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Bales Over Framed Floors
Although most of the straw bale homes I build
are built on a concrete slab, I do occasionally build on raised
floor foundations as well. Some people are interested
in a crawlspace foundation because they want to minimize the
use of concrete. Others
want to build over a full or half basement to house their utility
rooms and/or create root and wine cellars. People I hear
from often assume that a bale house cannot be built over a
basement or crawlspace. This is most likely because they
have never seen it done that way. The good news, for
those of you who want to build a bale house over a basement
or crawlspace, is that it is absolutely possible to build a
bale house over a framed floor.
Straw Bale on a Raised Floor
System
A raised floor system, either over a basement or crawlspace, is no different
than building a second floor on a bale home. Interior footings, i.e. footings
inside the span of the floor system, will likely be necessary to carry the load
of the floor system. In some cases, you may need to step these down into a lower
sectional basement for continued support as shown in the picture above. Once
the footers are poured and in place, you can build your stem walls. The stem
walls can be poured concrete or masonry block, depending on the look you are
going for in the finish product.
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With
the stem walls in place, simply add your floor system to the
top of the wall as with any floor system. Be sure to account
for the “dead load,” the
weight of the bales themselves, of the bale walls when calculating
your floor joist size and spacing and when considering how
many interior beams or girders will be required to split the
load across the span of the house. You may want to consult
with an experienced engineer when making the calculations.
I typically factor 40 pounds per square foot for the dead load
of the bales; however, this is a number your engineer will
have to accept or determine on their own. When
making calculations like this, most building inspectors will
want to see your actual calculations and ensure that they are
based on data, not assumptions.
Building a Secondary Bearing Wall
In
some cases, it may be necessary to build a secondary bearing
wall directly below the interior face of the bale walls to
transfer that load off of the floor joists. When used, the
extra wall creates a great place for running plumbing and electrical
in a finished basement making the installation of both systems
considerably simpler. If the interior bearing wall is
used, it will have to be placed on a bearing footing so more
concrete may be required. When
possible, I always prefer to either increase the size of my
floor joists or reduce the spacing between them instead of
adding more concrete. The
floor joists I use are engineered and are made of small, renewably
harvestable trees while concrete is an environmental nightmare
due to the inclusion of Portland Cement.
Summary
No matter what reasons
inspire you to build over a framed floor, you can build that
system with confidence. In fact, I
find it much easier to build over a framed floor than over
a slab. The biggest reason for this is that a
slab is usually going to be used as the finish floor which
means it must be protected throughout the construction process. This
is difficult and time consuming to do.Secondly, a framed
floor makes bracing exterior walls easier because nailers can
be attached anywhere on the interior floor system. The
biggest downside to building over a framed floor is that all
of the thermal mass properties of a slab floor are lost. In
every framed floor house I build, the owners choose a heat
pump over a radiant floor system. This is okay because
heat pumps are getting more and more efficient, but a radiant
system is still my favorite. As
with any choice in construction, there are always pros and
cons to be considered; it is nice to know that there are several
choices out there when it comes to foundations.
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