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BeOpt sanity check: Energy savings with external insulation

asked 2019-08-01 21:47:47 -0600

AdamH's avatar

updated 2019-08-03 12:00:52 -0600


I'm running energy use simulations with BeOpt on a single family house that I will be building.

Here's some brief details:

  • Climate zone 6 (Northern plains, US)
  • Single story main floor with full finished basement, ~1800 square feet
  • Simple site plan, rectangular with attached garage on SE side.

I'm trying to determine the best cost/performance tradeoff for the amount of insulation to install on the house. I've been using the annual site energy use and energy cost tabs to compare several different designs. I don't have complete cost information, so I haven't been using the parametric analysis and instead have created several potential designs:

  • Standard construction for this area (2x6 16", gas heat & hot water, normal fiberglass insulation)
  • Better energy efficiency (2x6 T-Stud 24" (full wall R value of ~24), ICF basement, gas heat & hot water)
  • Better energy efficiency build but with a minisplit heating system and electric hot water
  • Best energy efficiency build (2x6 T-Stud 24" with exterior R16 mineral wool, triple pane windows, minisplit heating)

There are some unmet loads with all of these systems. I'm also planning on some backup resistive heat for the bedrooms, and the main areas of the house will have electric fireplaces.

I'm not a huge fan of gas, but that's definitely the cheapest option according to this, using from 500 to 1000 bucks. However, what BeOpt is telling me is that the 'Best' build only saves about another 50 dollars a year in electricity vs the 'Better' build with the minisplit. This kind of blows my mind since the marginal cost for the 'Best' build is about $35k.

Have I messed something up here? I would have thought an R40 wall with triple pane windows would lose only around 60% of the heat of an R24 wall. Is my main area of loss the R20 basement in that case? I've tried adding extra attic insulation and tweaking other parameters, but it doesn't seem to change much. I'd appreciate a second pair of eyes on this to give me some suggestions on where to go from here.

Thanks, Adam

BeOpt file:

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answered 2019-08-06 17:52:44 -0600

Adam, there are a few things about your model that might be affecting the outputs.

1- The unmet loads are significantly large, especially for the heating days. Because your model is in a heating-dominant region, this can significantly affect your energy use. image description

2- Also, the selected size for the mini-split was a 1-ton system, which seems to be very small for this building. After using the autosizing and adding an electric baseboard to the models with a mini-split system, you can see how the indoor temperature is increased and meets the thermostat setpoint temperature during the heating days. Also, total energy use increased.

Before: image description

After: image description

3- Compared to the DOE prototype buildings energy use, your total energy consumption is very small and it could be why changing the building envelope properties doesn't show a significant impact. I compared the last two scenarios after reducing the thermal resistance of walls and noticed that the energy use increased by about 10% (mainly because of the changes in heating loads). However, because the total energy use is small, the energy-saving does not look significant (about 1600 kWh reduction in energy use for 12 cents/kWh = 190 dollars).

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Thanks for double checking my work. I noticed that the original baseline model with centralized heating and cooling had a large number of unmet hours, mostly due to basement heat. I wanted to keep things as apples-to-apples as possible, which is why I left out the baseboard heat for the last two options.

One note, which model did you obtain the 10% savings on? You mentioned that you reduced the thermal resistance, but the energy use increase?

AdamH's avatar AdamH  ( 2019-08-06 19:44:51 -0600 )edit

The last two models. I left the last model as the highly-insulated option and reduced the insulation in the other model. I didn't keep the models but I think the total energy use was about 77 and 70 MMBtu. The percentage in energy saving (about 10%) is relatively high but the actual energy use is very small if you calculate the EUI (energy use intensity).

Ehsank's avatar Ehsank  ( 2019-08-07 18:11:42 -0600 )edit

Another possibility is the amount of internal loads you've defined. If they're not insignificant, you may be heating with those instead of your heating system

Jim Dirkes's avatar Jim Dirkes  ( 2019-08-08 09:21:13 -0600 )edit

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Asked: 2019-08-01 21:44:41 -0600

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Last updated: Aug 06 '19