Week 2 - Passivhaus Design Blog

Passive Design

Active vs. Passive House Design

In order to fully embrace Passive house design it is important to first understand active design, verses passive design. Active design refers to houses that are reliant on fossil fuels to create heat, electricity and produce artificial light, which is simply not sustainable for the planet and for us. Passive house design exploits the natural resources we have such as sunlight and solar energy to heat our house. Active design was based off the idea that there is a limitless supply of these fossil fuels such as oil and gas, so it is okay to continue burning and using them, however now a days we know better and know these resources are in fact not limitless but finite and harmful to our planet and environment. The burning of these fuels or energy supplies emit harmful green house gases into our atmosphere (see paragraph and diagram on greenhouse gases), however if we put more emphasis on insulating our houses well and storing the heat we gain from the sun we can maximize our comfort while reducing our carbon footprint which is one of the fundamental ideas of passive house design.  

Maximize Solar Gain

The sun is a natural resource that we have an abundance of. Passivhaus Design exploits this resource by orientating our buildings and designing them in such a way that we maximize our solar gain within the home. By orientating our houses to face the sun all day long, while also having enough windows to allow the light into our homes, we become less reliant on artificial light and burning fossil fuels for heat. Below you can see an example of how I would orientate a building on our given site in Castletroy. The positioning of the dwelling is purely just to show how the dwelling would be best angled in relation to the road for maximum solar gain.

The sun rises in the east and sets in the west, by having the south facing facade with plenty of glazing and the main living space on this side, we maximize the solar gain. The sun both lights up and heats the house, we hope to retain this heat by fully insulating our house, ensuring it is airtight and stopping any potential for heat loss through thermal bridging (see heat loss paragraph).

Heat Loss 

Retaining heat in the home is very important in passivhaus design. For it to work successfully we must ensure all surfaces are well insulated to prevent the heat energy we have collected all day does not escape through the walls, roof and floor. Insulation in a cavity wall is usually 150mm, for a passive house the cavity is filled with insulation of 250-300mm to ensure the retention of heat.

Compactness is also extremely important to ensuring we minimize heat loss, with less surfaces for the heat to escape from the more the heat is retained in the structure. When you have additional add ons to the dwelling like a balcony, the concrete for the balcony has to extend out through the wall creating a thermal bridge where heat can escape. to stop thermal bridging we have to create breaks which often does not happen in practice. Below you can see diagrams of how the suns solar heat energy enters the home and heats the air and floor inside, and an example of compactness compared to a dwelling with plenty of added elements which create junctions where heat can be lost. The third diagram shows how we prevent overheating in the summer months, a simple overhang on the roof prevents the hot, high summer sun from overheating the house and making the inside temperature uncomfortably hot. This is a very simple permanent solution to prevent overheating, unlike blinds and curtains which rely on human interaction to work meaning they can fail.

Finally it is important the building is airtight, but with this we must also have a way to allow air to flow throughout the home, letting in fresh air and extracting the stale air while retaining the heat. This is why in passive design we must incorporate an air filtration system where the two airs never mix but retains the heat between them and in the home. I go into more detail about this under the paragraph healthy indoor environment/comfort. 

Climate

The reason we build houses is because the climate outside is not comfortable for us to live, if it were we would not need to build homes. This now leads to us having two separate environments, the natural environment and the built environment. In passivhaus design we try to bring as much of the natural environment into the built environment as as possible, an example would be the use of sunlight instead of artificial light. Here in Ireland the average outside temperature over the year is 10°C , humans are most comfortable at 20°C which is always our target temperature for inside the home. Climate is something that can change easily depending on altitude and latitude, an example of this is how the higher above see level you are the colder it can get. Depending on your surroundings microclimates can be created, such as a house in a valley may not get much wind, where as if it were built on top of the hill it would experience severe winds in comparison.

Healthy Indoor Environment/Comfort

The Psychrometric chart is what we use to determine the dry wall temperature of the air, the wet wall temperature of the air and the relative humidity. Human beings are at their most comfortable at around 20°C with a 40-70% humidity, which is why our target temperature for inside the home is 20°C. Below is a graphic explaining how to read the Psychrometric chart and where exactly we are aiming for.

In Ireland our average temperature year round can be said to be 10°C and our average soil temperature can also be said to be 10°C luckily enough. With that being the case it means we can rap our house evenly in insulation from top to bottom to maximize heat retention. Our proposed site is in Limerick and the closest reference point of data from Met Eireann is Shannon Airport and the figures are shown below.

The dwelling must also be made airtight to prevent drafts since any air movement over 0.1m/s will be felt on the skin. This creates the problem of the air going stale which is when there is a build up of carbon dioxide in the air and not enough oxygen. An air filtration system must then be introduced to create a steady flow of air throughout the home, replacing the stale air with fresh air while also retaining the heat in the home, this is done by not allowing the two airs to mix. Radon is a naturally occurring gas which is quite toxic. It comes from the ground and can work its way into the home, so a radon barrier must be put in place. Other things like DPC must also be put in to prevent water travelling up through the walls by convection, which could cause mold and growth to form.