What is a good R-value for a wall?

It depends heavily on climate and local building code, but a standard 2x4 stud wall filled with fiberglass batt is typically R-13 to R-15, and a 2x6 wall R-19 to R-21; colder climates often add exterior rigid foam on top to reach a higher effective whole-wall R-value. Always check your local building code's minimum before finalizing a design.

What is the difference between R-value and U-value?

R-value measures thermal resistance (higher is better) and is the everyday unit in the US and Canada; U-value measures thermal transmittance in W/m²K (lower is better) and is the unit most building codes outside North America actually specify. They are reciprocals of each other once units are aligned: U-value = 1 / RSI, where RSI is the metric equivalent of R-value.

Does adding more insulation always make a wall warmer?

Up to a point, yes, but with diminishing returns — each additional inch of the same material adds less benefit than the one before it, and once a wall is well air-sealed and meets local code, extra thickness usually saves less energy per dollar than fixing draughts, gaps and thermal bridges elsewhere in the same wall.

Is wall insulation common in Maltese homes?

Traditional Maltese construction relies heavily on thick limestone (globigerina) walls for thermal mass rather than added insulation layers, but modern builds increasingly add rigid foam or mineral wool to meet current energy performance targets, especially on roofs, where heat gain in summer is the bigger concern alongside heat loss in winter.

Insulation Calculator — R-Value & U-Value

Insulation Calculator: How Much R-Value Does Your Wall, Roof or Floor Need?

Before you buy insulation for a wall, roof or floor, you need one number: how much total thermal resistance does the finished build-up actually give you? Every layer in an assembly contributes — the insulation itself, but also the drywall, the sheathing, even a plain air gap — and it's the sum across all of them that determines how well the wall really performs, not just the headline rating printed on the insulation packaging. Get it wrong and you either under-insulate a wall that never quite feels warm, or over-spend on thickness a cavity simply can't fit.

This calculator adds up R-value layer by layer for any wall, roof or floor build-up: pick a material for each layer (fiberglass batt, mineral wool, rigid foam, spray foam, cellulose, or a plain material like drywall or an air gap), enter its thickness, and add as many layers as your real assembly has. It totals the R-value, converts it to the metric RSI and U-value equivalent, and gives a material cost estimate — so you can compare options before you commit to buying.

How to Use the Insulation Calculator

Add a layer for every material in your wall, roof or floor build-up — not just the insulation itself, but drywall, sheathing and air gaps too, since every layer adds some resistance.
For each layer, pick a material preset (fiberglass batt, mineral wool, EPS/XPS/polyiso rigid foam, open- or closed-cell spray foam, cellulose, drywall, air space, and more) or enter a custom R-value per inch if you have a manufacturer figure.
Enter that layer's thickness and choose its unit (inches, centimetres or millimetres) — the calculator converts everything to a common basis automatically.
Repeat for every layer in the assembly, in any order, then read off the total R-value, the metric RSI (m²K/W) and U-value (W/m²K) equivalent, and the estimated material cost.

The Math Behind the Calculation

Layers of insulation in series simply add: Total R-value = (R-value per inch of layer 1 x thickness of layer 1 in inches) + (R-value per inch of layer 2 x thickness of layer 2 in inches) + ... for every layer. R-value per inch is a material property (a denser, better-performing material has a higher figure), and because it is expressed per inch, any layer entered in centimetres or millimetres is first converted to inches internally so every layer is combined on the same basis.

Once you have a total R-value, it converts directly to the metric RSI (thermal resistance in m²K/W) by dividing by 5.6785917, and to a U-value (the figure most non-US building codes actually quote) by simply inverting the RSI: U-value = 1 / RSI. A higher R-value or RSI means a better insulator; a lower U-value means the same thing, just expressed the opposite way around — the two are simply reciprocals of each other.

Worked example: a stud wall with 3.5 in of fiberglass batt (R-3.4 per inch) plus 0.5 in of drywall (R-0.9 per inch). Layer 1: 3.4 x 3.5 = 11.9. Layer 2: 0.9 x 0.5 = 0.45. Total R-value = 11.9 + 0.45 = 12.35. Converting to metric: RSI = 12.35 / 5.6785917 = 2.175 m²K/W, and U-value = 1 / 2.175 = 0.46 W/m²K. That total — not the 11.9 from the insulation batt alone — is the true performance of the finished wall.

Pro Tips and Common Mistakes

At around €1.30 per board-foot-equivalent inch of material, the worked example above (3.5 in fiberglass batt plus 0.5 in drywall, 4 in of combined thickness) costs roughly €5.20 in material per square foot (about 0.093 m²) of wall or roof covered — multiply by your total area for a full project estimate.

R-value and U-value describe the same wall from opposite directions: a higher R-value (or RSI) is better, while a lower U-value is better — never compare an R-value figure directly against a U-value figure without converting one to the other first.
Common material picks by use: fiberglass batt for standard stud-cavity walls; mineral (rock) wool where fire resistance or soundproofing also matters; rigid foam (EPS, XPS or polyiso) for exterior sheathing, basements and below slabs, where its higher R-value per inch matters most in a thin space; spray foam for irregular cavities and where air-sealing is as important as R-value; cellulose for blown-in attic or wall-cavity retrofits.
A batt that doesn't fully fill its cavity, or is compressed to fit, loses R-value — manufacturers rate their product at its full, uncompressed thickness, so a squeezed-in batt performs worse than its label suggests.
Vapor barriers matter alongside R-value, not instead of it: in a heating-dominated cold climate, a vapor barrier belongs on the warm (interior) side of the insulation to stop indoor moisture condensing inside the wall; in hot, humid climates the logic often reverses. Check local building guidance rather than assuming one rule fits everywhere.
Air sealing (gaps, penetrations, poorly fitted batts) frequently undermines a wall's real-world performance more than a modest difference in insulation thickness — a well-sealed, correctly-fitted lower-R assembly can outperform a higher-R assembly full of gaps.