Sometimes I am asked about building with no gap
logs (zero gap). Although this is definitely possible, it is generally
not recommended. Instead, I suggest that you use at least a 1/4" gap.
Having at least a minimal gap(with chinking) allows for the logs to settle while keeping the notches tight. Also, some logs are sure to shrink, twist or bow after some time. If there is no gap to begin with, gaps would open up in various places and it would be difficult to seal them. Chinking a gap of 1/4" or more is much easier than trying to fill alot of thin gaps here and there.
Generally, the gap size should be selected based on how much shrinkage/bowing/settling is anticipated. If your logs are fairly green then it is better to have a bigger gap in order to accomodate more movement. 1 inch would probably be about the minimum in that case.
If the logs are kiln dried, resawn and then sealed, they should be very stable and a 1/4" gap should be fine.
Many cabins have fairly wide chinking and some people prefer that look. Gaps of 3-6 inches are not uncommon. A side benefit to using larger gaps is that it makes it easier to run wiring and mount electrical boxes.
A gap of 1/2" to 1" is generally good for chinking. The gap shown here is about 1 3/4". The larger the gap, the more expensive and time consuming it is to chink. I recommend using the modern flexible chinking products rather than mortar based. As the logs naturally move over time, the elastic sealant will maintain a good seal with no cracking.
Insert foam backer rod. The backer rod can be purchased in various diameters. It just needs to fit snugly. Recess it about 1/2". No need to nail it.
Apply the chinking (Log Jam, Perma Chink, etc.) using an air powered caulk gun. The standard caulk gun is really only practical for very small jobs and not recommended for a full cabin. I have used the bulk gun and 5 gallon buckets of chinking but found it is very difficult to draw the product into the gun and requires a lot of cleanup after each use. Now I only buy the chinking in 29 oz. tubes and use a pneumatic gun which eliminates both problems altogether. This is the one I have:
(In fact, I even used it to chink a remote cabin that had no electricity to power an air compressor. I brought in a small portable air tank and used a bicycle pump to pressurize it.)
Use a spray bottle to lightly moisten the chinking with water.
You can buy tools specifically made for smoothing out the chinking. Here, I am using a modified putty knife. It is pretty easy to spread and smooth the chinking. Be sure to force it against the wood along the top and bottom edges so it will form a good bond.