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Timber Frame Wood and Finishes
One of the first decisions you will make when you begin designing a timber
frame home is what wood to use. The most commonly used woods are eastern white
pine, oak, and douglas fir. There are many structural and aesthetic factors
involved.
White Pine is an excellent wood for timber frames, as it is extremely stable
and does not twist, bend and crack as much as oak. It is an attractive honey
color and has more knots than oak. White pine is susceptible to a fungus called
Blue Stain that causes blue striations in the timbers. Whether or not this is a
blemish or a character-adding feature is a matter of aesthetic choice. (See the
article on Blue Stain below.) You will want to determine your own opinion.
Oak timbers are darker and heavier than pine. They have more grain and fewer
knots. They also are stronger, and may be smaller in size to carry comparable
loads; however, they are less stable and consequently crack and twist more than
pine or fir.
The cracks are called "checks" and will occur in all large timbers. Very
rarely do they cause structural weakness if the proper grade of timbers has been
chosen for each application. Many people view them as adding character to the
timber frame, especially since checks are a feature of antique structures.
We use cypress wood for all of our porches and exterior trim. Cypress is
renowned for its durability. The heartwood in cypress trees produces oils that
help resist decay when exposed to moisture. These oils also provide a natural
deterrent to termites and other insects. While technically a softwood, cypress
is extremely strong and is commonly used as a hardwood. The timbers have an
attractive grain and a golden brown to reddish-brown hue. Cypress's excellent
qualities are beginning to recommend it to our customers for the main, interior
frame as well as exterior porches and trim. Its cost is comparable to douglas
fir.
With few exceptions, timber frames are cut from green timbers. This is
because it takes years to air-dry timbers: oak air-dries at the rate of only
about ½-inch per year, pine and fir at about 1 inch per year. Kiln drying takes
several months of great care, to prevent the extreme heat from increasing
checking and twisting, and consequently is expensive. A new technology of
microwaving the timbers in a kiln may be preferable, but it is even more
expensive.
As the moisture leaves the timbers, it causes shrinkage of the wood cells.
The shrinkage is in cell diameter, not in length, and this is true in the
overall timber as well. The timber will check and twist as this happens, but
this doesn't affect its length or strength. Timber framers know how to
compensate for the shrinkage in their design of the structure and joinery.
While recycled timbers are often extremely dry, re-sawing and cutting of new
joinery, plus humidity changes, can cause movement in the timbers. In any case,
people choose recycled timbers precisely for their antique look, which includes
checks and hand-hewn or weathered surfaces.
Once the timbers are locked into the frame, the tension prevents some of the
twisting and checking. It is advisable to avoid drying out the frame too rapidly
at first, especially during the winter heating season. If you make an effort to
keep the heat down your first winter and keep a humidifier going, you will be
able to reduce the degree of checking.
Building departments typically require that the frame design drawings
indicate the grade of timbers to be used in the frame and that the actual
timbers be stamped to certify that they meet the grade specified. There is a
very good reason for this requirement.
As we know, different wood species have different strengths. This is also
true of specific timbers cut from one species. Various conditions, such as the
number and size of knots in the timber, fungal rots that may be present in the
wood, or breaks in the wood fiber may reduce the strength of the timber. For
this reason, each individual timber must be inspected and graded for structural
strength.
When the designer does the structural engineering for a frame, he or she
selects the grade required to ensure adequate strength for the loads on that
member. These design values are established in the American Society for Testing
and Materials' standards. In most cases, the designer has the option of using a
higher grade of timber or going to a larger timber for a specific member in the
frame.
Certain types of wood get a fungus called "Blue Stain." White pine, which is
used extensively in timber framing, is one of them. The fungus that causes Blue
Stain begins to grow as soon as the tree is felled and feeds on the sap when the
wood is moist. Consequently it affects the sapwood, working inward through the
log or timber, but stops at the heartwood.
Blue Stain is not destructive to the wood and will not cause rot or
structural damage. In fact, blue-stained wood is used for cabinets because it is
considered by many to be attractive. These people are willing to pay a premium
for it. A high proportion of timber frame owners also enjoy the blue striations
in the timber frame, as it gives it more character.
