| |
Occasionally,
blistering of a chinked or caulked surface occurs. A
chinked area is more susceptible to blistersing due
to the larger joint size. After several years of investigation,
Sashco Sealants has uncovered most or all of the causes*
of this problem and can recommend ways to prevent it.*
At first it was thought that the chinking itself
was to blame. But the facts do not support this. For
example:
- The
same batch of product has blistered on one log and
not on another.
- It
is common to find blisters on one side of a house
and not the other.
- Blisters
occur above a certain point on a wall and not below
it.
- Some
blisters appear to be random on a chinking line
and others are in a straight row.
- Blisters
often reappear in the same place when repaired with
the same batch of chinking or even with a different
batch.
The Causes
are Site related and not product or batch related.
Causes of Blisters: The basic cause of any blister
is a build-up of gas between the log, the backing surface,
and the chinking/caulking. It occurs during The early
curing stages when The chinking is soft but has a skin
formed on the surface. Heat usually in the form of direct
sunlight causes the gas to expand. As the gas expands,
it pushes the chinking out, causing a "bubble" or blister.
This gas comes from water, wood sap, or even backing
material.
What Circumstances Create the Gas? Trapped Water
Vapor: As chinking/caulking dries, it gives off
water vapor. Any physical condition which traps the
resulting water vapor could cause a blister, especially
if the logs and chinking are heated by direct sunlight
here are some things that could trap this vapor.
Note: Sikaflex is not affected by water vapor
in the same as other caulking/chinking because it is
a polyurethane. Moisture makes it set up faster and
will not generally cause blistering.
- Crack
or indentation in foam or polyethylene backer rod
which doesn't go all the way through. Even tiny
spaces can trap enough vapor to create a blister
big enough to see.
- Air
bubble trapped during the application process against
non-porous backer material.
- Dead-end
crack in wood where moisture vapor can accumulate
and expand when heated. (This, most likely, is the
most wide-spread cause.)
Backing Material
- Backer
rod out-gassing. Closed cell backer rod has a puncture
and is squeezed by expanding logs. The gas used
to make the rod forces a blister in the chinking/caulking.
On day 1
the backer rod and sealant are applied in the cool part
of the day. On day 2 the sun comes out heats the logs.
This causes the backer rod to be squeezed, and forces
a small amount of gas in the backer rod to leave the
rod at a rupture or weak spot and make a blister in
the sealant. The phenomenon has been known to occur
in the caulk and sealant industry for many years and
occurs randomly and unpredictably.
 |
 Pitch Pockets:
Logs often have what is termed "pitch pockets." These
are areas of concentrated pitch, or sap, sometimes containing
several gallons! (Pitch pockets of 50 or more gallons
are known)! This sap (or its vapor) will follow cracks
in logs, coming out of the log almost anywhere. It especially
exits at the knots Where the sap flow was directed during
the life of the tree. Sap has a volatile component and,
when heated, forms a gas that causes blisters. Some
blisters appear to be in the dead center of the chinking/
caulking. Upon close investigation, however, most or
all of the bubbles are connected to a crack in the log
surface. The blisters are often the result of sap vapor,
which can come through the tiniest of holes in the wood.
Pitch pockets are often the cause of blisters, which
line up in a straight row. They are simply aligned with
the crack in the log along which the sap is traveling.
In the heat of direct sunlight some of the liquid sap
in the pitch pocket turns into a gas that is under pressure
and can form a blister as it tries to escape the log.
Prevention of Blisters: Some of these causes
cannot be controlled. Two factors can be partially controlled
- heat and backer rod selection/preparation. Since heat
normally is responsible for turning undetected gas into
unsightly blisters, it makes sense to control this element
as much as possible. Once the chinking/caulking has
become firm, it can then resist the low pressure from
the water or sap vapor and the blisters can then be
prevented.
 Control
the Heat:
- Chink
or caulk on the shady, cool side.
- Protect
walls from direct sunlight for a few days by covering
with a light colored tarp, cloth or plastic. Leave
plenty of room for ventilation. It is important
to cover the logs as well as the chinking/caulking
so the entire surface will stay cool.
- Stain
logs first. This can help to close some pitch pocket
holes before chinking/caulking.
- Although
rare, it is possible for excessive heat to build
up inside a log home during the early curing stage
and cause blisters. If this could occur, make sure
windows and doors are left open to allow cool air
to ventilate the interior.
Control
the Backing Material
- Use
blunt tools to install closed cell backing materials,
being careful to avoid puncturing or denting.
- Open
cell backer rod** may be used in place of closed
cell backer rod. These backer rods will not generate
gas and they can help dissipate any gas originating
from another source.**The weakness of this type
of backer rod is that it is more susceptible to
absorbing moisture if the chinking or caulking is
ruptured and driving rain is forced through to the
backer rod.
- When
chinking/caulking over flat insulation (styrofoam,
bead board or foil-faced foam board), surface cracks,
holes, or dents should be covered with a high quality
sticky mylar tape, 2" wide preferably. (The tape
will tend to prevent as much as 80% of the blisters
that would otherwise form.) Extruded styrene foam
(such as Dow Chemical's blue "Styrofoam") has a
greater tendency to cause blisters than white bead
board or foil-faced urethane foam.
- A
fresh blister can sometimes be eliminated by puncturing
the surface skin (to let the moisture vapor escape)
and pushing the chinking/caulking back into place.
This step can cause marring of the chinking finish,
but can be smoothed out 2-3 days later (after the
chinking has firmed up) with a small amount of Log
Jam applied and smoothed. Often the blister will
not reappear. To be effective, this action must
be taken within the first day after application,
while the underlying chinking/caulking is still
wet This method can be the most effective and efficient
since the blisters are caught very early (before
they become very large) and repaired quickly.
- More
thoroughly cured blisters must be cut off flush
and the cavity filled approximately one-fourth to
one-half full and allowed to dry. If the cause of
the original blister is still present, then it will
push the fresh chinking/caulking out into a shallow
blister below the surface of the surrounding chinking/
caulking. This may be allowed to dry and a second
application, blending with the surrounding chinking,
will complete the repair. (See the drawings below.)
If you can
cover a crack or hole with mylar tape, only one repair
step is needed. Chinking/caulking can be installed flush
with the surrounding material. |
