Solar access

A contribution to a comprehensive building code

fausto Simões



R. Ricardo Espírito Santo, 10 5º 1200 Lisboa




The Portuguese Building Code “Regulamento Geral das Edificações Urbanas” (RGEU), does not clearly consider passive solar. Thinking on the revision of RGEU, recast as a Building Code, as a "backbone" for other technical regulations up to now scattered, this paper presents a short synopsis relative to solar access of buildings  in regulations and concludes with some guidelines for the Building Code in this matter, considering  climatic conditions as well as the good use of the building stock and urban space prevailing in Portugal.

In these proposed guidelines, the measures for a suitable solar access are disentangled in three different orders concerning passive solar, daylighting and the biochemical effect of solar radiation, to be developed not only in the building design but also in the urban design phase of the Detail Plan (“Plano de Pormenor”), in compliance with specific technical regulations. Required design tools must be adopted and included in the curricula of the architectural schools.



The Portuguese Building Code “Regulamento Geral das Edificações Urbanas” (RGEU), does not clearly adress passive solar. Revealing a special hygienical care that prevailed in post-war Europe, the 1951 bylaw in force today without major changes, joins “the lasting exposure to the solar radiation effects” and hygienic ventilation (art. 58) and establishes (art. 59) a rough 45 degree solar access rule, for the utilisation of daylight and the germicidal effect of solar radiation. The 45 degree rule is applied to the rear court of multi-family or collective buildings in art. 62, but is reduced to a “2/1 rule” (about 63 degree) in art. 73, respecting the relationship between windows and walls facing them. The RGEU revision of 1960 proposed some minor improvements concerning insolation and planning requirements and the revision of 1990 linked RGEU as a “backbone code”, to other technical regulations, namely the thermal regulations RCCTE, “Regulamento das Características do Comportamento Térmico dos Edifícios” [1]. But these revisions were not implemented. 

Anyhow, concerning solar access, the Portuguese law, is not an exception in the UE, as we can read in the “European Energy” Internet document: “At present, the European figure of solar law, related to discipline adequately the solar access rights is very poor. On the contrary, different indications come from the United States.

The experience in the U.S. is to be distinguished for the plurality of the approaches adopted. But the tendency that emerges most often is to activate contemporaneously several mechanisms for the protection of solar access. In fact, frequently, to the decision of recommending (or imposing) local governments the adoption of “solar zones”, in order to prefigure an area-wide protection, is joined the predisposition of “lot by lot” approaches” [2].

Legislation like the California Solar Rights Act of 1978, gives guidelines for the establishment of solar easements that can be negotiated  between property owners.

One owner would receive assurances from the other that the sunlight which travels over neighbour´s propriety would always be available.

If such solar easements are properly recorded, the neighbour and subsequent owners, will be restricted in building or planting non exempt vegetation that could obstruct the sunlight.

Zoning Ordinances like the Solar Access Provisions for New Development of the Multnomah County, establish detailed guidelines and restrictions ensuring that land is divided so that structures or non exempt vegetation can be oriented to maximise solar access and to minimise shade on adjoining proprieties from structures and trees. In this case very detailed rules are given, especially for low density residential developments.

New Pattonsburg, Missouri; Soldiers Grove, Wisconsin; Port Arthur, Texas; Boulder, Colorado; S. José, California; Richmond, B.C.  among other cities, developed  legislation securing protected solar access, be it on a case-by-case basis or with the broad protection scope of an ordinance.





The case-by case procedure as the ordinance one require a technical basis to define and detail solar access requirements.

Solar access has to be disentangled in three orders of different measures because daylighting, the biochemical effect of solar radiation and passive (and active) solar have different requirements concerning solar access:

1.Functional daylighting is more tied to the sky  and exterior reflected components, so it is less influenced by orientation.

2.The biochemical effect of solar radiation, namely the germicidal effect and the synthesis of the “sunlight”  vitamin D which cannot be synthesised in absence of sunlight (ultraviolet B), are associated with specific wavelengths (UV and IR) and enough dosing of solar radiation does not necessarily occur in the cold Season.

3.Passive solar for thermal comfort and energy efficiency has specific requirements concerning solar access. To meet these requirements we will consider the following conditions and possibilities:

à  Passive (as active) solar require about six hours of direct radiation in a southward exposure for maximum efficiency.

à  Passive solar depends on local climatic conditions referred to thermal comfort requirements and on available  sunshine.

à  Passive solar can be a promote  or a restrict gain strategy. Emphasis on one of them depends on the building use (activity, clothing and internal gains) and type (form factor).

