Comparison of WINDOW 5 / THERM 5 and WINDOW 6 / THERM 6 Results for Specular Glazing Systems

Software/Versions Affected:

The WINDOW software program is used extensively in the fenestration industry to calculate the thermal and optical properties of glazing systems and whole window products. The version of WINDOW that is currently certified for the NFRC Ratings Program is WINDOW 5.2.17a. LBNL has developed the next version of WINDOW, currently WINDOW 6.3, which has the added capabilities of modeling complex glazing and shading systems (such as venetian blinds, woven shades and fritted glass).

However, the basic calculation method for non‐complex glazing products, also known as specular glazings, has not changed significantly between WINDOW 5.2.17a and WINDOW 6.3. The only difference between the two versions for the specular glazing calculations are the following changes:

  • Solar Heat Gain Coefficient Correction: This was a minor correction which resulted in a change of no more than one percent (1%) between the two programs. Calculations are now in accordance with equation 14 in Section 4.2.2 of ISO 15099
  • U‐factor Correction for Vertical Cross Section (Jambs and Vertical Meeting Rails): Two different fixes were made which affect the U‐factor.
    • The Lv (vertical length) and Lh (horizontal length) dimenions in equation 93 in section 6.6.5 of ISO 15099 were not clearly defined for cavities in vertical cross sections. Research in an accepted (but not published) paper comparing THERM with CFD showed discrepancies, which caused a review of the ISO 15099 algorithms. This resulted in a change to the algorithms such that the frame cavity dimensions are now correct for radiation calculations for cavities in vertical cross sections.
    • A frame cavity emissivity bug was fixed which very slightly changed the U‐factor for some cross sections. See the detailed description under the Whole Products section of this report.
  • CR Correction: Reference temperatures for CR calculations are now in exact accordance with NFRC 500. Temperature differences with W5/T5 CR calculations are less than 0.09 oC. In addition, the CR results are affected by the change in U‐value for vertical cross sections (see above).

Therefore, if WINDOW 6.3 and THERM 6.3 are used only for modeling whole products with specular glazing systems, in comparison to WINDOW 5.2 and THERM 5.2, the results for winter U‐value will be within 0.5%, the results for Visible Transmission (Tvis) will be identical, the results for Solar Heat Gain Coefficient (SHGC) will be within 1%, and the results for CI will be within 1.2%.

The first section in this report is a comparison of glazing systems only (not whole products), and the second section is a comparison of whole products.