How to build floor? Insulation necessary?

I am designing my strawbale/cob hybrid house. Not sure how I would best construct the floor... I would like to build as green as possible, so without concrete.
I'm considering an adobe floor. Is it necessary to have insulation under the floor? The adobe would act as thermal mass, but potentially would radiate too much downwards...? Since I'm using strawbale as insulation in walls, I was considering using them as well under the floor, but rot seems to be a big risk. Are there natural insulation materials I could use here?

Not sure if this would work, but maybe sheep wool insulation mixed with borax to keep the rodents from gnawing on it?
Sand and perlite (unsure of spelling) mixture possibly too for mass and insulation?

Use straw covered in clay slip. If you are doing a hybrid it is the perfect insulator as you will have it readily available. I believe some people use a layer of cardboard over the drain portion of the floor. I would do 6-8 inches of gravel--tamp--3-6 inches of road base--tamp--3-4 inches straw/clay slip--and then put your floor down either in one or two layers. More expensive, but if you have access to pumice that would be a good one for your drainage and insulator. People use rock wool as well, but definitely not economical to cob house builder. You won't need to worry about rodents in an earthen floor. There really isn't any way for them to get through 12" plus of tamped rock.

Where are you? What climate?

There are places where you definitely want insulation and places where you want to tie the floor into the constant temperature of the earth.

I wouldn't use straw in any form (bales or clay slip) under your floor. Aside from the rot risk, straw isn't rigid enough to support an earthen floor. It would crack constantly. Unless you built a wood framed floor with straw infill, but that doesn't make much sense to me.
The best option I have heard of is perlite. Attached is a short guide to installation. You don't even take it out of the bag it comes in, just lay them lightly on the ground and make sure they are well tamped. Then you'd probably put a layer of road base over that before your finished floor. This article claims an R-value of 3.13/inch, another source I read claims R-2.7/inch. So depending on where you live, 6 inches is probably a good goal. I don't know anything about where to buy it or what it costs, but I am looking into it for a house I'm building this summer.

R Scott wrote:Where are you? What climate?

There are places where you definitely want insulation and places where you want to tie the floor into the constant temperature of the earth.

Exactly! Interesting isn't it how much bad advice is given by people that don't have all the facts or knowledge. Besides this we need a proper soil test.

Thank you for the answers!!

I will build in Portugal, so in a relatively warm climate.
The thing is that I would like to have a comfortable indoor temperature by incorporating enough thermal mass (how much is enough...hmm?).

The clay-straw (wet or dry?) solution seems like the simplest and cheapest, but is it that insulating? Straw bales are so insulating because there's a lot of air in them, which doesn't seem to be the case when you mix with clay...?

Also, do you mean measuring the ground temperature when you say 'soil test'? Or do you mean testing the composition of the soil? If the latter, why would that be important?
Thanks!

i'm not seeing where theres bad advice here, just brainstorming ideas. when you brainstorm not everything you think of will work out, but thats the point of brainstorming...exploring the possibilities. actually i think some of these ideas are good.

i will trust people have enough critical thinking skills to separate out the good suggestions from the ones that wont work. i am particularly interested in this right now, as i have been reading and exploring similar things, different forms of insulation, and am also wondering what natural and/or cheap and not industrial materials, can be used as insulation below, or at grade, for a floor.

so i am curious to listen to whatever anyone may suggest, even if its not all the best ideas. of course it matters whats below the floor, particularly if its very wet, near the water table, or clay, rather than a hard, solid gravelly ground or whatever else.

something i was discussing with a builder friend is creating a thermal break under a floor under a rocket mass heat. he was suggesting two layers of plastic sheeting with a thick (4-6 inches) layer of ash between them. then 1-2 ft above that of earthen floor, clay/sand rocks or something along these lines.
that was kinda surprising and not even close to what i was originally thinking...more like the sand and perlite suggested here.

