Does my simulation sound accurate?

asked 2019-12-15 07:17:11 -0600

trevdawg122's avatar

Hello! I’m designing a home and contemplating energy trade-offs. It’s been a while since I’ve used BEopt but it’s my tool of choice because it’s easy to use in my opinion. The results I’m getting are a little surprising so I wanted to get some opinions. I’m using the EnergyPlus v8.8.0 simulation engine that comes with BEopt v2.8.0.0.

Central Florida, 2600 sqft under air, two-story Assume I simulated the home correctly and it meets energy guidelines for performance-based submittal (one area may be lacking but overall it passes energy code) Light-color metal roof, steel trusses, unvented attic, R-30 open-cell spray-foam insulation under roof deck Steel wall studs with R-20 open-cell spray foam insulation, R-3 continous rigid foam board over sheathing Low-E, double-pane, vinyl, gas-filled 0.26 U-value, 0.21 SHGC 5ACH50 (I’m kind of ignorant on airflow) 15 SEER heat pump(s)

Getting to the point, I’m seeing really small differences in the annual energy when I change parameters. For instance if I ditch the R-3 rigid foam and reduce the roof spray foam to R-20, I only consume 255kWh/yr more. That’s about $22/yr more. Another example, if I switch the walls from R-20 to R-10 the energy consumption goes up 182kWh/yr ($16/yr).

When I think about the cost savings of cutting the spray foam material in half or skipping the installation of rigid foam, that’s a whole lot more money than the above costs for even 50 years! Am I missing something?

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