Have you considered designing and building a really sustainable home?
Not one that is simply well-sealed and insulated, and therefore easy and more efficient to heat and cool using the ubiquitous air conditioning unit. Not just a 10 star home that doesn’t need heating or cooling, but one with a zero carbon impact when everything is considered and one that is naturally conditioned with plenty of ventilation, enjoying communication with outdoors and recommended minimum air changes (one to two ACH normal atmospheres) all year.
After all, in Australia we are blessed with great climates, clear skies and sunshine, and we love that connection between indoor and outdoor living for as much of the year as possible. There are few places in the world where it is easier to design and build a comfortable and healthy zero carbon building.
I recently started a zero carbon project for a client who wanted an earth home. For this, there are appropriate climate responsive design principles to embrace, assessment tools to help with design and specification, some really affordable renewable energy systems, and earth – the ultimate green building material.
I am not using the common lightweight/well-insulated/airtight approach to energy efficiency designed to minimise conditioning losses. I am more confident with the age-old design approach using mass-linked ventilation and appropriate climate responsive design to maximise gains from natural conditions. I want both efficiency and fresh air, so I need loads of mass to condition those constant air changes and I also wish to avoid the need for carbon intensive cooling altogether. Naturally, I will be using earth for thermal mass because earth ensures the walls are not just visually stunning, durable and maintenance free, but also that they have the lowest possible embodied energy.
The brief was for an earth two-bedroom home in a beautiful rural setting in southern coastal NSW. It will be a rural retreat for now and eventually a home for retirement. The client wants to be able to visit at any time of year and be fairly comfortable straight away, though she likes to light a fire for warmth and ambience.
From experience, earth walls will ensure cooling wont be an issue in this cool temperate climate. Cooling rarely is an issue with earth walls in most climates. The thermal mass and nighttime purging should be adequate in resolving high summer temperatures.
In winter, a slow combustion heater would keep the home cosy with a carbon neutral heating source. This would ensure carbon neutral space heating. However, a flying weekend visit to an unoccupied holiday home in winter may result in a cool day or two whilst getting the thermal mass up to a comfortable state. This is the only design challenge I see apart from in absentia operation of nighttime purging in summer.
I’m also keen to optimise design, improve the external envelope a little and diminish or negate the reliance on wood for space heating, so this project offers the perfect opportunity to experiment with this challenge.
There are some options to reduce or resolve the heating deficit. Insulate all of the rammed earth walls, insulate the southern walls that don’t enjoy improved effective R values through solar gain in winter, double glaze openings, trap enough free solar energy to compensate, use solar hydronic heating to supplement the solar design, or just wait for climate change predictions to resolve the situation.
I sketched a basic plan and enlisted the services of an architect from Ramtec, a rammed earth firm. The client liked the relatively simple plan that included a northern conservatory corridor and sun room that the main bedroom, kitchen/dinning and living areas can open into. This will be a powerful sun space that can also be a shaded, fully ventilated verandah outside the home. The sun space is narrow enough that the winter sun will reach onto and through glazing in the inner north earth walls.
Almost half of the rammed earth wall on the south is insulated by an attached lightweight-framed and insulated bathroom/laundry annex. The main bedroom is insulated on the south by a stairwell to the upstairs bedroom and cupboard beneath. A little remaining amount of living room has an uninsulated rammed earth wall facing south.
The finalised draft plans were subjected to a thermal assessment using NatHERS, an e-Tool life cycle assessment before a NSW BASIX assessment. Remember, we are looking at a result that achieves a zero carbon outcome, so therefore we must consider all of the embodied energy used to construct the building and its services, the likely lifespan of the building, maintenance and end of life impact and the size of the building and occupant capacity. Of course, we need to consider the efficiency of the external envelope and the energy for space heating and cooling but also all operational energy including lighting, cooking, hot water, waste water treatment, storm water, and potable water.
It is important to understand that though I am looking for carbon neutral space heating/cooling, I am not simply looking for a good star rating and I am not prepared to compromise on minimum ACH for efficiency. At the end of the day, we are trying to reduce greenhouse gas emissions, and so we need a tool that that considers carbon intensity and therefore reports using the common carbon metric kgCO2-e/person/m2/annum.
We used e-Tool Global consultants to guide us through the life cycle design assessment and optimise design and specifications for the building. It was interesting to discuss the implications of using high embodied solutions like double glazing, various types of smart glass, insulation in rammed earth, solar panels, solar powered hydronic slab heating, concrete slab and Colorbond roofing. We constantly needed to balance embodied energy and operational energy over the life of the building and consideration of climate.
Unfortunately, NatHERS had some problems with the plan due to some design features it couldn’t model. As is often the case with naturally conditioned buildings, it was difficult to achieve a good star rating though paradoxically less difficult to achieve an energy efficient building. How I wish it were more aligned with other assessment tool and reported with consideration to carbon intensity, and how I wish it could model conservatories and stack ventilation and a ventilation logic that needs to be used for operation of this building in summer. We have an assessment tool for conditioned buildings and now need a similar tool for optimising design in naturally conditioned buildings.
In summer, the conservatory would act and be best modelled as a fully ventilated verandah and in winter as a powerful sun space. The home is intended to achieve minimum air changes though maintain efficiency through mass-linked ventilation. The air changes will be conditioned by the thermal mass all year. At least minimum nighttime purging in summer will be maintained when the building is vacant with minimum air changes being vented through a loft Velux skywindow.
Naturally, maximum ventilation would be advocated at night in summer. The main house can be opened to the conservatory or closed off. We are looking for long periods of time where adaptive comfort is achieved relative to time of day and time of year.
The building complied with energy efficiency through NSW BASIX DIY method. A NABERS assessment should confirm a very sound as-built performance later.
We selected uninsulated rammed earth walls, concrete slab on ground with burnished finish and slab edge insulation, solar hydronic heating, PVs, gas cooking and instantaneous gas hot water, slow combustion heater, ceiling fans, LED lighting, double glazing in timber frames to all but the conservatory and inner conservatory glazing, some Velux skywindows with adjustable shades. We were all confident that the design could deliver on zero carbon outcomes.
When it is completed, we will assess the comfort and overall performance.