The Mud Home Guide to Natural Insulation

The top ten natural insulation materials, and how to use them.

If you live in a cooler climate (or indeed a hot one and need to keep the burning summer sun out), no matter which kind of house you build, insulation will be on your mind. Without a shadow of a doubt the most important areas to insulate are the roof (60% of your heat disappears out here), and the floor (at least another 10% of your home’s warmth is sucked into the ground).

But there are quite a few natural materials to choose from, and all are suited to different jobs. Some are naturally easier to use for roofing, while others work better under the floor. Here’s a quick guide showing the pros and cons, approximate R Value, and usage of each type:

R Value? Oh no, what’s that?
Insulation levels are measured in R values. Here’s a neat little infographic for US climate zones showing the kind of R values you would need for your roof, floor and walls for a conventional house in various climates. All building materials have an R value per inch. Bear in mind though, some insulating materials work in different ways (for example, if you compress certain materials like wool, then you reduce their capacity to insulate).

Cotton bats (sometimes mixed with wool) (R 3.7 per inch)
Pros: Easy to move about, easy to source, snug, clean and warm. Pleasant to touch. Not usually expensive.
Cons: Mouse heaven. These perform better in the damp than wood insulation, but worse than say cork. Can be squashed (and lose some of its insulation power)
How to use it:
For roofing pin it somewhere between your roofing boards and your waterproofing.
For flooring create a wooden framework under your floorboards and insert it in the gaps between the floor joists. Consider a vapour barrier beneath the bats and the substrate (if going straight on the ground).

Cotton and wool mixed insulation panels under my floor. Just for reference, I didn’t use a vapour barrier but laid cardboard beneath the panels, straight on the flagstones. This all worked well (and I’m in a very damp climate).

2. Wood insulation (R2.5)
Pros: Can be bought in easy-to-use panels or boards.
Cons: Can be difficult to source if you are in the sticks. Not the cheapest material either. And of course mice love it, because it's fibrous and soft (not solid wood). You can buy denser panels, but I'm guessing mice would still get into them. Wood insulation is also quite sensitive to damp. And of course a bit hopeless in a fire.
How to use it: If you want to keep the mice out of it, I recommend laying a metal mesh underneath it and over it. Lay the mesh first, slap the wood boards down, then wrap the mesh over the top and around. For roofing, wood insulation works best somewhere between your roof boards and your waterproofing. For flooring I’d only use it if I was sure there wasn’t damp anywhere in the equation.

Wood insulation going on my roof.

3. Cork boards (R 3.5)
Pros: Fireproof. Mice aren’t especially into it. Easy to install. If you get the sanded version it looks so good you may not wish to cover it.
Cons: Ants are into it, sorry to say. More expensive than some other options, and not available everywhere.
How to use it: Cork is super easy to use as it comes in sturdy-ish boards. For roofing it’s perfect. Tack it somewhere between your roofing boards and your waterproofing. For flooring you need a flat surface to lay it on. You could create a wooden framework under your floorboards and insert it in the gaps between the floor joists.

Cork is a superb, natural, fire-resistant insulating material.

4. Perlite (R 2.7)
Pros: This is a volcanic glass super for flooring. It copes well with damp, is sturdy, not prone to rotting, fireproof, and mouse-proof.
Cons: Can be hard to source.
How to use it: I don’t see how you can use this for roofing (perhaps by pinning up sacks of it, but it’s not particularly well-suited). Perlite really shines for flooring, though. You can lay a fat layer of these small rocks on top of some geotextile membrane, and stick your floor of choice onto it. If using under an earthen or limecrete floor, you’ll want another layer of geotextile membrane on the top of it to prevent your liquid flooring from sinking into the rocks before it dries.
 
5. Foamglas (R 3.4)
Pros: Foamglas is a bit like perlite, though inorganic, which is to say not natural. However it is probably more sustainable than its natural volcanic sister. It boasts the same advantages too. It copes well with damp, is sturdy, not prone to rotting, fireproof, and mouse-proof. It’s more readily available than perlite too, and although it’s inorganic it is breathable, which is important.
Cons: Not natural, but probably more sustainable in the long term than perlite.  A bit pricey, but getting cheaper as it becomes more popular.
How to use it: You can buy foamglas in panels or as small coal-sized rocks. The panels are best for roofs, the rocks for floors. You can lay a fat layer of these small grey rocks on top of some geotextile membrane, and stick your floor of choice onto it. If using under an earthen or limecrete floor, you want another layer of geotextile membrane on the top of it to prevent your liquid flooring from sinking into the rocks before it dries.
 
6. Pumice (R value not found)
Pros: Pumice has the same advantages as perlite and foamed glass. It copes well with damp, is sturdy, not prone to rotting, fireproof, and mouse-proof. It’s also very light, which makes transportation and wheelbarrowing a lot easier, which counts for a lot when you’re out in the sticks.
Cons: Can be hard to source.
How to use it: Best for floors. As with foamglas, you can lay a fat layer of these small rocks on top of some geotextile membrane, and stick your floor of choice onto it. If using under an earthen or limecrete floor, you’ll want another layer of geotextile membrane on the top of it to prevent your liquid flooring from sinking into the rocks before it dries.

Pumice.

