We’ve recently re-focussed our efforts on clay plastering. Clay plaster works brilliantly when applied directly to straw, as it allows moisture to permeate back & forth, effectively acting as a moisture buffer. It’s a healthier option compared to cement or gypsum plaster and will add significant thermal mass to the building – we have 7 tonnes of it to put on walls!
Some straw buildings treat the clay plaster as airtightness layer, but this can be problematic, particularly if there are tricky junctions. We use the clay plaster purely as an internal finish, and airtightness is dealt with by a separate membrane, installed to outside face of straw panels. More details on that are in my previous post.
We have sourced our clay plasters from a Czech company Picas, as recommended by our Slovakian plasterer Roman. I met Roman at an Ecococon event in Lithuania in May last year (short video from the event) and it was great when he agreed to plaster our house. The first few days were spent fixing reed mats to exposed timber elements – this will provide a good key for clay plaster. We’ve started with plastering the straw external walls, where two base coats will be applied before the final finish coat. We found our little fans a bit inadequate, so borrowed the neighbour farmer’s big fan to help dry the plaster out – it’s been working a treat.
We’ve installed a couple of DIY moisture probes that will enable us to keep an eye on humidity levels in the straw panels on both north and south facades.
After the first week of plastering Roman has finally finished the first base coat on all straw walls – a bit of a milestone. On to the second coat tomorrow!
It’s been fun having my Dad come over to help with timber ceilings in the main kitchen/ dining/ living space. Unlike plasterboard ceilings in the rest of the house, we’ve decided to have something a bit more special here. Hit & miss European redwood battens of varying widths give the ceiling a nice warm feel, as well as help with the acoustics in this large open plan space.
After a bit of a hiccup with the original timber order, we cut the battens to the correct lengths, sanded them down & applied a clear surface treatment. As we didn’t have a luxury of scaffolding inside, we invested in a finishing nail gun, which allowed me to fix the battens with one hand, while holding/ locating them with the other. This really worked a treat.
It was an intense week, but we are very happy with the results. Looking forward to starting clay plastering next week!
New Year has come & gone, it now seems like a distant memory. Our focus over the last few weeks has been on internal partitions. In a typical timber framed house this is usually done with gypsum based plasterboard. Standard plasterboard is relatively lightweight and is easy to work with. The downside is that due to its low density, partitions lined with it can often feel flimsy and sound hollow – something my dad would call ‘cardboard walls’. We have decided to improve the acoustic performance and thermal mass of internal walls by installing woodfibre insulation between studs, and by using heavy lining boards. Fermacell boards are made from recycled gypsum, paper and water and are twice as dense as standard plasterboard. This higher density should help moderate internal temperature peaks and troughs all year round. Another advantage of Fermacell board is its rough surface – it will be an ideal substrate for clay plaster skim layer.
It’s been great to have my brother coming over for a week to help out with the installation. With the linings on, we can now really start to visualise the spaces.
After the house has been made weather-tight, we’ve been focusing our efforts on first fix electrics and plumbing. You might ask: but how does that work with straw walls? Our strategy has been to run the main services in the dedicated service zone under the roof and distribute most of the final drops in internal partitions. For the very few power points and pipes on external walls we’ve taken advantage of the timber frame studs or installed ply pattresses, thus avoiding ‘chasing in the straw’. I know other straw builders have successfully chased into the straw and used timber pegs to fix the sockets to, but we’ve managed to avoid this without compromising the room services layouts (plus our electrician was happier:)
As there’s no mains gas in the area, and we didn’t want to go down the oil or LPG route, hot water will be supplied by a cylinder with integral air source heat pump (Ariston Nuos 200d).
We’ve used 15mm insulated copper pipes for domestic hot water, these also supply hot water to two towel rails in the bathrooms. There’s no other wet heating system in the house – the other source of space heating will be the wood stove, which we’ve described in the previous post. This should give us a much simpler and resilient setup compared to our current unreliable oil burner in the rental cottage.
…to be continued in 2017
Merry Christmas ❄️
No matter how good you think you are with paying attention to airtightness, you don’t really know where you’re at until the first air pressure test. It is a good idea to carry out an early test once the airtightness layer is complete and all doors & windows are installed and sealed, but before airtightness layer is covered up. It is an indicator of how much air passes through the external envelope. Our strategy for airtightness layer is wrapping the Ecococon straw/timber panels with vapour-open airtight membrane on the outside and a more vapour-resistant membrane to underside of roof structure. The wall membrane is sealed to damp proof membrane along the bottom, which in turn is sealed to concrete slab using a flexible sealant.
Paul Jennings came over to carry out the first air test. It was good to catch up with him, he was my airtightness tutor at the CEPH passivhaus designer course 5 years or so ago. Before the test started, we sealed up all the service penetrations (SVPs, ventilation ducts, electrical conduits). Paul then started to depressurise and pressurise the house with a small fan to 50Pa and take 10 consecutive readings for each. Early signs were promising. Nick Grant brought his wizard stick (smoke gun) and thermal imaging camera to detect any air leaks. We have identified a number of small leaks, mainly where the holes for Warmcel insulation were taped up, and around door thresholds. Surprisingly, the big sliding door performed relatively well, so well done to Smartwin for clever double seal design. The wood stove was the weakest point, under pressure the smoke was disappearing through the doors and up the flue, but in absolute terms it still performed well.
We were really pleased with the result of 0.3 air changes per hour (passivhaus compliance figure is 0.6 ach). Bearing in mind the unfavourable form factor of 4+ (long bungalow with a lot of surface area) and the fact that we used mainly membranes rather than rigid boards for airtightness plus fitted a wood stove, it really is a credit to the whole Mike Whitfield building team. Nick is still hoping to air test one of the future houses with a bike pump.
We can look forward to living in a draught free house that will be super comfortable!
We opened the house up for visitors last weekend as part of International passivhaus open days. It was nice to see friends as well enthusiats from further afield, so thanks everyone who made the effort despite the damp weather conditions – hope mince pies & mulled wine helped. I think it was quite interesting for the visitors to see our house under construction (warts and all) rather than a finished article. We received very positive comments, and some potential builders are now considering going down the ‘passivhaus’ route, or using more sustainable materials in construction.
This week we’ve had the whole roof insulated with recycled cellulose. The roof structure consists of 400mm deep timber I-joists, so the choice of insulation was quite critical in terms of achieving the right thermal performance and filling the gaps fully between individual joists. I am a fan of recycled cellulose, as it’s a sustainable product (imagine all those tabloids finally put to a good use!), it can moderate moisture movement, it fills every nook and cranny when installed and it greatly helps with acoustics – especially in our case as we’re using tin roof as a finish. Cellulose was pumped into individual bays between I-joists through small holes in the airtightness membrane. These were meticulously taped up after the installation to maintain proper function of airtightness layer. Timber battens to underside of membrane will form an independent service zone, so any installations will not compromise the airtightness.
We are expecting the first blower door test next week – this will give us an early indication of where we are with overall airtightness performance. Anyone wants to place a bet?
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.