Last week was quite eventful. Mike’s team finished the tin roof, started with installation of airtightness membrane to underside of roof structure and we had a wood stove installed.
Being a passivhaus with low space heating requirements, our house won’t need much heat to keep it comfortably warm, so putting a wood stove in is a bit of a luxury rather than necessity. The intention is that it will provide bulk of the space heating, topped up with towel rail in bathrooms. For simplicity, we have not coupled it with back boiler to heat water (hot water will be generated via heater with air source heat pump). There have been a couple of technical challenges to overcome – airtightness and air supply. We have used Morso S11-42 stove (with optional airtightness kit) in combination with Poujoulat Efficience triple wall flue to give us a completely room-sealed system. The external cavity of the flue supplies fresh air in from outside directly to the stove, and the inner flue takes the hot air out. This ‘pipe in a pipe’ system also means that there is only one penetration through the building fabric. The flue came with a proprietary airtightness plate with a flexible seal, the airtightness membrane will be taped to it.
It was exciting to see the first fire lit – installation crew from Poujoulat (flue) and Prince&Pugh Knighton (stove) did a great job. Will just have to be careful with keeping the fire under control with all that exposed straw inside!
We will be opening the house up as part of international passivhaus open days on 11 and 12 November – follow this link for more information.
Mike’s team have started on the corrugated tin roof installation, this is quite a delicate operation as we are leaving the underside of tin exposed around the edges.
We also started installing the lovely Smartwin windows. It took us nearly all morning to install the first one as there was a lot to go through – pre-taping for airtightness, fixing (using adjustable Essve screws), stuffing the gaps with sheepwool insulation, closing the external reveals with additional woodfibre insulation and finally external taping for windtightness. Eventually, the external timber reveals will fully cover the frames.
During the ‘unwrapping’ of timber/ straw panels we discovered that one piece got wet during the installation. We had a membrane going over this window with a taped joint, unfortunately there must have been a gap in the joint and water leaked down to the cill panel. Water and straw is not a happy combination, so we’ve put some fans on the panel trying to dry it out, but after a couple of weeks we were still getting new green shoots appearing. As we had to prepare the window opening for window installation, I decided to remove the wet straw from this panel. The gap will later be filled with Warmcel and capped off with woodfibre board so we can continue the clay plaster across. This incident was a reminder that straw is an organic material which reacts to humidity, but when this issue arises, it can be dealt with relatively easily.
This week’s weather has been kind enough to allow us install woodfibre boards on top of roof I-beams, and wrap the boards with windtightness membrane, held down by timber counterbattens. Larger timber battens will provide support for the corrugated metal roof. It’s good to have two layers of protection above the straw panels.
We have been tweaking the design of junctions between timber cladding and roof with Mike, and decided that it’s best to finish off installation of external woodfibre boards to walls and insect mesh to close off vented cavities before the corrugated roof goes on.
On Saturday Joyce & me finished off sealing the last charred cedar cladding boards, which was very satisfying to see. It’s now all stacked and ready to use.
There was a bit of excitement when a herd of escapee cows passed our site – I wonder if they wanted to have a peek at this straw filled passivhaus? 😉
As if the week wasn’t quite packed enough with action, we took delivery of our Smartwin passivhaus windows & doors on Friday. It’s been a bit of a logistical conundrum – due to recent wet weather the artic lorry could not park along the verge near the site, so we offloaded half the windows in a layby and the big sliding doors & main antrance door at the nearest industrial estate with a help of forklift truck. Those triple glazed sliding doors are pretty hefty at 300+ kg!
It was very satisfying to see the sliding door put in place only 9 days after starting the timber frame. All the planning, coordination and detailed design over the past few months has definitely paid off!
Hopefully next week is going to be dry enough to allow us install woodfibre boards on roof and provide the much needed protection for the straw panels.
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.