Passive Solar / Low Energy Buildings

Passive solar heat is a term used to describe the free heat gain described below which is, effectively, captured solar energy released as heat. We have all experienced this type of passive solar heating in buildings on hot days.

Passive Solar Heat

  

Solar radiation is transmitted indoors through windows and other glazing and is then converted into heat when it is absorbed by surfaces such as concrete, stone or brick walls and masonry. The surfaces then store and release this heat within the building.

Passive solar heat is a term used to describe the free heat gain described above which is, effectively, captured solar energy released as heat. We have all experienced this type of passive solar heating in buildings on hot days. A building can then be designed to make maximum use of these gains, by optimising the room shape, room size, glazing size and type room position and orientation (south facing rooms take most solar heat energy). By maximising the use of this free solar energy for heating by simply designing the building correctly, the building requires less input from a primary heating source (e.g. a boiler). Combined with high levels of insulation and airtightness, passive solar gains can provide more than 50% of a building's heat needs. It is also important to design against the possibility of unwanted heat gains which would lead to uncomfortably high room temperatures.

This section introduces two main topics :

  1. Overview: Buildings which maximise the use of passive solar gains, or passive solar buildings
    - "Passivhaus" buildings, or buildings constructed to the Passivhaus standard ( see also: www.passiv.de / www.passipedia.org)
  2. The A-Rated house (Refers to a Building Energy Rating (BER) of A1, A2 or A3)
 

1. Passive Solar Building Design

 

There are a number of important areas to consider when designing a building to maximise passive solar heat gains.

Site and building form
  • As close to a southerly orientation as possible
  • Minimise overshadowing of building
  • Aim for a compact, uncomplicated building shape
Internal layout
  • Position windows according to room functions (e.g. kitchen, living room, dining room facing south)
Glazing

Control the heat loss through glazing while maximising access to sunshine by:

  • Using windows with a low U-Value (high resistance to heat loss)
  • Minimising the number of north-facing windows
Walls / Roofs / Ceilings / Floors
  • Use exposed masonry where suitable in order to store solar heat in rooms with solar gain
  • Insulate the building above and beyond minimum requirements
Airtightness and ventilation
  • Aim for an airtight construction
  • Ventilation of the building should be controllable
  • Mechanical ventilation with heat recovery is viable in very airtight buildings
 

The Passivhause

 

A "passive house" building is often considered to be the ultimate low-energy or "passive solar" building, as it is designed to the practical limits in terms of insulation and in terms of using solar for heating. The Passive House concept, is an extension of the principles explained in the overview of passive solar buildings above, i.e. based on energy-efficient construction combining low heat losses with maximum use of free solar gains.

A Passive House requires as little as 10 percent of the energy used by typical European buildings, which means an energy savings of up to 90 percent. Passive Houses have high levels of insulation and an extremely airtight design with controlled ventilation including heat recovery system. Another important feature of passive houses is that of a design free from "thermal bridges", which means that the insulation all over the building is applied without any "weak spots" , which eliminates cold corners as well as excessive heat losses and moisture build up.

Any competent architect can design a Passive House. By combining individual measures any new building anywhere in the world can be designed to reach the Passive House standard. In order to correctly design a passive building, it is necessary to use the "Passive House Planning Package" or PHPP. This is a piece of software which calculates the optimum levels of insulation, glazing etc. based on a given building design. The major advantage of using the PHPP to assist with building design is that elements of the building such as glazing size/type, or insulation thicknesses/types can be tweaked to find the optimum levels.

The PHPP also takes into account the free solar heat gains, and heat gains from occupants, to calculate the final energy requirement of the building. It is important to note that the PHPP does not consider the Irish Building Regulations, and your architect or designer should also ensure that the design also meets these requirements. The Passive House Standard is also increasingly being used for non-residential buildings. A list of certified Irish passive house planners is available on the Passivhausplaner website.

The Royal Institute of the Architects of Ireland also have an architect service which is very useful, as you can describe your project brief and engage a suitable practice to assist.

Main Characteristics of a passive house

The main characteristics of a passive house that might seem to be "new" concepts, are in fact, very easy to grasp :

Insulation

The better the level of insulation is, the more energy and money will be saved. Other features are also positively affected by good insulation:

  • Heat losses through external walls and roofs account for more than 70% of the total heat losses in existing buildings, Therefore, improving thermal insulation is the most effective way to save energy.
  • Thermal comfort is increased and there are better options for effective heating
  • The risk of mould formation is reduced,
  • During summer months, good insulation also provides protection against heat.
Airtightness

Airtightness is another high-priority area in passive house construction, for the following reasons:

  • Building damage caused by water vapour present in air draughts can only be prevented by airtightness of the envelope, and building damage is mostly due to the absence of airtightness in the roof area.
  • Draughty living spaces and cold air pockets lead to a hard to heat building
  • Ventilation systems with supply air and exhaust air spaces only function if the building envelope is sufficiently airtight.
  • Airtightness results in better sound protection
  • Where a building is well insulated, unwanted air infilatration accounts for a high percentage of heat loss, thus reducing the overall effectiveness of the insulation.

