Two story load bearing straw bale structure? (Peterborough - Toronto)

I read a good book in 2005 about building with straw and have always loved the idea. I do remember it saying you cannot build a two story load bearing. I would like to build a square 20' x 20' two story structure (existing garage is there). First story is a garage at least 10' high, and second story is a summer seasonal bachelor pad with small bedroom and living room and bathroom with non functional kitchen (cabinets only since I can't get a permit for a kitchen). This is in Central Ontario Cottage Country Kawarthas (lots of straw building in this area apparently). Cold winters lots of snow and hot summers. There is already a cottage on the lot.

I wonder if I could build a second story on 4 piers - like telephone poles, or if there is a way to build two stories on load bearing bales (I doubt it). Can anyone here point me in the right direction? I wonder if there is an engineer / architect out there who I could hire to design the building.

Any advice greatly appreciated. Thanks in advance!

You could stick frame the outside walls of the first story and lay your second floor on those walls. I'm a big fan of fire separations between floors so the ability to drywall the underside if the second floor and protect it from fire is a truy great thing.

I would not build a new building on an existing foundation here in Canada. Especially one to be lived in. Strawbale is a superinsated building material so building it on a giant thermal bridge seems wasteful to me. I understand the desire to make use of the existing pad but often times it's more energy to keep than replace.

I should have mentioned the existing structure and foundation would be redone if this project were to happen. I'm looking for someone who might have an alternative to stick frame construction.

Alex Fournier wrote:I should have mentioned the existing structure and foundation would be redone if this project were to happen. I'm looking for someone who might have an alternative to stick frame construction.

there are all kinds of alternatives to stick framing, you can timber frame it, post and beam it, masonry, concrete... The list goes on and on, anything structural will work. Stick framing is cheap, easy and it gets the job done in a solid, structural, and long term way. If you have access to cheap or free materials with a structural value you can build the wall however you and your PE want.

I'm really keen on a fire separation between floors, garages/living spaces etc no matter what you're building. If you are doing a true second story then fire rating the floor can be the difference between life and death if something terrible goes wrong. If you're thinking more of a mezzanine style second floor then this is kinda pointless. If I was doing a wood floor on thicker posts and beams that will take some time to be consumed by fire then I would just line the underside of the floor with drywall or some other fire retardant material. It doesn't have to be drywall, but again its cheap and easy security. Some simple fire separation theory into a house can save the structure or occupants if things ever do go wrong. A kitchen or wood stove fire in a room that has wood panelling and a wooden ceiling between the main and second floors can quickly consume the building before you can control the fire, a simple fire separation material may only buy you 10m or 15 minutes but it could easily be enough to get the fire out before the structure is compromised.

If you're going to be pouring a new foundation then I highly suggest insulating under the foundation with EPS on the order of R40-60, this is a huge benefit to your building's thermal performance. You may want to allow for a larger foundation footprint then you initially think you'll need if you're only going 20'x20'. I'm seeing a few sources going towards simple structural concrete floors around 8" thick floating on 12" of EPS in our climate area for homes that are expected to perform at a higher level such as straw bale. This is becoming clear to be the most cost effective solution to both thermal mass and insulative performance.

Good luck!

I would be getting permits for all this and assume the insulation, and gas sealed fire rated drywall ceilings in a garage would be normal code.

I guess stick framing would be a good choice if the bales were cut and fitted around each wall. I wonder how well they would hold up to stucco. Normally a straw bale house holds up better without any kind of vapor barrier, which causes moisture problems in straw bale construction. Sticks would make it tricky. What kind of thickness would I need to support the extra weight of bales on the second story? Are we talking 2x6/2x8 instead of 2x4?

As to my original question, I mentioned using 4 telephone pole or something similar as pillars in each corner, outside of the straw bale walls, to hold up a second floor. I wonder what kind of beams I would need to span 20' without any additional supports.

Alex Fournier wrote:I would be getting permits for all this and assume the insulation, and gas sealed fire rated drywall ceilings in a garage would be normal code.

I guess stick framing would be a good choice if the bales were cut and fitted around each wall. I wonder how well they would hold up to stucco. Normally a straw bale house holds up better without any kind of vapor barrier, which causes moisture problems in straw bale construction. Sticks would make it tricky. What kind of thickness would I need to support the extra weight of bales on the second story? Are we talking 2x6/2x8 instead of 2x4?

