In a word, "durability" ... 1/4" will be a bit too easy to puncture, will
quickly suffer from moving equipment across, and will certainly warp more
easily with temperature/humidity changes.
1/2 would certainly work, but is simply not as durable to move shop
equipment over for any length of time.
The idea is to do it only once in your lifetime. ;)
I guess it depends on what kind of machines you have. if you have a
sheet metal and aluminum benchtop table saw you can go with a pretty
light floor. if you have or think you might want to get someday
heavier cast iron machines (which I definitely recommend) you will
quickly come to appreciate the added strength and stiffness of the
Most builders, interested in saving a buck, fool themselves into believing
the xMil Poly/plastic products sold in the construction business as
"moisture/vapor" barriers will last.
As the author states, an alkali soil will hasten degradation of this oft
used product. What he doesn't say is this degradation is further
accelerated with bacterial action in the warm, moist environment found under
As a builder, and because of the subsidence in this area of the Gulf Coast,
many municipalities are now mandating crawlspace foundations (the city of
Bellaire, TX is almost all mandated crawlspace in most parts), I spent a
good deal of time trying out various ways to deal with moisture issues in
crawlspaces. The above is exactly why I quit using "2,4,6 mil poly"
"moisture" barriers on soil under the houses I build on grade/crawlspaces
and switched to a polyethylene underlay with concrete mud slabs, and good
drainage and ventilation, to mitigate moisture problems.
Having made the decision to move and build, I am collecting knowledge.
What is the nature of the problem leading to the madate. Differential
settling? Something else?
How does the crawl space foundation mitigate the problem. Or does it
just make the future remedial work, if necessary, easier and less
With the mandates, do you have a choice of footing and block curb,
poured curb, or spread footings and piers?
Have been leaning toward a crawl space (depending on the final
location) however have only owned monolithic slabs, or in the case of
my Oklahoma residence, a three pour foundation, continuous footing to
frost line, curbwall, and then slab poured after the installation of
ductwork and plumbing and sand fill. Most of my current neighbors
have crawl space and there are some problems. One of my neighbors
actually has a catch basin and drain in his crawl space to divert an
underground spring. As long as the water is moving on, not standing,
seems to be OK.
, I spent a
Answer to both: subsidence, and a change in the 100 year flood plain map as
a result of TS Alison in 2001.
A crawl space foundation is usually a better alternative to a monolithic
foundation of great height with regard to cost (my own house is on a
monolithic slab on grade, 3' above grade ... I flooded in Alison ... never
It does that nicely, as a benefit, but it also has its downside .. movement.
Although the results of movement with a crawl space foundation is usually
cosmetic and rarely catastrophic, as often happens with a cracked slab on
That dog is wagged by the "soil survey/report" tail. You foundation choice
is generally, and strictly, limited to what the soil report dictates in many
Last crawlspace foundation I built here, in 2005, was drilled bell bottom
piers, w/grade beams penetrating 6" below grade, 3' above, and topped with a
This is the solution I've chosen to solve the prolbem in our flatlands:
Pictured is a drawing of what we call a "mud slab" under the house and
bounded by the grade beams (may be more technically known as a "seal lab" in
As I mentioned in a previous post, it is a layer of polyethylene, topped
with 3" of unreinforced concrete, with six "area drains", all within the
perimeter of the grade beams.
You can see the drainage slope arrows in the picture. We contour these, by
hand to insure this slope to each of these six drains. IOW, any water that
does manage to get into the crawlspace will not soak into the ground, will
not stand and will be immediately drained to the street, which is
approximately 2' below the foundation grade (the finished floor on this
particular house was almost 5' above street level. (the link above is the
mud slab portion of that particular foundation/drainage plan for this house)
In addition, we calculate the ventilation needs and spec the number of vents
needed based on the prevailing air currents, the location of nearby
These three elements in combination: mud slab, drainage, and ventilation, go
further than anything else I've seen/used to mitigate moisture problems
under a crawl space, which also effects your floors above.
I'm continually surprised that more builders in this area don't use this or
similar methods. My houses are in the $750 to $1 million range and most of
those who build in this range don't even bother with a vapor barrier on the
dirt under their crawl spaces ... walk by two years later and you can
actually smell the difference all the way from the street!
Let me know if I can answer any of your questions. Things are very regional
with regard to methods of construction, but every house I've built in the
last six years has been field tested and rated "Energy Star", so I'm fairly
versed in building to these standards, which are much more strict than IECC,
with the actual test results being the proof of the pudding.
You're most welcome ... just remember that the proudly proclaimed "built to
code" house is in fact a house built to the minimum possible standards that
the builder can get away with.
If you keep that in mind, any innate skepticism should immediately kick into
Just like a pilot who passed a check ride or a driver with the MV
At least as good as the minimum standards... Pass / Fail.
** http://www.bburke.com/woodworking.html **
Vapour barrier/retarder are the same thing...it's not actually a
barrier, so they're switching to the "retarder" terminology.
The purpose of the vapour barrier is to prevent condensation on the
framing/insulation. Hence it is usually placed on the warm side of
both, to prevent warm (and hence more moist) air from hitting the cooler
In cold climates it goes on the interior, in hot climates it goes on the
exterior. There is actually a narrow geographical band where no barrier
There's decent information at:
However, note the specific use of the term "walls" in the diagram ... this
is an important distinction/departure from the subject under discussion. :)
Good general info, particularly for "walls" and foundations ... they even
mention the "seal/mudslab" that I use over polyethylene in crawlspace
foundations as being a good thing ... but again, and unless I just missed
it, the main thrust of this article does not deal with, and indeed shies
away from, the subject of "floors" (to wit: no hyperlink on the word
IME, that's not unusual when dealing with floor "moisture/vapor
Good info, nonetheless.
How is the workshop built? Do you have a vapor barrier between the
ground and shed? If not I'd strongly suggest that you have a vapor
barrier between the shop floor and ground. Sheds built close to the
ground without a vapor barrier tend to have the joists rot, and
acquire a strong earthy odor. Higher moisture levels in the shop won't
help the rust situation either.
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