/ or thermal performance penalty. We’ve chosen Smartwin timber windows with aluminium external trims for low maintenance and warm feel from inside. We’ve used a few ground rules for windows design: keep things simple, minimise mullions, have opening lights only where needed, keep sizes to sensible (read: affordable) dimensions, provide adequate shading. It has been super exciting to visit the factory in Slovakia where our passivhaus windows & doors are made. They marry high performance with simple, elegant design – really can’t wait to see them delivered to our site.
After the mammoth task of concrete slab grinding, a small time window before the straw/ timber frame arrives has allowed me to get on with some prep work: cutting and staining roof rafters and battens that will be fully exposed under the roof. The idea is that these timber members will visually complement charred timber cladding on walls, so I’ve used thinned black OSMO natural oil woodstain applied to sawn Welsh Douglas Fir. This created a very nice effect of maintaining a good wood texture whilst providing a near black stain. Very happy with the results.
Concrete grinding took a bit longer than I originally thought. I don’t know what I thought actually, maybe it was wishful thinking that it would be all done & dusted in a weekend, but in the end it took 88 man hours to get the 100m2 slab to a finish I’m finally happy with. It’s not perfect, but the irregularities in the extent of revealed aggregate and areas where the float created some surface streaks add to the character. In terms of the concrete mix, we’ve used a small local concrete supplier who was flexible enough to allow us to choose a nice warm coloured aggregate mix (as opposed to standard mid grey used in Herefordshire), the rest is 100% cement, red sand and water. We would have liked to use some recycled content (GGBS) and plasticisers to make the concrete ‘greener’ and slow down the drying times, but unfortunately the small supplier wasn’t geared up for this. It did mean we had to work fast during the slab pour, and as a result some areas had gone off before they were floated, so ended up a bit ‘rough’ which, in turn, made the grinding a bit trickier. All in all, pretty happy with the results and hope the effort will be worth it.
So after the concrete slab was poured, rollered and tamped, we covered it up for a few days and started with grinding. The idea is that the slab will be the final internal finish. A day and a bit into grinding the machine had a bit of a fit and stopped working, later we found out one one of its gearboxes gave up. Replacement grinder turned up this week, so I can carry on. It’s quite satisfying to reveal what’s just a few milimeters under the surface…
Monday was a big day. After last week’s installation of Isoquick insulated raft system and necessary steel reinforcement, it was time to pour the concrete slab. We have chosen concrete slab as a base for our house for couple of reasons: it would provide useful thermal mass in an otherwise medium density construction and once ground down, it would provide a durable finished floor surface. Due to tricky site access we had to use a concrete pump and direct the pipes over our neighbour’s garden. Six mini mix lorries supplied 24 cubic meters of concrete with specially selected aggregate. Unfortunately this meant that we had to use 100% cement in the mix, which in turn made the mix go off really quickly – it certainly kept the team working hard until it was all poured, rollered with a giant steel roller and finally tamped with a big float and hand trowels. Can’t wait to start grinding the slab to reveal the aggregate…
AKA insulated raft foundation. Fundamental part of passivhaus design is eliminating thermal bridging and keeping the insulation layer around the house as continuous as possible. We have chosen Isoquick insulated raft system as it is pre-cut to the right shape and individual pieces interlock for greater stability, like giant lego blocks. Local legendary AECB builder Mike Whitfield and his team started on site on Monday 1st August. First they accurately set out the footprint of the house and laid drainage in the 150mm Type 1 stone, and followed by 40mm of gritty sand, which formed the base for the Isoquick system. We started the Isoquick with perimeter L-shaped pieces and finished off with central base, which was made up of two separate 125mm layers, laid in staggered directions. Special Compacfoam high density EPS pieces were installed to support door thresholds. All floor penetrations were meticulously sealed before the waterproof membrane was laid down. Two layers of steel mesh followed on top, providing reinforcement for 225mm concrete slab. There were some tricky details around recessed matwell and shower room – hopefully it will all be worth it in the end!