FWIW.... the average R value of softwoods is 1.41/inch. Hardwoods average 0.71/inch. To do an accurate heatloss calc for your home, you should measure the exterior walls at the average thinnest point of the logs, judging from your pics I would guess that to be about 6". So for the sake of some easy math lets say your walls have an R value of 8.5 on average. If you are investing in good windows they will be near that R value as well. That's OK as it will make your heatloss calc easier to do. All of the info from your heatloss calc will feed back into the selection of tubing size/tubing spacing as well as circ sizing, the latter being the most important to you due to energy (electrical) constraints. Not trying to discourage you, just want to make sure you size your system & all the zones in it in a way that will give comfort, as you said this will be your last house. Normally I would say that if you are in doubt you should oversize/oversupply the system & zones as you can always throttle back if you are too warm (hard to run a system past flat out if you are too cold). However in your case you will need to be accurate with your heatloss calc so you can design your system/zones to provide comfort while consuming as little power as possible. Your boiler supplier should be able to assist in this or at least refer you to someone who can assist. Hope this helps.