question about log sizing

[paddler]

I am in the process of cutting wood and designing a cabin for my family. it will be a horizontal, stacked log home. I am squaring logs on all four sides and using white spruce. Originally I was planning on using 8x8 for the horizontal logs. but, I have had several people tell me that was overkill. two of them currently live in log homes with 6" thick walls, and one said he grew up in a home with 4" thick log walls. I have been in their cabins many times and I know they don't struggle to stay warm.  So, my question is, how much practical insulation value do I lose out on by using a 6x6 instead of an 8x8? the cabin will be approx. 20'X28'

thanks for any input. this site has already been a big help to me.

[Brian_Weekley]

I'm no expert on log cabins.  However, my understanding is that softwoods like spruce only have an R-value ≤1.4 per inch (8 inch thick log only R11)?  That's below the recommended energy code requirements in your part of the country.  If it were my primary house that I was heating in MN, I'd probably want more insulation than that.  I guess it depends if you're planning to add additional insulation on the interior?  Personally, I would want to limit the amount of fuel I burned to heat my house whether it be wood, propane, oil, etc...

[beenthere]

they don't struggle to stay warm. 

If one adds enough heat to the inside of the home, then 2" thick walls would suffice. Depends on how much heat one wants to add. I suspect it is not a linear relationship to the solid wood wall thickness.
One could stay warm in a tent for that matter.

Another problem with 4" vs 6 or 8" may be the stability of the wood and warping as that wood dries more on the inside than the outside.

[davemartin88]

i believe there is a building code for log homes, ICC-400, that covers construction and talks to the insulation values of logs along with thermal mass which is what is more important in a log house. I believe the code recommends 8" logs. We live in a log house with 8" logs.

If I was sawing my own logs and had access to enough logs to make an 8" square, I know that is the way I would do it. As for overkill, easy to say if you're not paying the heat or cooling bill? Good luck with the build.

[gump]

I'm no expert on log cabins.  However, my understanding is that softwoods like spruce only have an R-value ≤1.4 per inch (8 inch thick log only R11)?  That's below the recommended energy code requirements in your part of the country.  If it were my primary house that I was heating in MN, I'd probably want more insulation than that.  I guess it depends if you're planning to add additional insulation on the interior?  Personally, I would want to limit the amount of fuel I burned to heat my house whether it be wood, propane, oil, etc...

It may have a lower R value but it has Thermal mass, so once heated up the logs will retain their heat.
The bigger the better I say, go with the 8"

[shinnlinger]

Another thought is unless you just got to have log walls on the inside, you could insulate and then dwall or plaster to lighten it up AND add R.   Many folks I know in log homes end up finishing the inside to something other than wood because they feel like they are living in a cave.   Putting a 2" piece of foam up or even stud walls with Fiberglass is an option.   If you want to endure a few winters first to pay as you go, you can do that too. 

[paddler]

we have talked about building in from the interior wall as a way to put foam and drywall on one or more of the interior walls. More as a way to add a bright painted surface than increase insulation. cost is definetely a consideration as we are doing all of this without a loan.

The only people that i have spoken with around here that have been disappointed with their homes not staying warm are people who timber framed and used log infill. They look pretty but have the potential to be pretty drafty.

[Brian_Weekley]

Heat transfer from hot to cold will continuously occur at a rate proportional to the R-values of the surfaces it is exposed to.  Thermal mass is certainly good to absorb some of the heat and radiate some back later (both to the inside and the outside).  However, the energy offset is minimal--logs do not create energy.  Any heat the logs absorb is due to the BTUs you burned to put into them.  Once the mass is brought to temperature and equilibrium is reached, the bottom line is the BTUs burned for heat must = BTUs lost through the building to the outside.  A building with lower R-values will still lose more BTUs than one with higher R-values.  So, if you choose to build a log cabin, I totally agree that you want the 8 inch logs--both for greater R-value and greater heat capacity.  However, I can't believe that a log cabin with 8 inch logs will somehow be more energy efficient than a building with higher R-values.  Not to mention log cabins are probably tougher to air seal compared to conventional construction.  Just because you can burn more fuel to stay warm doesn't necessarily mean you should...

For the same reason Shinnlinger brought up--my wife has told me that she will not live in a building with all wood floors, wood ceilings, wood walls, and decorated with wood furniture and wood cabinets.  She says that might be fine for a guy, a weekend cabin, or hunting lodge, but she would find it too dark and depressing to live in all year long.  No offense to the log cabin dwellers out there!

[mark3885]

We built our handcrafted loghome from 10" avg , red pine , 2100 sqft. floor space but equal to 4900 sqft by volume . We have 26' to the ridge in the great room , loft and 23ft to ridge in the kitchen,  dining room. We heat way more area than we should but inside is 2  cavernous rooms . We had played half court basketball , in the great room before we put in the gable end window walls. We heat with a Hearthstone Mansfield woodstove that is rated for 80,000 BTUs. The thermal  mass makes up for all the windows but is by far dark and dreary.  We offset the large heat loss with an R65 roof consisting of 9" of Poly-isocyrnate foam board 3/4" T&G ceiling and metal roofing on a 12/12 pitched roof. We built this with the materials we had or could source locally and cheaply. 8" Insulated concrete form basement with 9  ft to floor joist, with a 6" slab concrete floor over 2 " insulation board with 1500 ft of radiant floor pex tubing. Overkill throughout but that's the way I roll. ;D

[D L Bahler]

I would fall into the camp that recommends building out the interior. This is the best balance in my own opinion.

I think you will be the most cost effective to build a building with 4 to 6 inch walls (I like the appearance and proportions of the corners best with dimensions under 6 inches) which will be a far more efficient use of timber than going up to 8 inches. Like suggested by others, you can build this and have a moderately-performing home, then add an envelope to the inside later.

To add to the discussion of R-value vs. thermal mass,

Thermal mass is of benefit when you have a heat source in the equation that is inconsistent or has limited availability, like the sun. A house that makes use of thermal mass can be beneficial if it can absorb the heat from the sun during the day and release it at night, for example. But like pointed out, if you are designing around a wood stove or a modern heat source, you are going to want a high r-value.

If you want to have both, remember the thermal mass is going to work best on the warm side of the wall -meaning you want the thermal mass on the inside. That means wrapping the walls with an interior envelope is going to negate the benefits of thermal mass, instead leaving the logs to function only by their r-value.

[hacknchop]

When I first started out sawing timbers for a local square timber builder we used 6x12 with dovetailed corners then changed to 7x12 in order to meet the Rvalues for CMHC morgage by 1990 that changed to minimum 8" on outside walls so we changed again to 8x14.I dont know if that helps you make your choice but I have noticed that the care you take when building will make the biggest difference.

[woodworker9]

A good buddy of mine had a cabin in the northwoods of Wisconsin, an hour north of Hayward.  My guess is, similar climate to what you have.  He had a log cabin with 6" logs, and we were always pretty cold there.  Keep in mind, it was not a climate controlled cabin.  We fired up the wood stove when we got there for long weekends of snowmobiling.  So, not much thermal mass heat building up inside, since we were pretty much gone until nighttime. 

After about 10 years of dealing with this, he finally got tired of being cold all the time and finished out the interior walls with framing, insulation, and T&G knotty pine.  It made a really big difference in comfort at night.  We'd load the woodstove in the morning (a BlazeKing King model) and it's still warm inside when we get back from a day of sledding.

If you don't want to build out the interior and insulate, I'd be using the biggest logs I could finish the project with.  I hate sleeping in the cold.