186 COMMISSION ON BUILDING DISTRICTS 



thickness of the wall. For a given thickness of wall it increases with the 

 height and width of the window. 



In all the calculations relating to the amount of daylight entering 

 windows in this paper it has been assumed that the window pane would 

 fill the entire window opening. 



In an unobstructed window two and one-half feet wide and six feet 

 high set in a wall one foot thick the flux of daylight admitted is equal to 

 only 76.1 per cent of that through a window set in a wall one-half foot 

 thick. If obstructed by buildings on the opposite side of the street one-half 

 times the width of the intervening space in height the window set in the 

 one-foot wall would admit only 71.6 per cent as much daylight as the one 

 set in the half-foot wall. For obstructing buildings different times the street 

 width in height above the level of the window the amount of daylight 

 received by a window set in a one-foot wall as compared with that in a half- 

 foot wall would be as follows : one times the street width, 65.6 per cent ; 

 one and one-quarter times, 61.6 per cent: one and one-half times, 59 per 

 cent; two times, 52.2 per cent: two and one-half times, 45.3 per cent; 

 three times, 34.3 per cent ; and four times, 18.7 per cent. 



The curve in Figure 61 shows that obstructing buildings are more detri- 

 mental to the lighting of rooms than the thickness of walls. Inordinately 

 thick walls cut off a considerable portion of the direct daylight, but excess- 

 ively high buildings cut off more. 



An unobstructed window receives some direct light even though it is 

 set in an extremely thick wall but a window set in a comparatively thin wall 

 receives absolutely no direct skylight illumination when obstructed by a 

 high building. The exact point at which all such illumination is eclipsed 

 depends upon the thickness of the wall in relation to the height of the 

 obstructing buildings. Thus, a small window say two feet wide and four 

 feet high set in a one-foot wall receives absolutely no skylight if situated 

 at a distance three times the street width below the top of the opposite 

 buildings. A window two and one-half feet wide, six feet high and set in a 

 wall of the same thickness on the other hand receives direct illumination 

 from the sky even though it is situated four times the street width below 

 the top of the opposite buildings. This latter window if placed in a wall 

 one-half foot thick would receive some direct light even at a point six times 

 the street width below the top of obstructing buildings. 



The following table shows the flux of daylight in foot candles per square 

 foot admitted by a window two and one-half feet wide and six feet high 

 when set in a wall of different thicknesses and obstructed by buildings of 

 different heights : 



Thickness of 



Height 



of Obstructing Building 



in Terms of Street Width 















Wall in Feet 



None 



One-Half 



Once 



One and One 



-Half 



Twice 







6.600 



3,649 



1,933 



1,110 





699 



l A 



4.976 



1.440 



375 



114 





41 



l 



3.784 



1,029 



246 



68 





2! 



2 



2,280 



S40 



102 



20 





3 



3 



1,485 



299 



54 



3 





o • 



In all of these calculations only the direct illumination obtained from 

 the sky has been considered. The actual illumination in any particular case 

 exceeds that stated here on account of the reflected light received. When 



