How Straw Bale Houses Work

With the rising price and decreasing availability of lumber, straw has gained attention as a renewable resource that is regularly available as the byproduct of growing grains. Farmers use a little straw to fertilize the ground, but most straw otherwise goes to waste. Each year, 200 million tons of straw go unused in the United States [source: U.S. Department of Energy]. Straw is available in most parts of the country, which reduces transportation costs of construction. With more than 50 percent of all greenhouse gases produced by the construction industry and the transportation associated with it, these savings can be significant [source: Amazon Nails].

Although straw is cheaper than building materials, such as brick or lumber, constructing a straw bale home will usually cost the same as a conventional home because the wall budget is only about 10 percent to 15 percent of the total building budget [source: Mother Earth News Magwood, Mack, Therrien]. When you factor in other expenses, such as the foundation, the roof, and the doors and windows, the price of straw bale houses rises in line with more traditional houses.

However, you can squeeze out some cost savings depending on who builds the house. Straw bale raising parties, similar to barn raising parties, are a chance for a bunch of people to stack bales like building blocks. Little experience is needed to participate in building a straw bale home, and it can go fast. Many Web sites and DVDs are available to teach you how to build your own straw bale house, but it's important to realize what you can do on your own and where you might need a contractor's help.

The real cost savings of straw bale building relate to energy efficiency. The straw bales, finished by plaster, have a high R-value. The R-value measures the insulation resistance of the wall; straw bale walls provide an incredible insulation that can easily keep heat in or out, depending on your needs. A straw bale home can save up to 75 percent on heating and cooling costs annually [source: Morrison, Amazon Nails]. This represents a huge savings over the life of the house. These thick walls also provide excellent soundproofing. Straw bale building has been used for recording studios and for homes near busy highways.­

Photo courtesy StrawBale.com
The thick walls of a straw bale home provide
good insulation.

It might seem like straw bale houses pose a tremendous fire hazard, but they provide roughly three times the fire resistance of conventional homes [source: Morrison]. Loose straw is indeed flammable, but the bales are so tightly packed that they actually increase fire resistance. In a tightly packed bale, there's no oxygen, which reduces the chance for combustion. The plaster coating of the walls adds an additional fire-resistant seal. The National Research Council of Canada conducted testing where straw bale walls withstood temperatures up to 1,850 degrees Fahrenheit (1,010 degrees Celsius) for two hours [source: Mother Earth News Magwood, Mack, Therrien].

Hay Is for Horses

It's important to get straw, not hay, for a straw bale home. Hay contains leafy material that is delicious to a range of animals, while straw is not a food source. Tightly packed straw, combined with properly applied plaster, makes it hard for critters to get into straw bale walls. Even if they do, there's nothing for them to eat. Conventional homes, with nooks and crannies behind the walls, are more attractive to rodents.

Using straw should also ease the concerns of hay fever and allergy sufferers. Unlike hay, straw does not contain pollens. Rather, it's a natural, breathable material, one that proponents are quick to compare to modern building materials, which may contain formaldehyde and other chemicals. Lastly, hay also decomposes naturally, making the house likely to rot, while straw usually requires additional nitrates to decompose.

Reaping the benefits of a straw bale house involves more than just stacking straw willy-nilly. The home should be designed so that nature aids energy efficiency. Before building, figure out where the sun goes, how the wind blows or if there's water nearby. Where does the rain go when it falls? Think about how trees, shrubs and other natural barriers could block the wind or a heavy sun. These factors will determine how to best orient the house and design features such as windows.

Photo courtesy StrawBale.com
A post-and-beam house

There are two main ways to construct a straw bale home. The first technique is post-and-beam infill. In a post-and-beam design, the skeleton of the house is made up by a frame that supports the roof, while the straw bales are filled in as insulation. This method is usually more acceptable to building officials, lenders and insurers because it's closer to a standard form of construction.

The second method of straw bale house construction is load-bearing, also known as Nebraska-style, because it was employed by the first straw bale builders in the Sandhills. In this method, there are no beams, and the bales support the roof. The width and density of the bales make them inherently sturdy, and some people prefer this frameless method because it requires less construction skill and uses fewer resources. However, it does come with some design limitations. For one, homes can only be one story high, although with creative engineering, you can manage a loft. The length of the walls is limited; according to building ratios, you cannot have more than 25 feet of unsupported wall in any direction. Finally, windows and doors are limited to 50 percent of the wall surface area [source: Morrison], but double-check these numbers with local building codes and be sure to determine if one of these construction methods is required for your area.