However, if you do not like the look of Blue Stain, talk with your timber
framer about ways to control or eliminate it. They include using winter-cut
timbers, which are not as susceptible because the sap is down; or dipping the
timbers in a fungus-killing chemical, which will retard it for a while. Keeping
them well-aired while stacked will also help. Scrubbing the timbers with a
bleach mixture can reduce the visibility of the Blue Stain. Once the timber
frame is erected and the timbers are in a lower humidity indoor environment, the
Blue Stain will stop spreading.
Although a timber framed structure is often compared to a fine piece of
furniture, the finishing requirements and objectives for heavy timbers and
quality furniture are quite different. The finish itself should be different as
well.
Varnishes, lacquers, and urethanes work well on kiln-dried, dimensionally
stable furniture-grade lumber, but these same finishes would be impractical on
timbers, whose fibers and cells must "breathe" and move, due to varying moisture
content and stresses within the wood. Green timbers seek moisture equilibrium,
and the escape of moisture along with the chemistry of green wood will break
down or oxidize a finish that seals the wood completely. With re-sawn material
the same factors occur in reverse: the dry fibers need moisture and suppleness
from the finishing oils to reduce further checking and brittleness, as new
stresses and movement from re-sawing and re-joining come into play.
For either green or re-cycled material, there are three goals in the
finishing process. The first is to reduce the checking, drying, and shrinking
process in both the timbers and joinery. The second is to enhance the beauty of
the particular species of wood that is used. At the same time, the finish must
be easy to apply, and simple to maintain and re-finish over the years. Last, but
certainly not least, the finish should be relatively safe to both the users and
the environment.
Most commercially available finishes cannot adequately accomplish all of
these objectives for several reasons. Nearly all of these finishes contain
metallic driers which speed up the drying time of the finish, necessitating the
application of many thin coats that only serve to seal the surface of the
timber. The wood simply cannot stay wet long enough to allow the oils to be
absorbed into the cells, fibers, and checks of the timber. These driers,
although used in small quantities, are quite toxic. Also, most available
finishes are 70-80% solvent, leaving very little protective material on or in
the timber. Re-application only serves to seal the timber further because of the
rapid polymerization due to the driers. And the higher the solvent ratio, the
higher the VOC (volatile organic content) level in the shop. Some (but not all)
finishes out-gas aldehydes as they dry, although water-based finishes are much
safer in this respect. Typically, touch-up work, reapplication, and periodic
maintenance is a time-consuming process.
A good finish for timbers must be drier-free in order to allow time for
penetration and absorption, and to prevent rapid breakdown of a finish that
merely seals the surface. (On timbers, finishes that seal the surface only are
more prone to UV degradation, as well to oxidation for the previously mentioned
reasons.) Without driers in the finish, if properly applied, the finish will
definitely reduce checking and shrinking, even in green oak.
Secondly, a good finish for timbers must be able to be easily and rapidly
applied due to the large surface area of the frame. The excess should be easy to
wipe off without leaving lap marks and a gummy build-up on the surface. The
optimum solvent-to-solids (oils, waxes, resins) ratio is about 50/50. The
ingredients should be the purest grades in order to discolor as little as
possible over time. Such a finish is flexible, "living" in and not on the
timber, and is easily replenished through the years as needed without a lot of
surface preparation. I recommend application of one good heavy coat, filling all
surface checks and coating all joinery, prior to raising; and touch-up and
cleaning after drying in the frame, using more finish and a rough pad where
necessary to remove scuffs and abrasions, dirt, dust, and paint, and of course,
the inevitable footprints. Over the years, as the timbers appear dry and
"thirsty," more finish can be applied as needed to keep the timbers in the best
possible condition for generations to come.
Timber framed porches and exterior details are extremely attractive and bring
the timber frame outside, unifying the interior and exterior of your home.
However, the wood needs special attention, since it is exposed to insects and
the elements. There are some woods that will hold up better than others, such as
cedar, cypress or redwood. Cedar and redwood are prohibitively expensive for
most folks. Cypress, as mentioned above, is an excellent wood for exterior use,
and is available at a reasonable price.
