Thermal inertia as a negative

Hey all,

I know cob has great thermal inertia which helps to minimize big temperature changes throughout the day.

It seems like the exterior walls would hold the thermal inertia of the outdoor temperature, say 32 degrees. So if it's consistently cold outside for the whole winter, then isn't it harder to heat the interior space if the walls are always cold?

I can see how cob is great for interior items like a rocket mass heater, or passive solar floor, but I'm thinking that an exterior wall would be hard to warm and therefore make the interior cold.

Many cob structures have been built in climates to which they are unsuited. Popularity and the desire to work with it, has led to many convincing themselves that it's right for them. For those using free heat sources, the efficiency factor is less important. Sometimes it is insulated.

Hello Guarren,

I must second Dale response on almost all aspects...

There is more Clay-Cob-Adobe-Bousillage-Bajareque being built today than probably in the last 100 years in North-South America. I worry that much of it is out of context, built poorly, and promoted from a point of "romance," and "naivete," not sound natural building experience or understanding. Very much as is SB, (straw bale) which is often being built by novice without sound architectural understanding. Your concerns can be address, but you are wise to address the issue, especially for colder climates. Thermal breaks that can still maintain vapor permeability (breath) is crucial to clay mass walls in colder climes.

Regards,

jay

I agree. Oak Ridge National Labs has done lots of testing on thermal mass walls and most experts agree that they tend to only make sense in high desert climates that experience wide diurnal temperature swings. Thermal mass performs best when it is completely inside the thermal envelope (air barrier and continuous insulation layer).

http://web.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/

Almost all climates benefit from some insulation and the colder the climate the more important it becomes.

Its true that free fuel sources may benefit less but other than the sun, all fuel comes with a price. If its biomass (wood) that you heat with then you have to consider that wood gathering can be dangerous, hard on the body, takes time, can negatively distribute invasive insects, leaves less for hugelculture or building native soils, introduces more risk of fire and indoor air quality concerns.

We're in a high desert so maybe it's ideal for earth buildings, but I'm happy with thermal mass of our rammed earth buildings, uninsulated, in cold winters. The downstairs walls are 2 feet thick, the upstairs walls are 1'6". The whole north side is bermed into the earth (good south facing siting), about half of the south wall is covered with seasonal attached greenhouses all winter, and since we designed to a long E-W axis, there's only small walls on east and west. They don't have any insulation, are just cob-like rammed earth (ie not very tightly rammed), and they retain a good feeling throughout the year. We don't use any backup heat and we run activities all winter. Yeah, it gets colder than would be considered acceptable in the US, like low 60s and for the worst few weeks of winter, 50s. But then again our windows are terribly leaky so it would stay a few degrees warmer if they were better, and our roof is a traditional construction style, not highly insulated, which would also help with several degrees. We enjoy the seasonal thermal mass, of cool until July and warm until December.

Hi Rebecca,

It sounds wonderful where you live, and probably how you live as well. Part of the issue with descriptions like yours, and it was an honest one as I know your area, but it still feeds the romance of this building form, and not it's functional reality for most living conditions. As has been said dry desert regions of the world are going to respond well to this form of architecture, other location are going to have to be altered from the standard and better understood by the builder. I would even consider, as I am familiar with many of the Himalaya earth, timber and stone architecture, as well as the region (from hımış in Turkey, newari, bhatar, dhajji, leppa, and many more) all can often be better built as the region has lost many of the master builders over the last century. The structure you described, while still stay honest to the traditional style and materials, could be vastly improved.

I believe that is what Dale, Brian and I are trying to stress to folks. If you are considering cob architecture, it is a lot more complicated to build well than many are suggesting. It needs to be designed to the region, and there are vernacular and ancient traditional styles that are applicable but few understand or build in those styles today, which puts what they "do build" out of context, and often not as efficient, nor will they last as long.

What's the cheapest well insulated above ground construction technique out there?

Hi Guarren,

I am trying to break the habit of saying cheapest as when I use it I mean "not too good," or "modern." Lets say "less expensive." That is a hard one to pin down as it will be region specific and then you have other consideration like identifying whether less expensive also means easy to do from a labor-work perspective. I would feel like I was distracting from this post thread on "thermal inertia" if I got into that here. Perhaps you could start a new thread, and if you wanted I could expand the answer and/or get more specific questions regarding insulation types and their applicability to natural and traditional building.

Warm Regards,

jay

A cob house or other uninsulated thermal mass would tend to split the difference between nighttime lows and daytime highs.

Proper passive solar design strategies can certainly push a house towards warmer or cooler than ambient temps, but if the calcs don't add up, you've got a real problem on your hand that is going to require constant energy inputs in order to correct, whether it's heating or cooling - and heating or cooling a big thermal mass could be a major task.

If someone is new to earthen building, or building in a climate that has not had a lot of earthen building, there is risk of major errors. I agree that the romance may sometimes be overriding practicality - lots of cob & strawbale houses out there that have assorted problems - first and foremost with condensation & mold which can make a place completely unlivable.

We could use the permaculture advice that applies to planting exotics that are not tried-tried-and-true in a given area: Plant on a small scale with careful observation before moving to a larger scale. In other words, if you really want to try cob or other thermal mass in an area where it hasn't been done successfully, build a little kids playhouse or 10X10 guest bedroom first and see how it performs.

Cob structures can be relatively efficient even when not ideal climate-wise simply because they tend to be much smaller due to their labor intensive nature - less space/mass to heat/cool.

Cob-cordwood is something that offers a somewhat better r-value than pure cob. Light straw clay another option. One can do the north wall in something with better insulating properties and a southern wall in cob. Or one can do a thinner cob wall inside an envelope of straw bale.

