Planning a home insulation project? Enter what your wall or attic has now, pick the insulation you are adding, and see the new total R-value and how much the heat loss drops. Imperial units, the way US products are labeled.
The diagram is labeled with the same symbols as the input fields below.
At t = 0 the outdoor side steps to your design temperature. The cold wave creeps through the existing assembly and your added insulation while the heat loss settles onto the calculator's answer.
This preview solves a handful of lumped nodes. The NovaThermal engine behind ThermalResults.com (coming soon) runs the same physics on tens of thousands of nodes - full transients with phase change, radiation, fluid loops, and Monte-Carlo design envelopes, GPU-accelerated at 400× real-solver speed - and hands you review-ready margin reports.
R-value is the reciprocal of heat flow: R-38 lets half the heat of R-19 through the same area at the same temperature difference. That reciprocal is why the FIRST inches of insulation are spectacular and the last inches are marginal - going from an uninsulated R-4 wall to R-13 cuts conduction by two thirds, while going from R-38 to R-49 in an attic trims what is left by a fifth. This calculator makes that arithmetic visible before you buy material.
Real assemblies underperform the label. Wood studs bridge the cavity every 16 inches, attic hatches and recessed lights punch holes in the blanket, and compressed or damp insulation loses rating. Treat the computed reduction as the ceiling for the conduction path - and remember AIR SEALING usually beats the next inch of insulation, because infiltration is a separate loss this calculator deliberately leaves out.
To turn BTU/h into money: multiply by the hours at your design conditions (or use degree-days), divide by your furnace efficiency, and price the fuel. A whole-season estimate with real weather, thermal mass, and duty cycling is simulation territory - but for choosing between R-3.2 batts and R-6.5 foam at a given budget, this closed form is exactly the right tool.
US DOE guidance runs from about R-38 in warm climates to R-60 in the coldest zones. If you have less than R-19 up there today, topping up is one of the best-payback projects in the house.
Yes - layers in series add: existing R-13 plus 10 inches of blown cellulose (R-3.5/inch) gives about R-48. What does NOT add linearly is savings: each added inch saves less than the one before, because heat flow is 1/R.
Framing. Wood studs are only about R-1.2 per inch, so every stud bay boundary is a thermal bridge. A nominal R-13 stud wall performs around R-10 as a whole assembly - continuous exterior foam is the standard fix.