Passive solar design: Sunspaces

• A ‘Sunspace’ is a south-facing glazed area located outside of the main fabric envelope of the building.

• The space naturally heats and cools allowing daytime temperatures to raise higher and night time temperatures to fall further than the ‘comfort zone’ temperatures of the adjoining living space(s).

• The addition of a sunspace can realise significant gains in energy efficiency. This can amount to around 30% when compared with a direct gain equivalent (1), though this varies according to climate and latitude where buildings benefit from southern locations.

• Though unheated, sunspaces can provide additional living space when natural conditions make them comfortable.




+ simple traditional concept
+ construction details tend to be easier
- can shade adjoining areas of the façade
- can reduce natural light into adjoining spaces


+ the sunspace does not restrict daylight to adjoining windows.
+ anecdotal evidence shows that they are less subject to mis-use.
- self-shading
- less compact envelope to living areas


• In terms of design, whereas it is possible and profitable to use ‘lean-to’ sunspaces for thermal buffering to all heated accommodation, in practice most designs are limited to buffering the main living areas.



Functions of sunspaces


1 Buffer space


The sunspace functions as an intermediate space between the inside and outside of the building. By effectively adding another layer to the building envelope, the sunspace becomes a thermal buffer rather in the manner of air within a cavity wall.
A further effect of the sunspace is to shelter the envelope from wind chill and rain – this factor becomes increasingly important in northerly and exposed locations.


2 Pre-heated ventilation


Natural ventilation
Warm air can flow into adjoining spaces via openable vents located in the common wall at the top of the sunspace. Cool air is returned from the living spaces through lower vents to be heated as part of the convective loop. (‘Thermosiphoning’)

Mechanical ventilation
Mechanical ventilation can extend the penetration of pre-heated ventilation into areas of the house that are not adjacent to the sunspace. Heat is collected from the upper part of the sunspace and blown via ducting to other areas of the house.


3 Indirect gain: Thermal mass/ storage wall



Heat is transferred to the living spaces through a masonry common wall.

Opinion is currently divided as to the effectiveness of combining storage walls with sunspaces. Debate centres around the winter period when the wall has its greatest potential, yet when equally solar radiation is at its most diffuse.

Other limitations to be considered would be:
• much reduced efficiency when combined with an opaque roof to the sunspace
• overheating when exposed to direct radiation during summertime.
• lack of insulation qualities compromise efficiency during overcast conditions and at night.


4 Indirect gain: Remote thermal storage



Rock stores
As distinct from the immediate use of pre-heated air, thermal storage affords the capacity to store heat for future use. The traditional form of heat storage is the ‘Rock store’. Heated air from the sunspace is mechanically driven to containers of crushed rock. Heat transfer is then effected by one of two methods:
• Air is blown through the store into the space to be heated or
• The store is thermally coupled to the space and heat is transferred is through radiation and conduction.
Though the former enables a degree of remoteness between the store and the space to be heated, the latter method has the advantage of being simple, passive and is generally more popular.


In practice, rock stores perform reasonably well. However, very large amounts of rock are required to store relatively small am heat. For example, about 60 tonnes of rock are required to satisfy the storage requirement for a solar space heating system for an area of 100 square metres (2)

Phase Change Material (PCM)
An emerging alternative to using rock for thermal storage uses PCMs. PCMs work by storing energy in changing phase from solid to liquid – ie melting. (3)

PCMs are solidified inside spheres (commonly 75mm diam.) or cylinders stored in tanks. The PCM material is selected to change phase (melting and freezing) at specific temperatures suitable for space heating.
Compared with crushed rock, the sizing requirements are reduced by typically 90%.

5 Draught lobby


In its minimal form, the sunspace becomes a draught lobby. Heat is lost when doors and windows are opened to the outside. By providing sufficient space for the outer door to be closed before opening the inner door, the draught lobby functions something like an air lock.




Sunspaces are easily overheated in summer. The problem can be resolved by:

1 Shading from the sun.
- External solar shading from a roof overhang or adjacent louvers.
- Deciduous trees (beware though that leafless trees will, to an extent, continue to shade.)

2 Ventilating the sunspace by placing operable vents to the exterior through the roof of the sunspace. These can be automatically opened and should be coupled with vents at low level to enhance the ‘stack effect’.

Air permeability

• Air tightness around windows and doors should be particularly effective to reduce heat loss to the sunspace when that space cools to temperatures lower than the adjoining living area.


• If automatic vent operation is not provided, the building user needs to be fully aware of the need to control the vents correctly.

• Where automatic controls are provided, users should be able to set them for desired levels of comfort.

Thermal mass within the sunspace

The inclusion of thermal mass results from a clear design decision: - where the sunspace acts primarily as a buffer space, thermal mass extends the period of heating within the sunspace. - where the sunspace acts primarily as a passive solar collector and where there is a need for rapid convective transfer to the living spaces, air (and not mass) needs to be heated quickly. This process will be encouraged by the use of light and reflective materials.

Design and construction


Roofing the sunspace


Opaque roof
+ Provides insulation
+ Provides shading
- Restricts daylight to the living areas (though venting skylights could be a solution)


Glazed roof
+ Optimises solar gain
- Increased heat loss in overcast conditions and at night
- Prone to overheating
- Susceptible to leaking
- Prone to glare



• Unless where storage walls are used, the house wall to the conservatory should be of the same thermal performance as for other wall elements of the building envelope.

• All glazing within the shared wall should be high performance 'Low E' double glazing

• Sunspace glazing should be at least double-glazed. o If roof glazing is included it should be shaded at times of peak solar radiation

• For maximum performance, flanking walls should be considered to the sunspace.


Sunspaces - use and abuse

• Sunspaces are particularly sensitive to user misuse. When used correctly, sunspaces will make a significant contribution to energy conservation. However, when used inappropriately, they can equally increase energy inefficiency.

• It is imperative to inform users of the role of sunspaces. For example, anecdotal evidence relates that there are occasions where sunspaces prove to be so popular that users annex them into their living spaces - complete with carpets and electric fan heaters. The more 'room-sized' the space, the more likely it is to be adopted as permanent (rather than occasional) extra living area.

• In response, some designers deliberately design sunspaces with minimum floor space to deter their use as extra rooms.

• When undertaking predictive modelling, it might be considered wise to factor for a degree of 'irrational' behaviour with regards to the use of sunspaces.


(1) ‘A five-person living –dining room with generous direct gain double-glazing was compared with a single glazed conservatory, its combined vertical and tilted area of glass being twice that of the direct gain double-glazing.’ (Solar Architecture in Cool Climates, Colin Porteous, 2005)

(2) Australian Greenhouse Office, 2003

(3) The most common example of this phenomenon is the melting of ice to form liquid water-a great deal of energy is absorbed by ice at 0ºC to transform it to water, also at 0ºC. Conversely, an equally large amount of heat is released from water to form ice (i.e. freezing) at this same temperature.



• Solar Architecture in Cool Climates, Porteous with MacGregor, Earthscan, 2005
• Sustainable Solar Housing, Hastings and Wall, Earthscan, 2007
• Solar House, Galloway, Architectural Press, 2004
• The Whole House Book, Borer and Harris, CAT, 2005
• EcoHouse 2, Roaf et al, Elsevier, 2003

Further information

• BSRIA: Building Services Research and Information Association (
• CIBSE: Chartered Institute of Building Services Engineers (




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