R-Value Calculator: Measure Your Insulation Performance

R-Value: The Number That Decides Your Bills

R-value is how we measure a material's resistance to heat flow — the higher the number, the better it keeps heat in during winter and out during summer. It's the single most useful number to know about your walls and ceiling, because it's what your heating bill is really listening to.

Our R-value calculator lets you compare insulation materials and thicknesses side by side, so you can see what it actually takes to hit the efficiency targets you want — not just what a sales sheet claims.

What Drives R-Value

Two things set the R-value of any insulated assembly: the insulation's own per-inch rating and how thick you make it. Fiberglass runs around R-3.5 to R-4 per inch. Spray foam runs much higher — closed-cell can reach R-6 to R-7 per inch — which is why a skinny layer of foam can outperform a fat layer of fiberglass.

The calculator lets you plug in thickness and material and see the resulting total R-value for your walls, roof, or floor. That's the number your plan review should chase.

• More R-value = less heat loss = lower energy bills.
• R-value stacks additively as you add thickness.
• Per-inch ratings vary a lot: fiberglass ~R-3.5+, rigid foam ~R-5 to R-6, closed-cell foam ~R-6 to R-7.
• Whole-wall R-value (what it really is built) is lower than the R-value of a pristine sample — gaps, framing, and joints all leak.

The Log Home Twist

Here's where log homes confuse people, and honestly, the calculator needs a footnote. A solid log wall's R-value per inch is low — cedar is roughly R-1.4 per inch — but it brings something else to the table: thermal mass. That mass absorbs heat during the day and bleeds it out slowly at night, which can make a log home feel more comfortable than its raw R-value suggests.

What that means in practice: don't try to beat a 2x6 insulated stud wall's R-value with bare logs. Instead, value the log home for what it is, insulate the roof heavily (that's where you lose the most), and consider techniques like building a cathedral ceiling with a proper insulated assembly above the wood.

Energy Code Is Your Floor, Not Your Ceiling

Local energy codes set a minimum R-value that's really a starting line. In most of the Southeast, code expects solid roof/ceiling insulation and reasonable wall values, but the best-performing homes go well beyond it.

Run the calculator and aim deliberately above code — the cost difference to bump an assembly a few R-points is usually modest, and the payoff in comfort and bill-savings lasts the whole life of the house.

Test, Then Trust

R-value on paper is one thing; the real, built wall is another. After you frame and insulate, a blower-door test tells you how airtight the house actually is — and airtightness is where a huge share of real-world performance lives. An insulated but leaky home leaks its comfort right out the cracks.

Use the calculator to design good numbers, then verify the build with a blower-door test. Paper and reality should agree, and when they don't, you want to find out before you move in.