The effectiveness of solar passive principles
Improving health, freedom one house at a time.
Introduction
In May I presented the 7 key principles of solar passive houses - read Part 1 and Part 2 if you haven’t already.
Following up on many questions I thought it’s a good idea to present the effectiveness of the principles in a weighted list starting from the most effective to the least. So here you have it:
The effectiveness of solar passive principles
1. Continuous thermal envelope, thermal mass & thermal insulation
For starters, here’s an energy balance chart of an average house:
The thermal envelope components make up 50% of energy loss and these are the ones that should be addressed first, always.
This principle cuts up to 50% of energy losses regardless of location and it is the most cost-effective strategy to use too. Insulation is the cheapest element to install in any build (new or retrofit alike). You need a structure anyway so why not make it have more thermal mass if it helps?
Thermal mass is more important if your main challenge is cooling, thermal insulation is more important if your main challenge is heating.
Thermal insulation slows the heat travel through the walls, roofs and the foundation during the long winter days therefore cutting heat loss to a minimum.
A heavyweight building (rammed earth, masonry, stone) has high thermal mass that means it heats up much slower than a lightweight building in the summer therefore minimising need for the air-con cooling.
2. Adapted to local climate - building form factor
The key design concept regarding local climate is the form factor of the building. The colder the climate, the more compact the house must be. In hot climates we tend to see more single storey buildings with expansive plans.
There is a simple formula I’ll share here again:
Building Form Factor = Area of thermal envelope divided by usable area or “treated floor area”.
This ratio should be under 4 in order to achieve a low-energy construction in a cold climate. Most larger passive buildings have a form factor even lower than 3.
In order to achieve this I recommend every building over 120 sqm / 1300 sqft to have multiple storeys if it will be built in a temperate or cold climate.
As I said in part 2. of 7 principles…
I too had to compromise many times the initial building design on the altar of efficiency, to gain that extra 2-3 kWh of energy reduction, to get below the certification limit. As you might imagine this gets exponentially more difficult as overall efficiency gets better and the end result can leave both client and designer with a sour taste in the mouth.
Every kind of design will have a better result if it is infused by the local vernacular.
Local adaptation and building form factor can shave 20-30% off a building’s energy loss and the final design will be more appealing to all parties concerned.
3. Thermal bridge free design
Thermal bridges are less visible on paper than in real life, accounting for no more than 10-15% of energy loss. But they are an important health issue that affects almost all buildings today so that is why I put them so high.
Formation of mould and surface condensation is the second most important issue after fresh air in contemporary buildings. Eliminating thermal bridges lowers risk of asthma formation - especially in children who already suffer a growing asthma epidemic - mainly due to increasingly indoor lifestyle.
4. & 5. Air-tightness of thermal envelope and heat-recovery ventilation
Every good quality construction is at least moderately air-tight. This means it loses less than 3 times its interior air volume in an hour during a standard blower door test.
However if the strict Passivhaus standard is achieved (less than 0.6 air-changes per hour), it means the house is even more airtight and heat-recovery ventilation is needed.
But I can just leave the windows open and get fresh air, Solar, why do I need this heat recovery ventilation?
Because in cold climates air-tightness and heat-recovery ventilation can cut up to 25% of energy loss while providing constant fresh air. People in cold climates don’t leave their windows open in the long winter nights and that means that - if they stay indoors almost all day - they don’t get the proper amount of fresh air.
For example CO2 concentration over 1000 particle per million causes stuffiness of the air and drowsiness over longer periods.
Overall the first 5 principles can get you a long way:
You can see that heating demand is significantly down, internal heat gain and sunlight start to make a positive impact.
6. Quality windows & solar gain maximisation
You are probably surprised this is only on 6th place, given my whole philosophy revolves around sunlight. 🌞 But if you follow the charts you understand why.
The thing is, to keep the heat gained from the sun inside, you need good quality windows that have an overall positive energy balance - so they’ll lose less energy than they provide by the sun. Quality windows are the most expensive single component of the passive house building. The whole window+door order sometimes costs more than the structure itself.
So we usually leave tweaking the window sizes at the last step, after we’ve dealt with all the other steps. This way we minimise the risks of overdoing the size of windows just for the sake of solar gain.
Basically if a location has good sun exposure it can provide the 30-50% of the remaining energy demand from the sun (after steps 1-5. were applied). If it is in a heavily shaded area, like a forest or dense urban context, it can provide 10%, probably 20% max. It won’t be a passive house but it still will be a low-energy one with the same health benefits at the lowest energy consumption possible.
Ample sunlight combined with maximised efficiency can get you to the passive house level that = tiny heating demand:
7. Balancing it all in a holistic energy calculation
All of the above can be fine tuned infinitely but currently there is no better framework to reduce energy consumption in a building while maximising health and well-being benefits.
That is it for today, feel free to comment and ask anything about the topic in the comments! Next week I’ll share the first case study house, The Solar Saltbox that was recently finished!
I’ll leave you with one of my tweets:
Solar