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

Air transports pollutants and moisture.  To control pollutant movement you must first control air.But to control air you must first enclose air. That means getting rid of the big holes.  Once we getrid of the big holes, identify the building “boundaries” and get an “enclosure,” we can control airmovement within a home and between the home and the outside.

Then we can concentrate on the cold surfaces (get rid of them by insulating them) or make sure that the indoor air or the outdoor air never gets to the cold surfaces. This prevents condensation. Condensation, especially the kind we don’t see, can cause mold and destroys buildings.

Finally, we can limit indoor humidity and airborne pollutants by controlling air change between the interior and exterior. Dilution is the solution to indoor pollution that cannot otherwise be prevented or removed near the source.

Big Holes and Building Boundaries

The biggest holes in buildings often occur between basements, crawlspaces and living areas. Other major holes include bath tubs on exterior walls, ductwork in attic spaces or exterior walls, soffits, recessed lights, plumbing chases and chimneys.

Basements  These areas are part of a home, within the building boundary — despite repeated attempts over the years to disconnect them. They should be designed and constructed to be dry and conditioned. This is particularly important for basements because mechanical systems are always located in basements. Do not install mechanical systems outside of a home in unconditioned space unless there is no practical alternative.  If a basement is being used for storage or as living space, it needs to be kept dry to avoid mold and dust mites.

Basements should be insulated on their perimeters — they should not be insulated between floors. Ceiling basement insulation is a bad idea, especially if the basement is wet. If the basement is wet, make the basement dry. Don’t try to disconnect the basement from the home because it onlycreates problems, especially if you put mechanical equipment in the basement. The mechanical equipment connects the basement to the house.

Crawlspaces  There are different ways to build healthy crawlspaces.  Crawlspaces should be designed and constructed to be dry and pest-free. A dry crawlspace is good for the building’s inhabitants and good for the building’s durability. A dry crawlspace is less likely to have pests and termites.  Make sure you control rainwater, groundwater and provide drainage for potential plumbing leaks or flooding incidents.

Crawlspaces should not be used for storage.  Builders and contractors should use designs that discourage the use of crawlspaces for storage, and provide clear guidance to owners and occupants to avoid using this area forstorage.

Crawlspaces ideally should be designedand constructed as mini-basements —part of the house and within the building boundary (see Figure 23). They should not be vented to the exterior.  They should be insulated on their perimeters and should have a continuous sealed groundcover such as taped polyethylene. They should have perimeter drainage just like a basement (when the crawlspace ground level is below the ground level of the surrounding grade). Make sure there is good drainage away from crawlspaces.

Figure 23: Internally insulated concrete crawlspace with stucco wall above

While crawlspace venting has been viewed as good building practice and is still required by some codes, there is emerging consensus that an unvented crawlspace with insulation on the perimeter performs better in terms of moisture, durability and pest control.

Perimeter insulation rather than floor insulation performs better in most climates.  Crawlspace temperatures and relative humidity track those of the house. Crawlspaces insulated on the perimeter are warmer and drier than crawlspaces insulated between the crawlspace and the house. Cold surfaces that can condense water are minimized.

A major source ofsummertime highhumidity in crawlspaces is humid outside air entering through vents. When humid outside air comes into crawlspaces, the relative humidity goes up. Since crawlspaces are cooler than the outside, condensation may form on cold surfaces. Summertime ventilation in crawlspaces usually makes them wetter, not drier.

Wintertime ventilation makes crawlspaces colder and is not very effective at drying them. Additionally, wintertime ventilation increases the heat loss from the home — venting crawlspaces can waste energy, and can lead to freezing pipes and uncomfortable floors.

Note: The International Building Code (ICC) allows the construction of closed (unvented) crawlspaces. Contact code officials in the design phase to determine their requirements.

If it is not possible to treat the crawlspace as a part of the house such as in flood zones in costal areas or in dry climates where it is not necessary, it is important to construct the house such that the crawlspace is isolated from the house — outside of the building boundary. This can be accomplished by air sealing the boundary between the crawlspace and the house and by installing a vapor barrier on theunderside of the floor assembly (see Figure 24). This vapor barrier needsto have sufficient thermal resistance tocontrol condensation (both in summer and winter); as such, insulating sheathing is recommended in this location.  A similar approach is recommended for homes on piers (see Figure 25).