With the proper finish, white pine, douglas fir and oak may all be used for
the exterior. The exterior finish should protect the wood against mildew,
termites, and ultra-violet rays, which will discolor the timber, "graying" it
out. One way to do this is to spray on an insecticide, a borax product of some
kind, for example, and then top coat it with a penetrating oil finish. The oil
finish will need to be reapplied at regular intervals.
An attractive way to finish a porch or exterior trim is to use a tinted oil
stain for the penetrating oil finish--for example, a gray tint. This can be
applied and then wiped off, like a white wash. The result is a "pickled" look,
as the grain of the wood still shows through under the gray tint. This method
also has the advantage of reducing the visibility of any graying of the timbers
from UV rays.
by Bob Murray
Timber framers -- and homeowners buying their frames -- have made structural
insulated panel systems the preferred method of closing in timber frame homes.
There are plenty of good reasons for their choice. Timber framers pour their
hearts and souls into producing a frame that will be a work of craftsmanship and
beauty. They and the new owners want to protect that frame as quickly as
possible. With the use of insulated panels, the average-sized home can be
enclosed in a couple of days with a protective envelope that will keep all the
elements off the frame.
Structural insulated panels are placed on the outside of the timber frame, so
the entire frame is exposed to view in the interior of the home--unlike
stick-framing, which is filled in between the timbers. And because the timbers
are fully inside the insulated wall system, they are also protected from
moisture damage. With stick-framed walls, on the other hand, moisture can
penetrate into the wall cavities and cause rotting of both timbers and
intermediate studs.
Another advantage over stick-framed in-fill is that panels are manufactured
using OSB on the inside skin, which normally has sheetrock applied to it with
screws and/or nails. As the panels are manufactured using engineered materials,
they are very stable and will not shift. Because of this the owner will not have
a nail-popping problem in the future.
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EPS (Expanded Polystyrene) |
Polyurethane |
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3-9/16ths-inch foam core: |
R-17 |
R-24 |
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5-9/16ths-inch foam core: |
R-24 .5 |
R-40 |
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(As a general rule of thumb, polyurethane foam has a 60% higher R-value
than EPS of the same thickness.)
As a result, homeowners can begin saving money immediately by buying smaller
HVAC systems. And of course they continue to enjoy savings on energy costs,
anywhere from 40%-60%, depending on the size and type of panels they select, for
as long as they own the home.
I can't fail to mention the increased strength to be gained by the use of
panels. Both timber framers and their frame owners aim to build homes that will
stand the test of time. The use of panels provides significant additional
strength to the home. But first, a clarification needs to be made: two types of
panels are generally produced: structural insulated panels, which are used to
build homes that do not have any other structural component; and insulated
panels, which enclose the timber frames that provide the structure. The former
are called SIP's (Structural Insulated Panels) while the latter are simply
called insulated panels or stress skins, although sometimes they are all
referred to (inaccurately) as SIP's. True SIP's are structural, and since their
loads are carried evenly throughout, they are stronger than stick-built walls in
compressive strength and resistance to racking. In fact, according to a report
from the US Department of Energy (November, 2001) "Testing has shown that a wall
panel with two half-inch-thick OSB skins is nearly three times stronger than a
conventional 4-inch stud wall." This can make a big difference, especially in
parts of the country that experience severe weather. Houses built of SIP's have
stood up to windstorms, tornadoes and earthquakes that have knocked neighboring
stick frames flat.
Now that I've described the basic advantages of insulated panel systems for
use in building timber frame homes, let me list some of the aspects of
polyurethane panels that make them better than all the others.
- Most urethane panels are
produced using a Computerized Drafting System to design the panel system to
extremely close tolerances. The panels are produced and fabricated from the
design in quality controlled plant conditions. This means they arrive at
your building site already pre-fabricated and ready to go up without a
hitch.
- Urethane foam insulation
generates the highest R rating for panels. We generally recommend our R-26
wall panels and R-40 roof panels for home construction. This will reduce
your energy costs by a minimum of 50% over stick framing.
