Do these calculations seem reasonable?
I created an excel heat loss model for a proposed house design to help me select what steps beyond code compliance will have the most impact. I would appreciate any comments that more experienced people might have on my results. The house is about 2300 square feet above grade in climate zone 5 (about 100 miles from the zone 6 border). The second floor is about 1/3 the size of the first floor. The baseline code compliant model uses the following assumptions:
• 2×6 walls, 16 o.c., cellulose in cavity
• Attic cellulose R = 38
• Basement R = 12
• Windows U = 0.35
• ACF50 = 3
• 100 cfm exhaust only ventilation
The projected heating cost (95% efficient natural gas) came out to $997/yr. That is about 40% less than our current house of similar square footage built to 1994 standards.
Changing attic insulation from R=38 to R=60 saves about $23/yr.
Changing basement insulation from R=12 to R=20 saves about $41/yr.
Changing windows from U=0.35 to U=0.25 saves about $40/yr.
Adding 2″ of exterior foam saves about $86/yr (R-factor effect only).
Reducing air infiltration from ACF50=3 to ACF50=2 saves about $84/yr.
75% efficient HRV instead of exhaust only saves about $105/yr.
Changing from 16 o.c. to 24 o.c. saves very little, especially when in combination with exterior foam.
I don’t have cost numbers for each of these steps yet, so I cannot calculate a ROI. Most of these will also reduce air conditioning cost, but we don’t use our air conditioner very much. For some items, it is easy to associate the improvement with a particular building method or materials change. For others, like air tightness, that is not so obvious.
Do any of these numbers strike people as unexpectedly high or low? I was surprised at the HRV number because I read an article saying that HRVs don’t save enough to be justified on energy savings and must be justified based on comfort.
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Replies
Without running the math myself it sounds about right.
The lifecycle of insulation is at least 5x longer than an HRV, so you have to figure that in from a lifecycle economics point of view.
When viewed purely though a net-present-value-of-future-energy-savings lens you're missing more than half the real value of the higher performance house.
The comfort difference between a U0.35 and a U0.25 window is immediately obvious on a cold winter day, and window condensation problems are also much reduced.
The ice-damming and icicle potential of an R60 roof is much lower than that of an R38 roof.
The 2" of exterior foam enhances the moisture resilience of the structure immensely.
But even on a purely financial basis you should also consider the hedge value against future energy cost inflation in your "what if..." financial spreadsheets. The wholesale price of natural gas is currently trading near all time historical lows in much of the US, but it's unlikely to stay there over the full lifecycle of a house. If gas-grids get expanded/upgraded to take advantage of more cheap gas, the capital costs of those upgrades will show up in the retail delivered rates. Much of the recent years' gas glut has been due to the high cost of oil driving the fracking boom. Fracked dry-gas wells that have no liquids are usually not cost-effective at current wholesale prices. Now that the drilling rates have subsided with the price of oil, the gas glut will potentially dry up, and the wholesale cost of gas will go up. It's a crap shoot, but there's really no room for gas prices to go down very much unless oil prices reach levels where shale gas drilling rates ramp up again, but there's plenty of room for natural gas prices to rise. A typical fracked well with liquids is depleted of 95% of it's total-ever output in about 3 years. Even if production rates aren't falling fast right now, if oil prices stay at $60 or less, production rates WILL start slipping badly by mid 2017. (Feel like placing some bets?)
And, if you're financing the house, note that the capital cost of the building envelope upgrades is subsidized in the tax code via mortgage interest deductions, whereas utility costs are paid out in un-subsidized after-tax dollars.