BSP-040: READ THIS: Before You Design, Build or Renovate

Water

Water is a precondition for mold, insects, rodents and dust mites and is arguably the single most important factor in the design and construction of a healthy home. Water is the most important factor affecting the durability of a home and the most important factor affecting maintenance costs.

Control Water

The three most important sources of water requiring control are:

• Rainwater control
• Groundwater control
• Plumbing

One of the key elements to controlling water is the concept of drying. It is common sense to accept that things will get wet — especially homes under construction or under renovation. The problem is not that homes get wet; it’s how long they stay wet and how well they dry.  Homes should be designed to dry.

Rainwater Control

The fundamental principle of rainwater control is to shed water by layering materials in such away that water is directed downwards and outwards from the building or away from the building. This principle applies to assemblies such as walls, roofs and foundations, as well as to the components that can be found in these assemblies such as windows, doors and skylights. It also applies to assemblies that connect to walls, roofs and foundations such as balconies, decks, railings and dormers.

Layering materials to shed water applies to the building as a whole (see Figure 1).  Overhangs can be used to keep water away from walls. Canopies can be used to keep water away from windows, and site grading can be used to keep water away from foundation perimeters.

When selecting building materials, take into account that building materials may be exposed to rain or other elements during construction. For example, walls without roofs on them will get wet. It is not a good idea to build these walls with exterior paper-faced gypsum board since it holds water, a major concern with party walls or fire walls in multifamily buildings. Instead, use glass-faced gypsum board or other water-resistant alternatives.

Drainage is the key to rainwater control:

• Drain the site (see Figure 1)
• Drain the ground
• Drain the building (see Figure 2)
• Drain the assembly
• Drain the opening (see Figure 3)
• Drain the component
• Drain the material (see Figure 4)

Walls  All exterior claddings — brick, stucco, stone — pass some rainwater. As such, some control of this penetrating rainwater is required.  In most walls, this penetrating rainwater is controlled by a drainage plane that directs the rainwater downwards and outwards.

Drainage planes are water-repellent materials — building paper, housewrap, foam insulation — that are located behind the cladding and are designed and constructed to drain water that passes through the cladding. They are interconnected with flashings, window and door openings and other penetrations of the building enclosure to provide drainage of water to the exterior of the building. The materials that form the drainage plane overlap each other shingle fashion or are sealed so that water drains down and out of the wall.

Windows and doors are weak points in rainwater control for walls. If not flashed correctly they can channel water into walls. Steps for flashing a flanged window are detailed in Figure 3. In this case the window is installed in a stud wall with OSB sheathing, using housewrap as a drainage plane. The housewrap is stretched over the opening and then cut so it can lap the head flashing at the top of the window and yet tuck beneath pan sill flashing at the bottom of the opening. A panned sill flashing is installed in the bottom of the rough opening. It has vertical dams at the sides and back so that any rainwater leaking through or around the window is diverted to the outside of the housewrap below. Tape is used to seal the flange to the sheathing on the top, acting as a head flashing. Tape seals the side flanges to the housewrap on the sides. The flap of housewrap at the top is folded down over the tape and top flange and the end cuts are sealed with tape.

Figure 4 shows flashing details for trim added around the window. Framing a flanged window in this way makes it possible to remove the window at a later date without having to remove siding.

Reservoirs are materials that store rainwater on the outside of homes, acting like a sponge when it rains. Once the reservoirs get wet, the stored water can migrate elsewhere and cause problems (see Figure 5).  Common reservoirs are brick veneers, stuccos, wood siding, wood trim and fibercement cladding.

Figure 5: Reservoir problems with interior polyethylene—Inward moisture movement due to solar radiation

To handle reservoirs get rid of them or disconnect them from the building (see Figure 6).  Get rid of the moisture storage issue by back priming —painting all surfaces; back, front, edges and ends of wood siding, cement siding and all wood trim. This method is also effective for wood and fiber cement siding. If back-vented it is less important to back prime the siding. However, at least the bottom two feet should be back primed to avoid problems with back-splash.