Lastly, when designing keep in mind that any particular function, such as heating/cooling should be supported by multiple elements in the system - passive solar + rocket stove/cook stove + solar thermal pop-can heater, etc.

And here i am thinking I'm going to conquer the cob house market in the north east. I suppose i'm here to learn and needed a reality check. Between this and rain and lyme wall wash sensitivities it doesn't look good. I suppose breathing of the walls is a concern but i suppose our normal house doesn't breathe then anyway right. i think thats more for the natural living style. What if you went ahead and framed the inside and used pink deadly isulation, finish with sheetrock to at least deal with temperature? I suppose then all i need is some plywood on the outside and siding.

But again rain, snow and freezing, Is rain still surmountable still in my romantic little house? for breathing of the walls and limited durability of friendly paints and plasters could i use evil jimmy dean paint/ plaster on the outside and maybe get some weather resistance or will this inhibit the drying as well as being offensive to the whole aesthetic? I'd put concrete in the mix but don't think it will help with hand cobbed stuff, maybe some poured claycrete mixture as fill for more normal walls but then its not a little pottery house.

if we can not worry so much about rain, and if my insulation and proper insides with even official heating can handle cold, what about mold and any issues with an earthen wall against isulation and would i wany plywood even between my insulation and cob wall?

Sorry to hijack the thread i would ask them in another thread, i'm just getting a glimpse of what its all about after the people who turned me on to this kept telling me you can't do it here and i couldn't listen to why.

Hi Morven,

I don't believe I, or anyone else is necessarily saying "don't do it," in regards to earth architecture. What we are saying is:

1. Don't believe it is easy or low in labor...it is far for either of those.

2. Do not romanticize the craft as so many have.

3. If you decide on using earth based architecture, do not use it out of context, or the vernacular for you region, or climate.

4. Research it's many forms well and consider all comparative vernacular natural homes for your given climate/region before settling on a choice.

If you consider those four points, you should be fine with whatever you finally decide.

Rain is not your problem. Cob has proven itself in rainy Wales as well as many buildings in PNW. Earthhaven is technically a temperate rainforest, and they have lots of cob and strawbale, mostly problem free. The good boots and hat adage applies.

Long, very cold winters on the other hand are going to be a significant challenge, if you have them and must be designed for.

As for the northest, I have a few handprints in a cob building at Sirius Ecovillage outside of Amhearst, MA. Quite a nice building when I saw it finished, but I never visited in winter. Might ask some folks over there how that building performs. I don't know of a lot of other cob in the northeast, but I'm sure there are several tucked here and there.

Be very wary of mixing modern materials with cob - there is a high likelihood of creating some sort of moisture barrier that will end up trapping moisture in the cob, rather than retaining the natural breathability of cob.

I found some north east specific sites. There is some cob in Vermont (of course) and Canada. I am in a forest with rocks and mountain, these are fine building materials but I don't want to drag around large trees and boulders even if i get land with stone. Living in a castle would be neat but, Seems expensive and less possible to do on the cheap or at all but i'm sure it was never as easy as it seemed. If something cob will stand and just be cold they could be summer buildings or some kind of camp. Back to my aluminum spaceship idea, tunnels carved into rock, and mini log cabin. The other site also suggested it's not heating it and keeping the heat it's heating it to warm from a cold start. Living in it you never let it get so cold that you're going to have to heat the walls back up. Still requires energy and insulation but maybe not a huge drain.

If I framed an interior and insulated it, I would just be using cob in place of plywood and siding or just in place of siding (which it makes a horrible siding). It'd be more aesthetic like stucco and i could just do a thin layer (facade) to get the appearance of a cob house but then that would possibly need too much maintenance and restrict the rounded possibilities of working with cob. Lots to consider thanks for your help.

Morven Glas wrote:I found some north east specific sites. There is some cob in Vermont (of course) and Canada. I am in a forest with rocks and mountain, these are fine building materials but I don't want to drag around large trees and boulders even if i get land with stone. Living in a castle would be neat but, Seems expensive and less possible to do on the cheap or at all but i'm sure it was never as easy as it seemed. If something cob will stand and just be cold they could be summer buildings or some kind of camp. Back to my aluminum spaceship idea, tunnels carved into rock, and mini log cabin. The other site also suggested it's not heating it and keeping the heat it's heating it to warm from a cold start. Living in it you never let it get so cold that you're going to have to heat the walls back up. Still requires energy and insulation but maybe not a huge drain.

If I framed an interior and insulated it, I would just be using cob in place of plywood and siding or just in place of siding (which it makes a horrible siding). It'd be more aesthetic like stucco and i could just do a thin layer (facade) to get the appearance of a cob house but then that would possibly need too much maintenance and restrict the rounded possibilities of working with cob. Lots to consider thanks for your help.

If you wish to insulate a cob house, do it on the outside, not the inside... with some sort of breathing insulation A cob wall is 2 foot thick, there is nothing that says it must be a constant mix from inside of the wall to outside. The outside can be mostly clay straw and less sand, or as mentioned above straw bales can be added on the outside. Really, once the winter temperature stays cold, annualized heating makes more sense. Some derivative of the pit house where the living space is more firmly connected to the ground temperature below the frost line. There are a number of these (Wofati, Earthship, PAHS) including the concrete variations. I would not suggest an actual pit house unless you are really willing to acclimatize yourself to colder living.... I have seen children playing in the water in swim suits with 6C temperatures on "hot summer days" where most of the year is sub zero and yes I was wearing a coat So I know the human being can get used to much colder living than we are used to. I just don't think that is what any of us want.

BTW, Canada is a big place, climate ranges from rarely below freezing (here where -5C really cold) to never above about 6C(Frobisher Bay happens to be the coolest place I have lived) or colder. The cob buildings I have heard about in Canada, are on the wet coast.... spelling intended.