The effect of house length on the light transmissivity of single and multispan greenhouses

Abstract An investigation into the effect of gable ends on the light transmissivity of finite greenhouses is described. Initially, verification of a three-dimensional computer model of a greenhouse was carried out by comparing predicted results with those from an earlier experimental investigation, this used scaled physical models of single span greenhouses under direct and diffuse light conditions. The effect of increasing greenhouse length under both diffuse and direct sunlight conditions is then predicted for symmetric roofed E-W single and multispan houses, and for different combinations of two wall heights and two roof angles. Under diffuse light conditions, transmissivities near the gable end are shown to be up to 3% higher than those for corresponding infinitely long houses, and fall to within 1% of the asymptotic value at a distance of about 1 span width from the gable end. Under direct light conditions, transmissivities are shown to be different in detail for the two types of houses, for example, the influence of the gable penetrates more deeply into multispans (up to three span widths) than single spans up to two span widths), and functional differences occur along the house length. However, for E-W houses, the broad form of the gable influence on transmissivity is common to both house types, and shows an increasing penetration with increasing wall and roof angle, and a reduction of transmissivity near the gable at midwinter compared with an increase of midsummer. Tables are presented to indicate the house length required to provide at least a two-span length at the house centre over which the transmissivity is within 1% of that for an infinitely long house.