Question 1

What is the overall heat transfer coefficient (U-value)?

Accepted Answer

The overall heat transfer coefficient, usually called the U-value or thermal transmittance, measures how much heat passes through a building element or heat exchanger per square metre for each degree of temperature difference. It is expressed in W/m2K (SI) or BTU/(hr.ft2.F) (imperial). A lower U-value means less heat escapes, i.e. better insulation; a higher value means more heat transfer.

Question 2

How do you calculate the U-value of a multi-layer wall?

Accepted Answer

Add the thermal resistances in series: R_total = 1/h_in + sum(L_i/k_i) + 1/h_out, where L_i is the thickness and k_i the thermal conductivity of each layer, and h_in/h_out are the interior and exterior surface film coefficients. Then U = 1/R_total. This calculator does it for you: pick materials, enter thicknesses, and it sums every layer resistance plus the standard surface resistances.

Question 3

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

Accepted Answer

R-value (thermal resistance, m2K/W) measures how strongly a material or assembly resists heat flow, so higher is better. U-value (thermal transmittance, W/m2K) measures how easily heat passes, so lower is better. They are reciprocals: U = 1/R. R-values of layers add up in series, so insulation is usually quoted in R, while whole walls, windows and roofs are quoted in U.

Question 4

How do I convert W/m2K to BTU/(hr.ft2.F)?

Accepted Answer

Divide by 5.678263. For example, a wall with U = 2.20 W/m2K equals 2.20 / 5.678263 = about 0.387 BTU/(hr.ft2.F). Conversely, multiply the imperial value by 5.678263 to get W/m2K.

Question 5

How do I convert BTU/(hr.ft2.F) to W/m2K?

Accepted Answer

Multiply by 5.678263. For example, 1 BTU/(hr.ft2.F) = 5.678263 W/m2K. The conversion factor comes from the definitions of the BTU, the foot and the Fahrenheit degree: 1 BTU/(hr.ft2.F) = 1055.06 J / (3600 s x 0.092903 m2 x 5/9 K) = 5.678263 W/m2K.

Question 6

What are standard interior and exterior surface film coefficients?

Accepted Answer

For walls with horizontal heat flow, EN ISO 6946 gives surface resistances R_si = 0.13 m2K/W inside (h about 7.7 W/m2K) and R_se = 0.04 m2K/W outside (h about 25 W/m2K). Common simplified design values are h_in = 8 and h_out = 23 W/m2K (DIN 4108). This calculator defaults to 8/23 and lets you change both.

Question 7

What is thermal conductivity (k-value)?

Accepted Answer

Thermal conductivity (lambda or k, in W/mK) is a material property telling how easily heat conducts through it: copper (about 385) and steel (about 50) conduct fast, while insulation like glass wool (about 0.040) and polyurethane foam (about 0.023) conducts very slowly. A layer resistance is its thickness divided by k (R = L/k), so thicker layers and lower k give higher resistance.

Question 8

How do you calculate heat loss through a wall?

Accepted Answer

Q = U x A x dT, where U is the overall heat transfer coefficient (W/m2K), A the wall area (m2) and dT the inside-outside temperature difference in kelvin (same as degrees C). Example: U = 2.2, A = 20 m2, dT = 20 K gives Q = 880 W. Use Mode 3 above and it converts areas, units and temperature scales for you.

Question 9

What is a good U-value for walls?

Accepted Answer

Passive-house walls target below 0.15 W/m2K. Modern well-insulated walls reach 0.15 to 0.30, typical new construction 0.30 to 0.45, older buildings 0.80 to 2.00. A bare 200 mm clay-brick wall is about 2.2 W/m2K, which is poor. Most building codes now set maximum U-values, so check your local requirement.

Question 10

How do I convert R-value from SI to imperial?

Accepted Answer

Multiply the SI R-value (m2K/W) by 5.678263 to get the imperial R-value (hr.ft2.F/BTU). For example, R = 0.454 m2K/W becomes about 2.58 hr.ft2.F/BTU. It is the same factor as the U conversion, applied the other way around.

Question 11

What is the formula for the overall heat transfer coefficient?

Accepted Answer

1/U = 1/h_in + sum(L_i/k_i) + 1/h_out. The left side is the total resistance; on the right, 1/h_in and 1/h_out are the interior/exterior surface (film) resistances and L_i/k_i is the resistance of each solid layer. Invert the total resistance to get U. For a single surface, the local heat transfer coefficient is simply h = 1/R_surface.

Question 12

What U-value does a passive house need?

Accepted Answer

Passive House Institute criteria require opaque envelope components (walls, roof, floor) to achieve U of at most 0.15 W/m2K (about 0.026 BTU/(hr.ft2.F)). Windows must reach U of at most 0.80 W/m2K including the frame. Reaching these values typically needs 200 to 300 mm of mineral wool or equivalent.

Question 13

Is a lower U-value always better?

Accepted Answer

For insulation, yes: a lower U-value means less heat escapes, cutting heating and cooling bills. But material choice also weighs cost, thickness, moisture behaviour, fire rating and embodied carbon. For heat exchangers the opposite holds: a higher overall heat transfer coefficient means more efficient heat transfer.

Question 14

What is the U-value of a 200 mm brick wall?

Accepted Answer

With standard surface coefficients (h_in = 8, h_out = 23 W/m2K): R = 1/8 + 0.2/0.7 + 1/23 = 0.125 + 0.2857 + 0.0435 = 0.4542 m2K/W, so U = 1/0.4542 = about 2.20 W/m2K (about 0.388 BTU/(hr.ft2.F)). Adding 50 mm of glass wool (k = 0.040) drops it to about 0.62 W/m2K; try the preset above.

Convert	Multiply by	Example
BTU/(hr·ft²·°F) → W/m²·K	5.678263	1 → 5.678 W/m²·K
W/m²·K → BTU/(hr·ft²·°F)	0.176110	2.20 → 0.387
kcal/(h·m²·°C) → W/m²·K	1.162992	1 → 1.163 W/m²·K
m²·K/W → hr·ft²·°F/BTU (R-value)	5.678263	0.454 → 2.58
W/(m²·°F) → W/m²·K	1.8	1 → 1.8
cal/(s·cm²·°C) → W/m²·K	41840	0.001 → 41.84

Construction	U (W/m²·K)	Rating
Passive-house wall	< 0.15	Excellent
Modern well-insulated wall	0.15 – 0.30	Excellent
Typical new construction	0.30 – 0.45	Good
Older building, thin insulation	0.80 – 2.00	Poor
Bare 200 mm brick wall	≈ 2.20	Very poor
Single glazing window	≈ 5.8	Very poor

Heat Transfer Coefficient Calculator

Thermal conductivity table (k-values)

U-value conversion reference

How the multi-layer U-value calculation works (ISO 6946)

Typical U-values (W/m²·K)

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