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Artificial sky testing of architectural model

Thursday, 24 May, 2012
Steve Burrow
Number of Participants / Visitors / Audience: 
To test the light levels that will be available in the proposed reconstructions of the Bryn Eryr roundhouses, due for construction at St Fagans in 2013.

Experience in the wattle and daub roundhouses built by Peter Reynolds at St Fagans in 1992 has shown that there interiors are very dark (see [link]). The Bryn Eryr roundhouses are similar in shape to the earlier versions - essentially they are cones planted in cylinders - but they differ in other respects. First, the walls are much thicker - 1.5m of clom, compared to just a few centimetres of wattle and daub - which will affect the spread of light into the interior. Second, Bryn Eryr consists of two roundhouse built against one another. For the purposes of visitor-flow a doorway will be built linking the two "rooms" which will also have an affect on light levels, as will the roof structure which bridges the two roof cones.

It was decided to build a model of Bryn Eryr to test how these factors will affect light fall within the buildings. This work was undertaken with the help and facilities of the Welsh School of Architecture.

The model
The model was built to a design produced by Gerallt Nash, Curator of Historic Buildings, St Fagans. It was constructed at 1:40 using thick card. Off-white card was used to reflect the whitewashed walls of the final buildings and black card to reflect a smoke-blackened roof space.

Fig 1. Architectural model of Bryn Eryr (Iron Age roundhouses) used in study.

In order to test different lighting possibilities, 8 x 40cm square windows were inserted around the circumference of each building, just below the top of the wall height. These were designed to be blocked if necessary during testing.

Planned points of light entry into the building were: the two doorways, the eaves gap between roof and walls, the temporary windows. All unplanned gaps were sealed with tape.

During testing, the doorways of the model were cut away at a c. 45 degree angle on their inner face, to allow a greater light spread into the buildings.

No provision was made in the model to represent the presence of trees surrounding the St Fagans site although these may also have some impact on the overall daylight availability.

Tools and equipment
Testing was undertaken in the Artificial Sky facility at the Welsh School of Architecture with light levels set to simulate CIE standard overcast sky conditions.

Fig 2. Artificial Sky at the Welsh School of Architecture, Cardiff University.

A Hagner Architectural Model Light Meter was used to measure the light levels within the model. The photocells are 19mm high which translates to a scale height of 760mm, and an "external" cell was placed adjacent to the model at roof height, to provide reference light level values.

Fig 3. Hagner Architectural Model Light Meter.

Photographic recording was undertaken using a Nikon D3100 with 10-24mm lens (necessary to convey the scale of the space at close distances).

Sensor positions
Fig 4. Location of light sensors within model.

Configurations tested:

Configuration 1: Model as built including a series of 40cm square windows arranged around eaves of each house.

Configuration 2: Model as above but with eaves windows obscured.

Configuration 3: Model as config 2 but with door reveals cut away to the inside at around 45o on both houses.

Ref sensor luxSensor 1 lux (dlf %)Sensor 2 lux (dlf %)Sensor 3 lux (dlf %)Sensor 4 lux(dlf %)
Config 15,6000 (0)8(0.14)6(0.11)4(0.07)
Config 25,6000 (0)6(0.11)5(0.09)3(0.06)
Config 35,6002(0.04)7(0.13)8(0.14)7(0.13)

Table 1 Interior lux readings for 5,600 lux exterior sky condition (equivalent daylight factor% in brackets)

External luxSensor 1 luxSensor 2 luxSensor 3 luxSensor 4 lux
Config 110,000014117
Config 210,00001195
Config 310,0004131413

Table 2 Equivalent lux levels likely to be achieved under a 10,000 lux sky, (typical uniform overcast sky condition)

All of the internal measurements are close to the limit of the sensitivity of the instrument; consequently the absolute lux values must be treated with care. However differences between readings from the same cell, taken before and after interventions, will be more reliable.

The thickness of the wall construction and overhanging eaves results in very low light levels within each house, particularly away from the centre of the floor area.

New openings around the eaves designed to admit additional light did improve the daylight conditions but only slightly. However, “opening out” the reveals around the doors did have some impact on overall daylight performance; with the perimeter light levels being improved significantly.

Whether such an option is chosen in the final design will need careful consideration against the baseline provided by the archaeological evidence.

The figures obtained from the model are similar to those from the existing, Peter Reynolds roundhouses. Since these roundhouses have functioned well for many years (see [survey]), it seems likely that during the summer months the Bryn Eryr roundhouses will have light levels which are high enough for most basic activities but, the figures produced in these tests reflect external light levels. Given that the houses will be open to the public on winter's afternoons, light levels may, at some times of the year, be much lower, and no amount of adjustment to doors or eaves gaps will compensate for this.

With this in mind it would be sensible to build discrete secondary electrical light sources into the structures so that light levels can be increased to facilitate visitor flow, but returned to normal when trying to convey the mood of a prehistoric house.


Architectural model of Bryn Eryr (Iron Age...
Artificial Sky at the Welsh School of...
Hagner Architectural Model Light Meter
Location of light sensors within model