Window Film and Building heating

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I have several projects coming up dealing with window film savings on large glass area buildings. I need a primer to some of the results i am seeing.

Logically, if you install window film on glass, you will reduce the cooling load in that space, but would expect the heating load to rise. This, by the way, is in the temperate So Ca climate.

In BOTH cases, my improvement with window film does a killer job in reducing cooling. But is also saves about 7% heating !! I am going from a Shading Coefficient (SC) of 0.75 to an SC of 0.36.

Experiment. I reduced the SC to 0.01. AS boatload of cooling, but a 22% INCREASE in heating.

Then I tried a SC of 0.18. A median between the other two runs in space cooling savings, but now virtually NO change in space heating.

Why is there a balance point in heating? System is VAV w/RH.

This BTW was double glass.

I tried another building with single glass curtain wall. SC = 0.29. 3% reduction in cooling. Tried SC = 0.14. -1$ in heating. System is DDVAV.

Can ANYONE enlighten me as to what is going on?

John R. Aulbach, PE

John Aulbach's picture
Joined: 2011-09-30
Reputation: 1

One idea is to check the timing of your heating savings. Maybe you are saving reheat energy?

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7

I agree with Bill. It sounds like you are losing the passive reheat of the solar gain when you decrease the shading coefficient. We see this a lot in hospitals.

Cory Duggin, EI, LEED AP BD+C
Associate/Energy Engineer
TLC Engineering for Architecture


crduggin's picture
Joined: 2013-04-16
Reputation: 0

This question has been raised in the bulletin boards on several occasions, and is also
something that I saw in a project for the Pacific Northwest about 10 years ago.

I believe the answer has to do with the HVAC sizing. In a temperate climate, such as So
CA, HVAC sizing is driven by the cooling load that's in turn dominated by solar gain
through the windows. When window film is applied, the peak cooling loads will decrease,
resulting in smaller supply CFMs, which can also result in heating energy savings.


Joe Huang
White Box Technologies, Inc.
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Moraga CA 94556
yjhuang at for simulation-ready weather data
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Joe Huang's picture
Joined: 2011-09-30
Reputation: 406