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How do you seal and insulate a cob foundation?  RSS feed

 
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I have been in the planning stage of building a cob house in the high country of Colorado for a couple of years now. Once I think I'm all set to go, I read about something else that I haven't thoroughly researched. So . . . what is the best way to seal and insulate the foundation so I don't end up with water problems later on? I am planning to use a metal roof on the structure with an overhang of 4 feet all around. I have designed the house with support posts for the roof through the middle of the house and posts supporting the perimeter of the roof. Hope that make sense... The climate is fairly dry, but the snowfall is pretty heavy.
I would also like to know the best way to insulate a cob house. Will the insulation in the roof be enough or is there some way to insulate the walls? The entire south wall will be glass in one form or another.... a glass door and windows, to help with heating during the day.
 
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Hi Karen,
I've seen several different ways to do foundations and drainage under cob walls, it depends on climate and soils as much as the cob itself.

My favorite source for general weather detailing is Mike Wye in the UK: http://www.mikewye.co.uk/faqs.htm

There are some definite no-nos that apply to cob in particular:

- Don't use cob or earthen mortar directly in contact with the damp earth. I've seen damp spots that traveled 2 feet up a wall because a student helpfully filled in some mortar between two stones in the foundation. (Luckily, they just 'pointed' the crack, and the mortar could be removed). That particular site also had sandy soils subject to seasonal flooding, so there was lots of damp available.

- Careful with damp-proof membranes. They can trap condensation on the cold surface. Conventional wisdom for cob seems to be to separate the cob from any ground-damp membranes with a couple of courses of dry-stacked stone, brick, or what-have-you. Mike Wye is a particularly good source for this kind of detail.
So you could do a concrete or stone footing, then a layer of tar-paper or plastic, then another few inches of concrete or insulated wall blocks before starting with the cob.

- Don't try to 'waterproof' cob with concrete stuccos or other exterior waterproof materials. It traps damp and weakens the base of the wall. Instead, use dry-stone footings, or your good eaves design plus careful sloping, to keep water from contact with the base of the wall. Earth or lime plasters (with plenty of chopped straw) will shed water and protect the walls without trapping damp in the walls. A small amount of tile or facing stone on one side is sometimes OK, as long as the cob can breathe from the other side. But a full masonry footing wall, and then a transition to full cob above that, is better.

- You can also do breathable weather protection, like cedar shake mounted over purlins embedded in the plaster, or a separate shingled surface like a trellis (could create a little sheltered 'porch' or 'storage shed' under the eaves if desired). Graeme North in New Zealand has a nice white paper on primary weather protection, http://www.ecodesign.co.nz/

Climate notes: I don't know what part of Idaho you're in, so this is working from my impressions of the panhandle and nearby areas. Relatively dry, cold winters, heat spells and fire danger in summer, but enough water to grow huge crops of wheat, potatoes, and timber.
I would expect your climate, like ours, to have its highest ground damp during the few weeks of the spring thaw, when the ground may be frozen but meltwater accumulates on top from snow and roof drainage. The 4' eaves are what I'm leaning toward, to give us room to remove snow accumulation before it eats the walls.

- Slope the ground away from the walls, and have a plan for how to move snow and ice from the roof away from the walls. The 4' eaves sound like a very good part of such a plan. Will you be able to get in beside the eaves with a snowplow? Is there enough slope to ensure that piles at the edges of the roof will melt and drain away from the building? I'm thinking about including a big tank under one corner where a valley of our roof always dumps a load of snow, but there must be enough drainage to move any overflow away from the building perimeter.

- Create good drainage under walls and floors, or around the perimeter with floors graded to drain toward the perimeter. Large gravel or 'drain-rock' is often used to create stable drainage over undisturbed grade (scrape down, don't fill in, to level the drain pan). Research the usual methods for masonry walls, log cabins, or any relatively heavy wall structures in your region.
I've seen insulated concrete footings, slabs of reclaimed concrete or rock laid over a gravel french-drain, and dry-stacked rock walls up above the drip / splash / snow accumulation level and then start the cob above that.

- Footing insulation: I've seen 'casement' walls where the inside and outside are solid, and there's a layer of insulation between.
Insulative concrete blocks are also available - be sure that they remain insulative when in contact with ground damp, or learn what methods are recommended for damp protection. Some insulative concrete blocks use styrofoam, perlite, sawdust, or paper fibers for insulation; all but the styrofoam might tend to suck up some ground damp and lose some of their insulation value. There are also insulating 'sandwich' type products, I haven't worked with them but there's a lot of work going on in this area lately.
The perimeter of the footings is the most critical area to insulate. If you have insulation outside the footings and foundations, you don't necessarily need it under the floor as well. (assuming the ground stays dry under the house, due to careful siting and drainage).

