A Practice Guide and Plan Review Tool for Builders and Designers
Advance planning pays big dividends in preventing costly moisture problems.
Mold, condensation, structural decay, high indoor humidity, wet foundations, and ice dams. These are just a few moisture problems that are well known to builders, homeowners, and insurers. These problems are also largely preventable or controllable, if timely decisions are made and implemented.
This guide offers best practices for moisture management. It is a resource for designers, builders, remodelers and homeowners. The guide is organized by the three phases of building construction: Planning and Design, Construction, and Delivery. The key recommendations are organized by building system and listed with page references below.
Consumers are encouraged to read Section 4: Homeowner Guide to Water Management and Damage Prevention (begins on p. 93). This section outlines the quickest ways to spot common types of home moisture problems before they become dangerous and expensive. It describes what you can do to prevent moisture problems from developing.
Recommendations
ROOF AND CEILING SYSTEMS
Evaluate roof pitch and material properties when selecting roof coverings. (p. 4)
Apply bituminous adhesive taping on sheathing joints, use appropriately rated roof coverings, and fasten coverings per manufacturer instructions in high wind areas. (p. 7)
Use hail-rated shingles and remove old shingles (when re-roofing) in hail-prone regions. (p. 8)
Avoid concentrated or obstructed roof drainage pathways. (p. 8)
Minimize roof penetrations by using selected plumbing and HVAC technologies. (p. 8)
Specify flashing details for roofs, including kick-out flashing and other details, and incorporate in roo?ng contracts. (p. 10)
Design roof ventilation based on climate and insulation amount to prevent ice dams. (p. 16)
Size eave and rake roof overhangs based on climate. (p. 20)
Design a properly sized roof drainage system including gutter sizing, downspout sizing and downspout placement, based on roof pitch and local rainfall intensity. (p. 21)
WALL SYSTEMS
Consider a drained cavity weather-resistant envelope (WRE) system for most non-severe climates and building exposures, or select alternative WRE approach based on climate, site condition and target performance level. (p. 24)
Follow manufacturer’s installation guidelines for windows and doors. (p. 33)
Field-test repetitive installations on large projects. (p. 33)
Understand how windows and doors are designed to manage water. (p. 33)
Use third-party certified windows and doors. (p. 34)
Specify and verify wind pressure and impact resistance performance ratings for windows. (p. 34)
Specify flashing details for all windows, doors, and ledgers. (p. 37)
Supplement standard flashing details for additional protection against severe weather. (p. 40)
Specify and use appropriate sealants and installation practices for particular applications. (p. 45)
FOUNDATIONS
Figure 28 – Details to Separate Wood from Ground Moisture
Site Planning and Foundation Design
Create a workable site drainage plan prior to construction. (p. 46)
Provide a ?nished grade away from the foundation greater than the minimum (6″ in 10′) to offset back?ll
settlement. (p. 49)
Use a simple screening process to assess sites for moisture and drainage concerns. (p. 49)
Basement Foundations and Basement Walls
Include foundation backfill specifications on plans and in foundation contractor agreement. (p. 51)
Waterproof exterior walls of basements used for storage or living space. (p. 52)
Install horizontal reinforcement at top and bottom of foundation walls to control cracks. (p. 52)
Design basement insulation and finishes to dry towards the interior, especially where traditional finish practices (e.g. warm-in-winter vapor retarder) have resulted in moisture problems. (p. 54)
Use semi-permeable rigid foam insulation between the foundation wall and finished basement walls when using a basement finish system that dries to the interior. (p. 54)
Prevent warm, humid indoor basement air from leaking into finish wall and ceiling assemblies. (p. 54)
Separate basement wall finishes from the basement floor slab. (p. 56)
Slab on Grade
Provide a mounded foundation pad to achieve 8″ minimum clearance above exterior finish grade. (p. 57)
Use a sub-slab vapor retarder directly below slabs with a capillary break layer beneath the vapor retarder. (p. 57)
Provide for concrete slab crack control with wire or fiber reinforcement and control joints. (p. 57)
Install horizontal rebar as reinforcement to reduce foundation cracking. (p. 59)
Apply slab foundation insulation on the foundation exterior of slab on grade foundations. (p. 59)
Use moisture resistant finishes on new slabs where feasible. (p. 60)
Use slab insulation strategies when moisture sensitive finishes will be applied. (p. 60)
Account for top-of-slab vapor control before finishing existing slabs that do not have a sub-slab vapor barrier. (p. 60)
Crawl Spaces
Provide a lapped ground cover for all crawlspace foundations. (p. 62)
Provide foundation drainage and damp-proofing for crawlspaces that are below exterior grade. (p. 62)
Evaluate vented and non-vented (particularly for hot/humid climates) crawlspace ventilation strategies. (p. 63)
Wood Framing
Maintain minimum 8″ clearances to protect wood from ground moisture. (p. 64)
Match the treatment level of preserved wood to the application and exposure. (p. 67)
Store all treated wood in a protected, ventilated space before use. (p. 69)
Figure 33 – Air Barrier System Approaches and Important Features
WATER VAPOR CONTROL
Controlling Indoor Humidity
Provide increased whole-house and spot ventilation with dry outdoor air and add ventilation controls that automate spot exhaust when interior RH levels are a concern. (p. 74)
Protect building materials from exposure during storage and construction. (p. 75)
Moisture test wetted building assemblies during the construction process prior to close-in. (p. 75)
Properly size cooling equipment based on house characteristics and climate to improve water vapor removal. (p. 76)
Educate occupants on the RH impacts of homeowner habits. (p. 76)
Controlling Air Leakage
Consider the impacts on water vapor movement and water shedding that result from air barrier materials. (p. 77)
Seal major air leakage points such as attic hatches, mechanical chases and penetrations, and floor overhangs. (p. 77)
For cathedral roofs, focus carefully on sealing all air leakage points into the ceiling cavity. (p. 78)
Use an interior air barrier system in cold and very cold climates. (p. 80)
Use an air barrier system on the outside of the wall in moist/humid climates. (p. 80)
Vapor Retarders
In hot/humid climates exterior wall systems should dry towards the interior by locating vapor retarding materials on the outside of the wall assembly and keeping interior materials vapor permeable. (p. 83)
Educate homeowners in hot/humid regions not to limit the ability of walls to dry towards the interior by adding non-breathable interior finishes on exterior walls. (p. 84)
In cold climates exterior wall systems should dry towards the outside by locating vapor retarding materials on the inside of the wall assembly and keeping exterior materials vapor permeable. (p. 84)
MECHANICAL SYSTEMS
Size cooling systems with a house-specific load calculation using Manual J or a comparable tool. (p. 85)
Upgrade to variable capacity H/P or A/C to improve moisture removal. (p. 85)
Provide supplemental dehumidification to control indoor humidity in humid regions. (p. 86)
Use sealed combustion HVAC equipment. (p. 87)
Seal ducts to</= 5.0 CFM25/100 ft2 to reduce air leakage and moisture movement. (p. 87)
Design adequate return air pathways using multiple returns or jumper ducts and transfer grilles. (p. 87)
Provide whole-house mechanical ventilation appropriate for the climate. (p. 88)
Terminate all exhaust vents outdoors with appropriate through-wall or through-roof components. (p. 89)
Use exhaust duct runs that are as straight as possible and less than 25′ in length. (p. 89)
