Investigating Diagnosing Moisture Problems

The following article was published in ASHRAE Journal, December 2002. © Copyright 2002 American Society of Heating, Refrigerating and Air-Conditioning...
10 downloads 0 Views 1MB Size
The following article was published in ASHRAE Journal, December 2002. © Copyright 2002 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

&

Investigating Diagnosing Moisture Problems By Joseph Lstiburek, Ph.D., P.Eng., Member ASHRAE

ater comes in four forms: solid, liquid, vapor and adsorbed.1 All four forms can cause grief to building owners, designers and contractors. When water causes building problems, investigating and diagnosing the problem can be challenging because water constantly changes its form inside a building and within its materials. The investigator must hunt down the water by thinking like water.

W

The rules governing water movement are straightforward: 1. Water runs downhill due to gravity. 2. Air carrying water vapor goes from areas of higher air pressure to areas of lower air pressure. 3. Water in the vapor form diffuses from warm to cold driven by the thermal gradient. 4. Water in the vapor form diffuses from more to less driven by the concentration gradient. Okay, it isn’t always straightforward: 5. Water in a porous material diffuses on pore surfaces from more to less along the concentration gradient in the form of adsorbed water. When there is a lot of water and it fills the pores, it is sometimes referred to as capillary water. In this way it can move upwards against the force of gravity — or 36

ASHRAE Journal

move sideways over long distances. Basically, porous materials “suck.” Water always changes its behavior because its form is never constant. Evaporation, condensation, capillary suction, gravitational flow, vapor diffusion and mass flow of moist air are happening all at the same time inside building cavities and materials. Water plays clever tricks by changing forms and methods of movement along its flow path. We must fight back by knowing its tricks. We’ll begin simply with diagnosing and finding rain leaks. Then, we’ll explore water’s clever tricks. Rain, Rain, Go Away…

Diagnosing rainwater leaks is not complicated. If things are wet after a rain, it is probably the rain. Start at the wet spot and work backwards and upwards. However, there is a catch. Water likes to stick to things and can run a long way horizontally because nothing is ever completely flat. Wind can blow water uphill, even over items that are an inch or two high, such as sills, flashings and ledges. The best time to diagnose rainwater leaks is when it is raining. Since it doesn’t always rain when needed, use a garden hose to make your own rain. Some consultants use a spray rack to make it seem more “technical” and “scientific.” However, a About the Author Joseph Lstiburek, Ph.D., P.Eng., is a principal with Building Science Corporation, Westford, Mass.

w w w. a s h r a e j o u r n a l . o r g

December 2002

Moisture

‘When water causes building problems, investigating and diagnosing the problem can be challenging because water constantly changes its form inside a building and within its materials.’

Exterior Conditions Temperature: 80°F Relative Humidity: 75% Vapor Pressure: 2.49 kPa

Conditions Within Air Space Temperature: 120°F Relative Humidity: 100% Vapor Pressure: 11.74 kPa

Solar Radiation Strikes Wall Brick Veneer Is Saturated with Rainwater

Interior Conditions Temperature: 75°F Relative Humidity: 60% Vapor Pressure: 1.82 kPa

1 in. Air Space

leak is a leak, whether the water comes from a hose or a calibrated spray rack spraying calibrated water at a calibrated air pressure. In fairness to consultants, standardized tests can be helpful once the flow path is known and you want to determine if a leaking window meets industry standards. Be gentle with the hose. You don’t want the momentum of the water spray forcing water into the building. The secret is to mist the surface and let gravity do the dirty work. Sometimes a building only leaks rainwater when it is windy. So, you will need to make your own wind. Instead of blowing against the building from the outside, suck air out of the building from the inside to simulate wind-driven rain. Turn off the supply air, and crank up the exhaust. Rainwater leaks usually are straightforward to diagnose. However, finding moisture due to rainfall sometimes can be tricky.

Tar Paper Vapor is driven inward by a high vapor pressure differential between the cavity and the interior.

