Roof Insulation Options for Cob Houses

There are a variety of natural materials you can use for roof insulation in your cob home.
By Becky Bee
April 24, 2013
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With “The Cob Builders Handbook” you can learn to make cob, design your own cob home and set out to build it.
Cover Courtesy Chelsea Green Publishing
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Cob (an old English word for lump) is old-fashioned concrete, made out of a mixture of clay, sand and straw. Becky Bee’s  The Cob Builders Handbook (Chelsea Green Publishing, 1998) is a friendly guide to making your own earth structure, with chapters on design, foundations, floors, windows and doors, finishes and, of course, making glorious cob. From sod to straw, wool to wood, there are a variety of roof insulation options for cob houses. Learn about the pros and cons of each material in this excerpt taken from “Roofs: Insulation.”

You can purchase this book from the MOTHER EARTH NEWS store: The Cob Builders Handbook.

Insulation can be anything that creates still air spaces. Lots of roof insulation makes a house comfy. In most climates, insulating the roof is extremely important.

The temperature inside your house will always try to equalize with the outside temperature. A generous layer of roof insulation will help a lot in keeping heat inside when it’s cold out, as well as keeping the hot summer sun from making your house too warm. So insulate your roof!

How much insulation?

Check with local builders to find out how much is suggested for your area. They will probably tell you something like, ‘an R-value of 35’?! R-values are simply units of measurement used to describe how much insulation-value a particular material will provide. A higher number means a greater ‘resistance to heat-flow’. The building supply store can tell you what the R-values are for the insulation materials they sell. You’ll have to do more research to estimate the R-values of less conventional insulation materials. When in doubt, more is better.

Making a Space for the Insulation and Its Ventilation

Where there is a moisture barrier and a temperature differential on either side of it, you will get condensation! If you are using a roof sheathing that is a moisture barrier, regardless of which type of insulation you choose, it’s a good idea to create ventilation for it. Design the insulation space to be 2 or 3 inches deeper than the amount of insulation you plan to use. This extra space above the insulation is for air to flow through, taking any condensation with it. The top and bottom of this space need to be vented out of the roof through the cob walls. To make vents in the cob walls, bury a three-inch pipe running through the cob. Cover the pipe with screen mesh to keep critters out of the roof. The edges of the screen can be temporarily attached with a rubber band, then cobbed over along with the pipe.

For ventilation on the tops of gable roofs, there are all sorts of fancy spinning metal vents available at building supply stores. It’s easy to make your own venting, though. Leave a space at the ridge of the roof between the top pieces of sheathing and the tippy-top of the roof skeleton. This creates a vent opening along the entire top of the roof. Staple heavy-duty mesh along the opening to keep the critters out. Build a little roof (8 to 10 inch overhang) along the top of the roof over the opening, or buy the special flashing that’s made to keep rain out.

If you will be making a flat ceiling to create an attic space, make large vents from the attic space to the outside.

Mice and rats love to live in the attics or insulation spaces if they can get in. These places are warm, safe from predators, and have a wonderful supply of nesting materials. Your first line of defence is simply not to build or leave any entrances! Cob is so thick and hard the critters will have a hard time drilling through your walls. Use extra heavy duty mesh to cover all roof vent openings, and don’t leave any gaps where wood meets cob or where wood meets wood.

Insulation Options

Thatch The air in and between the tubes of grasses or reeds that make up the thatch are great insulation in themselves. The thatch is usually about a foot thick. In cold climates you may want to add extra insulation under the thatch.

Sod The earth, the plants, and their matted roots provide some insulation on a sod roof. It’s probably a good idea to supplement it with some other type of insulation and/or reflective material, because when the sod gets wet the insulative properties decrease significantly.

Straw If you use straw, make sure it’s straw and not hay. Hay is the nutritious part of the grass and is much more inviting to critters and mold than straw is. Straw bales placed close together in the insulation space have fantastic insulative properties. They are fairly heavy and require a fairly substantial roof structure of wood or steel to support their weight. They’re cheap. If the straw was grown organically and if it was kept dry and mold-free, it should be nontoxic. Straw bales will smolder if exposed to enough heat, but won’t really catch on fire if they can’t get enough oxygen. Loose straw has fairly good insulative properties, but one of its major drawbacks is that it burns well.

Light-clay straw One way to prevent straw from burning so well is to coat it in a clay slip. Put water, and the purest clay you have, into a big bucket or drum. Stir until the clay is suspended in the water — about the consistency of thin cream. Sand and stones will fall to the bottom. Push the goo through a screen to clean the mix and break down all the clay. This is called clay slip. Throw loose straw on a tarp and pour the clay slip on it. Use about the same proportions as salad dressing to salad. Two people can grab the corners of the tarp and toss the straw ‘salad’ and clay slip ‘dressing’ until each piece of straw is coated with the slip. Let it dry a little, then put it loosely into the insulation space. Use at least ten inches of this kind of insulation. It will provide you with about 2 or 3R of insulation per inch. Remember to leave a gap (at least 2 inches, more is better) over the straw-clay for ventilation.

