Solar Reflectance

Vinyl Siding Distortion, Cardinal Glass Industries, Bulletin #IG14 – 08/12

This bulletin was written by a major IG manufacturer who performed an extensive study on the distortion of vinyl siding.  It shares major results from the study and offers mitigation techniques.


  1. Vinyl siding distortion is the result of direct sun + reflected sun + ambient temperature. This combination of inputs is required for the siding to reach a temperature at which it will distort.
  2. The average deflection temperature of vinyl is 166°F (range of 142-192°F, depending on the vinyl type).
  3. The document list variables that contribute and goes into some depth on several of these:
    1. Color and solar absorption of the vinyl
    2. Distortion temperature of the vinyl (ave. 166°F)
    3. Cumulative time and temperature on the vinyl (1 hr @ 165° = initial distortion)
    4. Built-in strains in vinyl from forming the siding (relaxes when heated)
    5. Installation strains (distortion from expansion/contraction)
    6. Ambient temperature (warms siding without sun)
    7. Wind (cooling effect)
    8. Other heat sources (BBQ grills, A/C compressors, motors, etc.)
    9. Reflections from adjacent building materials (siding, doors, windows, asphalt, concrete, water)
    10. Orientation of building materials and siding to the sun
    11. Distance of reflective materials to siding
    12. Deflection of glass products
  4. Distortion results from a combination of variables and NO Single Solution is effective to resolve all situations.
  5. Building codes, the DOE, EPA promote and/or mandate high performance windows and doors to prevent solar energy from entering a home. Unfortunately, these products often reflect solar energy adding to heat buildup on adjacent surfaces.
  6. Replacing the distorted siding and/or window with the same product may not correct the issue.
  7. IG units are dynamic in that their gaps between panes contract and expand with temperature. They are constantly flexing.
  8. NFRC has gap reduction guidelines for IG units that manufactures follow. These guidelines are for the units as they leave the factory and NOT applicable to units in the field where altitude, barometric pressure, and temperature fluctuate.
  9. Capillary tubes are not recommended as a mitigation for IG unit deflection
    1. They may not eliminate all of the deflection
    2. Proper installation is required to preserve longevity
    3. It doesn’t address other issues (solar aborptance of siding, built-in strains, installation strains, proximity to other heat sources)
    4. Shortens life/durability of the IG
    5. Should only be used in high altitudes and dry climates
    6. Changes thermal performance if argon filling is used.
  10. Exterior screens can block reflection by 40%, but don’t address other contributors to vinyl distortion, and are not available or possible on all window and door types (ex. Fixed windows and casements)
  11. Two building conditions were identified as possible concerns:
    1. Opposite Wall condition—The glass in a window on a wall opposite the distorted siding has deflected and reflects focused solar energy to the siding.
    2. Inside Corner Condition—Two walls meet at 90° with a window one wall near the corner. If oriented so that the sun can directly radiate on the siding and reflect off the adjacent window at the same time, the effect is essentially a double of the incident solar heat.
  12. A case study of a home with distorted siding found that an Inside Corner condition with a window that met NFRC guidelines for deflection was able to heat the siding to the distortion temperature (166°F).

Considerations and mitigation

  1. Choose siding with pigments that have high solar reflectance (low absorption)
  2. Choose siding with minimal built in strain
  3. Review building design for heat buildup and select materials that will perform accordingly
  4. Consider building orientation and geometry to accommodate heat buildup (opposite walls and inside corners)
  5. Identify areas of high risk, (alcoves, overhangs, inside corners, etc.) and determine if adjacent windows or other building products are in play for increased solar reflection.
  6. Use trees and other landscaping to block solar gain
  7. Use awning or screes to block or filter solar gain
  8. Select siding with appropriate resistance to heat
  9. Use siding with low absorption (<40%) where reflections will be on the siding
  10. During construction store and install siding per manufacturer’s instructions
  11. Choose NFRC certified windows and doors
  12. Identify windows that could reflect on to siding and consider alternatives or mitigation with an external screen
  13. Glass manufacturer to follow NFRC guidelines for reduction of IG gap
  14. Use capillary tubes ONLY as a last resort to resolve IG gap issues contributing to solar reflectance. (These tend affect performance and shorten the IG units life.)

Again, no single solution is effective to resolve all situations. Due to the numerous variables, each incident should be approached for resolution on a case by case basis.

Making a Difference in Glass Deflection/Image Distortion SG-2000-92

This document addresses the issue of Reflective Image Distortion that is inherent in all insulating glass units.  This is as an optical phenomenon resulting from the law of “angle of incidence equals the angle of reflection.” When adjacent rays of light that are parallel to each strike the surface of a perfectly flat piece of glass, their reflections coming off the glass stay parallel. If the glass surface is curved, however, the once parallel rays reflect off the glass deviating from parallel. From a concave surface the rays come closer together and from a convex surface they move further apart.

This phenomenon is also in play when light that reflects off Insulating Glass units.  The paper reminds the reader that IG units are actually “flexible pressure chambers”.  The edges are sealed and when the elevation, barometric pressure, wind, and/or temperature change, the pressure inside IG unit changes also.  This results in a deflection of the glass lites that can be noticeable in the glass.

In addition, the lites of glass used to make an IG unit may be heat-treated for extra strength and/or safety reasons.  The process of heat-treating the glass can result in small waves in the glass that discernable reflective image distortion.

It is also pointed out that there is no way completely eliminate reflective distortion in IG units, and only a few methods to modify it.  Venting with breather holes, breather tubes, or capillary tubes is discussed, along with the pros and cons of each.  Of particular concern is that shortened IG live is expected if the venting device is left open.

Glass, Solar Radiation and Their Effect on Vinyl Cladding Materials, Biterice, et. al, Window and Door Magazine, February 2004