- As the Urethane is
injected into the panels, it seals itself to both OSB skins and completely
fills the panel. This provides an automatic air and vapor barrier. And, as
the panels are entirely filled, there are no air spaces for moisture to
build up, thus greatly reducing any opportunity for mold to grow in your
wall system.
- In most urethane panels,
your electrical plan is fabricated into the panels for you, some include
chases lined with conduit and wiring boxes. You or your electrician need
only fish the wires through once the panels are up.
- In some panels, your
electrical plan is fabricated into the panels for you, with chases lined
with conduit and wiring boxes. You or your electrician need only fish the
wires through once the panels are up. These wiring boxes are installed prior
to the foaming operation, furthermore, effectively sealing them and
eliminating any drafts through the boxes.
- Polyurethane panels are
manufactured using environmentally friendly products.
Insulated panels are relatively new in the centuries-old building industry.
But while they have been in use only for the past 50 years or so, these
"newcomers" are already tried and true. In fact, they have proven to be the
strongest, most energy efficient and most durable method of high quality home
construction. Polyurethane panels make the most of these advantages.
Panel Design
Panel design gives you the exact measurements for cutting and fitting the
panels. Panel producers provide the design to the builder if the panels are
going to be fabricated on site. Those companies that offer complete
pre-fabrication generally design and engineer the panels to much closer
tolerances. Many of the latter will also include your plans for electrical into
the design and put in the chases and/or conduit and wiring boxes during
pre-fabrication. The panel design generally does not include plans for plumbing
or heating and cooling, since these are generally worked into interior space, as
in conventional construction.
Accessories
These are the various fasteners required to connect the panels. Some panel
companies provide the accessories with the panels while others do not. The cost
of the accessories is generally 10-15 % of the panel cost.
Panel
Interior Treatments
If you are planning to use Tongue
& Groove Decking on your ceiling, the proper procedure is to install it first
and attach the panels over it.
If you are going to use sheet
rock on ceilings or walls, a furring strip should be nailed on the exterior of
the frame members before installation of the insulated panels. This leaves
spacing that will simplify installation of sheet rock. The dry wall installers
simply slip the sheet rock behind the timber frame braces and posts in the space
provided by the furring strip. The same method can be used to make it easier to
install wood paneling on the walls. Be sure to tell your designer what material
you plan to use, and which dimension (both sheet rock and wood paneling come in
different thicknesses) so the proper size furring strip will be indicated in the
drawings.
Exterior
Finishes
Panels allow for the whole range
of exterior finishes on roof and walls. This versatility is just one more of the
advantages of enclosure with panels, as it allows you to build a home that fits
into any setting. Neither EPS nor polyurethane foam contains formaldehyde or
CFC's.
On Roofs: You can use metal, shingle or shake roofs over the panels. Shingles
come in a wide range of quality. If you are using shingles, therefore, it is
best to choose a company that will warranty their product over panels. It is
also wise to avoid a dark-colored shingle in a hot climate.
At this point no studies have been completed to indicate that a cold roof will
increase the life span of composition shingles over panels. The typical panel
roof is simply covered with felt and shingles. Although this is theoretically a
hot roof, in fact, because of the extremely high R-Value of panel roofs, it is
more accurately defined as neutral. The insulation prevents heat from building
up in the interior and over-heating the roofing material.
On Walls: The panel walls can be covered with any kind of wood, vinyl, stucco,
stone or brick. Your siding manufacturer will provide specifications for
attaching the siding to the wall panels.
Dry-In
Enclosing a frame with insulated
panels is faster than stick-framing, but of course the type of panel used will
make a big difference as well. The pre-fabricated panels can be installed very
quickly and offer almost immediate protection for the timber frame.
Once the panels are installed,
you have only to get the roof covering over the panels and your home is dried-in
and safe from the elements. At this point, you have much more leeway in
scheduling the work to complete your home.
Finishing
Your Home
In general, finishing a timber
frame enclosed with insulated panels is not significantly different from
finishing a stick-framed home. However if the contractor is not familiar with
panel construction, he can consult with the timber frame company on questions
concerning finishing the house.
