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Could someone with experience explain the process of insulating a roof on a cob structure?  RSS feed

 
james Apodaca
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Subject says it all - I'm researching cob building due to the high availability of sand and clay in my area and have yet to see anyone explain the process of roofing a cob house in any specific detail.
Are the roofs your building on your cob houses traditional in function and construction?

Example: Ventilated soffits under eves to wick moisture away from the moisture barrier (that is the roof) utilizing Truss and Timber with insulated voids?

Traditional building practices leave room for an attic space with, generally fiberglass or blown cellulose, insulation between the trusses. I'm not noticing the same practice being used with cob.

A lot of what I'm seeing from places like CobWorks, Barefootbuilder and others appear to be flat wooden surfaces on some sort of sloped truss (be it 1"x6" or roundwood) abutted up to the load bearing walls with no air exchange or insulation.
I can't help but to think that these roofs sweat profusely due to temperature differences..



What am I missing?

I would like to plan on using tin sheeting on plywood laid on trusses leveled out onto the cob structure but my thought process won't deny the lack of ventilation and the possibility of condensation buildup - especially in our humid environment.

Also, how do you go about fixing the roof to the cob structure to ensure the roof isn't blown off in high wind conditions? I've seen adobe construction where anchors for the trusses are actually molded into the wall. But I've seen other videos/imagery which show the roofing process where the roof is simply laying on the walls with no explanation of anchoring..



Could anyone please clear this up for me? I feel as though I'm missing something of high importance here.

Edit: Just realized those pictures may be copyright.. If I'm violating some agreement that I didn't read please delete them.
 
Jay C. White Cloud
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Are the roofs your building on your cob houses traditional in function and construction?


Well yes, considering that cob and related earth structures have been here for about 10000 or more years. It may be necessary to understand that cobb is not the "new thing"...it is the original and what folks have been "fooling around with" in the last 100 years is the new stuff.

Example: Ventilated soffits under eves to wick moisture away from the moisture barrier (that is the roof) utilizing Truss and Timber with insulated voids?


That is modern thinking...

For one thing, there are so many different roof systems found above cobb and earth structures that it is going to be really hard to answer your question without you choosing a design modality...

It would be like asking about a car that you haven't even selected and bought yet...there is no context.

Most cobb structures are not always going to have a modern 'vented soffit and none (at least well built ones) will ever have a moisture barrier of any kind.

Traditional building practices leave room for an attic space with, generally fiberglass or blown cellulose, insulation between the trusses. I'm not noticing the same practice being used with cob.


Ummm...that is a little...ummm? ...?...backwards??

You mention traditional building practices...then write about attic space and fiber glass or cellulose with trusses? None of that is anywhere near "traditional" or remotely related to any vernacular structures as traditionally built, as most did not have insulation, or did not use it the way we are told we need to today...

Cobb is traditional...so it must be understood that way and in that context...This lack of understanding and intimacy with natural and/or traditional building practices is what is really causing some "bad" practices out there in trying to make these building fit modern goals. The goals can be achieved but not without treating the means, methods and materials in a traditional context.



A lot of what I'm seeing from places like CobWorks, Barefootbuilder and others appear to be flat wooden surfaces on some sort of sloped truss (be it 1"x6" or roundwood) abutted up to the load bearing walls with no air exchange or insulation. I can't help but to think that these roofs sweat profusely due to temperature differences..... What am I missing?


Short answer...yes...you are missing a lot. However, don't feel bad, it really isn't your fault. There is way more bad info, and misinformation out there on these modalities than there is a complete and comprehensive picture. Many of these folks have "stumbled" to where they are (some haven't even got a clue" so don't always paint (nor can) a complete picture.

I would like to plan on using tin sheeting on plywood laid on trusses leveled out onto the cob structure but my thought process won't deny the lack of ventilation and the possibility of condensation buildup - especially in our humid environment.


Well that is one way...but...that is not anywhere near a "natural and/or traditional build" and mixing modalities often leads to major failures. So pick modern "stick building" and contracting and the way it is understood, or begin to really open up to traditional and natural building systems and modalities. Folks here would love to help.

Also, how do you go about fixing the roof to the cob structure to ensure the roof isn't blown off in high wind conditions? I've seen adobe construction where anchors for the trusses are actually molded into the wall. But I've seen other videos/imagery which show the roofing process where the roof is simply laying on the walls with no explanation of anchoring..


Gosh...again...your cart is so far in front of the horse that I am not even sure you have a horse yet...and the cart is from a "box store."

We need location, size, goals, architectural type (and don't say Cobb, as that would be like me say I want food to eat...) one story, two? flat roof, pitched, traditional living, infill or structural wall, and the list goes on for about...hmmmm....at least 3 pages of print this size.


Could anyone please clear this up for me? I feel as though I'm missing something of high importance here.


I would love to, but lets start with the basics of "when, where, why, how," stuff first...

