People have built homes using straw, grass, or reed throughout history. During the late 1800s on the American plains however, straw bales houses were a matter of necessity; there was no lumber for construction. Now, with the rise in interest in sustainable housing, there’s been a revival in straw bale construction. It’s mostly been in the U.S., but in 1995, Bob Matthews, of the Institute for Social Inventions and author of The Complete Manual of Practical Home Building, built a straw cabin in the U.K. (possibly the first such building in Britain) and has been living in it ever since.
With straw often being a farm surplus product and very cheap (around 40p a bale, or £1.50 delivered), it’s inexpensive, and an easily renewable medium. Properly built, straw bale houses are fire-resistant, waterproof and actually pest free, with super-insulated walls.
You’ll need about 300 standard three-wire bales of straw to make a 2,000 square-foot house (186 sq. metres). You skewer them on rebar pins (or wood or bamboo stakes) to keep them firm. When you’ve finished, after adding plumbing and wiring, you seal and finish the walls.
You need to use bales that have a uniform size (about a metre long, half a metre wide), are well secured with two strings, and with very few seed heads. Make sure they’re compacted properly and dense – each bale should weight between 16-30 kilos – and dry (and be sure you keep them dry when you’re building, for obvious reasons). Even after you’ve finished the house, you need to be certain the centre of the bales doesn’t become wet through either the top or bottom – however, if the outside gets wet, that’s fine; it’ll dry out naturally.
Types of Construction
Use a “non-structural” or “in-fill” system for bigger structures. That means you make a frame first to support the roof, then pierce the bales with rebar as you rise, attaching the bales to the frame to keep your walls secure.
“Structural bale construction” is a little like Lego, where you stack the bales together in a “running bond” manner. After construction, you simply stucco the exterior and plaster the interior walls. However, since the bales compress as they settle, you need to leave settlement gaps above both windows and doors (how much depends on the number of bales).
Straw and clay construction is a little like cob. You mix clay and water, and then add straw, before packing it into a wooden frame.
Mortar bale construction is similar to working with giant bricks. You put mortar between the bales, and the mortar actually takes the wall’s weight (it needs to be quite thick – at least an inch on all surfaces of the bale). It’s a good method when you’re making a two storey house or a basement. About the only drawback is that the mortar creates cold bridges and thermal leaks. Again, use stucco and plaster to add to the structural integrity.
Finally, there are also pressed straw panels. Here you use straw that’s been compacted under pressure. You can also use the panels for floors and roofs if you wish.
In theory, at least, a properly constructed straw house, built where there’s good drainage, could last for centuries. You should probably use a different material for the roof, however (something more permanent), and build that roof at a steep angle for drainage.
If you’re building a straw bale house in Britain, the climate means you’ll have to be on a self-draining foundation (one of the best is a rubble trench). You’ll also need plastic or metal strapping to attach the wallplate (use anchor bolts) to the foundation.
Make sure your foundation goes well below the frost line, and then fill it with small stones before you put concrete in the rest of the space. You also have to be sure that water can’t gather at the point where the bales meet the foundation, which means you need to elevate the foundation by at least 23mm (much higher is better), and leave a good overhang (around 450mm) on the roof.
Obviously, after you’ve completed the construction, you still need to render, or finish, the place. Limewash is good, but very expensive. Earth plaster is an American favourite and durable to use both inside and outside. It’s non-toxic, and comes in many colours. Additionally, you have plenty of time to work it and it dries to a hard surface.
Rot and Pests
You shouldn’t have a problem with rot as long as you use dry bales. Remember, though, that all the paint must be permeable so moisture isn’t trapped in the wall. Believe it or not, you find fewer insects or vermin in straw bales than you do in wood, and once you’ve plastered the building, they can’t get in.
You might also be surprised to learn that straw bale construction is very fire-resistant; in tests it actually out-performed conventional building materials. However, in spite of all its advantages, getting a mortgage for a straw bale house can be difficult as can insuring it.