Interestingly, new 2011 building regulations prescribe a good level of airtightness.

Eliminating thermal bridges

Thermal bridges are junctions where insulation is not continuous , the main reasons to eliminate thermal bridges are :

  • They lead to decreased interior surface temperatures and in the worst cases this can result in high humidity in parts of the construction.
  • Where a building is well insulated, thermal bridges make up a high percentage of heat loss, thus reducing the effectiveness of the insulation.
"Passive" Elements

As well as reducing heat loss as much as possible through insulation, airtightness and eliminating thermal bridges, it is important to maximise the solar energy available to the building in order to contribute significantly to the low heat requirement of the building.
There are a number of areas to consider, which you will have seen already in the overview of passive solar buildings.

  • Compact form and good insulation
  • Southern orientation and shade considerations
  • Energy-efficient window glazing and frames

Note, one of the benefits of using the PHPP is that it can help find the balance between passive solar heat gains and overheating, so as to indentify the optimum form (shape), orientation, glazing type and size and shading techniques to use.

Controlled ventilation and recovering waste heat

We are accustomed to living in dwellings that have no controlled ventilation, and rely on the gaps and cracks to provide the air necessary, but this type of house loses a large amount of heat. An airtight construction reduces the infiltration of unwanted cold air and the escape of heated air, which is very important if a low-energy building is the goal. However, in an airtight building, where there are no gaps and cracks to supply air, there is a need for controlled ventilation, or controlled air supply to the building. For this reason, in very airtight buildings, there is usually a Mechanical Ventilation system installed to control air flow, and to ensure that the occupants have sufficient air flow for comfort.

In a passive house, where the heat requirement is so low, energy from waste heat (such as cooking, clothes drying, showers etc.) can be used to pre-heat the incoming air in the Mechanical Ventilation system using a heat recovery exchanger. This is also known as Mechanical Ventilation with Heat Recovery (MVHR). Note that in a house that isn't airtight, MVHR will be of no extra benefit to the occupants.

Building Services

Although a passive building is designed with very low heat losses and maximises free heat gains from the sun, there is always a remaining energy requirement which must be satisfied. A building meeting the requirements of the passivhaus standard (see www.passiv.de) does not need a conventional radiator system or underfloor heating. Because of its low heat demand, the heat can be distributed through the mechanical ventilation system, which allows the building services for heating to be very simple.

The following features are characteristic of building services in a passive house :

  • The remainder of the heat demand is supplied by systems such as:
    - A small heat pump (compact heat pump unit)
    - A small condensing boiler (compact condensing unit)
    - A small heat generator based on biomass fuel e.g. wood pellets
  • Heating and hot water production assisted by solar thermal - See Active Solar Heat Technologies.
Electricity use

In a passive house, at design, planning and construction stages, the main goal is to achieve a building that meets the heating requirements of the passivhaus standard. It is important to note that the passivhaus standard is designed with an overall low energy requirement in mind, and electricity use cannot be ignored as it can make up almost a quarter of energy used in our homes. For every unit of electricity energy we use in our homes, it takes 2.7 units of energy to produce and transport that to our house. This is known as the "primary energy". What we are using at our plug points is "delivered energy". The passivhaus standard limits the amount of primary energy that a house is resonsible for, and some of the common features are:

  • Low energy refrigerators, freezers, lamps, washers, dryers, dishwashers.
  • Low energy lighting
  • Energy efficient ventilation
 

2. The "A" rated house

 

A BER is an indication of the energy performance of a home. It covers energy use for space heating, water heating, ventilation and lighting calculated on the basis of standard occupancy. A BER is similar to the energy label for a household electrical appliance like your fridge. The label has a scale of A-G. A-rated homes are the most energy efficient and will tend to have the lowest energy bills.

This energy rating is calculated using software known as the Dwelling Energy Assessment Procedure (DEAP). Examples of building features which will help achieve a good Building Energy Rating are as follows :

  • Good levels of thermal insulation for windows, doors, walls, floors.
  • High level of airtightness
  • Arrangement of windows to facilitate passive solar gains and limit heat loss on northerly orientations.
  • No open fire chimneys
  • Thermal bridge reduction
  • Solar thermal system
  • Low energy lighting
  • Efficient heating appliance (see also HARP database)

It is important not to assume anything when planning an A-Rated dwelling, and to have a qualified BER assessor at hand to that a particular building design, including all of its services are entered into the DEAP software so as to provide an accurate rating. Most architectural practices will be qualified to use the DEAP software, and combined with a design tool such as the Passive House Planning Package, it is possible to design a building which is both passive and has a very high energy rating.

For further information and links on passive solar and low energy buildings, see also Solar Thermal Information/Resources.

 
 
 
     

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