Wool Combed wool has about the same ‘R-value’ as fiberglass. I imagine, in the not-too-distant-future, it will be easy to buy wool insulation commercially, as people become less willing to live with toxic fiberglass and chemically treated cellulose. Wool insulation batts are made in New Zealand but the U.S. requires toxic treatments before importation. Is it fumigated? If so, are you willing to live with it? Consider doing it yourself. Sheep farms often have dirty or scrap wool they’re happy to get rid of. Cleaning the wool is important, to discourage critters. You’ll need to brush (or card) the wool to fluff it up and make the airspaces that give it its ability to insulate. Cleaning and brushing the amount of wool you’ll need is a big job. You can put the wool in loose in your insulation space, or you may choose to first stuff the wool into agricultural/burlap bags or tubes.

Cork Cork is bark that can be harvested without killing the cork trees. Cork comes in granular form or pressed into sheets. Untreated cork seems to be a very non-toxic insulation material. It may be hard to find and will be expensive.

Vermiculite Vermiculite is natural chips of mica processed to make them puff up. It’s commonly used in gardening soils or to insulate concrete masonry. It is fairly expensive if you buy enough to insulate a whole roof. If you’re not allergic to it, it may be worth the cost. Any dust is bad for your lungs, so wear a mask when you are handling it. Vermiculite doesn’t burn. It will absorb moisture and make a nasty heavy mess before you realize there’s a leak!

Pumice Pumice is a volcanic rock that is so full of air pockets it floats on water. I have never tried it but if you live near a pumice deposit, you might want to try using it to insulate your roof.

Wood Sawdust or wood-chips have been used for centuries for insulation. You can imagine some of the problems: fire, rot, insects, etc. Recently, some low-toxic treatments for wood chip have been developed in Europe to minimize these problems and may soon become available elsewhere.

Seaweed When I was in Australia I heard that seaweeds that have flotation bubbles were used in a lot of the old homes for ceiling insulation.

Cotton Cotton is a possible insulation material, although the production of cotton is very hard on the environment. Let us know if you figure out a good way to use recycled clothes.

Ground up newspapers (chemically treated, blown in cellulose) In Southern Oregon, most of the roofs in new subdivisions are insulated with shredded newspapers that have been treated with something (probably toxic) to reduce their flammability. This is blown into the insulation space or attic with a big machine. If you decide to use untreated shredded newspaper, be aware that it is a great fire starter.

Soy milk containers Some friends in New Mexico insulated the roof of their earth home with soy milk containers. They cut them in half, washed them, and stapled them together. These were set into the insulation space face down, towards the heated house. They report that this works as well as the fiberglass insulation (about a foot thick) in an identical house next door. You could try using whole cartons.

Cardboard My grandmother’s house was built in the 1920s. It wasn’t super well-insulated, but it was reasonably warm in the winter with one woodstove. In 1995, the house was smashed by a big tree in a wind storm. Luckily my grandma was OK! I watched as the bulldozers tore down what was left of the house and I noticed that a lot of the house was made out of layers and layers of cardboard glued together. I didn’t see any mouse tunnels in it, even after 60 or 70 years!! Some of the glues used in cardboard might be toxic.

Fiber glass Fiber glass insulation is made out of yucky, itchy, sharp fibers that may be stuck in your precious lungs forever! They make your skin itch and they get stuck in your eyes and nose. Some brilliant company now sells fiber glass insulation in ‘poly bags’ which contain the fibers, and help protect people using the stuff.

Blown-in fiber glass There are big machines that can blow fibers of spun glass. This is usually used in a situation where there is an attic space. Do you think yucky, sharp fibers flying through the air is a good idea?

Silver bubble wrap This is what it says it is. It’s expensive and would require more than one layer to really insulate a roof enough. The reason I mention it here is that it is very flexible, so it works well for an organic shaped roof. It takes up very little space for the amount of insulating value it gives. It is also reflective. It could add an insulating and reflective layer to augment some other kind of insulation. It can be stapled onto the bottom of the sheathing and/or laid just above the ceiling. Pieces of this stuff lining the backs of window shades or against skylights really helps keep the heat in on cold nights. Bubble wrap is often used to wrap water pipes to keep them from freezing.

Rigid foam This is a commercially available product made out of urethane foam. It’s sheets of styrofoam-like stuff. It is extremely toxic when burning and probably not that good for you to live near even when it’s not burning. This stuff is expensive. One of the good things about rigid foam insulation is that you get a lot of insulation value in a small amount of space.

Anything that has trapped air spaces in it Use your imagination! What about old film canisters with their lids on? If you discover a natural or nontoxic recycled material that can be used for practical insulation, your name will go down in natural building history! Please let me know!

A Reflective Layer Reduces the Amount of Insulation You’ll Need

Shiny stuff reflects some of the heat back. Put the shiny side towards the heat that you want to reflect. This means shiny-side down toward your room to keep heat in where it’s cold, shiny-side up to reflect the sun’s heat away in hot climates, or both. Put the reflective layer between the heat source and the insulation. The heat will be reflected regardless of what is on top of the reflective layer. Silver bubble wrap and silver paper, both available at supply places, can be used for a reflective layer. One free source of reflective material is soy and rice milk containers, opened, flattened, washed, and stapled in place. In some countries, milk that doesn’t need to be refrigerated comes in those kind of containers. This is a useful thing to do with a hard-to-recycle product. Another way to reflect the summer sun is to paint the roof surface with a reflective white paint.

Read more: Discover more things that you can build with cob in Cob Construction: Shelves, Furniture and More.


This excerpt has been reprinted with permission from The Cob Builders Handbook: You Can Hand-Sculpt Your Own Home, published by Chelsea Green Publishing, 1998. Buy this book from our store: The Cob Builders Handbook.


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