- Thicker walls can both hold heat, and take longer to transmit heat. But R-value is not a great guide to thermal mass performance.
If cob's R-value per inch is something like common brick (0.20 per inch), then an R-value like R-36 would take a lot of wall - about 15 feet thick! In practice this is never done, about 3 to 5 feet thick is the max. unless you're doing an earth-bermed home of some kind. Architect's R-value page
The benefits of good insulative materials like straw-clay, foams, cellulose, or batting (fiberglass or wool) are that they can deliver similar insulation in much less space - if protected from wind and moisture. Cob with a lot of straw should be much better insulation than brick; here's a page that suggests light straw-clay should give the equivalent of about R19 per foot. www.designcoalition.org

- If your roof is structurally supported without reliance on the cob, you can do a lot of things with the walls to suit the climate.
I would consider a very insulative cob mix. In Idaho you can probably get a lot of straw pretty easily (we sometimes have to ship it from Idaho to get a reasonable price on straw in Washington). It can be applied as a straw-rich cob, as infill panels (coat the straw in clay slip and pack it into forms), or as ropes of clay-dipped straw woven between supports (pajareque / a straw version of bajareque or wattle-and-daub).

- Your climate includes some very cold periods, and cob is not typically considered great insulation. In northern Europe, traditional earthen buildings often had very thick walls (2 to 3 feet or more), insulative roofs (thatch), and might include a wrap-around porch or byre (animal shed) to form a buffer between the warm core of the home and the cold exterior walls. Turf was sometimes used to create a second exterior wall outside the first (e.g. viking-era turf longhouses).

Cob is great at storing passive-solar energy (especially when protected from outdoor temperatures by insulation). It's also very good at moderating the hot-cold swings of shoulder seasons, or averaging out the hot days and cool nights of summer.
- One modern natural building method that is sometimes used in interior climates is "cob-bale": a straw-bale wall (especially on the north side), plastered with 4" to 8" of cob on the inside for thermal mass, and a thinner plaster layer (2" or 3") on the outside for strawbale stiffening and longevity. You could do cob interior walls to capture that great passive-solar exposure from your glass wall, and make your own determinations whether the east and west walls should include strawbale, a layer of light straw-clay, or just thick cob.

- A full wall of glass sounds iffy for my understanding of Idaho climates: could lose too much heat at night, compared to how much you might gain during the day. Insulative curtains or shutters will help, of course, with that.
There are calculations for how much glass wall and how much thermal mass are suitable for a given climate and solar exposure - general rules of thumb are about 4-6" of thermal mass, about 6-7 times the area of windows that are letting in the light.
Here's a general intro to the passive-solar design process: http://www.sensiblehouse.org/nrg_passive.htm
a solar gain calc out of Seattle: http://www.susdesign.com/windowheatgain/
and here's one that seems to have some good DIY resources for any type of solar collection project: http://www.builditsolar.com/SiteSurvey/site_survey.htm

- Layered materials, such as screened porches, drapes, and hollow walls with plaster surfaces, are great assets for heat conservation. Each layer traps still air (a good insulator), and helps prevent drafts and leaks (removes heat through infiltration of cold air and loss of warmed air upwards). Cob is very good at sealing out drafts, and plastered surfaces are also good for sealing. Windows, doors, and any electrical or plumbing outlets tend to be harder to seal against cold air leaks or conductive bridging.

- Wraparound porches and wind screens are also a great idea, and might be easy to do under your 4' eaves between the posts. They can help protect the cob walls against any intense weather conditions, if needed, and also trap insulating air and block cold winds. If you have sideways rain from a particular direction, make sure that the eaves are relatively low on that side, or consider partial screens.

Hope that helps!
-Erica
 
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Hi Karen,

Erica has given you some generally good counsel on this...sorry that we didn't respond to you sooner. You got some great thoughts and details in Erica's post. For me, as a builder, I just am reluctant to go too far into details. To give detail one must understand the building site, have photos, and most importantly a design. Without these it is really difficult to give more detail, so generalities are about it. It would be great to read more and follow along as you start to fashion an actual plan, and design. Are you going to build yourself? Have you done any of this before? If not, do you have an experienced builder? A natural design/builder will answer many of these questions, and you can always check here for more certainty or to vet their approach.

Good Luck,

j
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