RE: GBlist: Mud Walls Give Poor Insulation

["Matt Duston" <mountainsun@pcisys.net>]

Hello folks - I think the writer of the below article missed an important
point.  I don't think anyone familiar with the physics behind R-values would
promote rammed earth or any other massive wall construction as an insulative
method BY ITSELF.  Almost by definition a material which would make good
thermal mass would make poor insulation, since it must be able to readily
transfer heat into its core and out again.  The author touches on the issue
when he says "Other studies have shown that walls with high mass combined
with high R-value can perform especially well but, unless the climate is
relatively mild, high mass is of little value if the wall does not also have
a high R-value."  But he doesn't elaborate on this, which is central to the
issue.

While I'm generally not one to promote masswall construction in cold
climates, it can have its place if properly utilized.  A high-mass
warm-side, married to a high-R cold-side, can be just the ticket for
moderating indoor temperatures.  The mass material absorbs heat from the
interior when the temperature climbs, and releases it when the temperature
starts to drop, while the insulative layer on the outside KEEPS the heat
inside where it belongs.  And if this technique is combined with a
alternative source of heat, such as a solar system, it can be a very
effective method of keeping a space comfortable at a very reasonable cost.

In my neck of the woods people often build uninsulated log homes, and say
"the thermal mass of the logs makes up for its relatively poor insulative
value."  Well that's just elk droppings.  Yes the walls will still feel warm
inside in the evening when they are radiating back all the heat you pumped
into them during the day.  But they are ALSO radiating the same heat to the
frigid outside at the same time.  So by morning there is a nice temperature
gradient through the inside of the logs, with the inside surface warm and
the outside surface cold, and your heat is flowing blithely out through
them.

A masswall has its value when the mass is no thicker than heat can
reasonably be expected to penetrate during a period when heat is being added
to the wall.  In a normal house (non-solar, heated by something like forced
air gas) that period is usually mostly the afternoon and evening hours, when
the heat is turned up and the occupants are moving around, cooking, using
lights and appliances, etc.  Then at night when they turn the heat down,
that stored heat (some of it free, from occupant activities) radiates back
into the home, so the furnace may not need to come on until the next
morning.  If you make the wall a little thicker you will get some carryover
into the next heating period, which may be nice first thing in the morning.
A little thinner and you will be sure of getting every BTU out of the wall
overnight that you put into it during the day.  But if you don't insulate
the outside then half of your heat will radiate outward instead of inward.

The temperature in a really thick masswall, which many people build, will
lag the heating period by some time period, depending on its thickness.
However any heat you pump into it can eventually make its way through the
wall's core and out the other side.  This is usually much colder than the
inside, and thus a much better heat transfer surface.  So the inside surface
will generally stay warm and comfortable to the touch, but at the expense of
putting a lot of heat through the mass to the outside.

So the bottom line is, IMHO, if you are in a cold climate, build a mass wall
if you like, but DON'T build it tremendously thick and DO insulate it just
as well on the cold side as you would any other type of construction.  You
will moderate swings in the inside air temperature without wasting any more
energy than you have to.

Matt Duston
Mountain Sun Engineering
Woodland Park, CO
719.648.0253

P.S.  Caveat:  solar heated systems are a whole different kettle of fish,
and may require more mass than a tempering system.  See discussions on this
list for gobs of info on mass for storing solar heat.


-----Original Message-----
From: owner-greenbuilding@crest.org
[mailto:owner-greenbuilding@crest.org]On Behalf Of Kristian Kicinski
Sent: Tuesday, January 09, 2001 10:45 AM
To: Greenbuilding
Subject: GBlist: Mud Walls Give Poor Insulation


I found this article intersting- any comments?
Reposted from  http://www.farmindex.com.au/goto.cfm/8509

Mud walls give poor insulation: CSIRO

                Thursday April 27 2000

                CSIRO researchers have made a surprising discovery in
tests on mud walls,
                finding they provide poor insulation.

                Latest tests by CSIRO have confirmed that rammed earth
walls have poor
                thermal resistance - adding hard facts to the debate
about their insulation
                properties.

                CSIRO says this raises the question, "Why do so many
Australians believe earth
                construction provides such a comfortable home?"

                The research, conducted by CSIRO and funded by the
Western Australian Office
                of Energy, also produced results that effectively end
the controversy over the
                accuracy of previously published data for the thermal
resistance of rammed
                earth walls.

                "Our findings were based on testing of two
commercially-produced rammed
                earth products, both of which turned out to have poor
thermal resistance,"
                says Mr Robin Clarke of CSIRO Thermal & Fluids
Engineering (T&FE).

                "CSIRO's tests confirmed that the overall thermal
resistance (R-value) of a
                typical rammed earth wall is less than R 0.4, the same
as the disputed values
                provided for by standard design tables.

                This is low compared to everyday insulating materials
such as glass fibre batts
                which typically come with ratings of R 2 or R 3," he
says.

                "The tests used sample panels 200 mm thick, about one
metre-square and
                weighing over 400 kg. These were tested in CSIRO's one
metre square heat
                flow meter apparatus - a new record for the size and
weight of panels tested
                on our rig".

                It has been suggested that the high 'thermal mass' of
rammed earth and mud
                brick constructions compensates for their low R-value.
There may be some
                truth in this for milder or warmer climates where
massive walls are able to
                smooth out hotter days and cooler nights creating a
comfortable average. For
                climates with long periods of cold weather, however, it
is the R-value which
                largely determines the heat flow through the wall".

                Other studies have shown that walls with high mass
combined with high
                R-value can perform especially well but, unless the
climate is relatively mild, high
                mass is of little value if the wall does not also have a
high R-value.

                "While environmental, aesthetic and cost considerations
may inspire people to
                build from these materials, our results show that simple
rammed earth walls
                may not perform as well as we would like to think," Mr
Clarke says.

                These results are important as Australia moves towards
                mandatory-minimum-energy-performance-requirements in the
Building Code of
                Australia for houses and other buildings.

                Local regulations already exist in Victoria, ACT and SA.
These regulations focus
                on R-value as the principal factor. For example, in
Victoria the minimum
                acceptable overall R-value for a wall is R 1.3. From
standard design tables (now
                confirmed as accurate), the rated overall R-value of
typical rammed earth wall
                would be less than 0.5.

                Mud brick is closely related to rammed earth but
slightly less dense and slightly
                better insulating. Based on the rammed earth
measurements, the data for mud
                brick must also be regarded as reliable, suggesting an
overall R-value for a
                300-mm mud brick wall of less than 0.6. It would require
only 20 mm or so of
                most insulation materials to achieve this same
performance.

                There is a current exemption in some regulations for
walls thicker than 180
                mm, not because of the acceptable thermal performance of
these walls but
                because established constructions such as double brick
and rammed earth or
                mud brick are difficult to insulate.

                Mr Clarke points out that their poor R-value would make
most earth
                constructions illegal if it were not for the temporary
thickness exemption.

                "In developing an energy code for Australian houses, we
urgently need to
                consider whether other factors (such as high mass) are
able to compensate for
                the low R-value. We are considering the use of
mathematical modelling to look
                more closely at the way heavyweight houses respond to
typical Australian
                climates," he says.

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For  instructions send  e-mail to  greenbuilding-request@crest.org.
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