Daylighting with Interior Windows

Claire Moloney
Consultant
March 3, 2014

Daylighting and Views

Our project has a regularly occupied room in the center of the floor plan. It does not have exterior windows, but it has interior windows that allow natural light from the surrounding rooms' exterior windows to enter the space. Can we use these interior windows to count toward daylighting in that space?

Answer:

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Divya Nambiar

March 4

1 users voted0 users voted

Answer:

For documenting the LEED daylighting credit requirement, you could use any of the four methods (simulation, prescriptive, measurement or combination) for documenting compliance. The quality of daylight varies with distance from the exterior window. So, one of the main issues to consider is the distance of the space served by interior window from the exterior window.
Simulation: For the spaces lit by interior windows you would need to check through simulation if the lighting levels are between 25fc and 500fc for theses spaces. Daylighting that reaches a space has the external reflected component, the direct sun / sky component and the internal reflected component. You would need a simulation model for the space lighting levels. If you had a lighting designer on the project he may already have a simulation on a software like AGI 3.2. If you don't have one, you need to check the cost and effort of getting one made against your need for the credit.
With the existing LEED calculation methods the interior glazing does not seem to be able to get much advantage as against the exterior glazing
Prescriptive: This method is more suited when you have a central core with the interior spaces around having access to external glazing. You would need to calculate daylighting based on the Visible Light Transmittance and the Window- Floor Ratio. High VLT values would give a high daylight factor. If the VLT factor is low, you would need a larger area to increase the daylight.If the height of window head from the floor for the exterior window is H, then you can assume that spaces within a distance of 2H from the glass of the window fall within the day lit zone. So you need to see if the spaces in question (served by interior windows) fall within the distance. Also, when drawing a slanting straight line connecting the top of the exterior window head to the 2H distance on the floor, there should be no obstruction from ceiling / any design element along this daylight line.
The lighting level available at the said interior spaces would be dependent on several factors - area of the internal window, area of external window, alignment of the interior window with external window, obstruction, Visible Light Transmittance of both the windows, the distance of the interior space mentioned from the exterior glazing, etc. Usually it is a little challenging to add the areas served by internal glazing or borrowed lights to the calculations of this credit.

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