Figure 24: Vented crawlspace

Figure 25: Pier foundation

In parts of the country where radon and pesticides in soil gases can be found, sub-slab passive ventilation is recommended (see discussion on Pressures). This also helps keep a crawlspace drier.

If possible, seal the vents in an existing crawlspace. Build new crawlspaces without vents. Where homes have both a crawlspace and a basement they should be connected together and treated together as a conditioned space (see Figure 26).

In many climates it is necessary to have a drying mechanism. One option is to passively connect the crawlspace to the house via floor registers or transfer grilles.  The incidental air change that happens between the crawlspace and the house in this manner typically provides sufficient drying.

In the event that it does not, drying can be improved using the house air conditioning system. A small amount of air can be provided to the crawlspace from the supply side of the air conditioning system; and a small amount of return air can be used to draw some air from the crawlspace (a large amount of the make-up air will come from the crawlspace. These ideas are illustrated Figure 27. In houses without air conditioning these methods cannot be used. Lastly, a small dehumidifier can be used to control summertime humidity levels in houses that do not have air conditioning.

Some existing crawlspaces are sources of pollutants that can’t be satisfactorily removed or controlled. The most practical solution is to install a durable fan to exhaust air continuously from the crawlspace to the outside. The fan should be rated for continuous duty and sized according to either ASHRAE Standard 62.2 (so that it also provides ventilation for the house if desired) or at a minimum rate of 20 cfm/1,000 square feet otherwise. This reverses the flow air, pulling air from the house into the crawlspace and then out of the building (Figure 27b).

To keep them dry all crawlspaces should have:

• Continuous, durable groundcover liner
• Rainwater and groundwater control similar to a basement if the crawlspace is below theground level of the surrounding grade
• Pest control measures as appropriate for the location
• Inside sloped to one or more low places for when a flooding incident occurs from a plumbing leak or rain entry — the low places should be either drained to daylight or a sump pump.

There are several ways to provide a durable groundcover liner. The option used depends on the resources available, the frequency of people entering the crawlspace to either store possessions (not a good idea) or to maintain equipment and the severity of the pest problem.

• Two inch (or thicker) “rat slab” (concrete slab) cast over a polyethylene groundcover provides the best option for durability and permanent control of pests.
• EPDM rubber roofing as a continuous lining.
• Overlapped, sealed and staked 10 mil polyethylene or 6 mil reinforced poly with runners to equipment or in areas that may be accessed (runners can be EPDM rubber or linoleum).
• Overlapped, sealed and staked 10 mil polyethylene or 6 mil reinforced poly if the crawlspaceis unlikely to be accessed.

One of the most important things to get right when constructing a crawlspace — especially one that ispart of the house — is to make sure it remains dry during the construction process.  Many builders install the subfloor and promptly forget about the crawlspace for weeks or months as the construction process progresses.  This is a huge mistake as the crawlspace is often wet, and significant mold and other moisture damage can occur during this time period.

It is critical that a groundcover be immediately installed (it can be a “sacrificial” groundcover — a temporary groundcover that can get covered or replaced by the “permanent” groundcover later) to prevent evaporation from the ground from damaging the structural frame. Groundwater must be kept out of the crawlspace throughout the construction process and the crawlspace must be protected from rainwater or dried immediately after a rainstorm. This can often be done rather easily with passive ventilation (leaving gaps in the subfloor) or by installing a temporary fan. Ponding rainwater on the groundcover should be removed using a shop vac or the groundcover can be punctured to allow the water to drain down into the ground — the groundcover can then be patched ifit is the permanent groundcover — or the hole left if it is the temporary ground cover.

Bathrooms  Bathtubs and shower enclosures are rarely draft stopped (air sealed) with rigid materials such as sheathing or gypsum board (see Figure 28). Most people forget that cavity insulation is just a filter or screen for air. Just leaving insulation behind a tub is like leaving your front door open –forever.

