Light Straw Clay

Light Straw Clay

Let's talk about light straw clay.

First off, I love it.

Secondly, what is it? And why is it worthy of love? Both of those questions have an answer.

Light straw clay is a bunch of straw (hence the "straw" part of the name) mixed with a clay slip (a slurry of clay and water, hence the "clay"); sometimes it's called slip-straw for this reason.

Is it light? No, dear reader. It is not. It is heavy.

Why, then, is it called light? Well, it probably gets its English name from its German one, which offers a clue on that note.

LEICHTLEHM. "Light loam" (German), a mixture of clay and lightweight organic or mineral aggregates used as a building material.

In any case, it's a lot lighter than cob. It's also used differently. Cob, which is made of clay and sand with just enough straw added to keep it from falling apart, is a load-bearing material that can serve as structure all by itself. You can use it to build a house, or a wall, or a pizza oven.

Home at Hollyhock

Light straw clay is not structural. It's an infill. If you've got a stick-built shed, for example, you could pack light straw clay between studs, up against the shell of the structure, and voilà. You'd have done that.

Which brings us to why. Why would you do that.

The answer is because you want to feel comfortable when you're hanging out inside the — erm, well, the shed.

Okay, so let's say it's not just a shed. Let's say you're converting a conventional shed into a tiny house. Let's say you want to live in it all winter, and you don't like to shiver.


Or let's say you're building your timber frame dream house and you want the walls around you to radiate the resilient warmth and stability of earth, while also keeping winter at bay.

Kaminzimmer Château d'Harcourt

Regardless of why you've decided to pack a wall with light straw clay, you can count on it to provide the interior with both insulation and thermal mass (which is an interesting combination, by the way, because those are two very different qualities which protect your comfort in very different ways — but let's not get ahead of ourselves). Point being, light straw clay does both.

It won't insulate as well as a straight-up straw bale, mind you, because, surprise! you've doused it with wet clay and compressed all its little straw tubes. However, it does provide moderate insulation. One inch of light straw clay has an R value of about 1.5. Compare that to wool (R-4, loose fill) or fiberglass (R-3, loose fill). Light straw clay is not the best insulator around, in other words; still it is an insulator.

It's also a source of thermal mass. That's because it's clay holding all that straw together, and clay happens to be one of the world's true heroes in the thermal mass game. As a result, a light straw clay wall will slowly soak up any heat and light it's exposed to during the day, then slowly radiate that energy back into the air during the night, making everything cooler when it's hot and warmer when it's cold. How nice.

Now. Why do I love this so much?

I'm a fan of light straw clay because I'm a fan of both timber framing and cob, which could, yes, theoretically go together, but aren't a match made in heaven. I see light straw clay as their angelic mediator.

Fiddleford Manor House, Hall Roof Timbers

I love the geometry of timber frame. I love the warmth and age of wood, and how a traditional timber building makes your safety visible: an emotional and physical shelter exuding joy and rest.

And I love the presence of earth, the steady, grounding quality of it, the stabilizing vibrance it gives to a space. Cob does this, but cob is best when built a foot or more thick, or, if thinner, dramatically curved. Neither of those arrangements is made to order for a timber frame building, which has leaner walls, and straight ones at that. Also, cob doesn't insulate.

But if, say, you were to build a timber frame house, and if you were to provide a web or shell of some sort between the timbers to stabilize a straw clay infill, and if you were, then, to infill it — well. Then your home would be composed of wood and earth.

It wouldn't just wear a facade of this kind of beauty; it would be the beauty.

With walls 10 inches thick, it would hold heat at an R value of 15 or so, which is as good as (or better than) conventionally insulated walls.

Your house would be cooler in the summer and warmer in the winter.

And your connection to the earth — to this forest, where you got the timbers, and to this land, which gave the clay — would be evident. Not just a pretty thought, but a lived experience, a quality resonant in the space around you.

Your house would sing.

Is there anything more joyful?

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