Controlling rain and ground water are the most important factors in the design, construction and retrofit of durable buildings and in the control of mold.

Rain Water

Water that comes from the sky - in the form of rain or snow - is something that every building must control. In wood frame construction the rain water control system typically consists of a rain-shedding layer, which we call cladding, and then, behind the cladding, a water control layer (or layers) that prevents water that passes through the cladding layer from penetrating further into the assembly. Water that collects on the water control layer - sometimes called "drainage plane" - is drained by gravity and conveyed back to the exterior by flashings. In addition to an effective water control layer or drainage plane, drainage from the assembly also requires a drainage space. Together, the water shedding layer, the water control layer, the flashings and the drainage space constitute the water control system (WCS).

The concept of water control and drainage applies to assemblies such as walls, roofs and foundations. The concept applies to the components that can be found in walls, roofs and foundations such as windows, doors and skylights. It also applies to the openings for the windows, doors and skylights. And it applies to assemblies that connect to walls, roofs and foundations such as balconies, decks, railings and dormers. Finally, drainage also applies to the building as a whole. Overhangs can be used to drain water away from walls. Canopies can be used to drain water away from windows, and site grading can be used to drain water away from foundation perimeters.

Knowing where the water control layer is in a particular assembly, component or building element is essential to maintaining continuous water control from one building element to the next. For example, understanding where to locate step flashing at a roof-to-wall intersection requires understanding the location of the water control layer in the wall assembly. In this guide, illustrations of retrofit details indicate the location of the water control layer in order to reinforce the awareness of the water control layer and demonstrate how the connections are made.

A word about project scope relative to water control:

Anyone undertaking retrofit of an existing home should be aware that good practice details are not necessarily common practice. For example, roofs can concentrate water where roofs intersect walls, yet kick-out flashings - flashings that direct or "kick" the concentrated water away from the wall - are very frequently absent. In many older structures, a continuous wood, cast stone, or stone sill serves as a flashing to direct water out of the window opening and out over the wall assembly. More modern windows, however, are seldom installed with pan flashing or, even proper head and jamb flashing. Other water management elements that may have been properly implemented initially, can fail over time and when these systems fail they can represent a significant moisture risk. For example, gutters with leaking joints, missing end caps or broken rain leaders can concentrate water on vulnerable assemblies. When evaluating the measures to be implemented on a project, it is important to include remediation of any failures or lapses in the water control system even if these are outside of the planned area of DER work..

A word about materials:

In order to control liquid water and effectively convey it back to the exterior, water control layers and flashing must be 1) properly lapped, 2) comprised of non-moisture sensitive materials, and 3) comprised of non-water absorptive or non-capillary materials.

Builders today have adhered flashing membranes that can facilitate robust water control and flashing. Modern "self-sealing" membranes can accommodate fastener penetrations without losing the water control function. An important consideration when using any type of peel-and-stick bituminous or butyl or self-sealing membrane is that these do not eliminate the need for proper lapping at the water control layer. Through thermal cycling, edges of these membranes tend to curl away from the substrate. This is generally not a problem on downward facing or vertical edges of adhered water control membranes. However, at the upward facing edges, this curl can create a lip that retains water in the assembly and concentrates water on moisture sensitive elements. It can also be difficult - or nearly impossible - to install a self-adhered sheet membrane without wrinkles. Wrinkles at the edge of a flashing membrane can act like tiny scoops that catch water and channel it to where it is not wanted. To manage these issues inherent with peel-and-stick membranes, ALL upward facing edges of bituminous or butyl membranes used in the water control system should be either lapped by another water control membrane or taped to the underlying water control layer with a reliable and compatible sheathing tape.

When using sheathing tapes to tape seams of insulating sheathing in order to create the water control layer, the tapes must be at least 2½" wide and must be recommended by the manufacturer for use with foam sheathings (or foil facings as the case may be). At horizontal seams, the tape should not be centered on the seam. Rather, the tape should be positioned so that about 2/3 of the width is adhered to the upper piece of sheathing and about 1/3 of the width is over the lower piece.

Ground Water

Ground water is the other important exterior source of water that the building must manage. Water in saturated soil against a foundation wall can be wicked to the building interior or to moisture sensitive framing elements through capillary action. Surface water or water below the surface near a building can work its way into a building through cracks or voids in the foundation. Because water is lazy, it will follow the path of least resistance as it flows down. Therefore, if water is present in the soil around the building and if the path of least resistance is through the basement, the basement will get wet. A water table that rises above the basement slab creates hydrostatic pressure on the slab and foundation. This pressure will force water through cracks and holes. It may also exert structural stresses on the slab and foundation.

The best techniques to manage groundwater include the following:

• Build the building with the foundation above the water table.
• Slope the grade away from the building on all sides with a slope of 5% or more for a distance of at least 10' from the building.
• Provide a capillary break such as damp proofing, washed course gravel, or heavy-duty polyethylene between the foundation and ground and between the slab and ground. The capillary break should be beneath the foundation wall as well as on the face of it.
• Provide free-draining back fill around the foundation and a perimeter foundation drain that drains to daylight, to a drywell or to a sump.

