CBD-161. Moisture and Thermal Considerations in Basement Walls
C.R. Crocker
As a result of extensive investigation of the performance of
roofs, windows and above-grade walls, there have been many changes
in their design in recent years. Such changes have reflected better
understanding of the principles governing movement of air, water,
water vapour and heat. One element of building, however, has not yet
received the same degree of attention. The concrete or concrete
block basement wall still presents problems. The primary concern of
the designer and builder has until now been to meet structural
requirements. Less attention has been given to the control of
moisture, air and heat flows. The standard design has often failed
to provide this control, mainly because of the compromises that must
be made with a wall that is partly underground. The occupants of
most buildings, however, have come to expect that this underground
space will be as acceptable for normal activities as other spaces.
The moisture problem causes the greatest concern. High heat loss
from non-insulated basement walls is also serious, but it is not
always recognized as a problem. Although solutions are not
difficult, they do require a new approach to the design and
construction of walls.
Moisture Problem
Moisture conditions in the soil around a basement wall can vary
from saturated to dry, depending on the soil type, topography,
height of the groundwater table, climate, season of the year, and
other factors. The site may be very wet and the foundation even
extend below the groundwater table. In such cases, where drainage is
not possible, it is necessary to construct a watertight compartment,
usually of concrete. Discussion of this type of structure is beyond
the scope of this Digest, which is concerned only with foundations
such as those used for houses and many commercial, industrial and
institutional buildings that are usually quite shallow and can be
drained. Basement walls of such buildings must be protected from a
hydrostatic head of water and must also be able to control the
movement of moisture by diffusion and capillarity from the soil into
the basement.
Control of Liquid Water
Groundwater levels fluctuate during the year, primarily as a
result of seasonal variations in rainfall or snowmelt. They are
usually at their highest in late fall and early spring. At some time
of the year most foundations, even shallow ones, will be subjected
to a hydrostatic head of water if provision has not been made to
intercept groundwater and remove it. The standard procedure is to
install drainage tile or perforated plastic pipe and crushed stone
around the basement so that water will not reach the under side of
the basement floor slab (Figure 1). Usually, the drainage tile are
located outside the footings, with the bottom of the tile at the
same level as the base of the footings. Connections are made through
the footings to the crushed stone under the floor slab. The tile or
pipe is normally covered to a depth of at least 6 inches with
crushed stone or other coarse granular material which acts not only
as a filter to exclude fine grained soil particles but also, with
the pipe, as a drain. The water collected by the drain is led to a
storm sewer, dry well, or to a sump from which it is pumped. When
properly installed this system works well, but to be fully effective
the drain must be placed so as to prevent ponding.
Figure 1. Well-drained basement wall.
For small buildings with a perimeter of less than 200 feet drain
tile can be laid level, except for the final section leading to the
storm sewer, dry well or sump. The drain should, however, be placed
on a carefully prepared smooth surface at the level of the bottom of
the footing. For larger foundations, a slight grade (6 inches in 100
feet) may be provided to assist the flow of water in the drains.
The footing drain should take care of water percolating down
through the soil from the ground surface as well as groundwater. If,
however, fine-grained soil is used as backfill, the downward
movement of the water can be restricted to such an extent that a
perched water table is formed at some level above the footing drain.
A substantial pressure can then develop, forcing water through
joints or cracks in the wall. The obvious solution is to use
granular material as backfill and if it is readily and economically
available it should be used. Granular material placed as a thin
layer against the wall would provide good drainage, but it is a
time-consuming and therefore an expensive procedure to place the
filter material correctly. Corrugated plastic sheets or mineral wool
roof insulation placed next to the wall prior to backfilling will
provide a drainage layer adjacent to the wall and prevent any
build-up of water.
The footing drain and the drainage layer adjacent to the wall
will prevent the development of a hydrostatic head of water. The
outer surface of the wall below grade is, however, in contact with a
very moist environment, with a relative humidity in the soil air at
or very close to saturation and, occasionally, water trickling down
from the ground surface or as the result of condensation at a higher
level. For most of the year the water vapour pressure in the soil
adjacent to the wall will be higher than that in the air in the
basement, with the result that water vapour will tend to flow into
the basement. The amount of water vapour that will pass through good
quality concrete is very low, but concrete made with a high
water/cement ratio is quite permeable. In addition, liquid water in
contact with the basement wall can be drawn by capillarity through
the wall. How serious this will be depends on the properties of the
wall material and the severity of exposure. Bituminous coating
applied to concrete and parging plus coating applied to masonry
walls will usually provide the necessary protection.
Another path for moisture is through the footing. To prevent
moisture migration to the interior by this route a membrane such as
polyethylene film can be placed on top of the footing before the
wall is cast. This membrane should subsequently be incorporated with
the membrane normally placed under the floor slab to provide a
continuous moisture barrier under the basement.
Many basements are wet because an excessive amount of surface
water gets into the soil adjacent to the building. This is the
result of improper grading combined with inadequate disposal of
run-off from roof surfaces. The solutions are obvious. First,
maintain a slope that will ensure positive drainage away from the
building. Clay surfacing over porous backfill can be used to
advantage here since it will promote run-off of surface water.
Settlement of backfill will require that additional fill be
deposited from time to time until the soil is fully consolidated.
This is particularly important if paved driveways or parking areas
are to be located next to the building. Run-off from such surfaces
can overload the footing tile around the perimeter if it drains
towards the building. Second, install eavestroughs and conduct the
drainage from roof surfaces away from the building, using extensions
to the downspouts and splash blocks. Run-off from roofs or paved
areas should not be led to the footing drain because of the
possibility of destructive uplift pressures developing under the
floor slab during severe rain storms that overtax the drainage
system.