If there is not a profit being made from copied pictures most of the net hosting protects reproduction...I fixed your links to them...

look forward to helping where I can...

j
 
james Apodaca
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Ha! forgive my bad terminology. I clearly meant 'modern' practices in place of traditional. I'm familiar with the way my father would build a house and put a roof on it, the way the army corps of engineers would build a house and roof it..

Please understand that I am trying to wrap my head around the most important part of shelter through that lense.

I understand that Cobb structures don't utilize moisture barriers, in the walls, however a roof can most definitely be defined as such. Unless we're talking about thatch.. That's a whole different story.

Which is why i ask. Maybe i didn't phrase it right.

Is it the "aspiration" of the walls and sheer thermal mass of the structure that keeps condensation from building on the ceiling/walls when the sun hits the roof..?

I'm just trying to figure out how the roofs on many of these structures are working without a ventilated, insulated, air gap. Seems to me the roof would have little thermal mass and heat up quickly.. Causing the ceiling underneath to sweat without the air exchange.

I haven't seen this addressed anywhere (Maybe it's not an issue). I just had to ask to satisfy my curiosity.

Edit; no clue why that posted twice.
 
Brian Knight
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Location: Asheville NC
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James, your thoughts and questions are excellent. Your desire to use available resources in a conscious manner is the same as those who were creating shelters 10000 years ago. I think 99.99% of ancient builders would be seeking the same wisdom and common thinking of "modern" building techniques.

Roof venting as a modern practice has an interesting history. It was originally intended to prevent the problems you describe, preventing the build up of humidity and condensation. Modern building practices, as I see them, handle this concern better by creating a vapor barrier between the house and ground interface (if there is one), an airtight ceiling, and spot or whole house mechanical ventilation. There isnt much science to suggest that roof ventilation is a good way of dealing with interior humidity.

Preventing ice dams are another good reason for including roof ventilation but again, an airtight ceiling with high values of roof insulation is more important. Considering that you are in a climate where mass-only walls might make sense, this shouldnt be a concern for you. Mass-only walls or not, high levels of insulation on the roof is a good idea for controlling the thermal influence of the sun and cold outdoor temps. This increases comfort and reduces the fuel needed to stay comfortable.

Most of the homes we build feature unvented roofs. I think traditional vented attics offer a bit more resiliency in that its easier to locate roof leaks. Still, with an airtight ceiling, the science of un-vented roofs seems to be fine to me.

The structural concerns are out of my expertise but there are many clever ways of tying down roof framing to walls. If this a major concern the walls should also be tied down to the foundation. 1/2" threaded rod from roof framing to foundation could be an option worth exploring.

http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-attic-venting

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation

http://www.jlconline.com/insulation/roof-ventilation-update.aspx
 
Bill Bradbury
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It seems that you need to look into bond beams. This is the portion of an earthen wall that ties the walls together and provides the attachment point for the roof. In my adobe home this is wood, but modern practice is reinforced concrete.
The reason traditional homes with no insulation or air space do not condense moisture is they are burning wood indoors in cold temps, moisture migrates in the direction of heat flow through the vapor permeable walls and circulating air currents prevent condensation on the ceiling.
Jay tried to point out to you that a complete system must be designed by someone with deep knowledge of both traditional and modern technologies in order to build a high performance hybrid home. Most of the traditional/modern hybrids are heading toward trouble with the very problems you have stated.
Have you looked into local vernacular systems of building? There are a few buildings in the US that are pushing 1000 years old, how long will your design last? We all copy and co-opt what others have done, look to the past and then interpret modern buildings through that lens.
Personally, I like to have a vented area between the hot roof surface and the insulation in the attic space. This can be incorporated into "vaulted" ceilings as well as any other.
 
Jay C. White Cloud
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Bill Bradbury wrote:Most of the traditional/modern hybrids are heading toward trouble with the very problems you have stated.


This is a very import point that Bill has shared!

Reinventing Wheels of architecture over what we know already works, Believing that "airtight" wrapped in plastic architecture is a solution, and not fully understanding natural/traditional systems and instead...relying on "new age" mechanically and technologically depended "concepts" is a pathway to disaster and often complete failure...in time...of most modern "hybrids." So unless we are designing a "space ship" lets get away from using the term "air tight" and try to use these traditional-natural modalities to achieve thermal efficiencies.

I have written it countless times and in plenty of locations that "draft proof" highly efficient homes can be built that are not mechanically/technology dependant, built from all natural materials and have been for a very long time.

"Breathing walls" (aka rain screen walls) and "cold roof" design has been around for centuries...perhaps even longer. So venting isn't somethin to ignore nor has it been. Few earthen style architectural forms use thermal mass as a roof treatment unless we are speaking of Wofati" and other fossorial architecture or of true dome and vault styles.

The architecture will have to have some type of environmentally sustainable high R factor insulation (many to choose from) if the above mass style is not employed.

When a final architectural choice is made, if it is within the area of natural building, I would be glad to help further and answer more questions. Until then much of this is only going to cover very generic principles.

Regards,

j
 
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