Preventing Mold and Other Moisture Problems

Size: px
Start display at page:

Download "Preventing Mold and Other Moisture Problems"


1 Preventing Mold and Other Moisture Problems A Community Workshop presented by William W. Hill, Ph.D. Department of Urban Planning, Ball State University Sponsored by the Community Outreach Partnership Center and East Central Reinvestment Corporation Topics we will cover Mold Fundamentals of moisture movement Relative humidity, absolute humidity & temperature Moisture transport mechanisms Bath fans Recessed lights Crawl spaces December 12, Common causes of moisture problems we will not cover Peeling paint is often, but not always, the result of a moisture problem Leaking pipes Roof leaks 3 4 Recommended reading Builder s Guide: Cold Climates, by Joseph Lstiburek. (1998). Published by the Energy and Environmental Building Association and available from the EEBA bookstore at Moisture Control Handbook, by Joseph Lstiburek and John Carmody (1993). Published by Van Nostrand Reinhold and available from the EEBA bookstore at Building Science Corporation web site Mold -- the mother of all callbacks Mold is a large and growing problem in homes across the U.S. Not at all unexpected by building scientists 5 6


3 Necessary but not sufficient conditions for mold growth Mold spores Nutrient base Temperature between 40 o F and 100 o F Relative humidity greater than 70% Mold is a moisture problem Mold is not contagious Mold does not come from infected building materials Mold does not come from the factory or the sawmill Building Science It s not rocket science But it is science Necessary but not sufficient conditions for a problem A source A hole A driving force Necessary but not sufficient conditions for a moisture problem Source Hole Driving force 18 William W. Hill, Department of Urban Planning, Ball State University, Muncie, IN

4 Moisture fundamentals Moisture moves in response to well understood physical forces. We call these driving forces The physics is the same no matter where you are The physics is the same no matter what the building Essential definitions Absolute humidity = actual amount of water vapor in air (See also humidity ratio, vapor pressure) Relative humidity (RH) = ratio of amount of water vapor in air to the amount that the air could hold at that temperature Dew point = temperature at which RH = 100% and condensation occurs (See also saturation temperature) Temperature and relative humidity Psychrometric Chart As air is heated, the amount of water vapor it can hold increases. If the absolute humidity remains constant, the RH will decrease. As air is cooled, the amount of water vapor it can hold decreases. If absolute humidity remains constant, the RH will increase. If the air is cooled enough, it will hit dew point at the water vapor will condense into liquid water Two questions we can answer with the psychrometric chart Why interior air is dry in winter Why is the air inside many homes dry in winter? How do walls get wet as a result of air leaking out of houses in winter? 23 24

5 The usual band-aid solution to dry winter air: a humidifier A better solution to dry winter air: seal the air leaks and stop the infiltration Other benefits of sealing air leaks: No moisture being dumped into the attic More comfortable home Lower heating bills How walls get wet from leaking air Insulation without an air barrier is asking for trouble Bath fans are needed for point source ventilation Ducts for bath fans need to carry the moisture all the way to the outside 29 30

6 Moisture transport mechanisms #1 Liquid flow #2 Capillarity/surface tension #3 Air movement #4 Vapor diffusion Moisture transport mechanism #1 Bulk moisture movement by gravity and momentum 32 Moisture transport mechanism #1 in a nutshell Buildings get wet when it rains It rains! Drain the rain off the building Direct it away from the building Provide a way for the building components to dry when they get wet Got to get these basics right! Poor guttering worse than none at all [Carmody & Anderson] 35 [Carmody & Anderson] 36

7 We ve known about the importance of perimeter foundation drains for many years 41 42

8 For homes in this area, the perimeter drain is quite possibly the most important moisture control detail Incorrectly installed housewrap 43 [Moisture Control Handbook, Fig ] 44 What s the purpose of the housewrap? Air barrier But it s also the drainage plane! It took the place of building paper Best attribute of building paper is that installers are thinking moisture control when they install it Think shingles when you install housewrap Pieces above must overlap those below, always directing water to the outside Details around windows are easy to get wrong 47 48

9 49 50 Moisture transport mechanism #2 Drip loops are important Capillarity and surface tension With no drip loop, the antenna lead acts like a pipe, directing water into the house 53 54

10 55 56 [Moisture Control Handbook, Fig 2-20] [Moisture Control Handbook, Fig 2-24