Again, the more sophisticated
pre-fabricated panels make finishing your home easier. Since they are
pre-fabricated, you will not have the time-consuming and messy process of
cutting the panels on site. The best panels have chases lined with conduit and
wiring boxes installed in the panels for your electrical. The rough openings for
windows, skylights and doors are cut and framed in with wood. The subfacia is
already installed on the roof edge. All of these features provide significant
time savings and make the contractor's job easier.
Moisture
While panels protect the timber
frame from moisture, they themselves require protection from moisture. There are
three key preventative measures. The most important is that the panels must be
properly sealed with foam during installation. As a second measure, you should
wrap the home in a vapor barrier. This step is not required by all panel
manufacturers, but is good building practice. The third measure-one the
homeowner can take-is to install an air exchange system to exhaust the
moisture-laden air that builds up in any tightly sealed structure. Some state
energy codes require air exchange systems in all new construction, now. More
will adopt that requirement in the future.
Indoor Air
Quality
An air exchange system will also
bring fresh air into your home and exhaust the stale air. This is recommended in
all tight houses, including those enclosed with insulated panels, because there
are no drafts or leaks to allow air exchange with the outdoors. Many products
found in the home, such as furniture, curtains, carpet, and cleaning products,
emit gasses that can built up and pollute the air inside the home unless there
is a mechanical air exchange system.
You can get an air exchange
system with a whole house air purifier, if you want to take that next step.
Off-Gassing
Insulated panels do not
contribute to the pollution of air with formaldehyde or other gasses. Neither
EPS nor polyurethane foam contains formaldehyde or CFC's. While trace amounts of
formaldehyde are found in fresh OSB, it is well below the limits established by
the U. S. Department of Housing and Urban Development. In addition, they
diminish to undetectable levels within the first few months. (HUD's limit is
0.2ppm for plywood and 0.3ppm for particle board. Fresh OSB is below 0.1ppm and
for this reason has been exempted from testing and certification.)
Fire
SIP's pass all the standard fire
tests required of wood-based construction. They contain no air within their foam
core, so the crucial element required for a fire to burn, oxygen, is absent.
This means that the fire will not run up the wall cavity. The sheetrock usually
used as the interior wall finish adds another 15 minute thermal barrier to
protect the SIP's and any other underlying structure. Urethane panels typically
have a Class 1 fire rating. Urethane does not burn.
Insects
Insulated panel systems have no
insect problems that are unique to them. The same insects that can infiltrate a
stick framed home may get into a panel enclosed house. It is advisable to take
the same precautions that one would take in building any home..
Keeping out carpenter ants and/or
other insect pests such as termites, may require a multi-pronged approach, which
includes treating the soil, putting in the metal shields between the foundation
and sub-floor, keeping flower beds away from the foundation and removing
overhanging limbs. It is advisable to talk with your local pest control
professional and your builder about which steps are most effective in your area.
Cost
The panel materials cost more
than stick framing with fiber glass batting, but less waste, cheaper labor costs
and the savings on heating and cooling systems often allow SIP's to stay
competitive in the long run. Prices vary region by region, so the best procedure
is to get competitive bids locally, figuring in the long term factors as well as
short term.
Wiring and
Mechanical
With the proper information and adequate planning, wiring a home enclosed
with insulated panels can be simpler and quicker than wiring a stick frame
house.
The most common method used in wiring insulated panels is for the
manufacturer to create horizontal chases (at two levels that are chosen by the
homeowner) through the foam insulation. The electrician then fishes the wires
through the chases. These chases save the time normally spent drilling through
the 2 X studs of a stick frame house in order to create pathways for the wiring.
An improvement in this method of wiring is offered by certain panel
manufacturers. In this option the electrical boxes for the switches and outlets
are pre-installed during fabrication. This saves the electrician time (and
therefore the homeowner money) and also provides a tighter, better-insulated fit
at each electrical box. The fabricator also lines the chases with PVC pipe,
which simplifies the wiring process. The wires will not get caught on the foam
insulation so they can be pushed, not fished, through the chases. This, once
again, saves on the electrician's time.
If it is not possible to make use of the existing chases, alternative methods
of wiring can be employed. These methods include running the wiring behind
baseboards, chair rails or wainscoting. However, these are options of last
resort due to the question of meeting local building codes and the additional
costs of material and labor.
