What is the difference between solar energy transmittance and SHGC?

As shown above, solar heat gain coefficient (SHGC) consists of two components:

  • Primary solar heat gain: the solar heat directly transmitted through a glass in its original solar radiation form.
  • Secondary solar heat gain: the solar heat absorbed by a glass and further transferred to the indoor space as heat and via all 3 heat transfer modes (conduction, connection and radiation)

The primary solar heat gain component is just the solar energy transmittance of the glass.

The secondary solar heat gain component is calculated as the solar energy absorptance of the glass multiplied by its inward flowing fraction. The solar heat absorbed by the glass causes a temperature increase of the glass. The absorbed solar heat flows to either the indoor side or the outdoor side. The fraction flowing to the indoor side is the inward flowing fraction.

For example, for a glass with 30% solar energy transmittance, 20% solar energy absorptance and 0.25 inward flowing fraction:

  • Its primary solar heat gain is 30%: 30% of the overall solar energy is directly transmitted to the indoor space
  • Its secondary solar heat gain is 20% × 0.25 = 5%: 20% of the overall solar energy is absorbed by the glass and 0.25 fraction of it is transmitted to the indoor space;
  • Its SHGC is therefore 30% + 5% = 35% or 0.35.

In summary:

SHGC = primary solar heat gain + secondary solar heat gain

Primary solar heat gain = Solar energy transmittance
Secondary solar heat gain = solar energy absorptance × inward flow fraction

Solar energy transmittance and SHGC are different. Solar energy transmittance is the primary solar heat gain component of SHGC only. The SHGC of a glass is always greater than its solar energy transmittance.

OTM Insights Newsletter: Issue 5

Our newsletter OTM Insights Issue 5 was sent to our customer on 27 Jul 2020. We are sharing some technical insights of accuracy of glass optical & thermal property test results in the newsletter. Click the image to read the full newsletter.

View past issues:

Choose OTM.

Work with the expert in Optical & Thermal Measurement solutions.

OTM Insights Newsletter: Issue 4

Our newsletter OTM Insights Issue 4 was sent to our customer on 27 Apr 2020. We are sharing some technical insights of optical & thermal property test of glasses from existing buildings in the newsletter. Click the image to read the full newsletter.

View past issues:

OTM Insights Newsletter: Issue 3

OTM Insights Issue 3

Our newsletter OTM Insights Issue 3 was sent to our customer on 03 Feb 2020. We are sharing some technical insights of partially fritted glazing optical & thermal property testing procedures in the newsletter. Click the image below to read the full newsletter.

View past issues:

New release of lab profile 2020 edition

We are pleased to release our 2020 edition of laboratory profile, which is a 56-page comprehensive document, with rich information on our capabilities and many technical insights.

What are in the laboratory profile?

  • A comprehensive overview of the laboratory
  • Detailed introduction of 10 popular tests
  • 14 informative technical insights articles

You may download a soft copy of our lab profile.