Assembly U-Factor Calculator

This calculator works out the U-factor of an opaque wall, roof or floor from its layers: up to ten materials from outside to inside, with the air films on each face, enclosed air spaces, and framing that bridges the insulation. It uses the material data and methods of the 2025 ASHRAE Handbook—Fundamentals, and where the methods disagree it reports the more conservative answer.

How to use it

Review the method below to make sure it suits your project, then:

1. Optionally enter the project and assembly names, who prepared it and the date. They head the results, so a printout identifies itself.
2. Choose the assembly type (wall, roof or floor), whether the exterior side faces outdoors or an unconditioned space, and, if the assembly has an air space, the conditions for it.
3. Enter the layers from the outside in. Pick each material from the list and enter its thickness; products with a fixed thickness, such as 1/2 in. plywood or 16 ga steel, fill it in themselves. For an air space, also choose the two surfaces facing it. For a product with a tested R-value, choose "Other material" (R per inch) or "Other product" (total R) and enter it. Do not enter the air films: they are added for you.
4. If studs, joists, channels, trusses or tube run through some of the layers, choose those layers under Framing, then describe up to three kinds of member and how much of them there is: by spacing, by total length (with the assembly area entered), or as a share of the area.
5. The U-factor, R-value and the working appear as you type. Print gives a clean copy of the inputs and results; Copy link keeps them for later.

Method

Layers in series. Without framing, the resistances add: R = R_{outside film} + R₁ + R₂ + … + R_{inside film}, and U = 1 ÷ R. A material's R is its thickness divided by its conductivity, R = t ÷ k, or for a product listed at one thickness, its listed R.

Materials come from Table 1 of chapter 26. Where the table gives a range of conductivity for a material, the calculator takes the high end, which the chapter itself describes as the conservative choice. The list shows each material's R per inch, or its R, beside its name. Steel is carbon steel at 26.2 Btu/h·ft·°F (314.4 Btu·in/h·ft²·°F), from chapter 33; sheet gauges are the manufacturers' standard thicknesses. These are generic values: where you have the manufacturer's aged or tested value, enter it as "Other material" or "Other product".

Air films come from Table 10, for nonreflective surfaces: outside, R-0.17 (15 mph winter wind); inside, R-0.68 on a wall, R-0.61 under a flat roof or ceiling (heat flowing up), R-0.62 under a 45° roof, and R-0.92 on a floor (heat flowing down). A surface facing an unconditioned space takes the same still-air film as the inside surface.

Air spaces come from Table 3, which sets an enclosed air space's R by its position, the direction of heat flow, its mean temperature and temperature difference, its thickness, and the effective emittance of the two surfaces facing it: 1 ÷ ε_eff = 1 ÷ ε₁ + 1 ÷ ε₂ − 1, with the emissivities of Table 2. The calculator interpolates between the table's thicknesses (0.5 to 5.5 in.) and emittances, as its footnote allows. Beyond 5.5 in. it holds the 5.5 in. value, and below an effective emittance of 0.03 the 0.03 value; both are lower than an extrapolation would give. An air space under 0.5 in. is outside the table and is refused. Polished or foil faces only keep their low emittance if they stay clean and dry, so choose them with care.

Framing. Studs and other members that run through some of the layers carry heat around the insulation. The calculator divides the framed layers into paths, each with its share of the area: between the framing, through each kind of solid member (its depth divided by its conductivity), and for a hollow tube, through its two walls and across its hollow, which is an air space between the tube's own steel faces (or the fill you choose). It then works the assembly two ways, both from chapter 27:

• Parallel path: each path runs from surface to surface on its own, and the U-factors are averaged by area: U = Σ (share ÷ R_path). This assumes no heat moves sideways between paths.
• Isothermal planes: the framed layers are averaged first, as one layer whose conductance is Σ (share ÷ R_{through the framed layers}), and that layer is added in series with the rest. This assumes the faces either side of the framing spread heat freely across it.

The real U-factor lies between the two. The calculator reports the higher U-factor of the two, which is always the isothermal-planes result, and shows the parallel-path result beside it. For wood framing the two are close. For steel framing they can be far apart, because steel conducts about 300 times better than wood: parallel path then understates the heat loss, and isothermal planes can overstate it, most of all where the framing is widely spaced and the faces are thin. Where they differ by more than about half, the results say so; a two-dimensional model or a tested value for the assembly would narrow the answer.

Results. U in Btu/h·ft²·°F, R = 1 ÷ U, the U-factor in W/m²·K (× 5.678263), and, with the area entered, UA = U × area in Btu/h·°F.

Example: a wood stud wall

Hollow-backed vinyl siding, 7/16 in. OSB, 5.5 in. glass-fiber batts (0.61–0.75 lb/ft³, k = 0.30) between 2x6 spruce-pine-fir studs at 16 in. on center, and 1/2 in. gypsum board.

Between the studs, R = 0.17 + 0.62 + 0.62 + 5.5 ÷ 0.30 + 0.5 ÷ 1.1 + 0.68 = 0.17 + 0.62 + 0.62 + 18.33 + 0.45 + 0.68 = 20.88. Through a stud, the batt's 18.33 is replaced by 5.5 ÷ 0.90 = 6.11, so R = 8.66. The studs take 1.5 ÷ 16 = 9.375 % of the wall.

• Parallel path: U = 0.90625 ÷ 20.88 + 0.09375 ÷ 8.66 = 0.0542.
• Isothermal planes: the stud layer's conductance is 0.90625 ÷ 18.33 + 0.09375 ÷ 6.11 = 0.0648, so its R is 15.44, and the wall's R is 0.17 + 0.62 + 0.62 + 15.44 + 0.45 + 0.68 = 17.98: U = 0.0556.

The reported U-factor is the higher, U-0.0556 (R-18.0). Show an example, above, fills in this wall.

Example: a steel roof with steel trusses

A 12 ga steel deck, 7 in. of closed-cell spray foam (k = 0.20, R-35), a 3.5 in. air space below it, and a 16 ga steel ceiling, on trusses with 12 ga (0.1046 in.) webs at 24 in. on center running through the foam and the air space.

The air space faces the foam (ε = 0.90) and galvanized steel (ε = 0.25), so ε_eff = 0.243, and Table 3 gives R-1.52 for a 3.5 in. horizontal space with heat flowing up at 50 °F mean and 30 °F difference. Between the trusses, R = 0.17 + 35 + 1.52 + 0.61 = 37.30 (the steel adds 0.0005). The webs take 0.1046 ÷ 24 = 0.44 % of the roof, and through one, R = 0.17 + 10.5 ÷ 314.4 + 0.61 = 0.81.

• Parallel path: U = 0.9956 ÷ 37.30 + 0.0044 ÷ 0.81 = 0.0320.
• Isothermal planes: U = 0.140.

The reported U-factor is U-0.140. The webs are under half a percent of the roof, but across the framed layers they conduct about five times as much as all the insulation beside them, and the steel deck and ceiling spread heat to them. Parallel path, which ignores that spreading, puts the U-factor at under a quarter of the reported one.

Limits

The calculator is for opaque, above-grade assemblies in steady state. It does not cover windows and doors, slabs and walls in contact with the ground, or the heat lost at corners, edges and penetrations where assemblies meet. Its data are generic; a tested or manufacturer's value for the actual product, or a tested assembly, takes precedence.