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118 - Eco Cabin

Garth Hood

New Brunswick, Canada

Portfolio: www.thoughtfuldwellings.ca

 

I am going to propose a project which will not meet some of the primary criteria for this project. I do this because I don’t think it is possible to build a low energy ecologically responsible building, for human occupation, that is healthy and will last (not rot prematurely) given the criteria given. I also think there are many reasons why a woodstove should not be the heat source of choice.

A well insulated, low energy building needs to be designed and built based on building science principles. There has been a great deal of experimentation (with good, bad & ugly results) since insulation was first added to the building process. Many things have been learned through this trial and error process. Several of the most important building science principles can be summarized by the phrase, “Build tight and ventilate right.” I have attached an article titled, “The Trouble with it – Building Science,” that explains the issues further.

I see two basic options and only one with a woodstove. The first one is to build a simple cabin with little or no insulation. This can be heated with (lots of) wood. In the analysis chart provided I have given, a small uninsulated structure would use 4.5 cords of hardwood to heat. This is a lot of wood but if the wood is readily available it could be sustainable. The wood could be burnt cleanly and a small woodstove’s heat output would match the needs for heating. The structure would also have very low imbedded energy.

The big problem with this solution is that it is not sustainable, or healthy on a mass scale. This amount of wood could heat 5 cabins insulated to a modest level or 64 built to the highest energy standards (Passive House levels).

The other option is to build the cabin to the highest easily achievable passive solar energy standards possible. This Passive House cabin would have a heating load equivalent to just 0.07 cords of wood per year (271 kWh). To achieve this level of energy efficiency the house requires mechanical heat recovery ventilation. Mechanical ventilation requires a small amount of electricity. Without mechanical ventilation occupant health will suffer and the life of the structure will be reduced. History has proven that super insulated buildings without ventilation will eventually (but likely sooner) build up moisture in the insulation layer. This leads to mould, mildew and eventually rot.

The house also requires a very small amount of added heat. During cold cloudy periods this cabin will require a maximum of 278 watts of added heat to stay at 20 ̊C. Heating this house with wood is problematic for several reasons. First there are no woodstoves available (that I could find) with a heat output less than 3,500 watts and this one was from Europe. The lowest output EPA approved stove I found in North America had a rated output of over 15,000 watts. Of course the stove can be run at lower levels but this will impact efficiency & emission levels. Even the European stove is more than ten times larger than needed for this cabin so using a woodstove will likely lead to overheating this cabin. The repeated opening of windows to cool the cabin defeats the purpose of the super insulation.

A second problem is that a woodstove can be starved for air in a super insulated building. A dedicated air supply will be needed. Woodstoves can also be dangerous with the untrained users that might frequent a cabin such as this.

I therefore decided to plan my cabin around a solar hot water heating and storage system. This system would collect hot water on sunny days to provide heat on cloudy days. The only drawback to this system is the cost. The cabin will also need a small photovoltaic system to provide electricity for the heat recovery ventilation system. There is also a new system that heats hot water with photovoltaic panels that could also work – with some of the electricity being used for ventilation.

My costs for equipment alone jumped over the $10,000 pricing criteria so I haven’t attached a detailed costing of my project. I suspect that this Passive House net zero cabin would cost between $20,000 - $25,000 to build. Equipment suppliers might be willing to give deep discounts or donations for and exemplary project with significant public traffic. This Passive House building should also last much longer as all the building science issues have been considered and only high quality Passive House materials specified.

I also at this time have not completed a detailed analysis of the carbon impact of the building. I see this as only half of the carbon impact. The impact of using a woodstove for heat (or any heat source) needs to be considered in the life cycle carbon analysis.

Thank-you for considering the ideas I have presented.

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