7. Sheep’s Wool (R 3.5-3.8)
Pros: Snug, doesn’t need much processing, and if you have sheep in your area then it’s an obvious, sustainable, and natural choice.
Cons: Wool has a bad reputation for attracting moths. Some say if you leave the tannin in the wool, this deters them, others disagree. And the tannin smells, so… I must admit I suspect the moth thing depends on other factors, because I have sacks and sacks of wool in my old barn (it’s been there at least twenty years) and no moths. The wool hasn’t been washed though, so the tannin theory may be correct.
How to use it: For roofing I’ve heard of folk filling bags or hessian sacks with it, and pinning it to the roof, or creating a wooden frame and stuffing it full of the wool. For flooring it’s easier. You can simply stuff it under floorboards, but make sure you seal it in well. Leave no holes for mice or moths.

Unwashed sheep’s wool. The tannin appears to deter moths.

8. Straw (R1.45)
Pros: Cheap as chips, easy to source (in many places).
Cons: Mouse heaven. Not as insulating as many of the other options, which means you might need half a metre of it to keep warm.
How to use it: I’m not sure about using straw in the roof. I suppose you could create a framework between your roof boards and your waterproofing, and ram it in there. But it would be difficult to seal it from mice, and you’d need a heck of a lot to really insulate well. For flooring you could potentially lay a vapour barrier, then stuff it under your floor joists, then add your floorboards. Laying a wire mesh all around would help deter mice. But yes, mice love straw. In my opinion, straw works best for walls. Slip-straw (a method of mixing a lot of straw into a clay slip, and then wedging it into a framework) is a good way of using straw for wall insulation.

Straw is easy to source, and cheap.

9. Papercrete with lime (R 2.6-3.2)
Papercrete is the method of mixing paper with lime to create a nicely insulating crete with a surprisingly high R value.
Pros: Very inexpensive, easy to make, pest-proof.
Cons: Messy to apply. Not for roofing.
How to use it: For flooring you could create a massive slab of it, and then add the floor of your choice over it. Works best as an insulating render though.
 
10. Hemp (R 3.5)
This is the trendy new kid on the natural building block. You can buy hemp loose, in flexible rolls like wool, or in boards. The boards are great for ceilings.

Hempcrete (or hemplime) (R 2.4)
Like papercrete, this is lime and hemp mixed. In some places you buy it already formed into blocks, which makes it easier to use.
Pros: Inexpensive, fireproof, mouse-proof and generally pest-proof.
Cons: Hemp can be difficult to source depending on where you live. Messy to use and apply. There have been issues in cool, damp climates of hempcretes not curing adequately. This is usually due to using a very soft lime putty. If you are in a cooler, wetter climate, consider a more hydraulic lime for this.
How to use it: Hemplime is better suited to floors and walls. For flooring, you can lay a fat slab of it on some geotextile membrane, and then add tiles, wood, or earth over the top.

Hemplime used in a Tudor house (see The Natural House at https://www.thenaturalhouse.net/hemp-lime-construct).

Extra: Cardboard (R 3-4)
The humble cardboard is a very useful material with a high R value. If you’re strapped for cash and need something to keep the cold out fast, you could do worse than use corrugated cardboard. I actually stuck this under my floor beneath some cotton bats.
Pros: Cheap, nay free if you head to the recycling bin. Easy to source, and easy to use.
Cons: No good in the damp, and rots very fast. You need a lot of layers to get a decent insulation value for a very cold climate. Not very durable.
How to use it: As it comes in boards, it’s really easy to use and can be tacked into place on a roof, for example. I’d see it as a short-term option, and I’d stick a vapour barrier under it if it’s going near a floor.
 
Earth, concrete, and insulation
People babble on about earth houses being bad in cold climates, but heck, earth is way way way warmer than Portland cement. I’ve experienced it firsthand, and the numbers agree. Concrete has an appalling R value (R 0.08). compared to earth (somewhere between R 0.15 and 0.2). This makes earth at least twice as warm as concrete. If you’ve got straw in that earth mix (which you probably will have) it’s automatically more insulating too. So, if you’re wavering about earthen houses, but have lived in concrete the rest of your life, you’re going to be pleasantly surprised. That said, for those in the seriously cold zones (below freezing for months at a time), earth houses will need insulating. More on earth and cold climates here.

Mud houses and the cold

​Insulation Summary
1. The roof is your priority.
2. The floor is your second priority.
3. Find any little draught hole and close it.
4. If you have a house with thermal mass (earth or stone) it is generally recommended to add insulation to the outside of the house walls.
5. But if you can’t do that for any reason, it’s still worth insulating the inside.
 
Best for roofs
Cork panels
Cotton/wool bats
Wood insulation boards
Foamglas panels
Hemp boards
 
Best for floors
Foamglas
Perlite
Pumice
Hemplime
 
Best for walls
Papercrete (lime-based)
Hemplime
Insulating earthen plaster (read more about that here)

How to insulate plaster

Everyone loves to be snug in insulation, including mice.

How to keep mice out
Mice are insulation-loving, upholstery-chewing, corner-pooping little scoundrels. The only real way to deter them from your insulation is to seal it. Thoroughly. They will chew through almost anything other than solid wood beams, stone, and wire mesh. Find every hole, crevice and gap and fill it with lime mortar or earth plaster (though they may even chew through the latter too, eventually).
 
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