leila hamaya wrote:i'm not seeing where theres bad advice here, just brainstorming ideas. when you brainstorm not everything you think of will work out, but thats the point of brainstorming...exploring the possibilities. actually i think some of these ideas are good.

i will trust people have enough critical thinking skills to separate out the good suggestions from the ones that wont work. i am particularly interested in this right now, as i have been reading and exploring similar things, different forms of insulation, and am also wondering what natural and/or cheap and not industrial materials, can be used as insulation below, or at grade, for a floor.

so i am curious to listen to whatever anyone may suggest, even if its not all the best ideas. of course it matters whats below the floor, particularly if its very wet, near the water table, or clay, rather than a hard, solid gravelly ground or whatever else.

something i was discussing with a builder friend is creating a thermal break under a floor under a rocket mass heat. he was suggesting two layers of plastic sheeting with a thick (4-6 inches) layer of ash between them. then 1-2 ft above that of earthen floor, clay/sand rocks or something along these lines.
that was kinda surprising and not even close to what i was originally thinking...more like the sand and perlite suggested here.

I think they have all been good ideas, just not necessarily the best for a given situation.

I like the ash idea (much better insulation than sand, less industrial than perlite) but could be tricky to get that much stored up and keep it dry.

i also like the ash idea, the two layers of plastic is the part i dont like. that may be unavoidable though, i have been reading about vapor barriers and all those who say its obviously the right thing, and then again others who feel strongly it is not neccessary and creates more moisture. but would probably be central, cause it would have to be dry ash.

I feel the same way. Like the idea of ash, but I don't like the idea of plastic. My ideal is a house that, when finally abandonded, can just go back into the earth without unnatural materials.
Probably not possible though... plastic seems necessary at least on the roof:-s

i could be wrong about what i think portugal is like, but i imagine it to be subtropical/tropical, really warm and never all that cold? barely any freezes, and not that much temperature extremes either way? or?

as r scott was saying earlier, you may want to "tie the floor into the constant temperature of the earth."
as this would pull out excess heat in the summer, and then release this heat months later once it was cooling off, many earth bermed structures work with these principles.

personally i want to go partly underground! but thats a whole ball of wax itself, and only suitable for certain areas without a lot of moisture, and other conditions. i am in the mountains with tons of slopes, so this is along the lines i am thinking. being underground, no matter how much you insulate, youre already tied into the earths mass and constant temperature underground.

Leila, I was thinking along the same lines.
It will rarely freeze in Portugal.
However, I am not sure what the ground temperature is exactly. This will be below comfort temperatures I assume. So if my internal thermal mass transitions continuously to the underlying ground, will it be hot enough? I'm trying to figure out exactly how fast the heat would be drained from my thermal mass.

i have also been trying to figure out the same things, and researching Annualized geo solar (AGS) and PAHS passive annual heat and other things related to geothermal. i am starting to get it a bit deeper, but cant say i have more answers than questions.

you can follow my half baked ideas here, where i have been wondering about similar things.
and if that isnt too much info already, heres some additional interesting links i just googled up -->

http://www.motherearthnews.com/renewable-energy/passive-annual-heat-storage-zmaz85zsie.aspx

http://www.norishouse.com/PAHS/UmbrellaHouse.html

but yes, ground temperatures are lower than what most people would consider comfortable temperature. i am rather acclimated to living in unheated places, but not to say its preferable. i find most people ideas of comfortable temps, like in peoples houses, rather too warm sometimes.
deep down the earth is the same temperature everywhere on earth, deep enough underground anyway. but closer to the surface has more fluctuations, depending on how much sun and warmth the ground is soaking up.
geothermal works as both heating and cooling, considering that the ground so many feet down is somewhere around 50 degrees ish....which could be made slightly warmer with one of these methods to warm up the ground below the structure. in many of those methods, insulation is put in a huge skirt around the perimeter of the building, to insulate the ground around the structure (and prevent moisture buildup), and no insulation is use directly underneath the floor. although actually theres many different things people are exploring, thats a common idea.

