I am looking for advice on building a guest cabin on my property. We are in Zone 5. Building regulations state that it has to be under 108 square feet from the outside of the walls, so I am hoping to make the walls only 6 inches thick. I am not terribly worried about the insulation of the walls as with that small of a space a tiny wood stove or rocket mass stove will keep it more than warm even in the dead of winter. The idea is to timber frame an octagon from 6x6 white cedar I have harvested from my property and then fill in the walls with 6 inch white cedar rounds and cement. For the foundation I am thinking of building a platform from 2x6 on deck blocks and covering with 2x6's. Please let me know your thoughts as I continue the planning.
I don’t see why that wouldn’t work. I would go heavier duty with the foundation than 2x6 though, if you are using 6x6 posts, why not 6x6 foundation at least for the rims, or 3 2x6 together to make bonded beams with pockets for our posts. In between 2x6 should be ok providing you have a short enough span (-8’). Though cedar is rot resistant, it isn’t as strong as other species. Consider that when building floor and roof. Put some sort of insulation in the roof and floor. Even though it’s a small structure, without insulation in the floor you will find it difficult to keep the floor from freezing without roasting people out. I’ve seen a few cordwood houses where they fill foamed the rounds in place, then used chinking to seal it up. I think I’d recommend that route over using only cement. The wood and cement freeze/ thaw differently and it will be prone to cracking. Use something more flexible between logs and it’ll likely hold up better in the end. It may cost more on the front end, but not in the long run.
If 108 sq ft, then the length of each side will be 56.75". The 6x6 will reduce that to something closer to 50".
I'm wondering how strong that wall will be - especially in resistance to lateral forces. I'm sure it can carry a lot of weight vertically A brick wall is of course stacked from 4" wide bricks so I had to go and see what we know about bricks. Turns out you can build a 35' high wall that's just 8" thick ... so 6" cordwood should be ok -- assuming the bricks are properly bonded! I didn't think that was going to be my answer...
I second the idea that concrete and wood will shrink differently ... and depending on how dry your cordwood is there will be shrinkage (and possibly cement-cracking expansion). Coating the outside with cob or plaster effectively seals any gaps that might open among the cordwood, and could give you a chance to embed something like chicken wire- which if attached to the cord wood and your frame can help provide tremendous strength.
You could also do some clay slip straw and or woodchip slip formed. Depends on what you have access to.
Maybe add some fiber to the mortar and keep the joints on the thick side, I can't remember the ratios for the size of the joints and strength.
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I've been researching cordwood construction for years as I plan to build a house using timber frame and cordwood techniques. I have a few books on the topic and the one thing that comes up in all of them is that the wood will shrink and expand and you will get cracks. The books give several ideas on how to minimize the cracks, but from everything I've read you cannot eliminate it with a solid in-filll like cob or mortar. That being said, the idea of using something like a spray foam to seal in the middle of the wood and then do some form of cob tuck point to give it the traditional aesthetic is the most solid idea I've heard of on small dimension cordwood building.
A couple other notes on the cordwood;
Softwood will have less shrinkage than hardwoods.
Adding sawdust to your mortar can reduce the cracking and shrinkage.
Cutting and stacking your wood for two years to cure before building the walls is highly recommended to minimize shrinkage. Logs laying for two years is not sufficient, it should be cut to your 6" length and stacked to dry.
That said, I'm really interested to see how this turns out as I've need contemplating a similar build as a chicken coop/poultry house. I may timber frame or roundwood frame it with sycamore and cottonwood and do the walls with the same using the spray foam to at least create a draft barrier.
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Cordwood wouldnt be my preferred method for a small cabin. I like the look of it, but dislike the maintenance of it. There is a fair amount of historic cordwood in my area, and it seems to all be used for additions on barns where a bit of air circulation isn't necessarily a bad thing, not on homes. I loath mice, so looking at all the gaps in the old cordwood makes me shudder.