The basic method for wiring ceiling fixtures and ceiling fans is to rout out
the tops of ceiling beams to create a channel in which to run the wires. This
procedure is done in the shop while your frame is being cut or on site after the
raising.
The lighting choices available to the timber frame homeowner are the same as
those used in conventionally built stick frame houses. There is, however, one
important difference to keep in mind when laying out your lighting plan. The
posts and beams that give timber frame homes their distinctive character can
also cast some big shadows. It is therefore important to decide where you want
to accentuate those shadows and where you want to eliminate them. The various
lighting fixtures -- surface mount, recessed, track lighting, hanging fixtures,
etc. - can be used alone or in conjunction with one another to produce the
desired lighting patterns.
Once the wiring method has been chosen it is necessary to develop a detailed
wiring plan that includes the path of the wires and the exact location of all
switches, outlets and light fixtures. The diagram can be marked on the home
plans by the electrical contractor or by the homeowner.
Since it is never advisable to run plumbing in exterior walls (due to the
reduction of insulation values and the danger of the pipes freezing) the
utilization of insulated panels does not impact a timber frame's plumbing
scheme.
Because of their customary open design, however, timber frame homes do
present specific difficulties in concealing the pipes. Keeping in mind a few
basic rules while laying out your floor plan and plumbing design will eliminate
most of these hurdles.
"Wet walls" typically must be constructed of 2 X 6 studs in order to
accommodate four and a half inch vent stacks and four inch drains. If, for some
reason, the partition walls will not accommodate the required pipes, or the
plumbing has to be run on an exterior wall, then a separate plumbing chase (a
false wall) can be built to enclose the pipes.
Upper level floors in timber frame homes often consist of 2 X Tongue and
Groove (T&G) decking. This method of construction saves money in both material
and labor and has the added advantage of creating an aesthetically pleasing
finished appearance. However, because the 2 X T&G forms both the flooring of the
upper rooms and the ceiling of the lower living spaces it leaves no cavity in
which to run horizontal pipe chases. To overcome this issue 1 X Tongue and
Groove is placed on top of the beams and a floor system is built up with 2 X
material to create a void for wiring, ductwork, and insulation.
Finally, a word of warning. It can not be stressed enough that, under no
circumstances, should any timber be cut or drilled through to accommodate the
plumbing. To do so would compromise the structural integrity of your timber
frame.
The first order of business in assuring year-round comfort in your new timber
frame home is to make certain that it is as energy efficient as it is beautiful.
Energy efficiency means savings for the homeowner not only in the long run
with lower fuel bills, but also in the purchase and installation of heating and
cooling systems. In many locations, a cooling system needs to be nothing more
than strategically placed ceiling fans. As for heating the structure, many
homeowners will utilize solar energy (active or passive) while others may want
to install a wood burning, or pellet, stove. These are excellent options that
can save money and be environmentally friendly; however, local building codes
(as well as the homeowner's lending institution) will almost certainly require a
back up heat source.
A forced air furnace, located in the lowest level, with properly placed
registers let into the floor of the main level, should handle the heating
requirements for the entire house. The heat simply rises to the upper levels of
the timber frame home.
If heat has to be delivered directly to specific areas in the upper levels, a
forced air furnace would not be a desirable option. In this case either a whole
house hot water system or electric baseboard heaters are better suited for the
task. As opposed to the space requirements for forced air ductwork, the pipes
used in hydronic systems are generally ¾" and therefore can be easily concealed
in partition walls. And finally, while the cost of electric heat per BTU is the
highest of any available system, the relative low cost and ease of installation
make this an acceptable choice if the heaters are only infrequently needed.
Radiant floor heating is a popular system for heating timber frames. Radiant
floor heat provides heat by convection, heating the floor and furniture first.
This is a very efficient way to heat a timber frame home.
As mentioned above, stress skin panel enclosure provides an extremely tight
and draft-free interior. While optimal in terms of comfort and economy, it may
reduce air exchange more than is desirable. For this reason, it is recommended
that you have an air exchange system installed along with your heating system. |