Where do straw bale homes work best?

The current straw bale home movement is primarily centered in the desert climates of the Southwest United States and Southern California. The method is most practical in locations with temperature extremes because the value provided by the insulated walls is a result of blocking excess heat or cold. With moisture a major threat to straw bale homes, you might think that wet climes don't lend themselves well to the method, but people have successfully built straw bale houses in the rainy Pacific Northwest and snowy New England. Humid climates, however, are not good locales for straw bale construction. The Straw Bale Registry maintains a database where straw bale owners can register their structures. The listings show structures all over the world, with the majority in the United States and China.

Straw Bale Types

Selecting straw is an important design factor. Straw bales come in all shapes and sizes, but the most common building blocks are two-string bales and three-string bales, so named because two or three lengths of wire or twine tie them together. According to the U.S. Department of Energy, they have the following measurements:

• Two-string bale Dimensions: 18 inches by 14 inches by 36 inches Weight: 50 pounds to 60 pounds
• Three-string bale Dimensions: 23 inches by 16 inches by 42 inches Weight: 75 pounds to 80 pounds

You can plug in your dimensions to an online bale calculator to figure out how many bales you'll need for building. An example of a calculator may be found at Harvest Homes.

You can buy bales at feed stores or directly from a farmer. It's important to select dry bales. Hand-held moisture meters can check the bale's moisture content; moisture content of up to 20 percent is considered safe [source: Morrison]. Look for bales with a golden color because lighter straw indicates less moisture. Bales also should be tightly strung, so that when you lift or drop the bale, it generally keeps its shape.

Once you've decided whether to use a load-bearing or a post-and-beam structure, your design choices look a lot like those of conventional home building. A straw bale home can use any kind of foundation or roof, but many people try to consider features such as solar-paneled roofs, which enhance the environmental friendliness and the energy efficiency of a straw bale home.

Photo courtesy StrawBale.com
A toe-up

To build a load-bearing home, the foundation is laid and the straw is stacked. Then, a roof frame is placed on top of the straw and the roof is assembled. For a post-and-beam straw bale home, all the structural elements, including the foundation, frame and roof, are put into place. Then straw bale is just filled in between the posts.

However, with either type of straw bale house, the bales need to be raised off the ground several inches so that they don't soak up moisture from the ground. Builders can accomplish by using toe-ups -- platforms made of lumber and gravel that attach to the concrete foundation. Nails or pins should be hammered into the toe-up, and then the bales are placed onto the nails, anchoring the bales.

Once the first round of bales is situated on the toe-ups, then the rest of the straw can be stacked just like blocks. Window and doors are inserted into the straw bales with wooden frames. Cutting, notching and retying bales are all a part of this process. To get a bale the size or shape that you need, baling needles, which are like large sewing needles, are used to cut the bale and retie it with twine. Chain saws are used to make the notches that will fit the wooden posts.

After the bales are stacked, it's time to break out your chain saw again! Chain saws can carve out architectural features such as niches, window seats or heavier features such as cabinets. Upper cabinets are too heavy to be attached to the plaster that will cover the straw, so pieces of lumber with spikes are inserted into the straw. They will support the cabinets when they're installed. Electrical cables should be encased in plastic sheathing, but they can be installed directly into the walls. Plumbing, however, should be kept out of straw bale walls when possible by using internal walls.

When they're finished stacking, some straw bale builders use giant poles of steel or bamboo and slide them in from the top of the bale to keep everything in place, while others just use pins and wire mesh. Either way, plastering is next. Stucco cement, gypsum plasters, earthen plasters and lime typically serve as the internal and external plaster. The first coat of plaster should be worked into the straw, followed by two additional coats of plaster. Contrary to what you might think, you shouldn't use a waterproofing material on the walls. Straw will work moisture out by itself, and the plaster needs to ventilate the moisture, as opposed to holding it inside next to the bale. Wall paints also should be breathable; examples of breathable paints include lime paints, silicate paints and some latex paints. Properly applying the plaster and paint will help to ensure that moisture doesn't affect the straw.

Photo courtesy StrawBale.com
The stairway before plaster

Photo courtesy StrawBale.com
The finished stairway

As we've mentioned a few times in this article, moisture is the biggest physical problem of building with straw bales. Moisture in the bales causes mold, which causes the straw to rot. You have to take precautions from the moment you buy the bales. A rainy day of construction could ruin exposed bales, so when the bales arrive on site, store them off the ground and under tarps.