Figure 6: Ventilated cavity

Rainwater falling from the eaves splashes the wall bottom. The siding and wall may warp, decay or become home for nesting insects. Back-splash problems can be reduced by:

• Features that protect the wall from wind-driven rain are:

-extended overhangs

-back priming and back-venting siding

-using housewrap

• Install gutters.
• Elevate the bottom of the wall enough to avoid splash.
• Install a bed of stone pebbles to absorb impact and drain rain away from foundation.

Back-venting brick veneers and installed over foam sheathings disconnects the brick veneer moisture reservoir from the home (see Figure 7).  Installing stucco over two layers of building paper or over an appropriate capillary break, such as foam sheathing, similarly addresses stucco reservoirs.

Figure 7: Drained cavity with condensing surface

Roofs  Roofs should be designed to shed rainwater away from the building. Steep pitches are better than shallow pitches. Crickets should be used to divert water away from chimneys and architectural features.

Roofs should also be designed to protect walls. Large overhangs are better than small overhangs or no overhangs.

Ideally, roofs should have simple geometry. The more complex the roof — the more dormers, ridges and valleys — the more likely a roof will leak. Penetrations should also be minimized or avoided. Outlet fittings for exhaust fans can be located in walls, gable ends or rim joists. A central exhaust fan venting all bathrooms needs only one outlet penetration. The exhausts for high efficiency boilers, furnaces and hot water heaters do not require a chimney and can be run through exterior walls.

Groundwater Control

Rainwater, surface water and groundwater will wick through concrete and masonry materials. This can be a problem in two ways: building materials touching the foundation may grow mold, decay, corrode or dissolve; or the migrating water might evaporate into the basement orcrawlspace and cause high humidity and/or condensation problems in the foundation and the upper part of the building.

The fundamental principles of groundwater control are to keep rainwater away from the foundation wall perimeter and to drain groundwater with sub-grade perimeter drains before it gets to the foundation wall.  This applies to basements, crawlspaces and slabs (see Figure 8A, B and C) regardless of whether they are newly constructed or undergoing rehabilitation.

Capillary Breaks  Concrete and masonry are sponges — they can absorb or “wick” water due to capillarity (see Figure 9).  This is the main reason that dampproofing (the black tar-like coating) is applied to exterior basement walls. Dampproof coatings come in a number of forms. Concrete and masonry walls must be parged before a dampproof coat is applied. Waterproof systems that can resist hydrostatic pressure also provide a capillary break. Dampproofing fills in the pores in the concrete and masonry to reduce groundwater absorption.  It is a capillary break. Under concrete floor slabs, the stone layer combined with polyethylene also acts as a capillary breaks.

Figure 9: Capillary rise through basement footing

Unfortunately, the capillary rise through footings is typically ignored. This can be a major problem if foundation perimeter wall are finished or insulated. In new construction a capillary break should be installed on the top of the footing between the footing and the foundation perimeter wall (see Figure 10).  This can be done by dampproofing the top of the footing or by installing a membrane at this location.

Interior Basement Insulation  Interior basement insulation and finished walls are very vulnerable to moisture problems. They create poorly ventilated cavities that are chilled by the surrounding earth. Rainwater or groundwater wicking through the walls or up from the footing has a hard time drying into the basement. The same is true for water condensing from warm summertime basement air. When this happens framed walls, fibrous insulations and gypsum board can stay chronically damp.

When adding an insulated wall to the interior of a basement, a layer of foam board can be used to prevent these problems. Interior foam board makes a capillary break between the concrete or masonry and building materials that may be damaged by water — like gypsum board, OSB and lumber. At the same time, it keeps warm, humid summertime air away from the cold basement wall.

By using foams with a perm rating greater than 1 the foundation wall will be able to dry to the basement interior. Then, a framed wall can be added with enough cavity insulation to meet (or exceed) energy codes. An inner layer of gypsum board finishes the system and adds fire protection. This wall must be able to dry to the interior of the basement — do not use foil or polyethylene vapor barriers or vinyl wall coverings. Latex paint on the gypsum board will provide a semi-permeable vapor retarder.

It is important the neither the wood framing nor the gypsum board touch the concrete floor or walls. Foam sill seal or foam board can be used between the bottom plate of the wall and the concrete floor to provide a capillary and thermal break. The gypsum board should be held at least a half inch above the floor. This is a good idea for all interior walls as well — a bit of water can flood the floor but no damage is done until it is deep enough to get to the framing and the gypsum board.