à  Finally,   passive solar  can  and  must be  coupled to  other bioclimatic strategies, according to local climatic liabilities and assets, so it can be a less imperative strategy, especially in mild climates that prevail in Portugal.

Solar access and exposure

For the purpose of this paper the solar access angle is the atg defined by the high and distance of a shading frontal  building or tree, referred to the base line of the lowest solar aperture in a wall. The solar aperture of direct or indirect gain systems, has a southward exposure and complementary thermal masses.

Considering only solar altitudes greater than 10 degree, for Lisbon (latitude 38.7 degree N), the solar aperture exposed to the South, more or less 15 degree, should have an unimpeded solar access between 8:30 AM and 3:30 PM true local solar time in the winter solstice, to maximise the utilisation of solar energy. Considering the winter solstice, this unimpeded solar access requires a maximum vertical shadow angle of 15.0 degree, for Lisbon; between 17.5  and 12.5 degree as we shift between 37.0 and 42.0 degree N, the extreme latitudes of Portugal excluding the Islands of Madeira and Porto Santo [ see Figure 1].

Figure 1. Shadow angle at the winter solstice as a function of  true local solar time and latitude

If the exposure of the solar aperture is extended to the SE and SW (i.e., South,  plus or minus 45.0 degree), as allowed by the Portuguese building thermal regulations, "RCCTE", the vertical shadow angle attains an extremly low value; 10.8 degree at 8:30 AM (or 3:30 PM) for the latitude of Lisbon.

This could be the solar access angle for passive solar, but such low value hinders a suitable density of urban precincts  by social and economical criteria. Such low angles strongly restrict the urban density, as clearly shown by a comparison between the floor space index  (the Portuguese “índice de construção líquido”) for the “45.0 degree”  rule and  for a 15.0 degree solar access angle, in courtyard or parallel flat block layouts [see Figure 2].

 Things are a little better if we adopt this low solar access angle only for the south facades in a courtyard layout (the Portuguese “quarteirão”), as it will be.

Research and projects after the Heiligenthall-Gropius model, consider the same  solar access angle for any orientation, even in a  courtyard flat block layout [3].

It would be more efficient to specify lower solar access angles for passive solar to south facades and higher solar access angles to other orientations, based on criteria for daylighting and the health effects of solar radiation.


Figure 2. Floor space index as a function of the floor number and solar access angle for residential areas

Anyway, taking into account the desirable urban density such low solar access angles can only be achieved and even exceeded,  in  low rise group housing,  with diagonal housing following a natural  slope.  A free solar access, a floor space index  greater than 1.5, more than 500 persons per hectare, privacy staying outdoors in secluded terraces and large views, can be attained with a landscaped  diagonal housing,  following slopes up to 40.0 degree. One of the rare (and fine)  modern  examples  of   such  diagonal   housing   in Portugal, is the first phase of Hotel do Mar, Sesimbra, designed by the Architect Conceição Silva for a 35.0 degree southward slope.

It is worthy to  note that,  in  Portugal,  for  instance in the case of Lisbon, it happens that higher southward slopes near 40.0 degree are warmer in winter and colder in summer than lower ones. But the current interpretation of the Decreto-Lei nº 93/90, 19th March, prevents the residential use of slopes greater then 30%, i.e., about 17.0 degree [4].

A healthy social life in a living  town can  be also strongly favoured by comfortable urban exterior spaces, but solar access of buildings and urban space may conflict with each other. For instance, in dense built up areas commanded by the street pattern, the E-W axis can be  better for solar gains of buildings, but streets along the N-S axis have a better equilibrium between both sides, concerning solar access in winter and summer. So, solar access and shading   in cold and hot seasons of urban exterior social space, would be properly considered using especial design tools such as those proposed by Malato [5] and others, available for computer aided design.


Solar access, local climatic conditions and building use


Generally, as concerns thermal comfort, Portugal  has a mixed but mild, well-tempered climate. According to the “RCCTE”, the heating degree-days (15°C) have a typical value of 1600 DD  for the coldest zone (I3), exceeded only at altitudes over 1000 m.

The hot and dry season has average temperatures in the comfort zone not exceeding 25°C, with somewhat large swings. There is available sunshine in winter and clear skies in summer. Shading is a must in summer and intermediate seasons. In winter, solar gains do not face the high thermal loads due to low exterior temperatures.  In most cases, with simple means, we can have the balance point near the average exterior temperature.

Service and commercial buildings, usually with somewhat high internal loads and even housing blocks with a low form factor, are less skin commanded, shifting  the priorities to the summer bioclimatic strategies. In medium and high density urban areas, with a desirable mixed use, floors near the ground can be commercial.

