Building a cob house on pre-existing garage structure

We hope to convert our garage into a studio and have found the cob method. But we have faced a few issues/questions:

(1) Our current garage has existing exterior walls (siding, vapor barrier, and wood studs), with a few tiny holes where air could sneak in, is it okay that we use cob as the insulation together with lime washes from the inside?

(2) We also hope to have a platform bed inside. We were originally thinking of just use strawbales and then patching the outside with cobs, but after watching some videos online, we are worries that the moisture inside could get into the strawbales and let insects grow. So we are not sure if we should stick to our original plan or to build structures inside completely using cobs. Any recommendations?

(for reference, we were thinking some inside structures like this home:    )

(3) we also have a pre-existing concrete floor and we are thinking maybe we should just use a vapor barrier to build other stuff upon (sand and then cob floor). We are in Illinois and weather can be cold and damp. So please correct us if this is not okay.

Thank you all!

I will try to post pictures along the way.

Welcome to Permies Lynn.  Cob is a very good patching/ insulation material.  Our house is totally compressed earth bricks with cob infill between the bricks and the studs.  The main thing is to make sure that the external surfaces are protected from rain.  I suggest linseed oil as one type of water proofing.   We are looking forward to seeing your progress pictures.

Paul Fookes wrote:Welcome to Permies Lynn.  Cob is a very good patching/ insulation material.  Our house is totally compressed earth bricks with cob infill between the bricks and the studs.  The main thing is to make sure that the external surfaces are protected from rain.  I suggest linseed oil as one type of water proofing.   We are looking forward to seeing your progress pictures.

Thank you so much for your reply. So new to this forum, I do not even know how to reply to comments...forgive me if I did this wrong.

Our garage already has vapor barrier and siding from the outside, so I guess my question is if I just do cob as insulation and lime wash +  linseed oil from the inside, will the wall unable to breathe and keep moisture within the cob over time?

Because most cob houses have the cob walls exposed to outside and inside both so that the wall can transport air and moisture, but in our case, the exterior was kind of blocked....

Thank you again so m uch for your reply and inspiration!

Lynn,

Cob will not offer much insulation - it's a dense material good for thermal mass. You should rather look at light clay (straw + clay).

For infilling stud framed walls I like light clay straw. Cob works great for really thick walls (1 foot+). With a 2x6 stud cavity you want as insulative material as possible (i.e. for natural materials light clay straw). The good thing is you can finish it with an earthen/lime plaster just like cob.

Regarding your concerns:

1) Small amounts of water getting past your exterior cladding isn't an issue. Cob or light clay straw are vapor permeable and will dry to the inside. Just use a binder(i.e. Portland cement) in your infill or plaster that will NOT(edit) hold on to moisture.

2) If you're talking about building a platform from straw bales I think it would probably work. If you're building on a concrete floor I would create a moisture break so water doesn't wick into your bales. Once the bales are plastered they're effectively protected from insects.  My concerns with a straw bale bed platform is there might be a tiny bit of give that causes the plaster to crack and the possibility of dusting.

And here's a video of infilling stud framed walls with cob and light clay straw:

 

Lynn Luan wrote:

Paul Fookes wrote:Welcome to Permies Lynn.  Cob is a very good patching/ insulation material.  Our house is totally compressed earth bricks with cob infill between the bricks and the studs.  The main thing is to make sure that the external surfaces are protected from rain.  I suggest linseed oil as one type of water proofing.   We are looking forward to seeing your progress pictures.

Thank you so much for your reply. So new to this forum, I do not even know how to reply to comments...forgive me if I did this wrong.
Our garage already has vapor barrier and siding from the outside, so I guess my question is if I just do cob as insulation and lime wash +  linseed oil from the inside, will the wall unable to breathe and keep moisture within the cob over time?
Because most cob houses have the cob walls exposed to outside and inside both so that the wall can transport air and moisture, but in our case, the exterior was kind of blocked....
Thank you again so much for your reply and inspiration!

I was looking at blocking the air gaps and not sure if any of the gaps were exposed to the outside or could get damp.  Cob efficiently moves moisture around to maintain a steady state.  As Cristobel says above, cob is not an insulator  It is a thermal transfer medium.  As the heat of the day hits the cob, it displaces the cool into the other side of the cob.  So, in summer, a sufficiently thick cob wall will cause cooling in summer and some heating in winter.
Lime wash is a great finish for cob walls.

You have done very well on the reply and comment.  If you want help with doing stuff have a look at this:  https://permies.com/w/how-permies-works

If you have a vapor barrier on the outside of the wall framing, any permeable filler material during the heating season will allow vapor to enter the wall and be stopped at the cold outer face by the barrier. This is bad. At worst, it could let the outer part of the filler get soaked and start to mold. What sort of vapor barrier are you talking about?

Glenn Herbert wrote:If you have a vapor barrier on the outside of the wall framing, any permeable filler material during the heating season will allow vapor to enter the wall and be stopped at the cold outer face by the barrier. This is bad. At worst, it could let the outer part of the filler get soaked and start to mold. What sort of vapor barrier are you talking about?

House wrap(e.g. Tyvek) it is considered to be a moisture barrier but vapor permeable. This article provides good explanation of vapor and moisture barriers.  

Tyvek would be fine; the question is what does the OP have on their structure.

Thank you all for your reply! I did not receive any notification sent to my email and thought there wasn't any responses until this weekend!

The exterior house wrap should be Tyvek but it was a diff and cheaper brand used by our contractor.

Here attached is the recent update!
We've finished the walls with some clay straw and modern insulation.
We started from doing the clay straw and then used chicken wire to hold them (our walls are 2*4, very narrow, and we experienced some clayed straw falling off due to gravity, so we used chicken wires... and it worked well).
Our zone is 5a in Illinois, it is very humid in the spring and early summer. We realized that the clay straws were not drying quickly, and we might also have insulation problems, so we switched to modern insulation to the north walls and west walls.

The current problem is that even the clay straw is too wet still and we already saw some mold growing.
But our hope is that once our weather turns hotter (we only started to enter summer this week), the mold can go away. If not, we will have to switch back to fiberglass insulation.

And due to my impatience, we will be starting the flooring soon.
Current plan is to use vapor barrier (6mil) and insulation foam. We are not sure about whether or not to use strawbales for the bed yet.

Lynn Luan wrote:
We started from doing the clay straw and then used chicken wire to hold them (our walls are 2*4, very narrow, and we experienced some clayed straw falling off due to gravity, so we used chicken wires... and it worked well).
Our zone is 5a in Illinois, it is very humid in the spring and early summer. We realized that the clay straws were not drying quickly, and we might also have insulation problems, so we switched to modern insulation to the north walls and west walls.

The current problem is that even the clay straw is too wet still and we already saw some mold growing.
But our hope is that once our weather turns hotter (we only started to enter summer this week), the mold can go away. If not, we will have to switch back to fiberglass insulation.

If you can I would run fans continuously for a while to dry out the clay straw. Sometimes a little mold will develop but it will die off once it loses access to moisture.