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Advertising supplement provided Jeld-Wen
Windows & Doors
Preservation Processes
There are a number of types of treatment
techniques utilized in wood preservation:
Pressure Impregnation uses vacuum and
pressure to obtain chemical penetration of permeable timbers,
while controlling the amount of preservative retained. The
timber must be free of stain and be less than 30% moisture.
Many treatment schedules are used, the pressure fluctuations
and timing being distinctive in each.
The Bethell process is the most important
of the treatment processes and achieves about 90% of the theoretical
maximum uptake in radiata pine. The Lowry treatment is designed
to achieve maximum penetration with a low retention of preservative.
Retention is around the 60% of theoretical maximum. The Rueping
process is used principally with preservative suspended in
hot oil such as creosote and PCP where a low net retention
is desired for some hazard categories. Net retention here
is as low as 40-50%. The Alternating Pressure method utilizes
repeat applications of pressure and vacuum to force preservative
into green wood.
Vapour
Phase utilizes the fact that some Boron esters boil
at low temperatures. The liberated gas can be drawn into timber
where it reacts with water and condenses. For this reason,
timber must be very dry (less than 5-6 percent moisture content)
or only low penetration is achieved. Treatment of framing
timber in the drying kiln is possible and can offer considerable
cost savings.
Vacuum
treatments utilize volatile organic solvents to transport
the preservative into the wood. It is designed to treat dry
profiled or machined wood. TBTO is typically used, being introduced
to the timber by either a double vacuum or low-pressure cycle.
The advantage of this technique is that there is no dimensional
swelling as associated aqueous treatments and wood can be
painted within a couple of days of treatment.
Diffusion
is used to introduce boron salts to green timber. The timber
is sprayed or dipped in the preservative solution and block
stacked (without fillets). The wood is then tightly wrapped
and left for a number of weeks during which the boron salts
diffuse into the wood. Thicker timber may require a second
dip to top up the salt levels. For this technique to be successful
the timber must have a moisture content over 50 percent. If
the timber surface dries out, the process may not work at
all, or be uneconomically slow. Another method using the diffusion
process is called double diffusion. This works in the same
manner except two successive chemical treatments are used.
The second chemical treatment (Na2Cr2O7/Na2CrO4 and Na2HasO4)
precipitates with the first (CuSO4) to form a non-leachable
preservative.
The Reynolds learned from their window
manufacturer: that despite common preservation techniques,
wood decay can occur, and that preventing “overexposure”
to moisture must include these steps:
- Eliminating exposure of the product to excessive wind
and water loads.
- Adding gutters or water breaks, awnings or barriers (natural
or artificial).
- Eliminating exposure of the product to irrigation and
sprinkling.
- Eliminating building design features that allow water
to pool at window (fenestration) openings or to come in
contact with the end grain of the wood like pop-outs, improperly
sloped sills, contact with brick, masonry, or stucco, insect
nests, or incorrectly installed flashings.
Homeowners, they are told, may find it
possible to repair doors or windows that have not been “structurally”
damaged.
A professional wood restorer using the
proper equipment, processes and materials MAY be able to repair
at a cost significantly less than the cost of replacing the
product. If the homeowner chooses to replace the product,
he/she is advised to choose products that are certified as
treated to the most current version of the WDMA IS-4 standard.
Homeowners are further advised to properly maintain window
products by:
- Painting with a high-quality exterior trim paint and
regularly maintaining the paint finish. Prior to painting,
all wood-to-wood joints where water can penetrate should
be sealed using a high quality sealant.
- Inspecting products at least twice a year, immediately
touching up any breaks in the sealant or paint film and
contacting the manufacturer at the first sign of any softness
or discoloration in the product.
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