Guideline I.5: Lighting and Daylighting

Electric Lighting:

During the schematic design phase, work with owner/client to understand electric lighting needs and develop a strategy to meet needs while conserving energy and maintaining a high environmental quality. For each regularly occupied interior space, document the light levels recommended for primary (and secondary) use type using B3 Electric Lighting Spreadsheet. This spreadsheet requires a room name, area, and program to be entered for all Regularly Occupied Floor Area, listed by room or space.. If the exact program is not included in the drop-down list, select the most similar available or the IES category space type that aligns with the room’s use (IES space type descriptions can be found in the spreadsheet tab, IES Reference Categories). After the program is selected, the IES recommended light level (in foot-candles [fc]) will auto-populate.

Representative Spaces for Electric Lighting:

Projects containing spaces that are significantly similar in size, program, and electric lighting design can make use of the representative space designator in the B3 Electric Lighting Spreadsheet. The entry should be indicated as a representative space with the total square footage included in the spaces and the rooms listed by number or name. For these spaces, the simulated electric lighting levels of a single room can be applied to the entire represented area.

As design progresses, computer simulations should be performed to determine the amount of light provided by electric lighting at the work plane, walls, and ceiling. As the results are entered in the spreadsheet, the compliance metrics will be calculated and indicated for each space. Because the contrast ratios are based on the light level at the work surface, this value must be entered first; then, the target light levels for both the walls and ceiling will be calculated and populated in the spreadsheet.

For contrast ratios between work surfaces and wall surfaces (excluding fenestration), the spreadsheet relies on the following calculation ratio:

For example, if the target average work surface illuminance is 40 foot-candles, the target wall surface illuminance will be, at most, 13 fc, so as to maintain the desired 1:3 ratio.

For contrast ratios between work surfaces and ceiling surfaces (excluding fenestration), the spreadsheet uses the following calculation ratio:

For example, if the target average work surface illuminance is 40 fc, the target ceiling surface illuminance will be, at most, 4 fc, so as to not exceed the desired 1:10 ratio.

When selecting and/or specifying light fixtures, ensure that the CRI and RoHS compliance criteria are mandated in the specifications and met by product purchases. This information can typically be found on manufacturer cutsheets.

Daylighting:

Glare control: During the schematic design phase, identify those façades that face within 45 degrees of east, south, or west, and the Regularly Occupied Floor Area adjacent. In 90% of Regularly Occupied Floor Area, glare control should be designed and specified with manual operation, or with automatic operation with manual override. Acceptable glare control devices are those that are operable by building occupants to address unpredicted glare and include interior window blinds, shades, curtains, moveable exterior louvers, movable screens, and movable awnings.[1]

During the schematic design phase, list all Regularly Occupied Floor Area, listed by room or space, using B3 Daylighting Worksheet.

Representative Spaces for Daylighting:

Projects containing spaces that are significantly similar in size, window-to-wall ratio, window-to-floor area ratio, and orientation can make use of the representative space designator in the B3 Electric Lighting Spreadsheet. Entries should include the area totals for the entire group of spaces represented and should list the included rooms by number and/or name. For these spaces, the simulated daylight utilization of a single room can be applied to the entire represented area.

During schematic design and design development, the daylight utilization for each regularly occupied (and/or representative) space should be simulated. Ensure that daylight models accurately reflect room size, height, window placement, glazing properties, interior surface reflectance, and permanently installed furnishings. Some software uses default values for interior surface reflectance; if these values are significantly higher than the design conditions (i.e., if there are a lot of dark surfaces), a different software with adjustable values must be used. Enter simulation results for sDA and/or DF in the B3 Daylighting Spreadsheet. As results are entered, the spreadsheet will automatically calculate the weighted average spatial daylight autonomy, or the percentage of floor area with a daylight factor of 1.0 or higher, and indicate compliance or noncompliance with the guideline.

