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 a separate, larger correction for steel framing.
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.
Unit System: Choose Imperial (inches) or Metric (metres). All values switch right away.
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.
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.2 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.
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.1 The math is simple:
R = thickness ÷ k
Thick materials with a low k value give the best R-value.1 Mineral wool conducts heat at about 0.036 W/(m·K), while concrete conducts about 0.9, so a thin layer of insulation can beat a thick layer of concrete.1
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.2 Even air gaps and air films add a small amount, so they count too.
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.3 With U-value, lower is better.3 Many building codes list one or the other, so it helps to know both, and a 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.2
To handle this, the calculator uses the isothermal planes method. It combines the framing and the insulation in a bridged layer side by side, weighted by how much of the layer each covers, then adds that layer in series with the others.7 Ireland's Part L guidance takes timber as 15% of a timber-frame wall, 8% to 9% of a ceiling and 11% of a suspended timber floor.7 Steel studs often cover 8% to 12%.
Steel is a much bigger problem than wood. Steel conducts heat at about 45 W/(m·K), against 0.10 to 0.14 for structural softwood lumber.1 U-value calculations use 50 W/(m·K) for mild and galvanized steel, the figure the tool uses for steel layers.8 An insulated steel-stud wall's overall R-value can be as low as half the insulation's R-value.2 Because of this, the tool applies a separate correction factor to steel-framed layers instead of the wood formula. A solid layer of insulating sheathing over the outside of the frame, called continuous insulation, is often recommended for walls for this reason.2
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.
- 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.2
- Mineral wool batt: about R-3.7 per inch.4
- Cellulose loose fill: about R-2.9 to R-3.4 per inch.4
- EPS foam board: about R-3.8 to R-4.4 per inch.5
- XPS foam board: about R-5 per inch.4
- Polyiso board: about R-5.6 to R-8 per inch.4
- Closed-cell spray foam: about R-6.5 per inch.5
- Softwood framing: about R-1.2 per inch.
- Dense 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.2
- Gaps and holes. Gaps around the edges of batts let air leak past, which significantly reduces R-value.4
- Wet insulation. Some insulation, such as cellulose, soaks up water, which reduces its thermal resistance and creates a risk of mold.4
- Air leaks. Most insulation does not stop airflow, so you still need air sealing.4
- 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, the 2021 IECC sets minimum ceiling R-values of R-30 to R-60, wood-frame wall cavity values of R-13 to R-30, and floor values of R-13 to R-38, depending on the climate zone.6 Always check your local building code before you buy materials.