Everybody be warm, this is an idea.
One of the big challenges with creating a tiny accessible eco-house is how to maintain a comfortable temperature within the home.
Keeping a house warm is the biggest single use of domestic energy in temperate climates, about 60% of our total energy bill. Energy saving bulbs and switching the lights off are little more than a feel good factor by comparison.
Maintaining temperature without consuming energy has high initial costs both in terms of money and space.
In terms of best practice, earthships and passivhaus (passive houses) design principles provide good guidance. However neither methods are 100% appropriate when considering mobility due to reliance on insulation, which require large amounts of space.
My thinking on this is in part from my desire to have a mobile house that is close to a passivhaus/earthship in it’s levels of energy consumption and comfort. The other part is from the sheer pleasure of a creative constraint that provides an interesting challenge.
It may be that no solution exists, but a combination of the below may allow for creation of mobile green buildings and allow for easier and cheaper “retrofits” or even potential solutions for renters burdened with the costs of heating poorly insulated homes.
I invite those with a superior knowledge of thermodynamics and building design to comment on these ideas and their viability. Please do so in a way that increases my knowledge of the topic, and understanding of the issues at hand.
The idea - Heat recovery house
The heat recovery house borrows heavily from the passivhaus idea outlined below. The core difference being that it compensates for a thinner layer of insulation with increased heat recovery.
Existing passivhaus designs recover heat from the air as it is replaced with fresh air from outside. I propose to increase the amount of heat recovered by also recovering heat that would traditionally escape from some of the walls and ceiling as well.
Like the passivhaus, the heat recovery house would be airtight when doors and windows are closed. Heat would be recovered by fresh air as it enters the space.
How a heat recovery system works
In principle a heat recovery system is very simple. There are many different ways to do it (see here for more info). Hot air flows though a conductive material transferring it’s energy (heat) to the material. Cold Air passes through the same conductive material (either at a different time, or a separate space in material (see diagram below). Heat is transferred to the cold air.
Heat recovery systems use greater surface area in order to maximise opportunity to transfer heat. Air flow is often assisted by a fan to pull air through the system.
How the heat recovery house works
The heat recovery house extends the surface area from which energy is transferred, whilst at the same time reducing the volume taken up by the HVAC (heat recovery system) unit. It does this by flowing fresh air through some of the walls and ceiling. Should the system work, then in principle it should be possible to recover heat that would usually escape through the walls, floor and ceiling. Recovering the heat and “recycling” it.
Wall panel design - for DIY construction
For early experiments wall panel should be as simple as possible. A layer of insulation (expanded cork), a layer of infrared reflective film, an air space (or 2 for ceiling) with flowing air, an aluminium sheet and wooden board or the frame. Plate and Fin aluminium could be used in future iterations to increase heat transfer if necessary.
The great thing about a mobile space is it’s easy to point the largest windows south.
Water filled Trombe wall
A Trombe wall is essentially a thermal sink - it absorbs heat and then slowly releases it over time. At it’s simplest it’s a giant water container painted black, although even transparent water tubes will work.
During transportation water could be emptied to decrease weight. Connection to plumbing system and radiant floor would also allow for heat to be easily distributed throughout the house as required.
Radiant floors are heated by pumping water through them.
Like putting a greenhouse on the south side of your house.
Solar air heater
Solar air heaters represent another simple opportunity for cheap heat gains. Black wall, conductive material and an air gap with air flow, producing warm outputs even in cold temperatures provided enough sunlight.
Given the small space we’re working with 6.2 m x 4.8 m x 2.2 m I think we should be able to provide for our heating needs with a combination of the above and skimp on the insulation. But would love to get some feedback on the feasibility of this idea.