Two daylighting metrics that may be used to comply with the B3 Guidelines are available as outputs from daylight modeling software:

• Spatial daylight autonomy (sDA_30fc/50%) is an annual measure that determines the percentage of the floor area that receives the minimum light level (30 fc) for at least 50% of the occupied hours during which daylight is available. Occupied hours vary by space use type and can generally be programmed into daylight simulation software.
• Daylight factor is the ratio of the illumination available inside a structure to the illumination outside, calculated or measured using an overcast sky condition. When calculating daylight factor for compliance with B3 Guidelines, note that the required result is the percentage of floor area with a daylight factor of 1.0 or higher. This is different from the average daylight factor in the space, which is what most simulation programs report. This value can be found in each of the software programs listed below, but may require specific settings for the simulation or a separate calculation from the results provided by the software. For further guidance on calculating the percentage of floor area meeting the requirement in the listed software tools, please reference guidance documents in the Additional Resources section of the guidelines or guidelines website.

Software tools capable of performing daylight analysis for B3 compliance include:

Other software tools may be used, provided the tool is capable of reporting sDA or percentage of floor area within a specific daylight factor range.

These calculations should be updated during the final design phase to match construction documents and represent the as-built condition as closely as possible.

Considerations for Buildings under 20,000 Gross Square Feet Conditioned Space:

Projects under 20,000 square feet that are considered small buildings under the B3 Guidelines program[1] can simulate their spatial daylight autonomy or daylight factor as described above, or can use the Small Buildings Method tab in the B3 Daylighting Worksheet. During the schematic design phase, list all Regularly Occupied Floor Area by room or space in the B3 Daylighting Worksheet. Representative spaces (described above) may be used when appropriate.

For the Small Buildings Method (WWR), enter the exterior window area, the exterior wall area, and the interior floor area for each area designated a Regularly Occupied Space. Window Area refers to the area of the entire window unit(s), including the frame(s). Exterior Wall Area refers to the interior finished face area of the exterior wall(s) in the space. Floor Area refers to the interior finished floor area for the space. The worksheet will automatically calculate the ratio of window-to-wall area and indicate compliance with the required threshold for the window-to-wall ratio. If the project includes any Regularly Occupied Floor Area that has no exterior wall area (a fully interior room), these spaces and floor areas should be listed in the B3 Daylighting Worksheet, Small Buildings Method tab, with 0s in the Window Area and Exterior Wall Area columns. These spaces are important for a more comprehensive understanding of the daylight conditions but are not included in the calculated ratios.

Update these calculations during the final design phase to match construction documents and represent the as-built condition as closely as possible.

Windows that are specified and installed in regularly occupied spaces should meet the visible transmittance (VLT) requirement of 0.65 or greater. Submit manufacturer’s cutsheets or other supporting documentation to meet this requirement. If the VLT requirement is in conflict with other glazing requirements (e.g., bird-safe requirements), please reach out to guidelines@b3mn.org to discuss an alternative compliance option.

If pursuing Recommended Guideline I.5-D, when selecting and/or specifying light fixtures, ensure that the criteria of Color Rendering Index of 90 is listed in specifications. This information can typically be found on manufacturers’ cutsheets.

If following Recommended Guideline I.5E, evaluate lighting plans and identify spaces with direct-only overhead lighting. Calculate the total connected lighting load in areas served by the direct-only overhead lighting, and if direct-only overhead lighting exceeds 25% of the total connected load, select different fixtures to meet the criteria.

If following Recommended Guideline I.5F, select or specify interior surfaces with reflectance to meet or exceed the values listed. This information can be found on manufacturer specifications, typically listed as a fraction of percentage light reflectance or light reflectance value. If the manufacturer’s data does not include reflectance, use the methodology described in the IES Lighting Handbook to calculate the surface reflectance with a product sample or installed product.

[1] Defining a small building under the B3 Guidelines program is noted earlier in this document in the Applicability section.

[1] This guidance aligns with credits available under LEED v4.1