Ductwork and Air Handlers  Avoid placing ductwork and air handlers in attics, air handlers in garages or ductwork in exterior walls because leaks are a problem. Leaky ducts and air handlers located in attics are one of the major sources of ice-damming problems.  During rehabilitation, move the air handler and ductwork from attic and exterior walls.

Wherever you install ductwork, ducts should be tight. How tight? Flex duct systems should leak at no more than 5 percent of their flow (as tested by pressurization testing at 25 Pa) and sheet metal duct systems should leak at no more than 10 percent of their flow (as tested by pressurization testing at 25 Pa).

Attics  Vented attics should have airtight ceilings that separate the vented attic from the conditioned part of the home. Soffits, chimneys, plumbing vent pipes, chases and dropped ceilings all require draftstops. Similarly, scuttleholes and drop-downstair openings should be airtight.

Utility Walls  Another big hole is the utility wall where it intersects exterior walls and ceilings or where exterior walls are padded out to provide space for utilities (see Figure 29).

Figure 29: Utility chase construction

Cold Surfaces

Condensation happens on cold surfaces.  One of the best techniques for controlling condensation is to eliminate cold surfaces.

Windows  The coldest surfaces in homes usually are windows. Windows should never (except for short periods in unusually cold weather) have condensation on the inside. This is a wintertime problem even in warm climates. In hot-humid climates, windows often sweat on the outside during the summer. Interior window surfaces must be warm.

At a minimum, windows must be double glazed with non-thermally conductive frames. Avoid aluminum extrusion windows without thermal breaks. Window frames should be constructed from wood, vinyl or fiberglass. Wood windows can be clad with vinyl or aluminum for maintenance reasons.

Older, single-pane windows will be cold and will have condensation on them. This problem occurs everywhere, not just in cold climates. They should be replaced, if possible.

In very cold climates (Zone 7 or higher; see Figure 21) window glazing should be low E (U-value less than 0.4). In all other climates, window glazing should be low E2 (spectrally selective, SHGC less than 0.4, U-value less than 0.4). The increased cost of such glazing is readily offset by a reduction in the size of the mechanical conditioning system.

Metal Studs  This type of stud is 300 times more conductive than a wood stud. Metal studs are prone to condensation and ghosting. Metal studs should never be used with cavity insulation because it makes them even colder. If metal studs are used they should be limited to interior walls or to the interior of rigid insulation assemblies. Metal studs should never be used below-grade unless they are separated from slabs with thermal breaks (“sill gasket”) and separated from foundation perimeter with rigid insulation.

Below-Grade Walls  The main problem with below-grade walls comes during the summer when warm moist air comes in contact with basement cold surfaces (see Figure 30). Basement walls should be insulated with non-water sensitive insulation that prevents interior air from contacting cold basement surfaces. The best insulations to use are foam based and vapor semi-permeable. Vapor semi-permeable materials allow the basement wall assemblies to dry to the interior. No interior vapor barriers should be installed in basements — ever — because they trap moisture inside the assemblies.

One of the worst assemblies for basement walls from the perspective of mold and moisture problems is a foundation wall that is internally framed and insulated with fiberglass cavity insulation and covered with a plastic vapor barrier. Metal studs only further aggravate the problem. Air gets in behind the framing and condenses on the cold surfaces. Alternatively, moisture from the foundation enters the internal framing. The moisture is trapped within the foundation assembly by the vapor barrier and deterioration occurs.

All bottom plates of below-grade frame walls should be thermally isolated from basement floor slabs with thermal breaks. Commonly available sill gasket (³/₈-inch thick foam by 3 ¹/₂-inches wide) is an excellent thermal break under bottom plates.

Concrete Slabs  If basement spaces or below-grade spaces (garden apartments) are designed and constructed to be occupied, continuous rigid insulation should be installed under concrete floor slabs to raise the temperature of floor coverings to control mold and dust mites. Alternatively, rigid insulation can be installed above concrete floor slabs and coupled with a floating floor (see Figure 31).

