Building with cob is a way to use soil as a natural building material. It is durable, ready at hand, and easy. People all over the world have built with cob for centuries.

But how to deal with humidity? Hot climates, as well as cold climates, can suffer from heavy rainfalls and humidity. For example, England has a cold humid climate, while Costa Rica has a hot humid climate. How can we protect our cob structures from becoming moldy?

In this article, we will break it down for you. First, we will deal with cob in a hot humid climate, and after that with cob in a cold humid climate.

Hot humid climate

Let us see what Christiano, owner of a permaculture farm El Zopilote has to say about building with cob in a hot humid climate.

In these two videos, Christiano explains it all to us.

Part 1: https://youtu.be/8FKX_QhlSrY

Part 2: https://youtu.be/ZcwYHSUifEQ

Hello Christiano, just when did you start building with cob?

‘About 15 years ago. A friend of ours introduced it to us. We tried it, and we never stopped. Cob is an excellent building material: it is pure soil, it is natural and it breathes. We are in Nicaragua, in the tropics, and we find that it works very well in this tropical climate.’

What kind of structures have you built with cob?

Nearly all structures here at El Zopilote are made of cob: the buildings for our dry toilets and showers. We have several tiny homes made out of cob, the pizza oven, several benches, the yoga decks, and my house.’

What are the challenges of working with cob in a hot humid climate?

‘The rain, and subsequently mold, could be a challenge if you do not deal with it aptly.

Stone foundation

‘It is important to use a stone structure as a foundation, of about 40 cm, or 16 inches, high. Just use rocks, for excess water to flow away. Stones do not retain water, whereas cob does pull the water up. So you need to elevate the cob structure with stones, for your cob walls to stay dry. This way, if the soil underneath the cob house becomes damp, the stones make sure that it will not be pulled upwards and affect the cob.’

Large roof overhang

‘The other main thing is making sure that you have a large overhang of the roof: two meters overhang is fine, I am not exaggerating. This way the walls of the structure are protected against rain. And, by having a large overhang, you will have more living space around the house. You will have a large verandah that you can use when it is raining. It works well in fine weather as well, because it provides shade. Open spaces work very well in the tropics, by definition.’

Sealing

What kind of finish do you use, to seal off the cob structure?

‘We use lime: this works for us. Every few years we paint the structures with lime, to add a new layer, and that prevents the walls from becoming damp. We have no mold issues.’

Insulation

Do you insulate the roof?

‘We use recycled plastic which we cover with plants that do not need soil to grow. The soil would make the roof too heavy. I love the green roofs, it looks so natural, and it works well.

We also have palapa roofs: we cover the roofs with palm leaves. We have a hundred and fifty royal palms, and still, we do not have enough leaves to cover all the roofs that we have on our property. That is why we also use recycled plastic with the plants on top.

The roofs that we have, keep our houses cool: we do not need air conditioning.’

Ventilation

‘We make sure that we have open windows, we let the air circulate throughout the cob buildings. This way our houses stay fresh and dry.’

Do you have some final advice to give to us?

‘Just remember the old saying and you will be fine: “Give a cob house a good pair of boots (foundation) and a good hat (roof), and she’ll last forever.”’

For more information about El Zolpilote, see: www.ometepezopilote.net

Building with cob in a cold humid climate

On the British coast, there are thousands of centuries-old cob buildings surviving in good repair. So building with cob in a cold humid climate can be done. But how to deal with humidity?

Just like building with cob in a hot humid climate, it is important to pay attention to foundation and drainage (build a high foundation of just rocks), large roof overhangs (this way the rain will not damage the walls), and lime plasters (for extra protection against rain).

Drying time

The main challenge with building with cob in a cold humid climate is the drying time of the cob. The cob walls consist of several layers and need to dry out to a certain extent, to bear the weight of the cob layer applied on top. If you have high air humidity during construction, it will slow down the drying time, and extend the time necessary to build the walls.

To tackle this problem, you can make the timber structures and the roof first. After this, you can continue with the cob. This way the cob walls that you are making are not exposed to rain. You can let them dry appropriately without them becoming wet.

Prevent flooding

Earthen walls can lose their strength if they are saturated with water for a prolonged time.
Choose a high site to build your house, out of all possibility of flooding.

Maintenance

If you maintain your cob structure well, then you will have far fewer issues with mold. Just repaint the house with lime every year, and make sure that you repair damaged spots of cob in the walls.

Final thoughts

Good news! We can build with cob in a humid climate, for sure, in hot humid climates as well as cold humid climates. We just have to pay attention to certain features, such as a high stone foundation, a large roof overhang, plenty of ventilation and lime sealing of the walls. Furthermore, by maintaining the cob structure, and by making sure the house is built in a strategic location, the humidity will not pose a problem. 

We hope that you will be happy to read this good news and that you will endeavor your own cob projects!