The 3 Things You Need to Do:
Believe it or not, mud homes work fine in wet climates if you build them right. What you need to understand at the beginning though is this: mud homes do not work like mainstream concrete and plastic houses, so throw most of your ideas related to them in the bin.
What do you need to consider when building a mud home in a wet climate?
There are three fundamentals you have to get right. If you do these three things properly, your mud home will be well set for damp climates. If you don’t do these three things, it won’t matter if you coat your house in concrete (ouch) or EPDM, you will encounter issues.
1. You need a rubble trench foundation. You do not want a concrete slab. I don’t care what the architect, the codes, or your mate Fred says, they are wrong when it comes to mud buildings. I feel like I’ve said it a lot, but if you’re new to mud building you may not have heard the news. Concrete wicks up water and holds onto the damp. It also doesn’t allow for water to evaporate. That is absolutely not what you want with a mud home. People have all sorts of erroneous ideas about foundations, but with mud homes the drainage is paramount. You want water getting away from your house as fast as possible. The rubble trench is the cheapest, easiest and by a long way the most effective foundation method for a mud house.
If you are building earthbag, you can create rubble trench foundations like this: https://www.themudhome.com/rubble-trench-foundations.html
If you are building another kind of mud home, then it’s basically the same, so still read that article. You can also watch this video, as it shows exactly how to do it. https://www.youtube.com/watch?v=eNN7T4fBQo4
2. The stem wall is very important. The stem wall protects the rest of your house from damp or accumulating rainwater, and stops your walls wicking up water. The stem wall is usually made of stone, and depending on your climate can be anywhere between 30-60cm high (above grade). If you are building with earthbags you can use gravel-filled earthbags instead (see the rubble trench article).
Normally we start the stem wall below grade (below the surface of the ground), so it acts as water resistant footings. Once the stem wall is sturdy and dry, you start to build your natural home on top of it. Some people advocate a vapour barrier between the stem wall and the cob/adobe. I never use one because as soon as a waterproof membrane is added, evaporation becomes impossible. Water will quite possibly seep into your wall one way or another, and if a vapour barrier is there, it prevents fast evaporation. Your wall will stay damp.
3. You need a good solid roof with wide eaves. The roof is the hat of the house, and I’d argue it’s the most important part of the structure. For a mud house in a wet climate, you want nice wide eaves to protect the walls (a good one metre long). If you do this, you are going a long way to protecting your earth plaster from the rain.
Other things you can do:
Backfilling around the stem wall is another great and simple way of keeping rain water away from your walls.
Installing decent guttering on your roof will help massively in preventing the ground below from getting soggy from rainwater run off.
Lime is the perfect substance for folk in wet climates. It “breathes” so it doesn’t obstruct airflow and allows any damp to evaporate. It’s also a natural fungicide. Use limecretes instead of concretes, use lime wash instead of paint. Use lime mortars for stonework. You may even want to add a bit of lime to your earth plaster.
What you don’t want to do:
Don’t cover your mud house in lime render or (OMG) concrete. Please. It’s a terrible idea. The clay in your mud house expands when it gets damp, the concrete or lime render doesn’t. It will eventually crack and fall off. Concrete will do something worse and stop the house breathing too, thus mitigating the half the point of cob/adobe in the process, and making your house damp and prone to mould.
Notes about natural plasters
People worry far too much about their earth plaster, in my opinion. I honestly don’t think it’s necessary or even a great idea to try and waterproof your plaster. The best thing to add to plaster to create a super strong but natural render is horse or cow manure. If you’ve built your footings and your roof well, your mud walls will only need a touch up every now and again, and frankly that’s quite good fun.
Extra Important Tips from Kristen Krash at Sueno de Vida who has built multiple mud houses in a cloud forest:
1. "Make the rubble trench foundation wider than the stem wall by 12-15 cm." This is a good point, and often happens by accident:)
2. "I would emphasize that people don't attempt to use plastic "moisture barriers" or any such thing in very wet climate. Water passes from areas of higher to lower concentration. Always and forever. Period. Meaning water is going to find it's way under or through your "barrier" and then not be able to evaporate. It will just keep moving to an area of lower concentration. As in inside your house.
3. "Don't build little nooks and shelves and such close to the ground level in your interior walls. I know it's tempting to be like "oh I'll just store my pots and pans under this table in this sculpted little nook, how cute!" Yes it is, for a season. Then you will be storing mold. The plaster will crumble from being damp. Keep your walls smooth and hard for at least one meter above the ground."
I so agree with this. I put my entire living quarters on the top floor of my barn for this very reason.
Resources and relevant links:
Kim’s strawbale hideaway in Scotland shows the foundations, stem wall, and roof you want for a mud house (or strawbale).
A great example of the rubble trench beating any other foundation method is how Gautam and Kim’s earthbag house fared in a monsoon. It also shows the perils of coating your mud house in a crete of any kind.
Kay’s balecob house showcase the rubble trench and stem wall very well.
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