Illuminance and Irradiance Studies:
The results of Daylighting Studies for the Proposed Hennepin County Brookdale Regional Center Remodeling and Addition in Brooklyn Center, Minnesota are based on Daylighting Illumination simulations of overcast sky conditions and an irradiance study using a heliodon device coupled with video capture equipment. The studies were conducted at the University of Minnesota, College of Architecture Daylighting Lab in Minneapolis. The areas studied were those that were of key importance in implementing successful daylighting strategies.
The proposed design performed satisfactorily having ample daylight in all of the newly constructed high ceiling spaces on a year round basis under varying sky conditions. The existing portions of the building which do not benefit from perimeter windows and which were restricted to ceiling heights of 10’ or lower enjoy only a marginal daylight level and will not have the same visual quality except for the locations with existing clerestories and where new skylights could be added.
Light Shelves Prove Effective
Light Shelves significantly improved the illumination uniformity and visual quality of the SE Library area where they were planned. Without light shelves, illumination levels from daylighting will vary by a ration of 5:1 highest to lowest while this ratio improves to 3:1 with the addition of light shelves. In addition footcandles levels near the windows with-out light shelves could often reach over 1500. For example, at 2:00 PM in June the probable illumination level at the Southeast windows on a bright overcast day would be a nearly intolerable 1400 to 1800 FC depending on the location. With light shelves the probable levels would be 1000-1200 FC next to the windows. In addition light shelves boost the illumination levels by 20-40% at the middle point, mainstreet, a full 50’ interior from the exterior curtainwall. In the table shown, the illumination comparison is expressed as the Daylight Factor, which is the percent of light available outside that reaches a particular location inside the building.
The measured benefits of light shelves include:
- Reduced glare and better quality, more uniform light
- Improved visual acuity and reduced eye strain
- Reduced heat gain and relief from direct sun
- Solar Control at windows is suggested
Even with the addition of light shelves, the highest daylight levels were near windows. The high level of illumination and the excessive contrast ratios between the light source and background can impair visual function. In order to maintain the optimal level of daylight, additional solar controls were needed. Recommended solutions in the non-public office and break area s, include operable shades or Venetian blinds. Recommended for the upper curtainwall glass above the light shelves is “fritted” glass, a light diffusing type of glass. The lower part of the curtainwall glass will require a special system which operates automatically to reduce brightness and provide relief from direct sun. See appendices for more information on specific shading and glazing products recommended.
Benefits of Clerestories
Cross lighting is crucial to getting good daylighting results, where daylight comes from two directions. This improves uniformity and substantially reduces shadows and veiling reflection. Clerestories further facilitate this by admitting light at a high level, yielding the deepest possible penetration of light to a central location in the building. Here the clerestory at Mainstreet boosts day-light factors from less than 2% to greater than 10%. Where the new and existing building come together, another clerestory maintains light levels at a 4-8% daylight factor where it would otherwise be 0-1%.
Skylights solve a difficult problem
Low ceiling areas in the library do not receive much daylight further than 10’ from the clerestory primarily because little light can pass under the low ceiling and over the 7’ high stacks. This can be substantially improved by adding diffusing or prismatic skylights at these areas. Two rows of 8’ x 8’ skylights in the low ceiling areas were modeled with impressive results. The skylights increased daylight factors by 3-4 times in most locations and as much as 10 times in a few locations. The only other source of daylight to this area is “borrowed” from windows in the adjacent Study Rooms through use of glass partitions.
Conclusions & Recommendations
- Light shelves were effective and should be incorporated in the design as proposed.
- Clerestories were critical to the interior areas of the North Library and Mainstreet.
- Skylights added to the existing low ceiling area of the library would provide a good daylighting solution to an area that will otherwise not benefit from any significant amount of daylight.
- Light controls in addition to light shelves will be necessary at most of the curtainwall perimeter glazing due to excessive brightness and contrast ratios.
- The glazing selected for this design is optimal for a day-lighting application of this type.
Irradiance (Direct beam) Study
As can be seen in many of the figures presented, most areas in the design receive direct beam radiation (sunlight) at some time or other during the year. This suggests the use of solar control strategies. For example, manually operated solar shades, solar optic lens film (SOLF) applied to the glass to redirect light more efficiently, prismatic glass or other refraction glazings having a similar purpose. Fritted glass, having an opaque or translucent applied coating can also be effective in certain applications. These coatings were available in a variety of patterns and custom patterns can be ordered as well.
Solar Control Problem Areas
All curtainwall glazings in the library and west facing clerestory windows in the library as well as clerestories in the Courts area will require some type of solar control. These were high-lighted in the foldout “Solar Shading Strategies Plan” included in this report. In addition all East, South and West facing windows in non-public areas will require manually operated shades or mini-blinds.
Daylong solar simulations show that without solar controls, direct light penetration through the curtainwall glass would cause egregious direct light problems earlier in the day over a widespread area and later in the afternoon in the library Lounge. Morning light coming through the East facing glass at the Teen area would also be a chronic problem. Other more isolated patterns of direct sun moving across an area from discreet areas in the west facing clerestories and curving Mainstreet clerestory can also be seen.
Other benefits internal and external
Energy savings as a result of these strategies will almost certainly be substantial, due to the level of effective daylighting throughout many spaces, because of the extended operating hours which in the summer will allow daylighting well into the evening. Another significant source of cost savings will result from peak load reduction by not only reducing lighting energy consumption up to 50% in some areas but also cooling energy needed to counter the heat given off by the light. Air pollution contributing to global warming will also be proportionately reduced, extending benefits far beyond the confines of the building beyond the boundaries of Hennepin County.
Back to strategies
Interior of SE Library curtainwall taken during Illumination Study in the artificial sky box. Interior Walls of sky box are the mirrored background.
Chart with Light Shelf performance comparison in SE Library. Red line indicates the comparatively poor uniformity of light levels across room without Light Shelves.
Contour maps help visualize overall daylight levels inside the space. The upper chart is without light shelves, the lower illustrates the improvement when light shelves are added. Reducing light at the window and increasing it into the center.
Illustration of daylighting scheme for library clerstories. Yellow represents southern direct beam radiation and blue represents norther diffuse light.
Photo of model on heliodon direct beam test apparatus. Heliodon simulates movement of sun across the sky for any given day of the year.
Camera View A: During Illumination studies in an artificial sky simulating an overcast sky condition.