Part II: Conservation FirstChapter 3. Seal, Insulate, and Cover

Adding Attic and Wall Insulation: Payback by R-Value

How to compare the next inch of insulation with the heat it actually saves in your climate.

Difficulty: Intermediate
Cost: Varies with area

published

Insulation slows conduction. The heat flow through a layer is the area times the temperature difference, divided by the R-value. Adding R-value helps, and the first R-value helps more than the last, because the math uses one divided by R. Going from almost nothing to R-11 cuts a huge fraction of the loss. Going from R-38 to R-49 cuts a small fraction. Both can be worth doing. They are not worth the same price per inch, and neither one works if air is pouring through holes the insulation is sitting on.

Seal the ceiling first. The air sealing weekend is that work. The degree-day guide is the weather number in the formula below. The bill baseline is how you check the answer a year later.

Difficulty

Level: Intermediate
Time: A weekend in an open attic, longer if the walls are closed
Skills: Working on joists, and knowing when wiring or a light fixture means stop

Cost estimate

Varies with area

Attic cellulose or fiberglass is often the cheap job because the cavities are open. A rough DIY top-up can land around a dollar or two per square foot of attic, and local prices move that. Closed-wall insulation is usually a contractor job and costs more per square foot. The calculator turns your own price into a payback.

Materials

Item	Qty	Notes
The existing R-value, or a measured thickness	1	Fiberglass batts are often near R-3 per inch. Cellulose is in a similar range. Mark guesses.
Insulation rated for the place	The area you measured	Attic floor, not stuffed against a hot non-rated light.
Baffles for the eaves	One per vented bay	So the soffit can still bring air across the roof deck if the roof is vented that way.
A sealed hatch	Already done	Do not bury an unsealed hatch and call the R-value finished.

The savings formula

Heat saved in a year, in BTU, is about the area times the change in one-over-R, times 24, times the heating degree days.

Change in one-over-R means one divided by the old R, minus one divided by the new R. For a jump from R-11 to R-49 that difference is about 0.07. Over 1,000 square feet, 5,000 degree days, and 24 hours, the heat saved is about 8.5 million BTU. A furnace at 90 percent efficiency burns about 94 therms to deliver that heat. At a dollar and a half per therm, the fuel is on the order of 140 dollars a year. A thousand-dollar job then has a simple payback near seven years. Change the degree days, the price, or the starting R-value and the story changes. That is what the calculator is for. It is not a quote.

Air leaks are absent from the formula. If the hatch is open, believe the formula less.

A thin attic loses a lot. A deep attic loses less, and the last inch loses less still. Seal first.

Attic work that does not create a new problem

Keep insulation off a fixture that is not rated for insulation contact, and do not fill the inside of a can light. Keep it off a flue by the clearance the flue requires. Do not bury knob-and-tube wiring. If the attic has vermiculite, stop and find out whether it contains asbestos before you add anything on top of it.

On a vented roof, baffles at the eaves hold a channel open from the soffit. Packing that channel solid can overheat the roof deck and invite ice dams in a snowy climate, because a vented roof deck is meant to stay cold. A hatch needs a gasket and a foam lid tall enough that you did not leave a bare square in the middle of the new insulation. Walk boards on joists if you will need to reach a far corner later. The insulation is not a floor.

The R-value comes from depth. A ruler and the bag's rated R per inch beat a guess. Fluff it to the thickness the bag claims. Packed or wet insulation does not deliver the label.

Walls

Open walls during a remodel can take batts fitted without gaps and without compression behind wires. Closed walls are a different trade. Dense-pack cellulose through drilled holes can work, and it can also miss a bay, soak a wall, or hit wiring. Lead paint on old plaster, and plaster that is not strong enough to drill, are reasons to hire this or to skip it. The payback on a wall is often slower than the same money spent on an empty attic, because the attic is easier and often starts at a worse R-value.

Foam board on the interior or exterior is a wall redesign. It changes the drying path of the wall and it has fire-barrier rules when it is left exposed. Do not glue a layer of foam over the inside of a wet basement and walk away. Fix the moisture first. Foam does not solve it.

Seal, then measure the R you have
Thickness and type. A bare ceiling and an R-30 ceiling are different jobs.
Run the sketch with your degree days and your price
If the payback is long, the attic may already be fine and the money belongs on air leaks or windows.
Protect vents, lights, and flues
Baffles, clearances, and no insulation on a fixture that is not rated for it.
Install to the depth you paid for
Markers or a ruler. Do not leave the hatch as a hole in the new blanket.
Leave closed walls alone unless you have a plan for paint, wiring, and missed bays
The attic is the first check.

Adding Attic and Wall Insulation: Payback by R-Value

Difficulty

Cost estimate

Materials

The savings formula

Insulation savings sketch

Attic work that does not create a new problem

Walls

Seal, then measure the R you have

Run the sketch with your degree days and your price

Protect vents, lights, and flues

Install to the depth you paid for

Leave closed walls alone unless you have a plan for paint, wiring, and missed bays

Further reading