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During the summer is it more desirable to have storm windows open, partially open, or closed?

I have old wooden, single-pane windows with aluminum storm windows and I have installed energy film on the windows. There is some draft if I do not weather seal properly each winter season.

Is the loss from drafts greater than the miniature hothouse heat gain from conduction through the single pane glass?

I have a central air unit properly sized for my home. Usually keep it at 26C and it usually kept at this temperature since my wife works from home.

I live at approximately 38N in the central mid-western US, with the seasonal highs of between 29C and 33C.

~15% of the southern face of the house has glazing and is subjected to constant sunlight after about noon. One window is constantly shaded.

LShaver
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RomaH
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    A few additional details to be able to answer this question adequately -- 1. Do you have whole-home air conditioning? 2. What latitude are you at? 3. What rough percentage of the equator-facing wall is glass? 4. Is this wall shaded at all (such as by trees or other buildings)? 5. Are you cooling the house during the hottest part of the day? – LShaver Jun 27 '18 at 21:23
  • Added your desired details. – RomaH Jun 28 '18 at 17:16

1 Answers1

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Keeping your storm windows closed in the summer will result in net energy savings due to reduced ventilation.

EDIT: model updated to eliminate cooling load during the winter.

Using eQUEST, a building energy simulation software based on research from the U.S. Department of Energy, I built a simple model of a roughly 500 m2, two-story home with 15% glass on the southern (equator-facing) side:

Simple house model

I then modeled three different changes caused by closing the storm windows to see how they affected total energy use for space cooling and space heating:

  1. Modest improvement in U-value of the southern-facing windows. Change from 0.50 to 0.38 (U-value is roughly the inverse of R-value). This would tend to reduce the rate at which thermal energy passes through the windows (so heat stays inside in winter, and outside in summer).
  2. Modest increase in solar heat gain coefficient. Change from 0.4 to 0.5. This is the hothouse or greenhouse effect, meaning that more of the sun's energy will be turned into heat inside the house.
  3. Significant reduction in ventilation. I assumed that the baseline ventilation rate is about 0.50 m3 per person, meaning that for every person in the house, 0.50 m3 of air is exhausted per minute. The closure of the storm windows is modeled as a reduction to about 0.25 m3/minute.

Here's the resulting effect on energy use for cooling:

Energy savings for cooling

Note that the measures are cumulative, so the final measure (in gray) includes the effects from the first two.

During the summer, the reduced ventilation reduces the need for the AC. The interesting thing is that in the shoulder seasons (particularly April and October), this increases the need for AC. Thus during the spring and fall it makes sense to open the windows in the morning and evening when the ambient air is cool.

And the same chart for heating:

Energy savings for heating

Here all three measures result in energy savings, as you'd expect. Note that the scale here is an order of magnitude larger -- meaning that keeping the storm windows closed saves a bit of energy in the summer, and a lot of energy in the winter.

Of course, as the saying goes, all models are wrong, but some are useful. Your best bet would be to test this yourself over a period of a few days, if you've got access to look at your electric meter reading periodically.

LShaver
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