11 [Moisture Control Handbook, Fig 2-26] Capillary rise in foundations Or Cedar Breather TM or similar product [Moisture Control Handbook, Fig 2-29] 63 [Moisture Control Handbook, Fig 2-19] 64 Preventing capillary rise in foundations Moisture transport mechanism #3 Air movement [Moisture Control Handbook, Fig 2-21] 65 66

12 The fundamentals: Necessary but not sufficient conditions for moisture transport by air movement Moisture sources for transport by air movement A source A hole A driving force Exterior: Hot humid air in cooling climates Interior: Wet crawl spaces, bathing, cooking, etc Principal driving forces for air movement in buildings Wind-induced pressure (positive on windward side, negative on leeward side) Stack effect (warm air rises) Pressure differences caused by the HVAC system 70 Stack effect -- a 24/7 driving force Warm buoyant air produces a pressure difference(driving force) 24 hours a day all winter long Creates the following pressure distribution between inside and outside of house: Positive pressure at top of house Negative pressure at bottom Zero pressure difference halfway between top and bottom (the neutral pressure plane) 72

13 Stack effect fundamentals Pressure is directly proportional to height and temperature difference The greater the pressure, the greater the driving force The greater the driving force, the more infiltration and exfiltration [Moisture Control Handbook, Fig 2-39] 73 Stack effect demonstration box Can be used to model any building The heating system is a 250 W light bulb Set to model a single story home on a day when the temperature difference between inside and out is 40 o F e.g., T inside = 70 o F T outside = 30 o F Stack effect demo -- questions Most important air leakage sites? Most important doors in a house? Solution to cold floors, frozen pipes in crawl space or basement? Solution to radon in the basement? Why does the CO from the attached garage get drawn into the house? How can failing to air seal the bottom plate result in an expensive call back? 75 How air pressure leads to wetting Recessed lights: Big holes in the worst possible place, the top of the house [Moisture Control Handbook, Fig 3-4] 77 78

14 Recessed light as seen from the attic Recessed lights in cathedral ceilings cause major moisture problems Results of 1993 Penn State tests on air leakage through recessed light fixtures Energy costs: $5 to $30 per light fixture per year Moisture: About 1/3 gallon per day Assumptions: Indoor air at 70 o F and 40% RH Attic at 32 o F 10 cfm continuous air leakage 20 fixtures => 180 gallons per month! One solution: Airtight fixtures Cost: About $5 more per fixture Source: EDU, January New construction Remodel 82 Airtight fixture for sloped ceilings Back to our mold-growing second story window William W. Hill, Department of Urban Planning, Ball State University, Muncie, IN

15 Source: Interior humidity condensing on cold storm Hole: Air leaks around primary window Driving force: Stack Moisture transport mechanism #4 Vapor diffusion effect Moisture transport by vapor diffusion Sources: Same as for air transport Holes: Not that critical; a vapor diffusion retarder that is 5% holes is still 95% effective Driving force: Vapor pressure Vapor diffusion & vapor barriers It s what builders and architects have been taught to think is important It s the least important of the moisture transport mechanisms I have never seen a moisture problem resulting from the lack of a vapor diffusion barrier I have, however, seen problems caused by a vapor barrier 88 Vapor diffusion is far less important than air pressure in moving moisture Source: Builder s Guide, Cold Climates, Fig III.1 89 Builder s Guide, Cold Climates, Fig III.3 90

16 Terminology is important: What s the purpose of that poly? Vapor diffusion retarder Does not have to be continuous or perfect Vapor diffusion = vapor pressure x area x permeability Air barrier Should be as continuous and perfect as possible Air under pressure will find the weak links -- the holes in the air barrier Drying potential 92 Walls and other building components need to be able to dry Assume they will get wet The primary moisture transport mechanism for drying is vapor diffusion Thus, while walls seldom get wet from diffusion, they mostly dry via diffusion They must be able to dry to either the inside or the outside 93 Vapor barriers in the wrong place are worse than none at all A vapor barrier (i.e., a vapor impermeable material) on both sides of a wall gives the wall no place to dry When in doubt, leave it out 94 A wall designed to dry to the outside A wall designed to dry to the inside [Builder s Guide, Cold Climates, Fig III.4] 95 [Builder s Guide, Cold Climates, Fig III.5] 96

