Developing Low-Conductance Window Frames: Capabilities and Limitations of Current Window Heat Transfer Design Tools

While window frames typically represent 20-30% of the overall window area, their impact on the total window heat transfer rates may be much larger. This effect is even greater in low-conductance (highly insulating) windows which incorporate very low conductance glazings. Developing low-conductance window frames requires accurate simulation tools for product research and development. Based on a literature review and an evaluation of current methods of modeling heat transfer through window frames, we conclude that current procedures specified in ISO standards are not sufficiently adequate for accurately evaluating heat transfer through the low-conductance frames.

We conclude that the near-term priorities for improving the modeling of heat transfer through low-conductance frames are:

1. Add 2-D view-factor radiation to standard modeling and examine the current practice of averaging surface emissivity based on area weighting and the process of making an equivalent rectangular frame cavity.
2. Assess 3-D radiation effects in frame cavities and develop recommendation for inclusion into the design fenestration tools.
3. Assess existing correlations for convection in vertical cavities using CFD.
4. Study 2-D and 3-D natural convection heat transfer in frame cavities for cavities that are proven to be deficient from item 3 above. Recommend improved correlations or full CFD modeling into ISO standards and design fenestration tools, if appropriate.
5. Study 3 D hardware short-circuits and propose methods to ensure that these effects are incorporated into ratings.
6. Study the heat transfer effects of ventilated frame cavities and propose updated correlations.