Although people didn't seem to like the idea of clay slip because the floor might crack, I think there is room to explore similar options since it is the most inexpensive and easily accessed. How about a perlite or straw rich cob mixture? I used perlite cob around my rmh and it insulates well while staying quite hard, that way you won't have to worry about your floor cracking. I agree with those who don't use any sort of moisture barrier, I don't think it is necessary as long as you have built up a good enough drainage layer. My climate is quite cold and I haven't felt my floor insulation is inadequate and I just used strawy cob.

Philippe Elskens wrote:Also, do you mean measuring the ground temperature when you say 'soil test'? Or do you mean testing the composition of the soil? If the latter, why would that be important?
Thanks!

Plastic Index, Moisture Content, Sieve Analysis, Shear and Compression, lab test Geo-tech reports are taken by the better natural builders to help reduce the guess work and financial losses from bad designs. We do not pay much for the test here in the USA, the issue is finding a lab to conduct them since too many builders have failed to get the proper test and when the building's fail and the clients sue the builders point the fingers at the labs so, now they only will do them for licensed professionals in commercial, no residential. Here due to foundation failures we repair all the time, code requires PI at a minimum. I get them all. The shear test is of utmost importance and requires test at the job site, that cost more.

You basically have three choices here, two to design a floor/foundation right,

Prescriptive: You find a local build and follow it to a tee.

You hire a pro that runs simulations like WUFI or BEOPT that have in it climate files with the local conditions including the soil test above and annual ground temps. The software that is based on many prescriptive builds around the globe will look at your heating and cooling options based on inputs about the building design too. In most cases inaccuracy is from inputs or people that make errors, not the software.

The third choice is to guess with no supporting data, in that case yours is as good as anyone's have fun!

BTW: My earth floors do not crack and my concrete designs are 100% natural. I rarely use a vapor barrier, only if in a very high PI or MC, and I place it in the appropriate location once I understand the soil at several site locations.

I have some experience to share in this.
I would say you want to insulate the floor from the earth where you walk. A cold floor always makes you feel cold. The foundation of your heater you do want attached to the earth. Heat transfer through thermal mass is very slow - it won't suck all your heat into the ground.

I have a cabin that is 26 x 26 feet or about 8 meters by 8. Standard plywood floor on wood joists on piers. In the center of the cabin is a very large fireplace of about 6x4 feet footprint on a foundation with brick exposed on all 4 sides (not up against a wall) giving about 150 sq. ft of surface area. There are a 2 air channels for convection airflow (hole near the floor with a "duct" through the brick to another hole near roof- cool air from the floor is heated and rises out the top hole). Of course in the winter the thermal mass absorbs heat from the fire and radiates it slowly. There's no electricity at the cabin so we rely on radiant heat and convection. But it's the summer I want to talk about.

In the summer it gets to the low 30's C with high humidity. Very uncomfortable with no A/C. The large thermal mass of the fireplace acts like a heat sink, removing heat from inside the cabin. Putting your hand on the stone and it feels like 5 or 6 degrees C cooler than ambient. It does feel like walking into an air conditioned space. It transfers heat so slowly that there is never a condensation problem even on the most humid days.
In the transition seasons, spring and fall, I will start one short fire to take the chill out of the thermal mass so the cabin doesn't feel damp and clammy after being sealed up for a week.

If I were living there full time I would say that the amount of mass exposed for cooling purposes could be a bit more to compensate for doors opening more often, cooking and body heat.
I'm planning to build a house along these principals:
For each 800 sq feet of floor space you want 200 sq feet of exposed thermal mass surface area on a foundation of 36 sq ft. This would be a 6x6 footing and an 8 foot high "cube" (6 foot walls x 8 high = 48sq ft x 4sides).
Or a concrete or block wall 12 feet by 8 tall. Your heater must be thermally connected to this "radiating surface" so you can control it's temperature.
No fire in summer and it removes heat from the living space into the earth. In winter your fire warms the mass and stops heat from being lost.