So, for SE  to SW exposures, we could  adopt a solar access angle not so stringent as  what was defined above, in more skin commanded low density residential areas. We  could accept a more loose application of a less stringent rule to more dense urban areas. This less stringent rule,  could be very simple, following the solar altitude at noon of  the winter solstice.

Starting from the general equation for the solar altitude at noon hn:


hn  = 90.0 - f + d



we can have the solar altitude at noon in the winter solstice hnw , taking it as the solar access angle:


hnw   = 66.5 - f



where f = latitude and d = declination. With this rule, the average loss of exposed area between 9:00 AM and 15:00 PM at the winter solstice is, in the worst case of latitude 42°N, less than 0.13 for a wall oriented due South.


    Solar access amongst the bioclimatic strategies


Solar access is indispensable to passive (and active) solar, but passive solar is only one of the bioclimatic strategies that we can use, according to local climatic conditions, in order to help ourselves to attain a desirable thermal comfort with simple means [6,7]...And, of course thermal comfort is a need among others.

We can balance lower solar gains with more insulation, a lower form factor, weather-stripping and wind protection, in winter. In summer we can use thermal mass, earth cooling, ventilation, radiant and evaporative cooling, according to local climatic conditions. But with strong hot seasons and clear skies the sunshading is a first choice. It can be done with shading devices on the south facade, but is better done with near buildings and trees for the West and East walls.

A comprehensive use of the bioclimatic strategies favours diversity and gives more freedom in the urban layout, including a more free, but conscious, adoption of a simple rule for the solar access angle, especially in urban central areas with mixed uses.




Passive (and active) solar requires the revision of the Portuguese building regulation “RGEU”, to respect solar access. The following preliminary conclusions are a contribution to this revision:

·      Solar access for passive (and active) solar, for daylighting and concerning the health effects of sun exposure, should be disentangled, because they require different measures;

·      Solar access for passive (and active) solar must be applied only to the  southward exposure, more or less 15.0 degree (or extended to more or less 45.0 degree). Requirements for other orientations should be established on the basis of daylighting and the health effects of sun exposure,  requiring   other studies to define specific rules;

·       Solar access for the southward exposure, in compliance with the eqn [2], could follow the rule:

ACE   =  66.5 - f

where ACE = solar access angle and  f  = latitude;

·      Solar access for passive solar should be mandatory in low density residential areas, and less mandatory, through a balanced use of the bioclimatic strategies and functional “stratification”, in medium and high density urban areas with mixed uses.  But in any case, solar access angle for daylighting or other health requirements should  be exceeded. This balanced use requires the detailed layout of the “Detail Plan”, as it is defined in Portuguese planning regulations;

·      Solar access and shading, as wind protection and natural ventilation of urban exterior spaces, should be properly demanded by law, to  encourage the staying outdoors in a liveable city. The rule and design tool proposed by Malato [5] for convenient solar access and shading of exterior spaces and others available for computer aided design, should be considered for the phase of the detail plan (“Plano de Pormenor”);

·      Diagonal housing on stable slopes, more than 17.0 degree and up to 40.0 degree, should be allowed through a more flexible interpretation of the Decreto-Lei 93/90;

·      Design tools for solar access and shading should be adopted and included in the curricula of the architectural schools.




1.  Ministério das Obras Públicas Transportes e Comunicações, Regulamento das Características de Comportamento Térmico dos Edifícios, Decreto-Lei nº 40/90, 6 de Fevereiro, Diário da

    República -I série nº 31,. 490-504.

2.  Committee on Energy,  Research and Technology of the European Parliament,    Relation of  Congressman  V. Bettini (Italy)   on the  use of  bioclimatic  technologies  for  residential  buildings and services  approved the 9th February 1994. Internet URL:

3.  L. Martin, L.March, M. Echenique, La Estructura del Espacio Urbano. Editorial Gustavo Gili, S.A. (Barcelona, 1975).

4.  A. Pereira, J. Bento, M. Gomes and N. Cerejeira, Metodologias de Avaliação da Reserva Ecológica Nacional (MAREN), Ambiente, Ordenamento, Gestão do Território e Sistemas de Informação Geográfica, Faculdade de Letras da Universidade de Lisboa-PROGEO  (Lisboa 1995) 33-41.

5.  J. Malato, Insolação dos Edifícios e dos Espaços Exteriores Urbanos. Comissão de Coordenação da Região de Lisboa e Vale do Tejo (not published).

6.  D. Watson and K. Labs, Climatic Design. McGraw-Hill Book Company (New York 1983).

7.  F. Simões, Desenhar com o Clima - Clima Conforto Habitação. Energia  Solar  e  Qualidade de Vida.    ISES,  SPES - RN    (Porto 1997) 303-308.