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Community and Q&A

Sanity checking BEopt’s modeling of exterior rigid foam benefits

insaneirish | Posted in Energy Efficiency and Durability on

I’m in the early phases of an extensive remodel to my 1916 colonial style home in New Jersey (Climate Zone 4A, closest weather model location Newark Airport).

As part of the remodel, I’d been planning on using Zip R-6 sheathing on both the new areas of the house as well as a way to retrofit exterior foam over the existing board sheathing. Seeing the relatively meager energy savings of this is having me rethink this option.

Here’s how I’m modeling the house right now:
Size: ~2,700 sq. ft. finished space
Walls:
R-15 fiberglass (this is a conservative midpoint for half the house being 2×4 dense packed cellulose and half being 2×6 w/ mineral wool, since BEopt can’t model differently per wall)

Modeled cooling loads (for some reason BEopt isn’t showing the modeled heating load when using a heat pump, but it is definitely modeling total energy use including heating):
No exterior insulation: 31 kBTU/h
R-6 exterior insulation: 29 kBTU/h

Modeled energy costs (using my actual energy costs):
No exterior insulation: $2,325/year
R-6 exterior insulation: $2,241/year

So, effectively we have a modest $84/year energy savings which does not offset the increase in material and labor cost even over a 30 year period. Does this modeling pass your ‘gut check’? I frankly had been expecting more of a savings for the investment/hassle.

Random relevant things:
– Even if I don’t use Zip R over the existing board sheathing, I will be air sealing that part of the envelope with a self adhered WRB.
– All the windows are being replaced, so I was prepared for the re-flashing that would be needed for exterior rigid foam.
– I plan to install a PV array which should offset about 50% of my electricity use.

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    You need to get heating loads. With cooling loads, there is little difference between indoor and outdoor temperature, the extra R value won't do much (for R value to matter, you need delta T). In most places, lot of your cooling loads come from infiltration, windows and roof.

    Once you have it set up for heating, the insulation should make a bigger difference. Overall, there is very little benefit to going over an R20 to R25 wall in milder climates.

    1. insaneirish | | #2

      > Once you have it set up for heating, the insulation should make a bigger difference.

      It is "set up" for heating loads. I think there's some sort of cosmetic bug in BEopt's ability to display, specifically, the calculated heating load in this configuration.

      It clearly shows a difference in overall energy consumption of the two options. The bulk of that difference in consumption is heating, as you suggested. The difference just doesn't correspond to that much money.

      > Overall, there is very little benefit to going over an R20 to R25 wall in milder climates.

      When thought about in the context of the reciprocal U-value, this makes sense.

      Perhaps some of my surprise is a product of recall bias. New Jersey can get quite cold, even for extended periods, and that's what one tends to remember. But I suppose in the larger picture of what overall fraction of time that represents, it's actually not as significant as it would be in a colder climate.

  2. Boosted139 | | #3

    I've had a similar experience with beopt. Also in NJ but northern zone 5. I've been modeling new construction of a ~3500 sq ft home with a heat pump and estimating 1 ach50. Going from "code" insulation to ~10/20/40/60 insulation "only" nets a 900kWh/yr (200$) savings. The only other savings is the HVAC sizing is reduced by ~20% (30.3 kBtu/hr to 25.7).

    Just as you are having trouble justifying the expense, I am as well. Sorry for not really helping, but just corroborating what you're seeing in in beopt. Curious as to others experiences and thoughts.

    1. insaneirish | | #4

      No apology necessary! Different data points are always helpful.

      Your example is pretty striking because it's a pretty big jump in effort to hit the numbers you're mentioning. If it were an either/or, I'd rather take that money and invest in a PV array.

      Thank you for sharing your analysis.

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