|
a PDF version of this document (143K).
Molds are part of the fungi kingdom and
all fungi have four basic requirements for growth: suitable temperature,
oxygen, food and moisture. Controlling moisture offers the best
opportunity to prevent mold growth on lumber and wood products.
Wood is a hygroscopic material, which
means it readily retains and takes up moisture. Under a microscope,
wood fiber resembles a series of hollow tubular cells which serve
to transport water and other nutrients in a living tree. When a
tree is harvested and sawn into lumber products, the moisture remains
in the wood cells until it evaporates to match the moisture conditions
of the surrounding environment.
In some circumstances, the moisture
in wood may contribute to mold growth, such as when the evaporating
moisture is trapped and condenses on the surface. Once the moisture
content of wood declines to less than 20 percent, it will no longer
support mold growth. Lumber used in construction typically dries
to below 20 percent moisture content before the structure is enclosed.
There are many ways to control the moisture
in wood and block the formation of mold. The best long-term protection
against mold growth is lowering the moisture content of wood to
below 20 percent and keeping other sources of moisture away from
the wood.
Unseasoned or green lumber has a moisture
content greater than 20 percent. This lumber will dry naturally,
usually during the framing stages, to match the surrounding environment.
If there is not sufficient airflow to carry the evaporating moisture
away from the wood, the trapped water may contribute to the formation
of mold.
Airflow in and around green lumber plays
an important role in drying the wood to below 20 percent moisture
content. Since most green lumber is used as framing material, the
wood typically dries in place as the outdoor air carries away the
moisture. In warmer climates, the framing can dry to below 20 percent
moisture content in just a few days.
Many mills in the West reduce the risk
of mold and stain on green lumber by applying anti-stain treatment
(also called sapstain treatment) to the wood prior to shipping.
Anti-stain treatments provide a thin coating of fungicide on the
surface of the wood. These fungicides are applied by dipping entire
bundles of lumber into a treatment solution or by spraying all four
surfaces of individual pieces.
Anti-stain treatments provide a microscopic
barrier against molds that last from three to six months, depending
on the chemical, the concentration used, the wood species and the
climatic conditions. The chemicals used for preventing mold and
stain are usually very mild and include many that are used on food
crops as well as in shampoos and paints. They are not designed for
long-term protection.
Some lumber buyers have responded to
mold concerns by switching from green to dry lumber. Using dry lumber
can reduce the likelihood of mold formation, but it will not guarantee
that the wood will remain free of mold. Lumber that is exposed to
moisture after it has dried can support mold growth.
Dry lumber may become wet through direct
sources, such as rainfall or condensation. Improper storage, such
as placing lumber bundles near puddles or other water sources, also
creates moisture conditions for mold growth. Even wet pieces inside
a bundle of dry lumber could generate moisture for mold. If the
wrapped bundle is exposed to direct sunlight, the evaporating moisture
could be trapped inside the wrapping and condense on the wood in
sufficient quantities to support mold growth.
Given the importance of controlling
moisture in preventing mold, proper lumber storage is essential.
The following recommendations are detailed in WWPA's Lumber Storage
TIP sheet:
Green lumber
- Move inventory quickly using the first in, first
out rule.
- Sticker green lumber to allow air circulation if
wood is to be stored for a long period.
- Place cover boards, extending beyond the ends of
the stack, on the top package of lumber to protect the wood from
the sun.
- Do not store lumber near standing water; ensure
the storage area has good drainage.
Dry lumber
- Open or closed storage sheds are preferable to
outside storage.
- If stored outdoors, cover dry lumber with tarpaulins,
canvas or paper wrapping.
- Repair any tears in paper wrapping immediately.
- Store high-grade dry items, such as shop, moulding
and millwork, in a closed, heated shed that has a clean, paved
floor.
You can find additional information about
mold online at these sites:
Mold, Housing and Wood
Western Wood Products Association
www.wwpa.org/lumberandmold.htm
Lumber Storage
WWPA Online Lumber Technical Guide (free registration required)
www.wwpa.org/techguide
Mold, Moisture and Lumber
Southern Pine Council
www.southernpine.com/mold.shtml
Construction Tips for Building Moisture Resistant
Houses
APA – The Engineered Wood Association
www.buildabetterhome.org
Also, see the other Fast Facts on mold and
wood products in this series:
No. 1 - What is Mold?
No. 2 - Preventing and Controlling Mold
No. 3 - Cleaning Mold on
Wood
|
Email: