With the recent heat wave in UK, it’s been interesting to experience internal comfort in our house. Although we haven’t fully moved in yet, we’ve been spending considerable time inside doing various finishing tasks. Over the last few days the external daytime temperatures have been gradually rising, peaking at a scorching 30 degrees C today. Night time external temperatures have also been going up, last night levelling at 17.5 degrees C. This pattern is likely to carry on over the next few days.
We’re really impressed with how our house has been coping with the hot weather. Internal temperatures have been fairly steady, gradually rising from 18.5 to 21.5 degrees C. We’ve been keeping the windows & doors shut and the ventilation unit on. The MVHR unit has a built-in automatic summer bypass, which kicks in at 21 degrees C. We can contribute the comfortable internal temperatures to a number of design features:
- exposed concrete slab with its high thermal mass acting as a giant temperature regulator (warning: this can work both ways!)
- 1.5m roof overhang along the south verandah means no direct sun hits the big glass = zero unwanted heat gains
- external roller blinds to main bedroom (south facing), and two windows to east & west gables
- high levels of insulation & airtightness in walls & roof (thermos flask effect in reverse)
- fully vented facades & roofing. It is worth noting the facades and roofing are black colour, but the vented cavities help isolate the hot surfaces from the thermal envelope, slowing the heat flux down
We don’t have the hot water system installed yet and have not been cooking/ living in the house, so internal heat gains have been minimal (bar the dehumidifier which we’ve temporarily turned off). We’ll keep an eye on the temperatures & relative humidity and will start proper monitoring once we get connected to broadband. The initial impressions are very positive though and we can’t wait to move in!
Every passivhaus home has a controlled mechanical ventilation system with heat heat recovery (MVHR), and ours is no exception. With expert advice from Alan Clarke and Green Building Store we have designed a cascade system based around Zehnder Comfoair 160 VV Luxe ventilation unit. This is the smallest unit from Zehnder and can supply up to 160m3 of fresh air per hour – plenty enough for the two of us and a medium sized dog. Another benefits are built-in frost heater and automatic summer bypass when internal temperature gets over 21 degrees C. We have kept the ventilation ducts dust-free during the clay plastering and only installed the terminals just before commissioning.
The ventilation system works best when properly commissioned, so it was great to have Alan and Nick coming over to help. Alan hired a nifty anemometer with self-balancing fan for accurate reading of flow rates, whilst Nick used manometer as an alternative way of measuring. We also installed a condensate drain, which wasn’t in place by default. A few hours later (including pub lunch & a game of ping pong) our system was set up. The first thing that struck me was how quiet the whole system is. The MVHR unit itself is just audible, but as it’s located in the Utility room it won’t be an issue. Supply & extract terminals throughout the house are totally quiet – a sign of good design and installation.
The next big step will be installation of hot water system and then we’re just about ready to move in – can not wait!
A few weeks have passed and we have finally (almost) finished the clay plastering.
After the application of base (body) coat to straw walls described in the previous blog post it was time to apply the top coats. Top coats consisted of two sub-coats, the initial 4-5mm fine clay coat and 1.5-2mm final coat, which was applied whilst the first coat was still moist. We have initially selected three different colours for the top coat (vanilla yellow, caramel brown and olive green) to differentiate various spaces in the house, but during the application we have opted for additional un-pigmented basic clay for the ‘feature wall’ in the main living space. We’ve also added finely chopped straw in the top coat for a bit of sparkle and a reference to our straw walls.
Normally it is not recommended to apply clay plaster to plasterboard because of poor bonding. However, as we’ve used fibre-based Fermacell boards for internal linings, we felt confident that this would work. We taped all the board joints with glass-fibre mesh and tile adhesive before a clay suck coat and two top coats were applied to internal walls.
Roman really showed off his skills, with beautifully crafted rounded window reveals and all surfaces expertly finished with small japanese trowels. It’s been an emotional process with ups & downs, but it has totally transformed the feel of the house, giving it a sculptural quality rarely seen in new build homes.
Meant to write this blog before the previous one on clay plaster.
We have thought long & hard about the ultimate feel of the interiors, what makes the spaces: materials, textures, colours, smells. It is quite fun to go through the process of breaking down and interrogating the elements and their relationships with each other. How is a door formed in a clay plastered wall? Are architraves and skirtings essential or purposeful? What does a door handle feel like in hand and how does it fit with the rest of the interior? We have decided to complement the simple aesthetic of the house form with pared down, but beautifully crafted joinery: solid Douglas fir doors, door frames, skirtings and window boards and simple recessed stainless steel handles – all supplied by a local joiner. We discarded architraves and instead opted to use chunky timber door frames and recessed skirtings as giant stop beads to finish the clay plaster to. This decision impacted on sequencing of the works; internal joinery had to be installed before the plastering started. We can’t wait to see how all these elements will come together over the next few weeks.
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.