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Re: GSBN:Fwd: CASBA_ Pea Gravel
Briefly, in response to John's comments...
I used to always start my thinking about such matters with "what
happens in a conventionally built wall or assembly? I know that long
ago I thought about the near complete lack of drainage detailing for
moisture entering typical wood framed wall assemblies. What's the
detail that lets moisture out? There typically isn't one. That doesn't
mean having one isn't a good idea! We're trying to make sure we're not
designing problems in that we could have designed out.
My suggestion for the foam on the inner half of the space a fairly
direct response to David's inquiry - which was about the rationale for
the drainage and how to provide the insulation that he'd like to have
there. It was based on how unlikely it seems that the source of liquid
water in a bale wall would be coming from the interior side. This might
be different in a hot humid climate with air conditioning, but we're
talking Northern California with its current climate. It's just hard
for me to imagine, other than at a bathroom with a tub or shower up
against that part of the wall, of a significant amount of liquid water
ending up sitting on that foam. Could happen, but as others have said,
at that point you have bigger problems. Of course, that strip of foam
could be less than half of the space as well and still give you a good
thermal break without having a large area of straw sitting on foam. You
could just put a vertical piece of 2" foam along the inside of the
interior 4x4 for instance and fill the rest of the space with other
material. I think a fill material over a layer of foam, with a smaller
width of foam insulation to the interior side should be relatively easy
to get approved.
And please don't get me wrong, I'm not a big fan of foam. I just
appreciate some aspects of what it does and does well. There are also
various types of permeable foundation drainage boards and matts that
could work in this space. And there are other materials; scoria as
Laura suggested, or pumice if either is available, and expanded clay or
shale which has been used in Europe as insulating fill under slabs and
could work for this.
All that aside, I have been a long and strong advocate of all manner of
approaches to making sure that the moisture that gets into wall
assemblies has a way to get out - all the way to the outside. I follow
the European and Canadian building science approach that assumes that
everything will get wet - that moisture will find its way in
eventually, and thus focuses attention on basic strategies to minimize
wetting but much more attention on maximizing drying, including
drainage. I've suggested to people that they not put a sill plate
gasket under the exterior sill plate because it is likely to do what
John is talking about, which is to create a dam that would hold liquid
water in. I like some sort of flashing pan under windows that extends
all the way out beyond the exterior plaster with a drip edge. I like
head flashings over windows and complete properly lapped flashing all
the way down onto the sill flashing. Here at the bottom of the wall
this is something we should always be thinking about...how do we not
trap water and moisture in the wall? There are lots of approaches to
accomplishing this and what's best depends on lots of things.
The question of being able to tell you have a problem is an important
one, and I'm not sure that any of the things we've been talking about
here address it directly. I advocate the installation of moisture
sensors in key locations so there is a way to periodically monitor
what's going on, as Laura suggested in her post.
From: John Swearingen john.skillfulmeans@...
To: GSBN GSBN@...
Sent: Wed, 12 Dec 2007 1:14 pm
Subject: Re: GSBN:Fwd: CASBA_ Pea Gravel
As long as we're going to full bandwidth on this, let's get into the
of drainage plain vs moisture barrier, and the subject of detailing,
something that DE touches on, but doesn't resolve. As he said, it
an issue with our beloved Sonoma County building department, which
creative interpretations of code issues.
The moisture barrier part...checking rising damp, is straightforward.
are two approaches, the capillary break (pea gravel) and the moisture
barrier (tar, paper, bituthane). We use both--a moisture barrier under
gravel. We prefer a liquid-applied "tar" because typically this is an
that wasn't covered by the plastic vapor barrier that went under the
It's a low tech and high tolerance issue--ie it doesn't have to be done
"Drainage plane" is, to me at least, a confusing term, and seems to
that there's a bunch of water to be drained. Usually, a drainage plane
designed to keep water from getting deeper into the building envelope,
with a rain screen, or a drainage mat under a roof.
In this case, though, if the "drainage plane" is seeing any water, the
is already over--moisture has come from higher up the wall, and I guess
purpose is to keep the foundation from getting wet? Help me on this
And a note on detailing: if it's not detailed to drain properly, this
"drainage plane" is actually more properly called a "moisture collection
basin", or MCB. And if the purpose is to drain the wall, where are you
draining it to, and how does that help your building?
The question I've been asking recently is: If your walls are getting
that the gravel is taking on water, wouldn't you want to know about it?
mean, some damage is very likely occurring, and the sooner you find out
less damage there will be.
We have previously detailed walls to discretely deposit their moisture
the outside, thereby saving expensive insurance claims for varnished
and mold in the walls. A few months ago, Dan, Dietmar and I examined an
owner-built building that had extensive leakage through cracks on the
rain-exposed side of the building at the second floor. The owners
eventually noticed the problem on the ground floor when water puddled
large storms. . In the meantime, the plywood box beam above was already
This caused me to think...hmmm, if that water were going somewhere that
occupants would have noticed earlier, the problem could have been
the bud. What is the down side of detailing so that the MCB drains
Oh, yeah.... What does David Arkin tell the building department? I
two of the suggestions so far would work...(1) alternating strips of
insulation and gravel, (2) insulation below gravel. I wouldn't favor
David's suggestion of (3) insulation on one side, gravel on the other,
because of the issue of putting bales on an impermeable surface.
John "What's that funny Smell? Swearingen
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