[GSBN] Cordwood walls

Ian Smith ian at lopezsmolensengineers.com
Wed Jan 7 22:00:36 CST 2015


As others have mentioned it seems as though a "single-wythe", non-rendered
cordwood wall has the potential to perform (thermally) about as well as a
dry-stacked stone wall.

But, on the R-value topic, I believe that in our codes here in the States,
log/timber walls are included in the definition of a "mass wall"
(N1102.2.5).  Not that they're as good as earth, etc., but as long as it's
a certain minimum thickness, maybe cordwood would factor in to your energy
calculations better as a mass wall then an insulation wall...?  Or both,
depending on thickness... ask the energy rater.

If the aesthetic desire persists, maybe go for a "double-wythe" wall with
insulation in-between...?  Or, just wrap the exterior of a single-wythe
with (more) insulation, an air barrier, and a render or rain screen (with
huge overhangs).

Ian Smith, P.E.
Boulder, CO, USA



On Wed, Jan 7, 2015 at 2:02 PM, <GSBN-request at sustainablesources.com> wrote:

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> Today's Topics:
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>    1. Re: Cordwood Walls (Chris Magwood)
>    2. Re: Cordwood Walls (Derek Stearns Roff)
>    3. Re: Cordwood Walls (Min Hall)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 07 Jan 2015 13:09:16 -0500
> From: Chris Magwood <chris at endeavourcentre.org>
> To: GSBN at sustainablesources.com
> Subject: Re: [GSBN] Cordwood Walls
> Message-ID: <54AD764C.4030008 at endeavourcentre.org>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> Hi Tom,
>
> Although it's something that cordwood enthusiasts don't like to hear,
> the best thing for cordwood walls is to plaster them on both sides. This
> prevents the otherwise huge amount of air leakage that has been evident
> on every cordwood structure I've seen. If not plastered, there is no way
> to prevent the cracks that open up as wood and mortar expand/contract at
> different rates. Surprisingly, the plaster coating seems to hold up very
> well.
>
> Separating the mortar as per John's note is one way to help with the
> thermal performance. We've also done some small scale cordwood buildings
> with hempcrete as the mortar, in which case we use a full mortar bed
> over the width of the wall. This gives the mortar a better thermal
> resistance than the wood in most cases.
>
> In general, hardwoods are avoided in cordwood buildings, as they shrink
> and swell more than lighter softwoods. Cedar is the cordwood of choice
> in our part of the world.
>
> Cordwood is not my first choice, but it can make use of abundant (and
> usually free) wood resources that aren't otherwise considered "useful"
> for building, and can be very low impact and owner-builder friendly.
>
> Chris
>
> On 15-01-07 12:47 PM, John Straube wrote:
> > My two cents, I dont have a lot of first hand experience.
> > I have heard too many rotten cordwood buildings to be cavalier and am a
> lot more careful based on those anecdotes.  That said, like straw bale, I
> would say the approach is to keep the wood dry and allow it to dry.  Big
> overhangs, capillary break at foundation, stay 8? above grade.
> > The r-value of different wood species is pretty well documented.  Expect
> an imperial R-value of around R-1/inch or a conductivity of 0.1 W/mK.  That
> said, using mortar around all the wood will trash the R-value by allowing
> the mortar to conduct heat.  It is not that uncommon in cold climates like
> the Canadian west for builders to limit the mortar to 2-3? on the inside
> and the outside and fill the gap in between with sawdust, perhaps lighty
> lime washed sawdust.  I would expect you would want walls that are at least
> 16? thick, and perhaps 24? (60 cm) to get good thermal performance.
> Upgrading windows, ceiling, and foundation should allow you to meet most
> stringent but sensible energy targets.
> >
> > Is the cordwood chosen for aesthetics? If not then, add insulation and
> ventilated and drained cladding on the exterior.  Still looks and works
> great on the inside.  Or vice versa, add insulation to the inside, but then
> be more careful about preventing rain wetting on the exterior.
> >
> > John
> > On Jan 7, 2015, at 11:54 AM, Tom Woolley<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
> >>
> >>
> >>
> >> Tom Woolley
> >> tom.woolley at btconnect.com
> >> 80 Church Road
> >> Crossgar
> >> Downpatrick
> >> UK
> >> BT 30 9HR
> >> 00(44) 28 44 831164
> >>
> >>
> >>
> >> _______________________________________________
> >> GSBN mailing list
> >> GSBN at sustainablesources.com
> >> http://sustainablesources.com/mailman/listinfo.cgi/GSBN
> > John F Straube
> > jfstraube at uwaterloo.ca
> > www.JohnStraube.com
> >
> >
> >
> > _______________________________________________
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>
> --
> Chris Magwood
> Director, Endeavour Centre
> www.endeavourcentre.org
>
>
>
> ------------------------------
>
> Message: 2
> Date: Wed, 7 Jan 2015 19:13:26 +0000
> From: Derek Stearns Roff <derek at unm.edu>
> To: Global Straw Building Network <GSBN at sustainablesources.com>
> Subject: Re: [GSBN] Cordwood Walls
> Message-ID: <6E4F9C22-5D3A-489E-841F-29160A941B18 at unm.edu>
> Content-Type: text/plain; charset="windows-1252"
>
> 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 t
>  he 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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> Message: 3
> Date: Thu, 8 Jan 2015 09:55:14 +1300
> From: "Min Hall" <minhall at xnet.co.nz>
> To: "'Global Straw Building Network'" <GSBN at sustainablesources.com>
> Subject: Re: [GSBN] Cordwood Walls
> Message-ID: <000e01d02abc$3c4111e0$b4c335a0$@xnet.co.nz>
> Content-Type: text/plain; charset="us-ascii"
>
> Hi Tom - A number of cordwood houses have been built in Golden Bay in NZ's
> South Island. Reasonably high humidity here, 2000mm rain and cold
> temperatures in winter. They are plastered both sides and a pumice mortar
> was used for laying up the walls to help with the insulation. I do not know
> of any testing done but they perform better than my initial scepticism
> foretold. Timber is pinus radiata.
>
> I'm not a great fan but I think the pumice rich mortar is a good idea and
> certainly helps.
>
> Cheers
>
> Min
>
>
>
> From: GSBN-bounces at sustainablesources.com
> [mailto:GSBN-bounces at sustainablesources.com] On Behalf Of Tom Woolley
> Sent: Thursday, 8 January 2015 5:55 a.m.
> To: Global Straw Building Network
> Subject: [GSBN] Cordwood Walls
>
>
>
> 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
>
>
>
>
>
>
>
> Tom Woolley
>
> tom.woolley at btconnect.com <mailto:tom.woolley at btconnect.com>
>
> 80 Church Road
>
> Crossgar
>
> Downpatrick
>
> UK
>
> BT 30 9HR
>
> 00(44) 28 44 831164
>
>
>
>
>
>
>
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