“Concrete carries λ (lambda)

Thermal transmittance

“Concrete carries λ (lambda)

And a pretty bad one indeed (about 1.63 W/m °C), but concrete isn't the only one; all materials have it and for architecture and comfort it is a very important value”.

If you understood this paragraph you can go back and change article, but if you have even a little doubt about what I am talking about; This is a must read.

I could start by explaining to you what the value of λ is, how this value helps us to know the TR (thermal resistance) of an enclosure and, in turn, its inverse: the U (thermal transmittance) of the same. But since I like things to be more dynamic, we are going to do a little exercise, so maybe you can believe me when I tell you that you can never stop knowing the λ of a material.

Let us begin! Let's say we have a studio; is a very simple, windowless little hex space (this is just to simplify the computation). Our studio has an envelope of 48 m2. As we like brutalism very much, we decided to build it, entirely, in reinforced concrete and to "protect" ourselves from the outside weather, we made sure that the walls were thick and dense; 40 cm thick to be exact. Suppose that when the fireplace our studio is at 22 ° C, at that very moment, outside, the thermometer reads 8 ° C. We can agree that our temperature difference is 14 ° C. As time goes by, we begin to get very cold , nothing remains of the heat from the chimney. Remember these values ​​that we will use later.

I think that, at this point, the problem is quite obvious, but I want to explain to you why this happens and, above all, how you can avoid it.

First of all, put your hand on the closest wooden surface you have, feel it and guess the temperature you think this material is at, then move it to a nearby metal surface and do the same, what material is colder? ? It is very simple, in fact, quite obvious.

But no, it's not the metal; Considering that the two materials are in the same environment, their temperature is more or less the same, so why do I feel one colder than the other? the main difference is in its thermal conductivity (λ), that is, in its ability to conduct heat; if a material has a higher thermal conductivity, it will let more heat through. In other words, metal is stealing heat from us much faster than wood.

So when we read that reinforced concrete has a λ=1.63 W/m °C, what it is telling us is that this material conducts 1.63 W (watts) per meter in relation to each °C.

Now that we know what this value means, we can proceed with the calculations. The next thing we have to do is divide our thickness, that is 0.4 m (note, it is important that the units are equal, so our thickness must be in meters) by the λ, this will give us R1 (resistance 1).

R1, R2, R3... = e/λ

By doing this division we would know that our R1 is 0.24 m2 °C/W. To get the TR we must add the R of each material that constitutes our envelope (R1, R2, R3...), and to this add Ris (interior surface thermal resistance, according to the climatic zone standard) and Res (exterior surface thermal resistance, also , according to the standard).

TR= Ris + R1 + R2 + … + Res

As we only have one layer, to know the TR we would add:

-R1: 0.24 m2 °C/W

-Ris: 0.11 m2 C/W (NRE-AT-8 standard)

-Res: 0.06 m2 C/W (NRE-AT-8 standard).

With which we would have a RT of 0.41 m2 °C/W. To know the U (thermal transmittance), we only have to do the inverse of TR, that would be;

U= 1/TR

Finally we could know that the U of a 40 cm thick reinforced concrete wall is 2.43 W/m2 °C.

Well, and this is... a lot or is it a little? Well, basically it's a lot, but I'm going to show you why, everything is in the units of the U: W, m2, °C, look how simple;

-W: 2.43

-m2: 48

-°C (thermal jump): 14

If we multiply the value of these three units, our result would be 1,633 W, which, when multiplied by 12 hours at night, would allow us to know that 19,596 Wh (watts/hour) were lost in our study. Let me repeat, we lost almost 20,000 Wh in a single night (this is a lot of energy). This considering that we do not have windows (I anticipate that the average U of the windows ranges from 3 W/m2 °C in double-glazed windows, to 5 W/m2 °C in simple windows).

Just to make a comparison, I am going to do (in a simplified way) the same process for the same envelope, but adding a 5 cm thermal insulator with a λ of 0.04 W/m °C.

Returning to the R, if we add to the entire previous process an R2= 0.91 of the new material (thermal insulation), it would give us an RT of 1.32 m2 °C/W and a U of 0.75 W/m2 °C. Multiplying this new value of U by the same m2 (48) and the same thermal difference (14), our loss would be 504 W and 6,048 Wh for the whole night. So simple and so fast. We reduced our losses by almost 70%.

This is not a paid commercial for insulation providers (although they should pay me) or a simple answer to heat loss in cold climates; in fact, all these calculations must be done many times to determine the correct layers for each of the enclosures and never failing to consider their embodied energy, in addition to the fact that the thermal behavior is influenced by much more than its envelope (ventilation, gains solar, orientation, internal gains and many others).

What is a reflection, is the importance of understanding how the materials we choose behave and the repercussions on comfort that they have in a space. Vernacular architecture made magnificent use of local materials to respond to a climate and enhance an interior feel; Along the way we have lost these foundations and we have become victims of an energy system that led us to transform materials into structural or aesthetic elements, leaving aside one of their great functions: to protect us from the outside temperature, but at what cost?

According to the International Energy Agency, in 2018, the demand for air conditioning required 10% of all electricity in the world, while the demand for heating required 23% of the world's final energy consumption, resulting in 18.4% of all the CO2 emitted in that year.

These numbers, in addition to being worrying, highlight what we have lost as architects: the ability to passively guarantee comfort, the ability to take what we have and turn it into something better.

Important note for the reader: it is important to mention that these calculations are made in a balanced state, that is, considering that the interior and exterior conditions were always the same; there are calculations to determine the variability and others of balance that consider more factors than the thermal conductivity, but we will leave those for another article ;)