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In Las Vegas: Why does adding a low-e tint (reduces u-factor from .65 to .5) to dual pane aluminum windows result in a negative savings?
Did the SHGC change as well? Was the change applied to all windows? Are you looking at the savings for the window improvement only or for the entire package of improvements?
Is this the SHGC the only change made to the window? Was it made to all windows or just one facing? Did you model the overhangs correctly? Where are you looking at the savings? Are you sure that the U-factor doesn’t change? You called it a “low-e tint”. A low emmittance coating would reduce the U-factor.
If everything is modeled correctly, then this means that the loss of solar gains in the winter is costing more than the the savings attained through cooling load reduction in the summer. The balance between the two will depend heavily on orientation and shading, but also on the all the factors in the rest of the model that determine the length of the heating and cooling seasons and annual loads.
Moderator note:
It turned out that the model had the film applied to the SW and SE facing windows, but not the NE and NW facing windows. Because of the orientation of this house, the average daily solar intensity in the middle of the summer is actually fairly even across the four facings. However, in the winter, the solar intensity is much greater on the two southerly facings that had the film applied. Thus, the improvement impacts a little more than half of the eligible solar gains in the cooling season, while impacting almost all of them in the heating season. This combined with the lower efficiency of the furnace, relative to the AC, led to an overall energy loss. However, due to the higher cost of electricity there was still a small positive dollar savings. Applying the film to all window increased the savings.
The new IECC codes requires lower SHGCs. Does this imply that Optimizer will find for Las Vegas this increases energy use?
This does not imply anything about general results for window films in Las Vegas. The results will vary considerably from home to home depending on the orientation, window distribution, thermostat set-point, etc. I expect that many homes will see both energy savings and dollar savings.
The IECC codes were not developed solely to save home owners money. They have a broader mission that includes reduced environmental impact. Reducing SHGC’s will always reduce peak electricity demand through cooling reduction and that can mean less need for the utility providers to fire up older inefficient plants. Also, when you consider all of the inefficiencies in electricity generation and transmission, the improvement to this home may have reduced global energy consumption, despite having a slight increase in point of use consumption.
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