No much different than a standard RMH heater except that you need much more surface area to passively remove heat in the summer months.

If you have electricity (thus fans) you could possibly make a "duct" through which you could force warm air past the earth attached mass and do with less surface area.
Using some kind of architectural block as a wall or laying regular block on their sides(maybe something that would look nicer) would create much more surface area for passive heat transfer.

Philippe Elskens wrote:Leila, I was thinking along the same lines.
It will rarely freeze in Portugal.
However, I am not sure what the ground temperature is exactly. This will be below comfort temperatures I assume. So if my internal thermal mass transitions continuously to the underlying ground, will it be hot enough? I'm trying to figure out exactly how fast the heat would be drained from my thermal mass.

I'm no expert but definitely have read up on the subject quite a bit in the past, so will try to help out a bit

Generally speaking, the subsurface soil temperature, 1 to 2 meters down, is an average of your seasonal air temperatures bumped up or down a bit by the amount of rainfall and sunshine you get. If you have summer highs around 30*C and winter lows around 10*C, the average would be 20*C...then if you get a lot of sun, the temperature might be more like 23*C, while a prevalence of clouds and rain might bring it closer to 17*C. Hope that makes sense. There are other factors that play into it, such as soil types (heavy and packed vs light and airy, etc), water table (high water table usually drops the temp a bit), the depth to bedrock (basically, the amount of soil you're dealing with), and even the amount of vegetation (think "shading" of the soil surface), but using your averages should give you a good idea of the baseline.

An actual rough measure, rather than just educated guessing, is as easy as digging down a meter or two and taking a temperature reading (if you can call digging deep holes in your soil "easy"). This, again, wont be perfect or completely reliable, but should give you a better idea. Do it on a cloudy day (but not when raining), when the air temperature isn't "hot" or "cold" compared to normal...you know, one of those days that just seem to reflect your climate's "average". Best would be in the early spring if you're most worried about the temp being too cold - you'll be getting your reading just as you're coming out of winter, before the sun starts to get strong and the soil temperature would be at its lowest for the year. A few test holes dug at different times of the year, over the course of a few years, would give you an even better idea, but remember that just by digging down, you've disturbed the soil and caused it to react differently, so a temperature reading from the same hole (or even within a few meters of it) would give you a skewed reading.

And keep in mind, with any indoor heating you do, the passive solar heating you'll get through sunshine coming in your windows and heating up your floors, etc, you're likely to see a temperature bump from whatever that initial ground temperature is anyway by at least a degree or two (C)

So, from what I understand of Portugal, you probably have a pretty comfortable soil temperature - it's certainly not a frigid/arctic climate, you get more than your fair share of sunshine with fairly low rainfall rates and, with the maritime effect, you probably don't see summertime temperatures in the "OMG I'M GONNA DIE!" range like, say, Morocco to your south does. A quick google search showed Algarve, Portugal with an annualized average temperature of 16*C/61*F, so depending on how your region compares, you can probably figure on your initial soil temps a little higher or lower than that. I find between 62F/16.7C and 68F/20C the most comfortable range myself and that would probably be what you'd get Consider yourself lucky!

For more detailed stuff with subsurface ground temperatures and what goes into it, here's a primer from everyone's favorite repository of alt-energy info: http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm

Right now I'm just doing research... There's a YouTube tutorial on earthen floors that talks about lining with plastic, using pumice insulation, and putting in cob. Another video showed people using cob in little bricks like how you'd put in a wall, but horizontal, so that seemed like the best option for me, probably heavy on the straw. However, I plan on using cob furniture or floor cushion type furniture unless something more stable like rock is put in underneath, like for the kitchen chairs.