Two Hybrid Timber-Bale-Cob Houses in Cantabria, Northern Spain
You may remember I visited a rather spectacular natural building complex in Cantabria a couple of years ago. You may remember two straw bale houses in the process of being built. Those houses are now complete (yippee!) Robert Alcock from Abrazo House gives a brilliantly detailed rundown on how they were made, and shows us around.
If you're a regular follower of the Mud Home blog you'll have already read a bit about Abrazo House. At this ecological learning centre in a tiny village in the green mountains of Cantabria, we've spent the past fourteen years building natural homes and doing permaculture with the help of hundreds of volunteers from all over the world. You can read all about the project in our free ebook: (http://abrazohouse.org/en/book/)
In 2016 we decided to apply our hard-won experience to a further natural building project: to create two new, beautiful and efficient eco-houses on another plot of land in the same village, with the aim of attracting like-minded people to live and work in this amazing part of the world. The houses are now complete and are on the market. (http://abrazohouse.org/for-sale/) For all you natural building fans out there, here's a quick rundown on the essentials of the project.
The site is 5000m2 of south-facing terraces with young woodland (planted after we bought the land in 2005) and a stream, in a small village in Cantabria. The "urban" zoning of the land meant we could get permission to build two homes there, and we went down the legal route of architect's plans and municipal permits. (NB We've never run into legal issues because of the unconventional nature of our buildings.)
Because of the steep, south-facing site, we decided to cut away into the hillside and build earth-sheltered houses. A key design element is the addition of a semi-enclosed garage to the north, in between the house proper and the hillside: basically doubling your useful space for just the cost of the roof.
The choice of materials was based on ten years' experience of natural building in this bioregion. Of course there would be loooads of cob: we love working with cob, it's cheap and easy to mix with our well-honed rotavator technique, and it makes gorgeous organic shapes. But we wanted these houses to be completely passive solar—not needing any additional heating in winter—and cob alone isn't quite warm enough to do that even in our mild climate, so we went for a hybrid construction: straw bales laid on edge (35cm thick) with a good 15cm of cob inside and out for protection and thermal mass.
A notable feature of our local landscape is the humungous areas of eucalyptus plantations. These non-native trees are mostly used for making paper, but they actually make a very good structural timber which is very durable if treated with borax solution. So we decided to erect a timber frame structure and green roof first, and build the bale-cob walls afterwards, allowing us to work under cover. We cut and peeled our eucalyptus trunks on a friend's land, less than 1km from the building site. Using them in the round meant stronger beams, and saved us an expensive trip to the sawmill.
Earthmoving and Foundations
On site, our digger crew scraped away the topsoil—which we piled up for use in mixing the cob later on—and found that the underlying subsoil wasn't soil at all, but rock. This meant a lot more expensive digging, but it did have two benefits: a nearly infinite supply of stone for building retaining walls and foundations, and a very solid base for the houses, with no need to pour a concrete foundation. In some areas we could go straight up from the bedrock; in other places we built a brick pier for the posts to rest on.
Timber Frame and Roof
Due to administrative delays we weren't able to start building the timber frame until December 2016, but once we got going it went up in just a couple of weeks. With the main frames up and temporary supports in place, we put on the rafters and the roof during the winter, luckily blessed with good weather, and were ready to start filling in the walls by March.
To keep the bale-cob walls dry, you need a good stemwall. Despite having plenty of stone on site, our stemwalls are mostly built from termoarcilla, a specially insulating type of brick that interlocks like Lego, because it's way quicker: we only used stone on the visible exterior walls. We filled in between the two layers with expanded clay pellets (arlita) — a lightweight insulating pellet that's a bit like Rice Crispies.
On top of the stemwall we put a thin layer of cob, and then it was bale time. It was pretty easy to keep the bales straight and stable by tying them to the post-and-beam structure; in some places, we strengthen them with bamboo poles tied through the wall from inside to outside. At this stage we just left a big enough gap for each window or door, added a wooden lintel and kept going with the straw bales. It's important to take into account that the straw bales will settle over time, so the lintels must be able to move with them. (Our lintels could have done with being a bit stronger, too.) Later on, we would come back and hang the doors and windows from the lintels, filling in around them with slip-straw.
The first layer of plaster we added to the bales was a clay slip—just clay soil and water mixed by hand and foot in a pit (an excellent way to make friends and get incredibly muddy), and applied to the bales by hand.
Electricity tubes get fixed straight on to the straw bales. Then it's time for the cob plaster—layers and layers of cob plaster until the walls are weatherproof, thick and straight.
On top of the cob we applied Ecoclay, a commercial earth plaster—basically a much finer version of cob. Then came the finish layer: gypsum plaster inside the West house and an extra fine commercial clay plaster in the East house, with lime plaster on the exterior of both houses for strength and durability.
On the market
The two houses are now on the market, together with 5000m2 of land with wooded terraces and a stream. The East house is furnished as a small family home, while the West house is slightly larger, with a loft and two bathrooms, and would be ideal as a studio space for working, running courses or as a rental property. We believe this is an exceptional property for the right people, who are looking for a place to realise their dreams of a life in harmony with nature. Please take a look at our webpage (http://abrazohouse.org/for-sale/) or check out our promo video (https://youtu.be/mgMiTj5ujCA) and get in touch if you want to know more.
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