Slab-on-Grade Construction  Capillary control also applies to slab-on-grade construction and crawlspaces (see Figure 11).  Monolithic slabs need plastic ground covers that extend under the perimeter grade beam and upwards to grade. Additionally, the portion of the slab edge that is exposed to the outside must be painted with latex paint to reduce water absorption and a capillary break must be installed under perimeter wall framing.

Figure 11: Capillary control for monolithic slab

Interior perimeter drainage can also be used in new construction — particularly where impermeable rigid insulation is used on the interior of the foundation wall. This allows rigid insulation of greater than 1 inch to be used. And, if foil-faced rigid insulation is used — with the appropriate flame-spread and smoke-developed rating — it can be left exposed (i.e. interior gypsum board does not have to be installed as thermal barrier for fire protection; see Figure 12).

Figure 12: Interior drainage—New construction

In renovations, the conditions under aslab may be difficult to determine, or once they are determined, it may be found that a stone layer or polyethylene is not present. It may be necessary to provide “top-side” control of water and vapor. This can be done several ways.

If salts are not present in the ground, epoxy coatings or chemical sealers may be used.  Salts can migrate through the slabs by osmosis, damaging floor coatings. If salts are present, spacer systems that provide vapor control and drainage can be used over the top of existing slabs (see Figure 13).

Figure 13: Slab top-side vapor control—Airspace approach

A “floating floor” (see Figure 14) can also be used where moisture flow upwards is small — or where a finished wood floor (or carpet) is to be installed over a slab.  Rigid insulation and plywood are installed on top of the slab. In this assembly extruded polystyrene should be limited to ³/₄-inch or less so that the slab can dry upwards (floors are different than walls with respect to permeability limits).

Figure 14: Slab top-side vapor control—Semi-permeable floating floor

Carpets should never be installed directly on below-grade slabs unless slabs are insulated (below or on the top surface). Carpets on uninsulated slabs are cold, resulting in sufficiently elevated relative humidities within the carpet to support dust mite and mold growth.

Exterior Drainage  It is always better to stop groundwater from getting to a foundation wall. Exterior perimeter drainage is always preferable to interior perimeter drainage.

However, in renovations exterior perimeter drainage may not be present or may not be practical or possible.  In such cases, interior perimeter drainage can be used and connected to an interior sump pump. Interior sump pits/crocks must be fitted with airtight gasketed covers to prevent soil gas entry.

This interior perimeter drainage may be combined with an interior drainage layer.  Where an interior drainage layer is used, it must be gas tight and vapor tight relative to the interior  (see Figure 15). Another technique is to use an exterior impermeable material to minimize rain and groundwater entering below-grade spaces (see Figure 16).

Figure 15: Interior drainage—Renovation

Figure 16: Using an impermeable skirt outside

Plumbing

Do not put plumbing in insulated exterior walls and ceilings. Insulation in a wall cavity holds water like a sponge. Soaked insulation dries slower than an empty wall cavity andwater from the leak is less likely to beseen inside the house. Insulated wall cavities experience greater temperature swings which put more expansionand contraction stress on pipes and, in cold climates, may result in frozen pipe leaks. The result can be plumbing leaks and breaks that cause significant water damage and that can cause mold growth. To prevent such problems, whenever possible avoid putting any cold or hot water supply pipes, steamlines, hydronic heat pipes or air conditioner condensate lines — nothing that carries water — in outside walls.

Put plumbing in interior walls or in floors (see Figure 17). When the plumbing leaks, which it will, the leak can be found and fixed.

Figure 17: Locating plumbing pipes

Figure 18: Single-throw shut-off

Bathrooms  Since plumbing leaks and there’s lots of plumbing in bathrooms, there will probably be leaks in bathrooms — particularly in the — “wet areas.”

Don’t use paper-faced gypsum board or “green board” products (it is just paper with a green color) in wet areas such as tub and shower enclosures.

It’s best to use cement board, fiber cement board or paperless gypsum board, or do it the old-fashioned way with cement plaster.

If gypsum board is used, keep the gypsum board up off floors everywhere at baseboard locations.

When the inevitable leak occurs, this space reduces the chance that the paper-faced wallboard will come into contact with water from a leak and suck it up into the wall.