It is always a good idea to install a dehumidifier in all basement spaces. The dehumidifier shouldbe plumbed directly to a condensate drain.

Other Preventative Measures  All cold water pipes should be insulated to control summer condensation.

Wood framing details, particularly in corners, should be constructed “open” to allow the installation of insulation and reduce thermal bridging (see Figure 32 and Figure 33).  Headers should be designed to accept insulation, as shown in Figure 34.

Figure 32: Corner framing

Figure 33: Interior wall at exterior wall

Attic insulation at exterior walls should be protected from wind wash by placement of a windbaffle (see Figure 35).

Indoor Humidity and Airborne Pollutants

Indoor humidity and airborne pollutants are both controlled by ventilation. There are two kinds of ventilation, both necessary for a healthy home:

• Spot ventilation deals with point sources of pollution such as bathrooms and kitchens(see Figure 36).
• Dilution ventilation deals with low-level pollutants throughout the home.

This ventilation is in addition to the use of operable windows.

Kitchen and Bathroom Fans  Every home needs to have exhaust fans from kitchens and from bathrooms. In kitchens, recirculating fans should be avoided because they become breeding grounds for biologicals and a major source of odors, and in all cases allow grease vapors to coat surfaces throughout the home. Kitchen range hoods must be exhausted to the outside to remove moisture, odors and other pollutants.

Bathroom fans must exhaust to the exterior, even in bathrooms with operable windows — no exceptions. Low sone fans (less than 3 sones) are recommended because they are quiet (so they are more likely to be used) and more durable (to make them quiet they must be made durable).

Clothes Dryers  Clothes dryers must be vented to outside because they are huge sources of moisture, as well as pollutants.

Dilution Ventilation  Dilution ventilation can be provided three ways: exhaust, supply or balanced (see Figure 37).  In all cases, it should be continuous and fan powered. ASHRAE Standard 62.2 should be followed to establish dilution ventilation rates for all homes, new or renovated.

The key to dilution ventilation is good distribution. Outside air should be provided throughout the house. Forced air duct systems can be excellent distribution systems (either by directly providing outside air or by mixing interior air). Where duct distribution systems do not exist, multiport exhaust strategies can be used.

Most individuals are comfortable where relative humidity is in the 20 percent to 65 percent range (see Figure 38).

During the coldest part of the winter, indoor relative humidity should be kept low but in a comfortable comfort range as shown below. During summer months, indoor relative humidity (in air conditioned buildings) should not exceed 70 percentfor extended periods of time (more than several days). In hot and humid climates this may only be possible with supplemental dehumidification (a stand alone dehumidifier plumbed to a condensate drain), especially in small units with very little solar heat gain.

Formaldehyde and other emissions from particle board can be harmful. To reduce emissions from particle board surfaces, reduce the amount of particle board. Use wire shelving in closets. Wireshelving iseasy to cleanand permitsair circulation. With kitchen and bathroom cabinets constructed from particle board, the exposed particle board sources can be sealed with 100 percent acrylic paint or clear sealant.

Pressure Changes

Air pressure differences cause air to move, which is both good and bad. Air change is good when it causes dirty air to be replaced with clean air. Air change is bad when dirtyair is brought into a home. The bad air change is commonly associated with high negative air pressures (suction) in homes. High negative pressures can draw pollutants (such as radon and soil gas) into homes from below-grade. They can also cause problems with furnaces, boilers and water heaters. Additionally, high negative pressures can cause smoke and odors to be drawn from neighboring units.

High negative pressures can be avoided in several ways. The first is to seal forced air ductwork, particularly on the return (or suction) side (see Figure 39). The second is to provide air pressure balancing between rooms (transfer grilles) when forced air systems are used (see Figure 40).  The third is to compartmentalize (isolate) multifamily units and high rises to limit airflow between floors.

Good duct layout and proper placement of air handlers provide thermal comfort and avoid the introduction of polluted air into homes (see Figure 41).

An effective way to use air pressure differences to capture and to vent pollutants is through a sub-slab ventilation system (see Figure 42). It is a good idea to construct all floor slabs, both below-grade and on grade, with sub-slab ventilation.