Suppose I was building a house a couple of hundred years ago and I was using hay or leaves for insulation. How would I know how tight or loosely to pack it? What are some of the rules that apply to insulating? Lets say I have a 12" space to work with.
Since “non-moving” air is one of the best insulators the idea is to get enough insulation in there to keep the air from circulating while also avoiding having the insulation so dense that it becomes its own method of heat transfer.
The basic rule is that different materials have different levels of convection.
That said, if wind blows through your wall then convection is sort of irrelevant since as much as you might try to build up the warmth of the air inside your structure (assuming we’re in a cold climate) it will just be blown away with fresh, freezing air.
Generally, the first rule is to try and contain your air as effectively as possible. Igloos work as a shelter, despite themselves being freezing.
Once you’ve accomplished air containment, figuring out the relative convection of your materials follows. Air is fairly insulating and is also a part of your set of materials to work with unless we’re presuming that we’re living in a vacuum.
Quite likely, your best answer will be to form some sort of relatively air proof wall and to seal that in with a second relatively air proof wall. That said, without any sort of glue or plaster, it might be hard to build something wind-proof. Your best bet might be to weave thin slices of the wood into two thin walls and pack the middle with leaves. Sufficiently dense, they should block the flow of air and you’ll have 12" of packed leaf insulation - which is whatever it is. It will be better than nothing.
Two layer brick walls would be good insulation, wouldn’t they? That’s why Practical Pig’s house was so snug.
This is from memory…
What others said is pretty accurate. Here are a few fussy adds:
The heat transfer through the air will include convection, which itself includes advection (air movement) and conduction. People often use “convection” as if it means “advection”. You can pretty nearly eliminate advection, but you’ll still have conduction, so you still have convection.
There’s a dimensionless number, the Rayleigh number, that characterizes the conduction advection mix. Your insulation material needs to keep the Rayleigh number big enough, to do its best.
As someone pointed out, the solid part of the insulation can increase heat transfer by its own thermal conductivity. The thermal conductivity of solids is generally orders of magnitude bigger than of gasses. It also matters how the solids are oriented; for example in a wall cavity full of leaves, you hope the leaves tend to be vertical. If they were horizontal they’d tend to conduct heat better in the horizontal direction.
There’s also radiative transfer of the energy of heat (though technically it’s not heat while it is radiating). Insulation typically slows this by having incomplete absorption and emission of electromagnetic energy at each interface. Many layers of metal foil, textured so they have few and tiny contact points between layers, or separated with nonmetallic separators, can be good insulation (this construction is typical of the big containers for liquified gasses such as nitrogen and oxygen and air).
One last marvelous detail: the Smoluchowski effect causes the thermal conductivity of gasses to effectively drop to zero as the dimensions over which they are conducting become as small as the mean free path for the gas in those conditions. Conductivity is actually a large-scale model, which works fine as long as the gas molecules primarily collide with other gas molecules. The model breaks down if the average distance the molecules travel between collisions (the mean free path) becomes large relative to the dimensions of the container they’re in. In this case, the molecules primarily collide with the container walls. Bringing the walls closer together does not increase the heat transfer, as it would if the molecules are colliding with one another. To exploit this effect, you can choose insulation with very small void spaces in it, known as microporous insulation. Or, you can lower the pressure of the gas, which increases the mean free path.
Normally when hay is used for insulation, it’s in the form of bales, not the loose stuff. And, Googling, bales are typically 14x18x35 inches, so your twelve inch space is going to be hard to work with.
The OP limited us to hay and leaves (though, for some reason, I thought he had said “sticks and leaves” when I wrote my post - so I gave the wrong answer).
Forget about my example, that was just what was on my mind when I posted, I was watching a guy insulate his house with hay between 12" rafters and I was trying t figure out the best way to install the hay, loose or tight. Interestiung discussion some good info.
Nitpick: Straw, not Hay.
Straw is a good insulator because the stems are hollow. When formed into bales, the R-values can be upwards of 50, depending on the bale’s orientation. In practice a straw-bale wall is probably closer to R-20 or 30.
What is straw not hay?
Also, insulation isn’t enough. A comfortable house needs a lot of well-insulated thermal mass. In straw-bale homes, the inside of the bales are usually coated with mud or plaster to provide this thermal buffering.
Straw is the shafts from wheat or rice, after the grains have been harvested. Hay is whole-grass (or alfalfa). You don’t want to use Hay, because:
- It’s valuable.
- It attracts vermin.
- It’s not as good an insulator.
As pointed out by @Hampshire and others, the basic idea behind wall insulation is to try and minimize the air from moving around (convection) and to try and minimize heat conducting through the material. Which means, for any insulation material, there’s an optimal density: if the density is too low the air will move around, and if the density is too high (which can happen if it is compressed) heat will conduct through the material.
As a practical point, there’s also the issue of moisture. If the insulation is damp for whatever reason, it will lose its effectiveness.
Well, I don’t want to use either straw or hay but the OP apparently does.
Paper on the insulating properties of thatching, btw:
A couple of hundred years ago, hay balers hadn’t yet been invented. So the size of not-yet-existent bales wouldn’t have been an issue.
You’d need material dense enough that it wouldn’t all fall down to the bottom of the wall cavity; at least in the walls. If what’s being insulated is the ceiling, this is less of an issue; but if you pack the straw, or whatever, really loosely, it’s still going to settle into a denser pack over time.
Hay balers have been around since the late 1800s
Yup. And it’s not the late 2000’s yet; by about 50 years.
The first automatic hay harverter/baler was commercialized and offered on the market in 1874.
In the 1860s, mechanical cutting devices were developed; from these came modern devices including mechanical mowers and balers. In 1872, a reaper that used a knotter device to bundle and bind hay was invented by Charles Withington; this was commercialized in 1874 by Cyrus McCormick.
From Baler - Wikipedia
So, 150 years almost exactly. Certainly less than OP’s “couple hundred years”, so depending on how seriously you take @HoneyBadgerDC’s lack of precision, we could be talking about bales (if “couple hundred years” is taken non-literally) or loose straw.
They wouldn’t have been common the instant they came on the market; that would have been at least a few years later. Although, granted, if we’re not taking the “couple hundred years” at all literally, bales could have been meant.
I have a friend who’s a farmer and a historian and she always gets annoyed at depictions of 1700’s and most-of-1800’s scenes that show hay bales in them.
I also don’t know what size the bales were during those early years; or even whether this was at all standardized. I believe modern balers are at least somewhat adjustable.