Could you make your logs into 56" pieces and just do log infill between your vertical posts? You could even put the logs through a saw to square up 2 sides to use less chinking and provide better insulation. I used to live in a log house where part of the house had been done that way, with notched verticals and a bit of a taper at the end of the logs so they slid in place, and it seemed to work pretty well and be fairly easy. Just an option. You could use thin 6" logs to keep your floor space if you chose (or square them fo 6").
One of the benefits of cordwood is that you can take a giant tree, use low technology to reduce it to sizes that humans can easily deal with and then (sort of) glue it back together. But if the longest wall is 56" and you have trees of the right diameter (upper portion of the tree?) then those logs are pretty easy to maneuver. And they'll seal better, etc.
I am also planning a cordwood cottage in northern Maine. Due to the significant freeze-thaws that occur up here, I am looking for an alternative method of stabilizing my 24" cedar logs. So far, the only idea I have is to stack the logs and then fill in the spaces with a foam. The stability would come from the logs own weight and the use of some log nails.
Are you stacking 24" rounds or 24" split wood? Split or cord wood is angular and thus stacks with less deadspace, but with rounds you have a LOT of deadspace to fill. That's a LOT of foam. Foam is expensive. Foam is full of nasty things and discussions of foam get you moved to the space dedicated to toxic ick. Better to find smaller rounds, split wood, branches, etc to use to fill.
Although not structural, mosses can be used to clink. I'd think cob would be preferable, adding a lot of mass as well as tightly bonding the wood. Adding sawdust or saw chips to the cob would give a thermal expansion coefficient closer to that of the wood.
Eliot, the logs are 24" long. Their diameters range from 6" up to 14". I am still dealing with the question of what to fill the spaces with. Yes, the foam is expensive. I am considering laying in some fill as I stack the logs. Saw dust & borax, sticks, etc. Moss, too, could work, though I have to gather that. But my general idea is to then come with the foam and sort of just 'plug' the ends of the spaces. Mortaring the logs does not appeal to me mostly because it is really tedious and time-consuming. Messy, too. And I am doing this by myself- me and my arthritis and bad back, lol. I might find a helper, but it is to be pretty much a one-man job.
A mix of log diameter sounds good... get some branch wood and you can build something pretty tight.
Foam as a way to air-seal the outside is possible, and if you have to use the stuff then at least you're minimizing. Mortar is a mess, its heavy, it has a lot of embedded energy and it doesn't work well ... so yeah, don't do that!
My concern is that what you're talking about, in its pure form, is a bunch of roundwood stacked and held together with some nails. Sealing the ends creates empty spaces in the middle - and I'm not sure if that's good (dead air is insulation) or bad (a place for critters to run, air movement, minimal contact). I know that your wall built with cob filling all the spaces would almost certainly perform better (thermal mass, air sealing, mechanical bonding) - but the question isn't "which is better,?" but "can you get a functional wall with rounds and foam?"
Over short distances, some types of foam can definitely act as a structural glue but I'd want to minimize the span of the foam to increase its strength. On the outside, the foam will be hit with UV light which the foam doesn't like (unless you get a UV resistant version). In that case you'll have to come up with some sort of screen (cob or paint?). I'm not sure that's making construction easier...
Eliot, I certainly do have an experimental aspect to my plans! I have been struggling to find an alternative to the cob-mortar in cordwood building for a long time now. I may yet find that there is no real good practical way aside from mortaring. The number one 'problem' is that neither rounds nor splits really sit perfectly upon one another. Mortar is what allows the cordwood to be built upon itself and turn out so visually and structurally nicely. But I am intent on re-inventing the wheel here. I will be proceeding to build by merely stacking the rounds and splits and I will decide the 'mortar' fill once I start. I may have some trial and error. I will be doing the main, insulating, fill as I go, and the question of how to fill/close the outside and inside end openings will have to be resolved once I have a stack to play with. There are good chinking products these days. And there is always a cob mix that will do the job. I am not so concerned with insulation as I am with structural integrity. Some nails or pins might have to be employed here and there. In any event, the cottage will have a good-sized masonry heater in the center and I expect no issues in heating the place what with using 24" logs.
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