During design and construction, take special care to keep water out of the home. Just as the toe-ups on the foundation provide protection from moisture below, roof designs that incorporate overhangs will provide protection from above. Windowsills and joints must be carefully sealed. These methods will keep out the liquid dangers posed by rain or snow, while using the natural, breathable plasters mentioned in the previous section will keep moisture from the air moving through the home. Cracks in these plasters are the primary maintenance issue for straw bale homes so that moisture doesn't accumulate in the walls.

Photo courtesy StrawBale.com
Straw bale home before plaster

Photo courtesy StrawBale.com
The final product

Because straw bale building is fairly new in the construction world, one of the main challenges of building a home in this way relates to how the outside world sees it. Someone interested in straw bale building will likely have to jump through more hoops than if building a conventional home. Building codes might not account for straw bale methods. Conservative banks, lenders and insurance agents may not want to take a chance on financing such an experimental method.

If you want to build a straw bale house, particularly in an area in which they're not common, you might have to work more closely with building officials to get plans that will meet codes and pass inspection. To gain financing and insurance, be ready with data to help explain straw bale building to someone who may not have heard of the method before. You may have to hire consultants to look over your plans and methodology and to vouch for you with the institutions.

How do I find a straw bale contractor?

With straw bale construction still fairly outside mainstream construction methods, it might be difficult to find a contractor knowledgeable about it. " The Last Straw," the quarterly journal about straw bale building maintains a list of resources. You can also check out this database of green builders: http://directory.greenbuilder.com/search.gbpro. It might be easier to look for contractors to provide individual services, such as plastering or roofing. Just as you need to be well versed in straw bale building to deal with banks and insurance companies, you'll need to know how to work effectively with the contractor

As for reselling a straw bale home, little data exist, maybe because straw bale owners don't leave them. Some evidence suggests that straw bale homes might receive a lower resale value estimate than conventionally built homes [source: Mother Earth News Magwood, Mack, Therrien]. However, this is another place where an educated owner/builder is important; explaining to a potential buyer the value of a well-insulated home may help someone decide to pay more than the appraised value.

Say you're interested in straw bale building, but you're already settled in your house. You can still use straw bales for additions and retrofits.

Photo courtesy StrawBale.com
Straw bale construction Straw bale additions are accomplished by tying the addition to the existing structure. After laying each row of straw, place a lath, which is a sheet of metal mesh, over the bales and attach it with dowels or landscape pins. Then, fold the lath at a 90-degree angle and staple it to the frame of the existing house. If you've built a straw bale addition with wooden frames, then the frames can just be attached directly to the frames of the house. Plastering the addition is completed in the same way as straw bale construction.

A contractor can assist with matching up the exteriors of the house and the addition. In laying foundation for an addition, consider the thickness of both the bales and the plaster so that the finished addition will line up with the existing house.

It's possible to wrap your existing home in straw to achieve better energy efficiency. As in building a new straw bale home, the design for a retrofit must ­consider how to prevent moisture from getting into the walls. Straw bale s feature large roof overhangs to protect the walls from rain, so a retrofit may require changing the current roof of a home. This may be accomplished by changing the slope of the roof and extending it.

You may have to create a larger foundation for the house so that the foundation can support the additional width of the bales. This new portion of foundation can be attached to the existing one with epoxy (or anchor) bolts [source: Morrison]. Then, toe-ups should be built to keep the bales off the ground as they are in house construction. However, instead of stacking the bales and then plastering, as is done in straw bale construction, the bales should be dipped in an earthen-based plaster before stacking. This way, plaster is on the bales before they are set up against the home. The bales also can't be tied right up against the house because of the risk of moisture from the siding of the existing house getting into the straw bales. Use a wire netting between the house and the bales to create ventilation [source: Morrison].

When you have bales on the outside of an existing home, you'll have to do a little work on windows and doors; otherwise, using bales 18 inches wide will set windows and doors into deep wells. You can avoid this extra work by retrofitting with the bales on the inside of the house, which is done in much the same way; however, doing so would reduce the space within the house by the width of the bales.

Although creating a larger foundation, changing your roof line or undertaking any of the other tasks necessary to incorporate straw bale construction may seem prohibitively expensive, remember to weigh those costs against the energy savings achieved over the lifetime of the house.