I am in the beginning stages of designing a rammed earth house in Eastern Canada and I have questions regarding insulation and wall thickness. I read that an 18" rammed earth wall would have an approximate R value of 6. I was looking at some 2" thick rigid insulation to use in the interior of the walls with an R value of 10. I was wondering if anyone has used similar materials in a cold climate and maybe could hep me with a couple questions:
1) Should you Double up the rigid insulation to make it 4" thick with a value of R20?
2)If so, would you increase the thickness of an 18" wall to compensate for the thicker layer of insulation?
3) Is there a minimum thickness recommended for the rammed earth wall on ether side of the rigid insulation?
If I kept my wall 18" thick and took away 4" for insulation, that would leave me with a 7" thick wall on either side. Somehow I feel like that isn't enough... I would appreciate insight from anyone with knowledge and experience on this. I'm sill learning! Thanks folks!
I'm planning a build on the other side of the country and I considered rammed earth, but decided against it because of the insulation factor. Sounds like you're planning a single wall with a layer of insulation on the inside, and not a double wall with insulation between them. A double-wythe wall is probably the best solution, but doing that in rammed earth strikes me as being a tremendous amount of labour (maybe less so you're using blocks). If you're just insulating one side of a rammed earth wall, seems to me you'd be better off putting the insulation on the outside, allowing the wall to act as thermal mass.
Personally, I've pretty much settled on a light clay straw slip approach, which gives you a single wall with good insulation values. The process isn't a world apart from rammed earth, using slip forms and such, although they are usually timber-framed. It also takes quite a while to dry, which could be a problem in particularly humid areas.
There's more info about light clay straw elsewhere in these forums, such as this thread:
Thanks for the reply. I am actually planning a double wall build with the insulation in the middle. I was commenting that at 18" I would only be left with 7" of rammed earth on either side of the insulation. I am wondering if that is thick enough and if there s a "magic number" for wall thickness that will ensure its structural integrity. I have yet to find clear info on this. If you have any thoughts I would appreciate your insight. Thanks again!
7" does sound thin for a rammed earth wall, but I suspect a lot depends on the nature and amount of stabilizers, such as lime or cement, that are used. I've also heard of rammed earth walls being reinforced with rebar, and there might be some way to brace the 2 walls together to increase their strength. While it doesn't have figures for the minimum thickness of a rammed earth wall, you may find this PDF document interesting:
Unlike insulation dynamic mass is difficult to quantify since it works under a different mechanic with lots of variables vs. steady state U-value or R-value that you find a lot of info on like min thickness per climate. Dynamic mass should be ran through a dynamic processing model like WUFI with better chances to get it right, and then verified by test. At a minimum I'd suggest calculating a u-value (thermal conduction) for your climates max cold temp differences, the inverse of that is r-value, then I reduce it by 30% for mass benefit to get a min thickness at the core insulation. Guessing is risky. You do not add soil stabilizers unless your soil type needs it, nor rebar unless in a seismic or wind zone that requires it, and I'd never use steel. The other variables that determines thickness is compression, flex modulus, and deflection. Thicker the better for getting wide load distributions to the ground especially if on weak soil with low compression and shear.
In the US goggle NM Rammed earth code or call their building and safety office for the latest and/or amendments. Follow it, it has some good guidelines but be careful it is more for SW climates. 18" is minimum, most use 24" with a R-10 insulation core as a thermal break. I would not use foam, I would use Roxuls IS Board @ 3" (staggered 1.5") for R-12 core. I have success using a siloxane (sand/natural silicone sealer in wet climates). All these thicknesses include insulation or what is referred to as "whole wall r-value and/or DBMS (Dynamic Benefit Mass Systems" " that includes the insulation, thermal mass, and any other layers. I'd run a test in the climate and check for thermal bridging. There are many cases of sucesfull CAN rammed earth like SIRE you could contact for help: http://sirewall.com/ some with little to no HVAC load in freezing temps, or net zero and off grid.
I'd also suggest reading books on the subject there is a lot to know.
Thanks very much everyone, you have given me a lot to think about. I have a couple more questions if you have time... I was planning to use steel rebar to reinforce, but you say you would not use steel. Can I ask why and what you would use instead? Also, I am wondering about the best route to attack the foundation? The house plans I have called for a rammed earth footing for the foundation... However, living in Canada as I do I'm concerned about freezing and thawing. My first thought is to use the rammed earth but to insulate the outside of the foundation. I was hoping to avoid pouring a traditional concrete foundation but it may be necessary... The battle of the frostline... Gotta love it! Any thoughts?
Steel will rust jack but may be required by code. A better option is fiberglass or bassalt rebar I was referring to for RE if needed. There is nothing wrong with concrete, rammed earth is concrete. Some believe the portland cement as a binder harms the atmosphere due to it's kiln temp which about 1000 F higher than lime that probably has just as high of an embodied energy if one would do the calculations. OPC can be reduced with pozzolans like fly ash, magnesium, geopolymer cements, to make "natural concrete" some of the best foundations world wide.
Since RE is not cheap most do not poor 26" footings at frost depth unless the soil requires it. Again, refer to your local and other codes mentioned above. Siloxane prevents freeze/thaw, efflorescence, water damage, etc...
I'm looking at going AWOL in a few years when my youngest is done school. SLC seems like the best option and I'm looking at some of the courses offered by http://endeavourcentre.org. Do you have any experience with those courses or SLC? Book learning and seminars are a great starting point but hands on real life is the best. Depending on the situation, I'd be interested in offering some hands on help for the experience.
I just noticed your post - I'm still a few years away from having the funds to get started, but it gives me plenty of time to learn and make plans. It'll be my first experience with SLC, so I'm sure I'll need all the help and advice I get if/when the time comes. Odds are it'll be in BC somewhere, but I'd also be interesting is getting some experience beforehand if circumstances allowed.
Interesting, I'd prefer to build one in BC, possibly the Nakusp area. Finances may force me to stick to the Blairmore area so I can keep my job. For the next year or two, I'll be doing some experiments and keep the neighbors curious. Well, if you ever Lethbridge way we can get together for a chat.