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OC Foam in Exterior Walls

djhaus18 | Posted in Energy Efficiency and Durability on

I am a homeowner buying a new house in the Boston area, climate zone 5. The finished area of the house will be in the 5000-6000 range including the basement and attic. I got involved after some work was already done. Namely the roof deck has been sprayed with open-cell (OC) foam. I have a separate discussion going with Martin and Dana on that topic.

The question now is related to exterior wall insulation which needs to be decided. The builder is planning to do the following:

1) Closed-cell (CC) spray foam on interior basement walls and basement rim joists
2) OC spray foam on rim joists between first and second floor
3) OC spray foam on garage ceiling (living space above)
4) OC spray foam on partition wall between garage and house
5) Fiberglass batts in house/garage exterior walls (with appropriate air sealing)

I’ve been offered the opportunity to upgrade to OC foam all of the house exterior walls or OC foam only the exterior wall stud bays with utilities in them and batt the rest. The cost of each upgrade is $3000 and $800 respectively.

My question is whether either option is a worthwhile upgrade? I’ve seen some studies that suggest between 16-22% improvement with heating costs, but that is comparing an all batts house with vented attic to an all OC foamed house with unvented attic.

Using the utility bills from another similar house this builder constructed, if I assume OC foam in all exterior walls gets me an 8-11% improvement in annual heating bills it would take about 20 years to recoup the cost at today’s heating rates.

So I’m leaning against the upgrade. Thoughts?

-dave

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    David,
    Compared to properly installed fiberglass batts, open-cell spray foam does not give you any improvement in R-value. However, it may give you an improvement in airtightness.

    The key question is: What does your contractor mean when he or she promises that the walls with fiberglass batts will get "appropriate air sealing"? If it really is appropriate air sealing -- (that's rare) -- then you are all set, especially if you get a Grade I installation of fiberglass batts that meets the requirements of fiberglass batt manufacturers (also rare).

    Ask the contractor who promises "appropriate air sealing" to put his money where his mouth is. Ask if he'll sign a contract promising to meet a specific air leakage target, verified by a blower-door test.

  2. Tim C | | #2

    Today's heating rates are probably about as good as they're going to get, so 20 years is your worst case scenario. And you're probably financing that 20 year payback with a 30 year loan, which means your net mortgage+utility bill is cheaper from day 1 and will keep being cheaper from there on out. So I'd say you make a pretty strong argument to go for it.

  3. Dana1 | | #3

    If they caulk the framing to the sheathing, the bottom plate to the subfloor, and between the top plates batt insulated walls can be tighter than ocSPF in the stud bays wiithout caulking at the bottom & top plates.

    Predicting the future cost of energy is dubious game to get into. That sais...

    With the steadily falling cost of solar PV and wind power many in the investment banking sector are predicting energy price deflation broadly (not just in electricity) to set in before 2030. Just as nobody in 1983 was predicting that oil would hit it's inflation adjusted all time low price 1998, a mere 15 years later, nobody in 1998 was predicting that it would hit it's all time record high price in 2008, just 10 years later. But oil is a commodity, with high price volatility, wind & solar are technology, technology with fairly consistent year on year "learning" trends, with price and efficiency improving consistently over time. If you want to hedge the bet, heat with heat pumps (another technology with a learning curve) rather than getting sucked in by the current record-low price of natural gas ( a commodity with even more volatile pricing than oil.)

    In a Boston climate the marginal cost of heating with better class air source heat pumps is already competitive with natural gas, but the up-front cost of the equipment is higher. While roughly half the power generated in New England is currently fueled by natural gas, the other half is not. A doubling in the wholesale gas price has a far greater impact on homes heated with gas than those heated with heat pump technology. Gas may gain some market share in the grid generation mix in the near term, but will be fading in the intermediate & long term, given the exponential rise in installation rates of both PV and wind, accompanied by the continued cost trends.

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