Insulation R-values explained: R, RSI, U-values and thickness

An R-value measures how well a layer of insulation resists heat flowing through it: the higher the number, the slower your home loses heat in winter and gains it in summer. The catch is that "R-value" means different things in different countries. The US uses imperial R-values, most other countries use metric RSI, and building rules often quote the inverse, the U-value. The insulation calculator converts between them and works out the thickness a target needs, and this guide explains the numbers.

What an R-value measures

Heat moves from warm to cold, and insulation slows the flow by trapping still air or gas in fibres, foam or loose fill. Thermal resistance depends on two things: how thick the layer is and how readily the material conducts heat, its thermal conductivity λ (lambda). The relationship is simple: R = thickness ÷ λ. Double the thickness and you double the R-value; choose a material with half the conductivity and you get the same R-value from half the depth. That is why manufacturers quote λ on the pack, and why thin foam boards can match much thicker rolls of glass wool.

R, RSI and U-values

The US R-value is in ft²·°F·h/BTU. The metric equivalent, often written RSI, is in m²·K/W. The conversion is fixed:

• 1 RSI = 5.678 US R, so RSI 2.5 is about R-14.
• 1 US R = 0.1761 RSI, so R-13 is about RSI 2.29 and R-49 is about RSI 8.63.
• U-value = 1 ÷ RSI, in W/m²·K. It measures how much heat gets through, so lower is better.

Mixing the systems up is the classic mistake: an "R-5" product in Australia or New Zealand is RSI 5, which is about R-28 in US terms. When comparing products from different countries, convert first.

Thickness and materials

Using typical conductivities, here is roughly what one inch (25 mm) of common materials provides in US R-values:

• Glass wool (fibreglass) batts: about R-3.3
• Loose-fill cellulose: about R-3.6
• Mineral (rock) wool: about R-3.9
• EPS board: about R-4.1; XPS board: about R-4.4
• Closed-cell spray foam: about R-6.0
• PIR or polyiso board: about R-6.6

So reaching R-49 in an attic takes about 15 inches (380 mm) of glass wool but under 8 inches of PIR board. In metric terms, 270 mm of mineral wool, a common UK loft depth, gives RSI 7.3, about R-41, and a U-value of about 0.14. These are typical figures; the λ or R-value on your product's label is what counts.

How much you need

There is no single right number. Recommended levels depend on climate, on which part of the building you are insulating, and on local building regulations, which set minimums for new work. As one example, ENERGY STAR's guidance for adding insulation to an uninsulated US attic ranges from R-30 in the warmest climate zone to R-49 in zones 2 and 3 and R-60 in zones 4 to 8. Walls, floors and basements have lower targets because space is limited. Outside the US, check your national guidance and building code, which usually express targets as U-values. Lofts also need ventilation to stay dry; the attic ventilation calculator works out the vent area.

Layers and real-world losses

R-values of layers in series add up: 100 mm of old glass wool (RSI 2.27) topped with 170 mm of mineral wool (RSI 4.59) gives RSI 6.87, about R-39. U-values do not add; convert to R, add, then invert. Real performance is usually lower than the label because compressed batts, gaps and joists that bridge the insulation let heat bypass it, so lay rolls loosely and cross the second layer over the joists.

Returns also diminish. Heat loss is the U-value times the area times the temperature difference, so a 100 m² loft with a 15 °C difference loses about 660 W through 100 mm of glass wool but about 206 W with 270 mm of mineral wool. Doubling to 540 mm would save only another 100 W or so. Over a day, the first upgrade saves around 11 kWh at that temperature difference; What is a kilowatt-hour, and how do you work out what an appliance costs? explains what that means on a bill. For summer cooling, the BTU calculator sizes an air conditioner for a room, and the area calculator measures awkward spaces.