[GSBN] Cordwood Walls

Derek Stearns Roff derek at unm.edu
Wed Jan 7 13:13:26 CST 2015


Perhaps you will get comments from supporters of the cordwood method, but to me, the approach makes it hard to attain most goals of prudent construction.  For example:

Air sealing.  A good air barrier is critical to an efficient, healthful, and robust house.  Cordwood construction makes it very difficult to create an air barrier.  All wood shrinks as it dries, and it shrinks unevenly.  This leads to a complex network of cracks and checks.  Every piece of cordwood is likely to have multiple cracks that create air pathways across some portion of the wall’s cross section.  The pieces of cordwood are laid up in a mortar or other matrix, and sealing the interface of dissimilar materials is always difficult.  Since the cordwood in the wall will continue to expand and contract with moisture cycling every day and every season, the chosen sealing material will constantly be stressed, and is likely to fail.  Which brings us to:

Moisture.  The function of a tree’s cellular structure is to conduct moisture along the axis of the grain.  This is the axis that cordwood construction uses to connect the outside world with the inside of the house.  This orientation maximizes moisture movement, which increases wood expansion and contraction, as mentioned above.  While a certain amount of moisture vapor exchange can be a good thing in a wall, the inevitable cracks in the end grain can lead to moisture problems, due to moist air moving into the wall.  A more severe problem occurs if rain water is able to strike the outside of the wall.  The cracks will serve as funnels, straws, and channels, to conduct water into the wall.  If the interior of the wall is filled with sawdust, the water will saturate the sawdust near every leak, leading to gaps, air pockets, and very slow drying.  Or lack of drying, in the 100% relative humidity level that Tom mentions.  Mold and decay are fairly likely in the interior of the wall, if the sawdust is saturated.

Longevity.  The heartwoods of different species of wood have varying levels of resistance to decay.  However, the sapwood of all common species is not rot resistant.  Cordwood construction, as normally practiced, includes all the sapwood.  The sapwood is very susceptible to a variety of fungal and microbial attacks, and is also loved by various insects.  Termites are the most famous, and I question offering them super-highways into the home, with continuous fast-food eating options all along the way.  But even if you can defend yourself against termites, there are plenty of other beetles and insects, many of which can fly, which are eager to dine on sapwood, especially when the endgrain is exposed.

Energy efficiency.  Wood has a nominal insulation value of R-1 per inch (0.176 RSI), which is quite low among insulating materials.  The cracks that are always present in cordwood compromise this figure, and the likely air infiltration lowers the energy efficiency still further.  The mortar between the cordwood pieces is likely to offer even less insulation.  So a mortared cordwood wall is built of a mediocre insulator, in a matrix of an even poorer insulator, offering large-scale thermal bridging.  To improve insulation, many people choose to fill the interior of the wall with sawdust.  In this method, we have a double-walled structure, in which the cordwood constitutes the thermal bridges.

To address the problems above, some people choose to plaster over the inside and/or outside wall surfaces.  This certainly helps, but begs the question of why include cordwood at all.  Proponents sometimes claim that this is an inexpensive building method, and if wood is cheap in a given location, then the initial costs might be reasonable. although labor will likely exceed expectations.  However, the costs of maintaining the structure, or remediating insect attack and microbial/fungal decay, may put the ownership costs off the charts.

Finally, visual aesthetics is a commonly given reason for choosing cordwood.  Everyone is welcome to like what they like, but a cordwood wall has few visual analogs in the history of world building.  Cordwood is visually very busy.  It is irregular, and generally lacking in a flowing pattern or visual rhythm.  A cordwood wall is fairly dark to start with, and tends to darken with age.  Most people across the world and throughout the ages have preferred the opposite aesthetic choice for their homes, on each of these indices.  This leads me to suspect that many of the people who think they love the look of cordwood may find that they get tired of it.  If that happens, there is little that can be done to modify the appearance, excepting covering it up.  More generally, buildings that can’t change and evolve aesthetically tend to get neglected and knocked down, sooner or later.

The factors above convince me that cordwood is antithetical to the stated goal of high insulation standards, and the implicit goals of building longevity and reliability.  In terms of rot resistant species, larch is often mentioned as a good option in European construction.  However, the presence of the sapwood, the cracking, and the exposing of the endgrain all compromise whatever rot resistance a given species might have.  The year-round 100% relative humidity mentioned guarantees wood decay, and no sealing method can keep that moisture out.

I think your instincts and concerns are well-supported, Tom.  It’s probably clear what my advice to the client would be.

Derek

Derek Roff
derek at unm.edu<mailto:derek at unm.edu>


On Jan 7, 2015, at 9:54 AM, Tom Woolley <tom.woolley at btconnect.com<mailto:tom.woolley at btconnect.com>> wrote:

We have  been asked to advise on building a cordwood building in Northern Ireland

The site is in the Sperrin Mountains which has nearly 100% RH all year round
average rainfall is well over 1000mm per annum though I am surprised it is not higher

There are two issues
The building will need to meet reasonably high insulation standards under the building regulations
I have found articles claiming that cordwood gives good insulation but I find this hard to accept

Log cabins on the west side of Ireland rot in a few years where the end grain is exposed but maybe cordwood would do better?

What is the best timber to use ?  Oak is indigenous here but not available commercially

Any help much appreciated

thanks and Happy New Year

Tom

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