As to my original question, I mentioned using 4 telephone pole or something similar as pillars in each corner, outside of the straw bale walls, to hold up a second floor. I wonder what kind of beams I would need to span 20' without any additional supports.

I would build the strawbale kindal like a balloon frame so the bldg would be bales from the foundation to the roof and then just frame up supports on the main floor level to support the second story, or you can do a typical post and beam/timber frame structure with bales stacked all around the structure. Structural hay bale or non structural you will need to build something to support the second floor. 20' is a pretty long run, it would take a substantial beam to span that far and support a second floor above.

Your next step is gong to be linking up with either an engineer or architect to work out the structural details, engineer might be the better way to go but if you want design input architect. I know of a Canadian engineer who might be a good fit you can email me if you want his contact info.

Thanks for the tip. I'd like to get more specific answers for my original questions. Not just random guesses and suggestions.

Hi Alex,

I'm new to this forum, so my response may be a bit late to be of any help... But, I have built numerous 2-story, load-bearing bale homes in Ontario. From something smaller than what you're proposing to a 2,400 square foot house, there's nothing to stop you from doing two-story load bearing. Your biggest challenge will be keeping yourself dry during the process, but a 20x20 is feasible to tarp. We've done load bearing sb before by building a scaffolding all the way around the exterior of the building, and then a scaffolding tower inside the centre of the building. It's then very easy to make a temporary roof with 2x4 rafters that rest on the scaffold. You'll need the scaffold to efficiently stack the bales and plaster anyway.

I'd suggest that you make the walls in two stages, so that there is a box-beam wood plate between the first story walls and the second. This plate will allow you to precompress the first floor bales to make the whole thing very stable (and maybe even do the scratch coat of plaster), before moving on the second floor. Your floor joists can hang from this plate, which means they aren't going through the wall assembly and ruining your insulation value and making plastering really difficult.

Chris

Chris Magwood wrote:I'd suggest that you make the walls in two stages, so that there is a box-beam wood plate between the first story walls and the second. This plate will allow you to precompress the first floor bales to make the whole thing very stable (and maybe even do the scratch coat of plaster), before moving on the second floor. Your floor joists can hang from this plate, which means they aren't going through the wall assembly and ruining your insulation value and making plastering really difficult.

Chris

Hey Chris, I like your idea of doing that separator between floor levels. What else would need to be done to precompress the first level of bales? Do you usually make the box beam as wide as the bales, and insulate inside the box? I am wanting to build a small two story cob house with bales on the north walls, and have been wondering how to go about it.

Chris, correct me if I'm wrong on this but wouldn't this system create a thermal bridge all around the perimeter of the second floor?

The bales can be pre-compressed with metal straps. The same that are used to pack skids and crates. Or nylon. The book I read also says the roof can be put on, and the bales can be left a year to compress naturally, then plastered. Not sure if that works in all climates (unless maybe the overhang is wide enough?).

Chris I'm glad to hear it can be done! The box beam sill plate is 2x4s sandwiched between ply? Stuffed with bales / insulation? I suspect there is more to it than just using joist hangers. Thanks so much for the reply! Can you tell me who your engineer/architect was? How was the inspector with 2 stories?

Sean Rauch wrote:Chris, correct me if I'm wrong on this but wouldn't this system create a thermal bridge all around the perimeter of the second floor?

Wood is an insulator. Every inch has an R value of 1. The box is normally filled with straw or insulation.

I understand wood is an insulator but its also a pretty awful insulator. My point is that you would need the wood to be 30"-50" in order to match the R value of the Straw bale at 18". So anywhere that you have wood making contact with the exterior through to the interior of the wall system you have a thermal bridge. I'm sure its done all the time but to me especially in cold climates adding thermal bridges to a straw bale house is kinda defeating the purpose.

A typical box beam for a load bearing straw bale wall is 2x4 or 2x6 on edge on the interior and exterior edge, with plywood top and bottom. The box is insulated, more typically with a batt type insulation than with straw, since the straw is hard to pack into such a small space effectively. So it's not a thermal bridge (well, the thin plywood is a tiny thermal bridge, but thermal images barely show it).

We usually use nylon banding straps for precompression. The idea isn't to crank down really, really hard, but more to achieve a level top plate and firm up the wall.

Top mounted joist hangers are what we'd usually use for the floor joists.

We've used a number of different structural engineers in Ontario for 2-story load bearing, including Kris Dick in Winnipeg, Blackwell-Bowick in Toronto and Tim Krahn in Campbellford (who we are using for all our work now).