Introduction
The R-Value Calculator helps you find how well a wall, roof, or floor keeps heat in or out. You build the assembly layer by layer, and the tool adds up the R-value of each part. It then shows the total R-value and the U-value for the whole assembly.
Start by picking a building element and a common assembly, like a timber frame wall or a truss roof. The calculator fills in the layers for you. You can change any layer's material, thickness, or conductivity (k). If a product has a tested R-value, type it in and it will be used instead. You can also click a real insulation product from the list to drop its R-value right into the layer you picked. Once you know the R-value you need, our Insulation Calculator helps you work out how much material to buy.
Studs and joists let heat pass faster than insulation. This is called thermal bridging. Tick the framing box on a layer, choose timber or steel, and set the framing percent. The tool uses the isothermal planes method for timber and an ASHRAE zone correction for steel, so your numbers stay honest. If you are still laying out the frame, the Stud Calculator and Framing Calculator will give you the stud count and spacing that feed the framing percentage here.
Results update as you type. You get a total R-value, a U-value, a performance rating, a bar chart by layer, a cross-section view, and full step-by-step math. Switch between imperial and metric units at any time. When you are done, print the summary or save it as a PDF for your job file.
How to use our R-Value Calculator
Pick your building part, choose an assembly, then set up each material layer. The calculator gives you the total R-value, the U-value, a performance rating, and a full layer-by-layer breakdown you can print or save.
Building Element: Click Walls, Roofs, or Floors. This changes the assembly list, the layers, and the insulation products you see.
Construction Assembly: Pick the build type that matches your job, like a timber frame wall or a steel truss roof. The layer stack fills in for you. For roof jobs, the Roof Pitch Calculator and Roof Area Calculator pair well with this tool.
Unit System: Choose Imperial (inches) or Metric (metres). All values switch right away. For quick length swaps outside the tool, try the Mm to Inches Calculator or the Meters to Feet Calculator.
Assembly Name: Type a name for this build, like "Exterior Wall — Type A". It shows on the summary table and the PDF.
Material Type: For each layer, pick the material from the list. Layer 1 is the outside face and the last layer is the inside face.
Thickness: Enter how thick that layer is. Leave it at zero for air films.
Conductivity (k): Enter how well the material carries heat. The tool divides thickness by k to get the layer R-value. Default values are already filled in.
Or enter R-value directly: Type a tested R-value from the product label. This replaces the thickness ÷ k math for that layer. Clear the box to go back.
Framing members checkbox: Tick this if studs, joists, or rafters run through the layer. The tool then uses the isothermal planes method to count thermal bridging. The Floor Joist Calculator and Rafter Calculator help you confirm member sizes and spacing.
Framing Material: Choose Timber or Steel. Steel loses more heat, so the result drops more.
Framing Area Percentage: Enter how much of the wall or ceiling area is framing. Use about 15% for timber studs and 8–12% for steel studs or ceiling joists.
Add Layer: Click this to add a new layer at the bottom. Use the arrow buttons to move a layer up or down, or the trash button to delete it.
Insulation Products: Select a layer first, then click a real product to drop its R-value into that layer. Click "Show All Products" to see every brand.
Calculate: Results update as you type, but you can click Calculate any time to refresh them.
What Is R-Value?
R-value tells you how well a building material stops heat from moving through it. A higher R-value means better insulation. Walls, roofs, and floors with high R-values keep heat inside during winter and outside during summer. That means lower power bills and a home that feels more comfortable all year. To turn better insulation into dollars, run the numbers through the Heat Loss Calculator and the Electricity Cost Calculator.
How R-Value Is Found
For most materials, R-value comes from two things: how thick the material is and how fast it moves heat. That second part is called thermal conductivity, or k. The math is simple:
R = thickness ÷ k
Thick materials with a low k value give the best R-value. Foam boards and batts have a very low k, so a thin layer can beat a thick layer of brick or concrete.
Adding Layers Together
A real wall is not one material. It may have siding, a wrap, insulation, drywall, and thin films of still air on each face. Heat has to pass through all of them, one after the other. So you add each layer's R-value together to get the total for the whole assembly. Even air gaps and air films add a small amount, so they count too. When it is time to order those layers, the Drywall Calculator, Siding Calculator, and Brick Calculator handle the quantities.
R-Value vs. U-Value
U-value is the flip side of R-value. It shows how much heat passes through, instead of how much is blocked. You find it with U = 1 ÷ R. With U-value, lower is better. Many building codes list one or the other, so it helps to know both. Our dedicated U Value Calculator works the problem from the U side if that is what your code table asks for.
Thermal Bridging and Framing
Studs and joists break up the insulation. Wood and steel move heat faster than batts do, so heat takes a shortcut through the frame. This is called a thermal bridge, and it drops the real R-value of the wall.
To handle this, builders use the isothermal planes method. It works out heat flow through the framing and the insulation side by side, then blends them based on how much of the wall is framing. Wood studs at 16 inches on center usually cover about 15% of a wall. Ceiling joists and steel studs often cover 8% to 12%. If you need the stud count for a given wall length, the Stud Calculator gets you there fast.
Steel is a much bigger problem than wood. Steel moves heat hundreds of times faster, so a steel stud wall can lose over half of its rated insulation value. Because of this, steel framing uses an ASHRAE correction factor instead of the plain wood formula. Adding a solid layer of foam board over the outside of the frame, called continuous insulation, is the best fix for both wood and steel.
Imperial and Metric R-Values
The two unit systems use very different numbers for the same product. An R-13 batt in the United States is about R-2.3 in metric. Always check which system a number uses before you compare products or check code. A general Metric Conversion Calculator is handy when specs arrive in mixed units.
- Imperial R: ft²·°F·hr/BTU
- Metric R (RSI): m²·K/W
- To convert: Imperial R ÷ 5.678 = Metric R
Typical R-Values by Material
Some rough numbers per inch of thickness, in imperial units:
- Fiberglass batt: about R-3.2 per inch
- Mineral wool batt: about R-3.4 per inch
- Cellulose loose fill: about R-3.5 per inch
- EPS foam board: about R-3.9 per inch
- XPS foam board: about R-4.8 per inch
- Polyiso board: about R-6.0 per inch
- Closed-cell spray foam: about R-5.9 per inch
- Softwood framing: about R-1.2 per inch
- Concrete: about R-0.08 per inch
Things That Change Real Performance
The R-value on a label is tested in a lab. In a real house, a few things can lower it:
- Squeezing insulation. Stuffing a thick batt into a thin cavity cuts its R-value.
- Gaps and holes. Missed spots around wires, pipes, and outlets let heat slip past.
- Wet insulation. Water moves heat well, so damp insulation performs poorly and may grow mold.
- Air leaks. Insulation slows heat, but it does not stop moving air. You still need air sealing.
- Cold weather drift. Polyiso loses some R-value at very low temperatures.
What Counts as Good
Target R-values change by climate and by local code. Cold areas need much more than mild ones. As a general guide, exterior walls often aim for R-13 to R-21, attics for R-30 to R-60, and floors over crawl spaces for R-19 to R-30 in imperial units. Always check your local building code before you buy materials. Once the envelope is set, size the mechanical side with the BTU Calculator, the AC Tonnage Calculator, or the Mini Split Sizing Calculator, since a tighter, better-insulated shell usually needs smaller equipment.