17 A wall with no place to dry Vapor diffusion from the outside can cause problems This is a primary moisture transport mechanism in the South (cooling climate) It can be a problem anywhere with brick veneer walls built with a vapor diffusion retarder on the inside (as per code!) [Builder s Guide, Cold Climates, Fig III.17] Sun on brick => inward vapor diffusion Why the increase in mold problems behind brick veneer in new homes? Polyethylene vapor barriers installed under drywall as per code New exterior sheathings are more permeable than plywood Poor detailing in brick work [Moisture Control Handbook, Fig. 3-22] Latest recommendation on poly vapor retarders from Lstiburek Polyethylene should not be installed on the interior of any assembly with the exception of above grade walls and ceilings in locations with 8,000 heating degree days or greater (Muncie has 5900 HDD) There is one place where a vapor diffusion retarder is absolutely essential -- as a ground cover in the crawl space

18 Foundations The cause of many moisture can be traced to the foundation -- crawl space or basement Why crawl spaces are a problem more often than basements While homeowners wouldn t tolerate a wet basement, crawl spaces are... Out of sight, out of mind Building codes, code interpretation and common practice do not properly address moisture control in crawl spaces 105 A damp foundation can provide an almost unlimited source of moisture for distribution throughout the house Let s apply our understanding of building science fundamentals to understand how poor foundation design and detailing can result in mold in the attic (and elsewhere) 106 Moisture transport, foundation to attic Source of moisture: rain, snow melt and groundwater [Bulk or liquid moisture] Poor site drainage, no perimeter drain allows wetting of foundation [Liquid flow] No capillary break allows groundwater to wick to surface of soil and concrete/block wall in crawl space [Capillarity] No vapor barrier on soil and blocks allows moisture to enter air in crawl [Diffusion] 107 Foundation to attic, continued Lack of continuous air barrier allows moist air to be carried by stack effect to the attic [Air transport] Moist air cools in attic, condenses on roof sheathing [Condensation and/or elevated relative humidity] Conditions are ripe for mold growth and rotting of roof sheathing, etc. 108

19 The bottom line on foundations -- both crawl spaces and basements 1 Pay careful attention to site grading, guttering and downspouts 2 Install a good perimeter drain, surrounded by pea gravel and filter fabric 3 Install a capillary break on top of the footing (damp proofing) and over the ground (4 inches of pea gravel) Bottom line, continued 4 Install a quality vapor diffusion retarder (6 mil poly) on the ground and up the inside walls 5 Insulate and air seal the perimeter walls, including the access door 6 Provide a continuous air barrier in the house above 7 Run all condensate lines to outside 110 [Builder s Guide, Cold Climates, Fig. 4.17] 111 [Moisture Control Handbook, Fig 5-15] 112 Moisture control details, short basement Short basement with concrete floor

20 Perimeter drain must drain to daylight or a sump What s the difference between a crawl space and a basement? Answer 1: About 4 feet Answer 2: Building code requires vents in the shorter one Crawl space venting is not a solution No amount of venting can deal with poor surface water drainage Crawl space vents: Conflicting code requirements Thou shalt install operable vents in crawl spaces -- CABO One and Two-Family Dwelling Code Requires minimum net free area of 1/150 of crawl space area, 1/1500 with an approved vapor retarder Thou shalt insulate crawl spaces -- CABO Model Energy Code (MEC) Requires either R-19 floor insulation or R-10 walls

21 The science behind crawl space venting requirements Ventilation cannot do the job Requirement dates to 1942 FHA Property Standards None of the research in Britton s [1948] seminal paper appears to support the recommendation for venting Conclusion: There is no technical basis in the literature for current or past crawl space ventilation requirements Source: Bill Roses 1994 ASHRAE paper Evaporation cannot remove the amount of water that comes into a crawl space that has poor site drainage and is lacking a perimeter drain Worse, venting provides the appearance of a solution, which prevents builders, architects and code officials from focusing on real solutions 121 Crawl space ventilation does not work and can make matters worse Psychrometric Chart In summer, venting crawl spaces will add moisture to the house Outside air in summer is hot and humid. Use the psychrometric chart to see why that air can only add moisture to the crawl space In winter, vents, if not closed and made airtight, will result in higher heating bills 124 Normalized annual heating bill using $.85 per therm Effect of crawl space vent position on space heating (research on one house over seven winters) $600 $500 $400 $300 $466 $200 $100 $0 Vents open Closed & insul $487 $377 $330 Vents open Closed & insul Closed, not insul $453 Closed & insul $401 Vents open 125 $537