How Long Until an Attic Insulation Upgrade Pays for Itself?

How does this calculator work out the payback?

Three steps, and only one number in them did not come from you.

First, the share. ENERGY STAR publishes an average share of heating and cooling cost saved by sealing and insulating a home in Climate Zone 5, which is Longmont's zone on the federal DOE climate-zone map rather than a figure the city publishes. That share is for a whole seal-and-insulate job, so the calculator scales it by how far your ceiling actually moves: the fractional drop in conductive loss between the level you picked and the target you picked. A house already close to the target gets a small share of that average; a house at bare joists gets nearly all of it.

Second, the money. That scaled share is applied to your own annual heating and cooling cost, off your own bills. Not an average bill, not a typical Longmont bill — yours.

Third, the division. The quoted price, less whatever rebate figure you entered, divided by that yearly saving. The answer is in years.

Two things about the inputs are worth stating plainly. Current-R bands are scored at their midpoint rather than their floor, because scoring at the floor would inflate every answer on the page. And the yearly saving is an estimate built on an average, not a quotation — what matters is the shape of the answer, whether this is a three-year decision or a thirty-year one.

Why is the rebate amount a field you fill in?

Because a rebate figure printed on a page like this one is wrong the day the program revises it, and the reader has no way to tell. The program's own page is never stale. So the field ships empty — no default, no placeholder, no example — and the number that goes in it is one you read yourself, dated the moment you read it.

There are two places to read it, and they do not publish the same kind of number. Xcel Energy pays a share of project cost against a per-measure maximum. Efficiency Works pays envelope measures by the square foot of treated area — which is why this calculator asks for your square footage even though the payback arithmetic does not use it. Which of the two you can read for your own address is a separate question, and it turns on your utility accounts rather than your mailing city: Efficiency Works is open only to Longmont Power & Communications electric customers, and Longmont Power & Communications does not serve every Longmont address.

The Efficiency Works air-sealing step is set by how much measured air leakage the finished work removes, so nobody — this calculator included — can tell you in advance which step your house will land on. It is settled by the test after the work. Any figure named before then is a guess.

For current Xcel rebate amounts and program rules, see the Xcel Energy insulation and air-sealing rebates page. For Efficiency Works eligibility and current incentive values, see Efficiency Works home rebates. Eligibility may depend on which electric utility serves your address, program funding levels, and qualifying product specifications.

What happens if you leave the rebate field empty?

The calculator answers anyway, and says what it did.

With the field blank, the payback you get counts nothing from either program. It is the answer to a narrower question: how long the energy savings alone take to cover the quoted price. Whatever the programs your address actually reaches will pay comes straight off the quoted price, so the real payback is shorter than the blank-field figure — never longer.

That makes the empty-field answer a floor rather than a guess, and a floor is the more useful number to hold a decision against. If the arithmetic already works without the rebate, the rebate is a discount on a decision you had already made. If it only works with a rebate figure you have not read yet, that is worth knowing before the paperwork rather than after.

Why doesn’t the square footage change the payback?

Because it is already in there, once, and counting it twice would be wrong.

The ENERGY STAR share the arithmetic rests on is a share of a home's own heating and cooling cost. Your bill already reflects how big the house is — a larger house has a larger bill, and a share of that larger bill is already a larger saving. Multiplying by square footage on top of that would count the size of the house a second time and inflate every answer for every larger home.

Square footage is collected for a different reason entirely: it is the number you need in hand when you go read the Efficiency Works figure, because that program pays envelope measures by the square foot of treated area. Walk up to that page without it and you cannot turn what it says into a number for your house.

One scoping note, because a payback figure is not a survey of every route a Longmont-area home might take. Colorado runs its own Weatherization Assistance Program for income-qualified households, delivered through a local agency rather than through a utility, with its own rules and its own paperwork. It is not a rebate you enter in the field above. Ask the body that runs it.

Take the next step

What happens after you request a quote?

A rebate-aware quote starts with the utility question, because it decides everything downstream: which measures are covered, which testing has to be scheduled before work starts, and whose paperwork the rebate travels on. Have a recent electric bill, a recent gas bill, and a measured attic depth ready for that conversation.

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