[Greenbuilding] log homes

[Chris Green]

Rob Tom wrote:
> On Fri, 18 May 2007 23:08:36 -0400, Janet L. Hobbs  
> <janethobbs at hobbsink.com> wrote:
>
>   
>>> Last I knew log home "manufacturers" were of the opinion that their  
>>> homes have great R values,
>>>       
>
> Wood, when dry has R-values between ~ 0.7 and ~1.4 per inch depending upon  
> the species.
> It can be worse but not better.
>   
Logs are round, so you have to determine the average thickness of wood 
for the whole wall. The logs might be 14" in average diameter, but the 
total wall will only average maybe 12.4" in thickness. So a solid log 
wall without windows or other openings would run about R-17.3 at the 
best of times, using Robs' highest R-value. That's a purely theoretical 
assumption: it is highly likely that heat will flow faster through the 
thin parts at the notches, where there may only be 5 or 6" of 
thickness.An energy system tries to reach equilibrium. If you had a 
really sensitive infra-red camera, you would see ribbons of colder areas 
where the notches are, and warmer areas where the wide parts of the log 
are. Air convection currents  next to the logs would transfer more 
energy from the better insulated thick areas to the less insulated thin 
areas.
So you have to downgrade the theoretical R-value by a certain amount. 
I'm not sure what that amount would be.
Throw in a few nice windows and you're whole wall R-value drops 
proportionately.
How well the log structure was crafted also comes into play, as it does 
with everything else. Having taken log building training, I know it is 
quite difficult to get a consistently tight compression seal between the 
logs in the Scandinavian notch style of construction, so air leakage is 
a major concern. The same goes for the dovetailed square log style.
That means even more energy losses.
Somewhere along the line, the logs will have cracks in them.
And so-on.
One could balance out the lower efficiency of a log wall to some extent 
by increasing the R-value in the roof, perhaps including a living roof 
on top of that, and carefully designing and siting the house for solar 
heat gains, pv-powered ground-source ("geothermal") heat, etc..
<snip>
> What he did was proactively implement what many log home and cordwood home  
> dwellers do reactively after having lived in them for awhile.
>   
Yeah, I found the log houses I've stayed in kinda chilly too.
However, none of these were designed or sited for optimal solar heating. 
I suppose that a solar heated log house is possible, but not where you 
have 70' tall trees right next to the house, and/or you're on the north 
side of a mountain.

The energy efficiency of a log house all boils down to where the house 
is sited, how well designed, and how well crafted it is. The E.E.R.E 
states:
> In a log home, the wood helps provide some insulation. Wood's thermal 
> resistance or resistance to heat flow is measured by its R-value. The 
> higher the R-value, the more thermal resistance.
>
> The R-value for wood ranges between 1.41 per inch (2.54 cm) for most 
> softwoods and 0.71 for most hardwoods. Ignoring the benefits of the 
> thermal mass, a 6-inch (15.24 cm) thick log wall would have a 
> clear-wall (a wall without windows or doors) R-value of just over 8.
>
> Compared to a conventional wood stud wall [3½ inches (8.89 cm) 
> insulation, sheathing, wallboard, a total of about R-14] the log wall 
> is apparently a far inferior insulation system. Based only on this, 
> log walls do not satisfy most building code energy standards. However, 
> to what extent a log building interacts with its surroundings depends 
> greatly on the climate. Because of the log's heat storage capability, 
> its large mass may cause the walls to behave considerably better in 
> some climates than in others.
>
> Logs act like "thermal batteries" and can, under the right 
> circumstances, store heat during the day and gradually release it at 
> night. This generally increases the apparent R-value of a log by 0.1 
> per inch of thickness in mild, sunny climates that have a substantial 
> temperature swing from day to night. Such climates generally exist in 
> the Earth's temperate zones between the 15th and 40th parallels.
>
Source: *http://tinyurl.com/2cxthw*

 As for 'green,' buying a log home kit from a North American dealer then 
erecting it on top of a 50-60 story tower in Tokyo (this really 
happened) isn't very green compared to winching selectively cut logs out 
of forest on your own property in the Appalachian or Ozark mountains.
So there are a lot of variables to consider all around.

Would I consider building a log house? Well, maybe...

Cheers,

Chris Green.