The requirements of moisture control can be summarized as
follows:
-
Place drainage
tile or perforated pipe and crushed stone below floor level at the
base of the footing and dispose of the water to a storm sewer or
drywell.
-
Cover the
footings with a moisture barrier.
-
Apply a
damp-proof coating to the exterior surface of the basement wall.
-
Install a
drainage layer from near grade level down to the footing drain.
-
Provide and
maintain a positive grade away from the building.
-
Control run-off
from roof surfaces and paved areas adjacent to the building.
The Thermal Problem
Basement walls are traditionally built of high-conductivity
materials - concrete or concrete block - that provide very little
resistance to heat flow. The total resistance (R) of the above-grade
portion of an 8-inch concrete wall is only 1.5 units (1 inch of
polystyrene or mineral wool equals 4 units). Fortunately, the
below-grade portion has a higher thermal resistance because of the
insulating effect of the soil. Six feet below grade thermal
resistance has reached a value of about 20, which is considerably
better than that of a well-insulated frame wall.
The proportion of heat loss that occurs through the uninsulated
basement wall of an insulated and weatherstripped building can be
very high, depending upon the amount exposed above grade. The heat
loss from the heated but uninsulated basement of an otherwise
well-insulated typical frame bungalow may be 25 per cent or more of
the total heat loss from the house. There is no question of the
value of insulating the walls of a heated basement.
The optimum amount of insulation is related to many factors such
as cost of energy, and labour and material costs of providing for
and installing the insulation. To obtain the equivalent resistance
of an insulated frame wall 12 to 14 units of resistance must be
added to the above-grade portion of the basement wall and continued
for at least 18 inches below grade. For new construction, at least
this amount of insulation should be specified. It may be difficult
with existing buildings to add as much insulation because it
requires 3 to 3½ inches of space, but even modest amounts will
provide substantial savings in energy. For example, 4 units of
resistance (1 inch of polystyrene or mineral wool) added to the
above-grade portion of the wall will reduce the heat loss through
that portion by 70 per cent. This large reduction is possible only
because the uninsulated wall has such a high conductivity. The heat
loss with 1 inch of insulation is still 2½ times that of an
insulated frame wall.
There are a number of advantages in applying insulation outside
the structural elements of a wall (CBD 50). For a basement wall this
will keep the concrete at a reasonably uniform temperature
throughout the year and reduce the possibility of condensation on
the inside surface. The below-grade portion of the wall can be
insulated either by continuing the insulation down to the required
depth or by placing the same amount of insulation on or just under
the ground surface and extending it out from the wall, as
illustrated in Figure 2. The latter is the preferred location
because it maintains the soil adjacent to the basement wall above
freezing, thus promoting drainage through the soil; and avoids the
possibility of frost heave damage that exists with insulation fixed
to the wall below grade. Insulation selected for this purpose must
be resistant to moisture (for example, extruded polystyrene) and for
the above-ground portion must be protected from solar radiation and
physical damage. Stucco, paint or asbestos cement sheets have been
used on walls for this purpose, with crushed stone, patio slabs or
soil employed to protect insulation placed on the ground.
Figure 2. Exterior insulation of basement wall.
Placing the insulation on the outside can simplify the interior
finishing of basement walls. If adequate control of water at the
exterior has been provided, the interior can be left plain or, for
aesthetic effect, painted or wallpapered. Panelling can be installed
on furring strips, leaving a ventilated space behind the panels. If
insulation must be placed on the inside, it should be protected from
moisture or be inherently moisture resistant. Batt-type insulation
must be protected by a vapour barrier and the below-grade portion by
a moisture barrier (CBD 13). Insulation on the inside should be
installed tightly against the concrete to reduce the possibility of
air circulation, which could lead to condensation. Walls constructed
of hollow masonry units should be insulated to the floor to overcome
the effect of convection currents in the core spaces.
Heat is also lost from a building as a result of air leakage into
the basement. The pressure differences that exist between the inside
and outside of heated buildings promote air infiltration through
cracks or openings at the lower levels. This cold air picks up heat,
rises and subsequently escapes from the upper levels. The caulking
of joints and the use of weatherstripping around window units
greatly reduces air infiltration and thus heat loss. The most
vulnerable part is usually the joint between the foundation and the
superstructure, particularly in wood frame buildings. Thus, an
essential part of controlling heat loss from basements is the
prevention of air infiltration.
The requirements of heat loss control can be summarized as
follows:
-
Caulk the joint
between sill and foundation and around window frames.
-
Weatherstrip
basement windows.
-
Insulate headers.
-
Insulate the
basement wall, preferably on the outside.
Many basement walls that do not conform to these recommendations
are performing well and may continue to do so. It would be very
difficult, however, to guarantee that even these basements will
always perform satisfactorily under unusually severe conditions
simply because of their past records. In the meantime, the incidence
of dampness, musty smells, rotting wood and flooding in basements is
so high and the consequences so unpleasant that every effort should
be made to construct a basement that will be trouble free.
Finally, it should be pointed out that not all dampness is due to
improper design or construction. Ventilation of basements during
hot, humid weather, particularly in late spring and early summer,
often leads to condensation on cool wall and floor surfaces. The
solution is to limit ventilation during such weather and to
dehumidify under particularly severe conditions.
Originally published 1974.
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