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Can continuous exterior insulation possibly increases energy use?

Running my model (~4000SF, 1 story home, Climate Zone 5A , ~6000HDD) through BEOpt, using 16″ dense pack celluolose celing, insulated slab with perimeter insulation, I get higher energy use if I add continuous R-6 Polyiso exterior insulation to my 2×6 framed walls. Is this a bug? Could this be due to reduced energy gain in the winter? Is there a possiblity of continuous rigid exterior insulation INCREASING energy use?

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Replies

  1. Trevor Lambert | | #1

    I would be very surprised if BEOpt factored in winter solar gain, even through the windows, let alone through the opaque walls. Even if it did, there's no way that removing insulation from the walls is going to result in a net gain in heat in the winter. Seems like a bug is the only explanation, other than the obvious possibility of user error.

  2. Expert Member
    Zephyr7 | | #2

    I agree with Trevor. The only energy reduced by removing exterior insulation is the energy expended by the installers of that insulation. In winter, you're losing more heat through your walls than you gain -- that's why insulation is necessary.

    I'm guessing some kind of bug in the way BEOpt is doing the modeling.

    Bill

  3. Expert Member
    Dana Dorsett | | #3

    Is the higher energy use reported as cooling energy use, or is it heating?

    1. Expert Member
      Zephyr7 | | #4

      Are you thinking that a chimney-like effect was helping to exhaust our heat prior to the better sealing/insulating, and that’s where the “uses more energy after insulating” comes from? It’s seems very counterintuitive that the extra insulation would result in increased energy use for either heating or cooling.

      Bill

      1. Expert Member
        Dana Dorsett | | #5

        It's not about chimney effects and infilration, it's about solar gain, internal heat sources, and conductive losses offsetting the cooling loads during the shoulder seasons. Some amount of overnight heat loss through the wall assemblies lowers the amount of cooling energy needed during the day. This effect is more apparent in larger buildings/ commercial construction, where the optimal R point is at a much lower value than what it is with single family home type buildings. The higher the R-values, the lower the heating/cooling balance point, and at some R-value it's increasing the annual cooling load more than it's reducing the space heating load. (At a PassiveHouse located near me in zone 5A they needed to actively cool or open windows even February to stay comfortable.)

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