Building Physics Fun Fact

I sat at an aluminium desk yesterday. Well, not for long... brrr 🥶

You probably have noticed how some materials (like wood) feel warmer to the skin than others and I guess I don't have to tell you that you shouldn’t stick your tongue against a lantern in winter...

Yesterday's unpleasant experience brought back a lesson I learned from my building physics classes that explains why different materials can feel warmer or colder to the touch, despite being at the same room temperature.

🔍You probably already guessed that the sensation of warmth or coldness is related to specific physical properties of the materials. Those properties determine what temperature exists at the moment of contact between two materials with two different temperatures.

🖩 We can even approximate that temperature with a simple formula.

First, we need the contact coefficient (𝑏) of the materials in question: It can be derived with the formula 𝑏=sqrt(𝜆𝜌𝑐). This coefficient helps us understand how effectively a material can transfer heat upon contact, taking into account its ability to conduct heat (𝜆), its density (𝜌), and its specific heat capacity (𝑐).

To then approximate the contact temperature between two materials the following formula is used: 𝑇contact=(𝑏1𝑇1+𝑏2𝑇2)/(𝑏1+𝑏2), where 𝑏1​ and 𝑏2 are the contact coefficients of the two materials, and 𝑇1​ and 𝑇2​ their respective temperatures.

This formula basically gives us a weighted average of the temperatures of the two materials, where the weights are their respective abilities to transfer heat.

If we now fill in the values for aluminium, wood and ...yes, indeed: 'skin', we get the following:
- 𝑏 aluminium: sqrt(200*2755*880) = 22020 [kg/Ks^2.5]
- 𝑏 wood: sqrt(0.14*800*1880) = 460 [kg/Ks^2.5]
- 𝑏 skin: sqrt(0.5*1000*4200) = 1450 [kg/Ks^2.5]

Let’s assume a temperature of 17°C for the table I leaned on yesterday and 30°C as skin temperature. We then get a contact temperature between aluminium and skin of 19.7°C and for comparison between wood and skin of 27.4°C, which explains the difference in sensation.

And yes, that same principle explains the shocking chill when your tongue touches cold metal in winter - which can literally freeze your tongue to the surface!

Do you want to explore more on building physics to understand how these everyday experiences connect to sustainable building design?

What other phenomena have you noticed in buildings that you would like to understand?