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Tubular Daylighting Device Performance Indicators Cheat Sheet (DRAFT) by [deleted]

This is a draft cheat sheet. It is a work in progress and is not finished yet.


If you are consid­ering TDDs for your building, consider some key indicators of perfor­mance before purcha­sing. Dr. Abdelaziz Laouadi, Research Officer at the National Research Council Canada, explains that the following four indicators are the most important factors in a TDD’s perfor­mance.

1) Visible (or Light) Transm­ittance (VT)

Higher VT ratings will ultimately be better, with typical VT values of current products ranging from 0.15 to 0.55.

2) Spacing to mounting height ratio (SR)

The higher the SR, the better to minimize the capital cost of TDDs.

However, the SR should not infringe on the applicable building fire code requir­ement for skylight spacing. For ideally diffusing TDDs, the SR varies from 1 to 1.5 to provide acceptable illumi­nance uniformity on horizontal work planes.

3. U-factor

Lower U-factors (inverse of thermal resist­ance) will more effect­ively reduce heat loss in climates that require more heating. Multi-­plane collectors work best when insulation is placed at roof level in commercial buildings. Typically, U-factors (in Btu/h•­ft2­•de­grees F.) range from 1.39 (R-0.72) to 0.37 (R-2.7).

4. Solar Heat Gain Coeffi­cient (SHGC)

For climates that require a lot of heating, TDDs should have a medium to high SHGC. But most TDDs have a medium to low SHGC (<0.45).