Clothes Washers and Water Heaters  Clothes washers can leak, especially the rubber hose connections.  Reinforced hose connectors should be used. Locate clothes washers in rooms with floor drains, floor finishes that can be wet mopped and a raised sill in the doorway.

Especially when they are old, water heaters can leak, pressure relief valves and drain pans for water heaters leak.  Water heaters should also be installed in rooms with drains and with floor systems that have floor coverings that are not water sensitive.  In warm climates, it is best to install them in garages so when they leak they don’t cause much damage.  Never, ever install water heaters in attics.

Because clothes washers leak and water heatersleak, shut-off valves that can be used to isolate these devices should be provided.  These shut-offvalves should be easily accessible and visible. Don’t hide them behind the clothes washer or hotwater heater. Single-throw shut-offs (see Figure18) for clothes washers are effective and convenient.

Air Conditioning Systems  These systems also can be sources of plumbing leaks.  Besides cooling air, they are dehumidifiers that remove moisture in the air by condensing it on a cold coil and draining the condensate to a drain or to the outside.  This involves plumbing and drain pans and therefore leakage.

Note: Installing air conditioners in attics where they can leak is as risky as installing water heaters in attics.  Wherever air conditioners are located, their drain pans must be constructed andinstalled so that they drain.

Drying

Walls  Walls get wet from both the outside and the inside. And, in many cases, they start out wet due to the construction process. Therefore, walls should be designed to dry. Walls that are designed to dry to both sides (see Figures 19 and 20) typically perform better than walls that are designed to dry only to one side. All walls should be designed to dry to at least one side.

Sheet polyethylene is an almost-perfect vapor barrier because it does not allow any moisture inthe vapor form to pass through it.  This is a great feature during cold winters, but is a counterpro-ductive feature during summers and in climates that require cooling.  Although many homes withpolyethylene vapor barriers in walls may not be experiencing problems due to trapped moisture, these homes may be at a higher risk for such problems, which can cause mold, than walls that dry more easily.

In new construction and significant renovations when there is a choice of vapor retarders, paper-faced cavity insulation can and should be used in place of plastic interior vapor barriers.  Alternatively, cellulose cavity insulation can be used in conjunction with latex paint on gypsum board.

Building codes do not call for the installation of vapor barriers.  They call for the installation of vapor retarders — and only in cool and cold climates or colder (5,400 heating degree days or greater = Zone 5 or higher; see Figure 21).

Note: A vapor retarder is defined by building codes as a material that has a vapor permeability of 1 perm or less (as tested by ASTM E-96 Test Method A — the desiccant or dry cup method).  Sheet polyethylene has a vapor permeability of 0.1 perms, which is 10 times less than what is called for in the building codes.

Installing interior polyethylene as a vapor barrier on wall assemblies should be limited to very cold climates (9,000 heating degree days or greater = Zone 7 or higher).

Roofs  Roofs should be designed to dry, meaning they should be ventilated.  It is possible to design and construct unvented roofs, but this should be done only with professional design and analysis.

Installing polyethylene vapor barriers invented roof assemblies should belimited to cool and cold climates orcolder (5,400 heating degree days or greater = Zone 5 or higher).  Even in Zones 5 or higher, polyethylene vapor barriers are not required in vented roof assemblies. Building codes do not call for their installation, only for a vapor retarder.

Basements  Basements should be designed to dry to the interior (see Figure 22).  This principle is often in conflict with some common misapplied energy conservation and moisture control practices — for example, the use of sheet polyethylene as an interior vapor barrier.

Sheet polyethylene (or vapor barriers) should never be installed on the interior of interior basement insulation assemblies or on the interior of interior insulation in below-grade wall assemblies in any climate as it prevents drying to the interior.  The exception is where drainage is provided between the interior vapor barrier and the assembly (i.e., exterior to the vapor barrier; see Figure 12 and Figure 15).

Impermeable interior finishes should be avoided, such as vinyl wall coverings or oil (alkyd) based paints. In a similar vein, vinyl floor coverings should be avoided on basement floor slabs or on slab-on-grade construction unless a low water-to-cement ratio concrete is used (less than 0.45) installed directly over a polyethylene vapor barrier — and only where slab edges are protected from capillary water (see Figure 15).