Attic Assembly R-Value Calculator
Stack two insulation layers — say batts under blown-in — and this tool adds their R-values to give the assembly total, because for layers in series the resistances simply sum.
Calculator
Stacking 3.5 in of fiberglass batt (~3.2 r/in) and 8.0 in of blown cellulose (~3.5 r/in) gives about R-39.2 (layers add). Framing members are a thermal bridge, so the whole-assembly R is a little lower.
Attics rarely hold a single, uniform blanket of insulation. A common upgrade leaves the original fiberglass batts between the joists and adds blown cellulose or fiberglass on top. When insulation layers sit one above another in the heat-flow path — "in series" — their R-values add. This calculator takes two layers, each with its own thickness and material, and returns the combined assembly R-value, along with the R contributed by each layer so you can see where the resistance comes from.
The material selector carries the R-per-inch for each option, so you only enter the depth you can measure with a ruler or an insulation-depth marker. That turns a tape-measure reading in the attic into a number you can compare against your climate zone's recommended R.
Formula
Rtotal = (t1 × Rpi1) + (t2 × Rpi2)
where t is a layer's thickness in inches and Rpi is that material's R-per-inch (batt 3.2, blown fiberglass 2.5, cellulose 3.5, open-cell foam 3.7, closed-cell foam 6.5). Because the layers are in series, you add their resistances — you never average them.
Worked example
Take 3.5 in of fiberglass batt (3.2 R/in) with 8 in of blown cellulose (3.5 R/in) on top:
(3.5 × 3.2) + (8 × 3.5) = 11.2 + 28.0 = R-39.2
The batt contributes R-11.2 and the cellulose R-28.0, for an assembly total of about R-39.2 — a solid mid-zone attic, a little short of an R-49 target if you are in a cold region.
Thermal bridging and compression
Thermal bridging. The number above is the R-value of the insulation itself, in the cavity between framing members. Wherever a wood joist or truss chord crosses the assembly, it conducts more heat than the insulation around it, so the whole-assembly R (averaged over framing and cavity) is somewhat lower than the cavity R. For a rough planning figure that is fine; for an energy model, derate for the framing fraction.
Also remember that compressing insulation lowers its R-value: batts stuffed into too small a space, or blown-in matted down by stored boxes, deliver less than their rated R. Keep each layer at its full loft.
Frequently asked questions
Do you add or average R-values of stacked insulation?
You add them. Layers in series (one on top of another in the heat-flow path) sum their resistances, so R-11 batt plus R-28 blown-in is R-39. You only average R-values when comparing parallel paths, such as insulation versus framing across an area.
What is thermal bridging in an attic?
It is heat taking the easy path through the wood framing, which has a much lower R-value than the insulation beside it. Joists and truss chords act as small "bridges" that lower the effective R of the whole assembly below the cavity R this tool reports.
Can I mix insulation materials?
Yes — mixing batts and blown-in is one of the most common attic upgrades, and their R-values still add. Just avoid stacking two vapor retarders (for example, faced batts over faced batts), which can trap moisture between them.
How do I measure existing attic insulation?
Push a ruler or a marked stick straight down to the drywall at several spots and read the depth. Multiply each depth by the material's R-per-inch, then enter the layers here. If you cannot tell the material, fiberglass is usually pale yellow/pink batts or fluff and cellulose is gray, dense, paper-like.