help with wall design & general design for round pole timber frame

Hey all,
I'm building a round pole timber home - starting this year- with a designer from Whole Trees in Madison, WI and another builder who is familiar with straw bale builds. The house is shed style roof, south facing with 2 exterior bents and one central. 24'x36'. Small lofts on E and W wide that you can only stand up in for a couple feet. I'm familiar with the issues that can form with south style sheds that have the back of shed draining to uphill side, but I do plan to have gutters on this side, french drain, bring in sand/stones, sculpt land. It's to be built on a S-E facing (dry and windy) slope probably with W and N small berms (a few feet). I'm wary of berms, but the builder's house is bermed with concrete and has lasted a long time. Wood stove with backup propane (rarely used hopefully). Slab on grade. I will attach initial drawings but they are being edited.

What I'm not so familiar with is the wall systems and I'm having trouble getting a hang of it. We originally planned on doing straw bales as it's a dry south facing hill site, but lots of folks I'm asking say straw bales can have major moisture issues in the temperate climate of southern WI.  I know it depends on the design/build, and I'm leaning toward no bales on N wall where there isn't much sun, and maybe straw/clay on S wall where it will be dimensional lumber due to number of windows and bales were never the plan there. 2x8's and infill with cellulose is other option on S wall but then it's a question of plaster+cellulose in contiuity with E-W bale walls, or wood siding over cellulose and dealing with the interface of wood siding/plaster. I'm mainly having trouble deciding on E-W (originally bale) wall design, and options for N wall. S wall is easy with dimensional lumber. I'm considering doing a double wall infill system but I'm having trouble grasping the details on how this works with round pole timber. I guess my question is 2 parts.

1) Firstly, I would appreciate any tips on keeping moisture issues in check with straw bale in temperate climate. What I do know is overhangs are important, toe ups, vapor permeable walls (interior less so than exterior), some sort of ventilation system although we are off grid, are ok with small fan but do not want to rely on big batteries forever. I know alternatively you can have an air barrier on interior wall but that seems tricky. I'm concerned about condensation on interior of walls in summer. The designer also wants to put bales on roof for insulation, which is cool but unless we know exactly where air is flowing I'm concerned about moisture getting trapped. We will also be building with green timbers, although we could let it stand over winter with roof to dry. I assume most people do wrap around bales?

2) Secondly, I'm having issues grasping this built up infill wall for timbers which is another option we are considering. I think we would want timbers exposed to inside, not outside, but do some people enclose timbers in wall once dry? Can I use wood siding on outside of round pole timbers as the sheathing, then have light framing on inside and fill with cellulose? What could be issues here if timbers are dry? There would have to be vapor barrier on inside wall right? I don't understand with the double wall how it can be built on the outside of the timbers without majorly increasing overhang and therefore using dimensional lumber as load bearing. Or why people would build the double wall on the inside and expose timbers. A My friend who has a timber frame double wall system says he used foam board to help build a secondary wall to use cellulose.

Any tips on this stuff would be greatly appreciated!
-Courtney

Courtney

https://permies.com/t/120/38510/berm-sheds

Is a  link with some serious discussion of various roundwood topics in action, so to speak. You can read forward or back and see if anything helps. Or you could give a shout to Kyle Neath and see if he has a few moments to discuss or refer you.

Or others with actual knowledge may show up here and hit the bulls eye. <g>


Cheers,
Rufus

Hey thanks Rufus,

I have followed that topic - I was previously thinking wofati style. I will see if Kyle Neath can possibly lead me in the right direction. I'm looking for books, website, experiences, etc!

