you've ever enjoyed a glass of iced tea on a hot summer day,
you've experienced this "condensation" first-hand.
Moisture collects on the glass when water vapor in warm, moist
air condenses on the cold surface. Something similar could
happen in your chimney. Warm, moist air hits the cold chimney
wall and water droplets form on the inside of your chimney.
Condensation was not a problem with your older, less efficient
furnace because more heat escaped up the chimney. According
to nature, these hotter vent gases could carry more moisture
straight out your chimney.
Not so with modern furnaces and their lower emission temperatures.
Moisture can seep into cracks in the bricks and mortar. In
cold climates, the moisture freezes and thaws, and can cause
chunks of tile and chimney to break loose. In the worst-case
scenario, this debris could actually block the venting system,
causing a potentially harmful situation. When not vented properly,
excess moisture damage may not be limited to your chimney.
Water could even drip back into your furnace, causing it to
corrode. As it seeps through porous mortar joints, this
moisture could also ruin neighboring drywall.
Water Damage and Your Masonry Chimney
As odd as it may seem, water causes more damage to masonry
chimneys than fire. Think about it for a moment. All the brick
and other materials that make up your home are protected by
the roof and eave, all that is, except your chimney. The chimney
bravely sticks up above the roof constantly exposed to all
the elements; rain, snow, and freeze/thaw cycles.
A masonry chimney is constructed of a variety of masonry and
metal materials, including brick, mortar, tile, steel and cast
iron. All of these materials will suffer accelerated deterioration
as a result of prolonged contact with water. Masonry materials
deteriorate quickly when exposed to the freeze/thaw process
in which moisture that has penetrated the materials periodically
freezes and expands, causing undue stress. Water in the chimney
also causes rust in steel and cast iron, weakening or destroying
the metal parts.
penetration can cause interior & exterior
damage to your home and chimney including:
- Spalled and broken brickwork
- Deteriorated metal or masonry firebox assemblies
- Rotted adjacent wood and ruined wall coverings
- Deteriorated central heating system
- Cracked flue liner systems
Preventing Water Damage
a Chimney Cap
Chimney caps, also called
rain covers, are probably the most inexpensive preventive
measure that a homeowner can employ to prevent water
penetration and damage to the chimney. Chimneys have
one or more large openings (flues) at the top that collect
rain water and funnel it directly to the chimney interior.
A strong, well designed cap not only keeps this water
out, but will also prevent birds and animals from entering
and nesting in the chimney.
Caps also function as spark arrestors, preventing
sparks from landing on the roof or other nearby combustible
|Repair or Replace a Damaged
The chimney crown, also referred to as the chimney
wash, is the top element of a masonry chimney. It covers
and seals the top of the chimney from the flue liners
to the chimney edge. Most masonry chimneys are built
with an inadequate crown constructed from common mortar
mix, the same mixture used to lay the bricks of the chimney.
This mortar is not designed for and will not withstand
years of weather abuse without cracking, chipping or
deteriorating--situations that allow water to penetrate
the chimney. In fact, most sand and mortar crowns crack
almost immediately after installation because of shrinkage.
A proper chimney crown should be constructed of a
portland cement based mixture and cast or formed so
it provides an overhang, or drip edge, projecting beyond
all sides of the chimney by a minimum of two inches.
This drip edge directs the runoff from the crown away
from the sides of the chimney, helping prevent erosion
of the brick and mortar in the chimney's vertical surfaces.
There are also some modern waterproof, non-shrinking,
cement-like coatings for repairing damaged mortar crowns
that seem to work fine.
|Repair or Replace Flashing
Flashing is the seal between the roofing material
and the chimney. Flashing prevents rain water from running
down the chimney into living spaces where it can damage
ceilings or walls, or cause rot in rafters, joists, or
other structural elements. The most effective flashing
is made up of two elements, the base flashing and the
The base flashing is an L shaped piece of metal extending
up the chimney side and under the roofing shingles.
The counter flashing, which overlaps the base flashing,
is imbedded and sealed in the chimney's masonry joints.
This-two element flashing allows both the roof and
the chimney to expand or contract at their own rates
without breaking the waterproof seal in either area.
|Waterproof Your Chimney
Most masonry materials are porous and will absorb
large amounts of water. Common brick is like a sponge,
absorbing water and wicking moisture to the chimney interior.
Defective mortar joints or the use of improper mortar
or brick can greatly increase the tendency to absorb
and convey water to the interior of the masonry chimney
Several products have been developed specifically
for use as waterproofing agents on masonry chimneys.
These formulas are vapor permeable which means that
they allow the chimney to breathe out, but not in.
Thus water that has penetrated the chimney, or moisture
that has originated from inside, is allowed to escape,
while the waterproofing agent prevents water from entering
from the outside.
Paint, or any non-vapor-permeable water sealer, should
never be used as a waterproofing agent because it will
trap moisture inside the chimney, accelerating deterioration.
Reprinted with permission from the Chimney
Safety Institute of America, www.csia.org