Retrofit situations seldom present opportunities to implement these techniques. The retrofit, therefore, must make the most of opportunities presented by the existing situation. Rather than trying to resist water penetration through foundation walls and slabs, the primary water control strategy for interior retrofit of foundation walls and slabs is to anticipate water penetrating through the foundation and provide a means to direct it to drains. It may also be possible in retrofit to relieve water pressure on foundation walls and slabs by installing an interior foundation drain below the level of the slab.

Water wicking through the basement slab can be controlled by using a coating on top of the slab - such as an epoxy paint - or by creating a drainage space and capillary break above the slab - such as by installing a well sealed, high- density polyethylene dimple mat above the slab.

Special Consideration: Protecting the framing sill from capillary moisture

Controlling water that wicks from the soil through the foundation wall can be more complicated. The major complicating factor is that water sensitive materials - in the form of a structural wood frame - are typically sitting on top of the foundation wall. Measures to control liquid water and insulate the foundation wall at the interior side also reduce the ability of water to dry from the foundation to the interior. This increases the likelihood that water will be wicked further up the foundation wall to where it could impact the framing. In new construction, placing a capillary break - such as sheet metal or a bituminous membrane - between the foundation wall and the wood framing effectively protects the wood framing from water that is wicked through the foundation. This happens to be the best approach, in terms of robust water control, for retrofit as well.

Capillary breaks have been successfully installed between foundation walls and framing sills in existing buildings. This is done by jacking the frame - working a small section at a time - just enough to slide an ice and water membrane or sheet metal capillary break between the foundation wall and framing.

Whether inserting a capillary break at the top of the foundation wall is necessary is a matter of judgment. It depends on an evaluation of the risk factors, the sensitivity of the structure, and the risk tolerance of the building owner or stakeholders.

Risk factors pertinent to water wicking through the foundation to the structure include the following:

1. The foundation wall is constructed of capillary (wicking) materials. Brick and concrete are capillary materials. Mortar and grout are capillary materials. Some stones are capillary materials. Igneous stone, like granite, is not a capillary material but one should be wary of foundation walls with granite facers and rubble or brick infill.
2. Liquid water is in contact with the foundation wall. No water, no wicking problems. Liquid water can be in contact with the foundation wall if the water table is above the bottom of the foundation, if the soil adjacent to the foundation is saturated (not well drained), if surface water is not drained away from the foundation, or if the exposed part of the foundation wall is subject to splash-back from roof drainage, for example.
3. Water vapor condensing in or on the foundation wall. This is a variation of the second risk factor. It occurs when moist interior air is allowed to contact cold foundation surfaces or when foundations are insulated with vapor permeable insulations.
4. Limited drying of the foundation wall. Generally speaking, foundations do not dry into the ground but they can dry into the air. If the exposed exterior surface of the foundation wall above grade is less than 1½ times the thickness of the foundation wall, then the drying of the foundation wall is limited. The drying of the foundation wall would also be limited if vapor impermeable coatings or coverings are applied to the above-grade portion of the foundation wall.

Where insertion of a capillary break is not feasible and risk factors are moderate, framing resting on the foundation should be insulated in a way that promotes drying to the exterior. If water can dry from the framing at a faster rate than water is wicked to the framing, then moisture will not accumulate. Part 2 of this guide, contains special case details that show insulation at the base of a frame wall in a manner that promotes drying of the framing sill (see "Special Case: Provide Drying for Framing Sill" in the Exterior Frame Wall- to-Foundation Wall Transitions section).

Special Consideration: Protecting brick foundations from freeze-thaw damage

Brick foundation walls present a special consideration for retrofit. Some older bricks - but not all - can be susceptible to freeze-thaw damage if exposed to excessive water and cycles of freezing temperatures. Foundation walls are typically subject to wetting from the sources discussed above. In existing pre-retrofit homes, uninsulated brick foundation walls are typically protected from freeze-thaw damage by heat from the building or by waste heat given off by inefficient heating systems. When the heating system is replaced with a more efficient system or when the foundation wall is insulated to the interior, the foundation wall becomes colder. This may increase the risk of freeze-thaw damage for brick foundation walls. The risk is only increased if water wets the brick. Whether the brick foundation wall will develop freeze- thaw damage is determined by how wet the brick gets. Control the water and you control the risk.

The following minimum measures are recommended to protect brick foundation walls from water:

• Slope grade away from foundation walls to prevent surface water from wetting the brick.
• Provide overhangs to prevent rain from falling on wall surfaces.
• Provide and maintain gutters and rain leaders to direct water away from building or provide a drip trench of gravel and surface drain at grade to reduce splash back and convey water away from the foundation.

Additional measures to prevent water from soaking the brick should also be considered. These include:

• Provide free-draining back fill and a sub-surface drain to the exterior of brick foundation walls.
• Provide a capillary break - e.g. damp proofing, dimple mat, or gravel wrapped in filter fabric - between below-grade brick and wet soil.
• Paint the above-grade brick with latex paint to prevent the brick from soaking up water from its surface but still allow some drying from the brick.

It is far better to manage the freeze-thaw risk by keeping the brick from getting wet than to attempt to manage the risk by keeping the brick warm with lack of insulation or wasteful heating systems.