-Courtney

Timberframes are great works of art when built correctly.  I’d hate to go through all the work of building one only to hide the structure inside a wall.  My opinion is that the insulation and siding, both interior and exterior, should go outside of the frame so it is exposed to the interior.  I know that this poses some additional challenge since you are using round timber instead of square.  If it were mine to do, I would probably take some dimensional lumber and scribe fit a board to the exterior side of your frame, then insulate in between.  You will probably end up framing in the whole wall so start in one corner and pick a point of reference on the corner of your first scribed boards and stick with it.  Make that board plumb from that reference point. Then do the other corner on the same wall.  Use some masons string along the 2 corner boards and scribe every post along the way.  For the space between the posts, frame it in too but you won’t need to scribe anything, just make the 2x framing material flush with the outside edge of the scribed boards (your masons line). Then you can insulate in the framing, with most of the insulation on the exterior side of the timbers.  Use a vapor barrier on the interior side of the building and some sort of house wrap outside before you close it in with siding.  DONT forget to insulate your corners and rim joists!  If you really want to make it tight, use Building Envelope Sealant instead of staples to adhere the vapor barrier, but it’s a messy petroleum product, so you may not want to.

Some issues you will face with using green beams are shrinking and checking.  Depending on the species, your beams will shrink up to 3/4” or more per foot of diameter once dry.  But, they shrink very little in length.  If you frame to the exterior of the timbers, then most, if not all of your insulation will not be affected by this shrinkage.  You will however see the shrink along every log if you do your interior siding in the first couple of years.  One way to manage this is to go ahead and put on your interior siding and then scribe fit a piece of trim along each log.  Pin it with some finish nails and in a few years when the logs are done shrinking, pull them off and move them over to hide the gap.  If you frame in between the logs, then you will develop air leaks between the log posts and the insulation as they shrink, which may take several years to be fully dry.  The best way I can think of to mitigate that if you do chose to frame in between the posts would be to leave a gap between post and stud and fill that space with some sort of resilient insulation that will expand as the post shrinks.  Fiberglass or rockwool would do. Pack it in the gap at first, it’ll expand as the post shrinks.

There’s nothing you can do to prevent checking altogether , but you can slow it down and direct it a little.  On the exterior side of each log, if you make a saw kerf an inch or 2 deep that runs the entire length of the log, any checks that develop are more likely to form there on the outside of the log and not on the interior side.  Also, once you get a roof on the thing those logs are going to do a lot more drying.  Running a humidifier inside the building for a year or two once enclosed will help.  Running a wood stove will invite more checks though.  The key to slowing checking is to slow the rate of drying.  

To extend your roof overhang without having to get too structural with your double insulating walls, simply extend your roof purlins.  Does your design run the purlins beyond the frame, or do they terminate at the outside posts?  It’s more common for log cabin to have purlins that extend way out beyond the walls, but a lot of timberframes have the purlins and rafters end at the posts and beams that support them.  Then the roof gets framed over sized in dimensional lumber like a regular house.  I prefer to extend my purlins all the way out to the gable ends.

T-

Hey T! A lot of golden useful information here. I agree about showing the timbers inside the house, but with the central bent and the roof rafters showing it looks something like this:
https://wholetrees.com/portfolio-item/albertson-residence/

This house is very close to the design we will have. They did some sort of infill between timbers and still have the timber frame feel inside.

Excellent suggestion to use rock wool or an expanding insulation around the timbers! I will take you up on that. We are definitely aware of the shrinkage and checking that will happen as the timbers dry, and we will just have to keep tightening as the drying happens. Good point about slowly drying- we will insulate for the winter and maybe not live in the house until next year, so they will have a chance to dry slower hopefully. This is off-grid so no chance for a humidifier. I was concerned about moisture affecting the insulation, but I was assured that won't be an issue either.

The other reason I don't want to wrap the timbers is that the larger your wall, the less overhang you get. I'm not 100% sure if the perlins will be extended because we haven't decided on roof design, but I know the rafters I have seen like in the example link only extend to edge of timber frame, then smaller extension rafters are placed at the ends.

Logs shrinking is huge and more huge than most people can wrap their mind around. I visit a buddy in WA every year or so; he build a log house built about 10 years ago using large logs. He marked certain walls and created marked blocks to place against others. Some of his shrinkage is approaching 2" over a 8' height. Now that is probably an anomaly based on his particular situation (possibly because he built with large logs), but... Do check and double check the expected shrinkage and don't bet anything important on it being "slight".


Regards,
Rufus