All natural build - roof insulation

Hey yall! I'm brand new here, but have been doing reading here and elsewhere for years now. I've long dreamed of building an all natural, passive solar home for myself that would entail a small living space and a large workshop. More recently I'm getting to a point where I may actually have the cash to start building things, so I'm trying to get realistic about what is possible. To that end I really want to start with building a small workshop of bale and cob with a round-pole living roof to figure out how viable my plans might be. I live in Western Massachusetts and the space I have available isn't viable for passive solar, so I'm starting with a fully enclosed and insulated build. My plan is straw bale walls, packed with cob and covered with about 6 inches of cob on either side (also known as bale-cob), with corner cob pillars (2'x2') and cob pillars at minimum every 6 feet along the walls. The roof would have a 1:12 pitch and it would have earthen floors. All on a concrete bound stone stem wall (1.5') and a rubble trench foundation with a drain pipe at its base. Heating would be accomplished with an RMH with a cob bench.

I have extensive construction experience and have worked with natural materials in the past for exterior landscaping projects, just never to build a structure. I would be doing all of the work myself with no hard limit on how long it takes me.

The big thing I have yet to figure out is how to insulate the roof. I'm totally fine with leaving the ceiling open, especially since this will be a workshop. I've found suggestions here to use spray foam or to lay foam board above the roof cladding, but under the EPDM liner, but I really want to avoid those materials. I'd prefer to use wool or rockwool, but I'm concerned about moisture. If I use one of those materials secured with battens or furring strips to keep it against the roof will that be adequate to keep it dry? Are there other downsides to that? Any other suggestions are very welcome!

Also if anyone has thoughts on other aspects of my plan I'd be more than happy to hear those too.


Thanks so much!
Keira

I appreciate your resistance to foam. It is horrible stuff.

Condensation will occur. The task is to foresee where it will occur, and provide egress. Ventilation and drip escape are the two things that come to mind. If your insulation is flat against the roof surface, it may not readily dry when condensation forms. An air gab between insulation and membrane may be considered.

The 1/12 slope in wet area is going to be an obstacle.

I am assuming you plan to use rubber roofing for your final roof surface?

Can you put roof boards above your rafters, then a vapor barrier, then your insulation, then possibly another layer of roof boads and finally your rubber roof?

The insulation could be straw bales or or loose straw.  

If the roof was a bit steeper maybe you could put the rubber roof directly above the rubber roof material.  But on such a flat roof any unevenness in the bales would cause puddling.

I am doing something similar on my house (using 12 inches of second hand foam instead of a natural material)  It sounds like a convoluted system but every part serves its purpose.  This is how mine is being built.
  Rafters- support the roof
  Roof boards- interrior surface of ceiling and prevents roof from racking side to side
  Rubber roof -used a temporary roof, vapor barrier and prevents stuff falling through the gaps in roof boards
  2x4 vertical furring- placed directly above each rafter and screwed to rafters with 16 inch screws.  Holds the foam down, anchors roof to rafters, and provides air gap from eaves to ridge to prevent moisture.
  2x4 horizontal purlins- provides a surface to attach steel roof to.
  Steel roof-final roof surface.

I may include a layer of radiant barrier  between the vertical and horizontal 2x4's to act as moisture control and a radiant barrier.

When I first started designing the system I thought it was crazy but once I did some research it is a pretty common way to do it if a person wants the rafters exposed AND wants the roof insulated.

Keira, I have a balecob house in Montana with 4" of cob on outside and 6-8" of cob on the inside. While this structure is a joy and works extremely well with maintaining temperatures. I've realized that the amount of mass on the inside is overkill for such a structure, especially when in combination with an earthen floor. If you were deciding on balecob instead of purely bale based on the desire to maximize your U values inside, then a mass floor with thick clay plaster is definitely adequate. I know you didn't ask for advice on this, just thought I would mention it.  Though, I've walked away from my house in the dead of winter for a week and only had the temp drop from 75F to 55F with constant negative temperatures, so all that mass does do something.

As for the roof insulation, I suggest a moisture barrier directly above the ceiling and followed by your choice of batting, sheathing, and epdm on a cushion of some sort. Normally I would say that the cushion layer could be polyiso foam, but you could use anything really.

Hi Keira,

Welcome to Permies.

Hi Keira,
Very cool to hear about your ideas, balecob appeals to me very much as well. I'm also in MA (although Eastern) and very interested in learning and working on natural buildings - let me know if you'd like any help with your buildings.
Stumbled upon your thread while looking up ideas on roof insulation as I'm planning a small project and/or retrofit. Have you considered HempWool? I've seen this used for roofs. Product guide: https://www.hempitecture.com/_files/ugd/86419d_0f6e16e6e3ba48e687958ff5806e8b9d.pdf
- Emily