For Day, an eco-home is not just low-energy. It must have low environmental impact in general and also be comfortable and healthy to live in, with plenty of fresh air. Keeping cool in summer is as important as keeping as warm in winter. He doesn't forget sensory issues, particularly light and noise, and also accessibility. For example, he recommends allowing space for wheelchairs to manoeuvre and solid structures to support hand rails and chair lifts in critical places. This allows the house to be adapted to our needs as they change over time.
I was particularly impressed with his ideas for landscaping around the home. For example, narrow passages between buildings accelerate air flow and increase heat loss from walls, whereas windbreaks can lower heating bills. When walls are covered with climbing plants they provide a layer of insulating air in winter, and help to keep the wall cool in summer. However, plants that have rooting tendrils such as English Ivy can be very destructive - apparently Boston Ivy and Virginia Creeper are more wall friendly. Plants with twining tendrils are the least damaging but they need a supporting framework. This should be at least 50mm from the wall so that shoots can grip it and grow thickly. Also this leaves air space next to the wall so that it can dry easily. There is one potential disadvantage though to balance against the insulating effect of greenery on the wall - it makes your upper storeys accessible to wildlife such as spiders and even rats. You may need to protect yourself against unwelcome visitors.
Day is a fan of community homes with shared facilities. He says that equipment sharing typically reduces electricity consumption by 35-40%. Presumably we wash our clothes less often if we have to go to a shared laundry to do it! Another advantage of shared facilities is extra space (think of the floor footprint of all those washing machines saved). However, you do need a 'resident led management structure' to deal with issues of upkeep and maintenance. Having shared parking is easier to manage and has the useful side effect of making life more difficult for burglars as it isn't obvious which cars belong to which house and so who is or is not at home.
There are chapters full of useful detail on issues of air tightness and insulation with lots of diagrams (mostly intelligible) about how to layer materials to make an airtight and well insulated envelope - even to the detail of what to do about the ends of floor joists and how to insulate around windows. All of this is good advice, to my knowledge.
For heating systems, in terms of climate change impacts he favours wood pellets and logs and strongly dislikes electricity because it has the most carbon emissions. Of course this is true at the moment but electricity emissions will halve by 2020 and halve again by 2030 if our emissions targets are met [2] by which time it will be considerably cleaner than gas. How long is our eco-house supposed to last? He talks about use of wood stoves both as a source of radiant heat in a room and to heat water for radiators in other rooms. Wood stoves should not be run too slowly, as this makes them inefficient and produces smoke. To avoid the need for this you can use accumulator tanks of water to store heat, or use the stove itself as a thermal store - for this he recommends soapstone facades as they retain heat longest.
The objection to reliance on electricity carries over into his dislike of 'active' ventilation systems such as mechanical ventilation with heat recovery - another area on which we disagree, as I have very good experiences of living with MVHR. As well as needing electricity to run he says 'fan driven air is never as invigorating as fresh air'. Well I agree with the statement - because fan driven air isn't cold (at least it isn't in our house) - but I don't see that as a disadvantage. If I want to be invigorated I go for a walk.
On the air quality issue, Day is deeply concerned about air-borne toxins such as radon and formaldehyde. He has a table giving the rates of formaldehyde emission from various building materials such as MDF and chipboard. However he does not indicate what this means in terms of levels of the pollutant in indoor air, or what is regarded as safe. According to the WHO [2], levels at or below 200 µg/m³ are unlikely to cause harm even over long periods, though lower levels may cause irritation. Over a large range of studies of indoor air quality (in many countries) the average levels were generally well below 100 µg/m³ and even the maximum levels mostly below 200 µg/m³. Incidentally, smokers subject themselves to levels considerably higher, but smoking is bad for you in lots of other ways too. The WHO agrees with Day that levels are higher where wall or floor coverings are new, by which the WHO means less than one year old, though Day claims high levels last for 3-5 years. Also they agree that ventilation is very important to keep levels healthy. I can't say that formaldehyde is never an issue - but I fear Day overstates the problem and following his advice to the letter will cause you unnecessary expense.
On the radon issue, Day has concerns about emissions from walls incorporating breezeblock and recommends demolishing these or sealing the surface to keep the radon out. However in practice the contribution from building materials is generally low compared to background levels [4]. The main source of radon is the rock beneath your home. (Public Health England publishes a map showing the areas in the UK that are worst affected).
I could go on, but I think you have the flavour of it now. The book is well structured, easy to read in sections and has lots of good and useful advice. However, it is a bit hard to digest in bulk and some of his advice is, in my view, unjustified scare mongering. I do recommend you read this book - but only with critical faculties engaged.
[1] The Eco-Home Design Guide
Principles and practice for new-build and retrofit
Christopher Day
Green Books
https://www.greenbooks.co.uk/the-eco-home-design-guide
[2] Power sector scenarios for the fifth carbon budget (Committee on Climate Change) October 2015
[3] WHO Guidelines for Indoor Air Quality: Selected Pollutants.3: Formaldehyde (WHO) 2010
[4] Building Materials and Health (United Nations Centre for Human Settlements) 1997
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