Gypsum Sheathing Fiberglass Insulation Polyethylene

Interior Gypsum Board

Figure 1: Inward moisture movement due to solar radiation.

inwards out of the brick into the airspace where it turns into vapor. It is unlikely that the airspace is free from mortar droppings or that it is vented at the top and bottom. The cavity may be drained, but it is seldom vented — although it always should be.2 It may be vented accidentally — this accidental venting Rain on the Brick Sun Trick saves a lot of buildings. Up to now, the story has been simple. Let’s complicate it by Now, the water vapor goes inwards, traveling along the splashing rainwater on a brick veneer. Brick is a sponge that temperature gradient and along the concentration gradient. wicks water into itself because it is porous. The mortar in the How far it travels depends on what is in its way. If the vapor joints between bricks is also a sponge because it is porous. runs into something impermeable, such as foam sheathing or Cracks between the brick and the mortar also are “pores” that a rubber membrane on the backside of the cavity behind the wick water. Remember, porous materials suck. Think of a brick brick veneer, the vapor won’t travel far. If it hits foam sheathveneer as a moisture reservoir that is ing or a rubber membrane, the vapor filled during a rainstorm. condenses, turns back into a liquid So now a wet sponge is on the outside and runs down the backside of the airof a building. The sun comes out and space, and with any luck, to a flashbeats down on the wet brick on the southing where it is directed out of the wall west side and heats the water in the to the outside. brick. How hot? Probably 120°F (49°C). What happens if the water vapor Let’s go to the psychrometric chart. Find runs into a building paper or a where 120°F (49°C) crosses the saturahousewrap that breathes? The heattion curve (100% rh). Hey, it’s off the driven vapor blows through it easily. chart. Next, go to the steam tables. Wow. Parapet fresh air intake. The building paper or housewrap Any guesses what direction the water in probably is installed over gypsum the brick wants to go? Did I mention the building this brick sheathing, which is highly permeable to vapor. So, the vapor veneer encloses is air-conditioned? diffuses right through it. Next, the vapor comes to the fiberThe brick is wetter and hotter than the rest of the wall and glass cavity insulation, which can’t stop the vapor because it’s wetter and hotter than the inside. Water in the brick is driven permeable too. The vapor goes all the way in until it hits the December 2002

ASHRAE Journal

37

plastic vapor barrier. It was a bad idea to place a plastic vapor will do. All we need is outside air to have a dew-point temperabarrier on the inside of a brick veneer wall. The vapor con- ture above the temperature of any surface inside the interior denses on the plastic vapor barrier and runs down the wall to space. Actually, we don’t even need that. Anything that leads sit in the bottom plate track. Now, we have a full range of to a relative humidity at a hygroscopic surface above 80% problems to choose from: corrosion, mold, odors or staining. works well for mold growth. Alarmingly, it is common to have The same effect occurs by installing a vinyl wall covering the de facto fresh air intake of many buildings be the underside of the parapet flashing. (Figure 1). Just ask the hotel industry about this practice. This “sucking on the exterior wall” trick can be accomplished In this example, water started out as a liquid governed by gravity (the rain on the brick veneer), and was pulled into the in many ways. We don’t always need a dropped ceiling return porous brick veneer by capillarity. It was driven from the brick plenum. We sometimes can use an interior wall that thinks it’s a veneer and converted into a vapor in the airspace behind the duct.4 Hotels and nursing homes are famous for this type of brick by the energy added by incident solar radiation.3 Once in problem. When this is coupled with vinyl wall coverings, even the cavity, it traveled along the concentration gradient and on interior walls, mold grows behind the vinyl. thermal gradient through the wall assembly materials by the Fan coil units often are enclosed in less than airtight gypprocess of vapor diffusion until it condensed back into a liq- sum board enclosures, and rooftop exhaust fan systems rarely use airtight ductwork or airuid at the interior of the exteAir Barrier System Not Present rior wall assembly. Once in tight shafts. The negative presto Prevent Air From Being Exliquid form again, it ran down sure f ields developed by tracted From Roof Assembly suction from these fans can the wall to the bottom plate. Corrugated Metal Roof Deck extend to exterior walls, drawTwo rules can prevent this ing air out of exterior cavities common problem: Membrane Roof Brick Veneer Rule One: Never install a inward (Figures 3 and 4). Rigid Insulation vapor barrier on the inside of In this example, water is pulled out of the cavity or from a wall assembly that has a Building Paper Interior Gypsum moisture reservoir cladding the exterior ambient air in va4 3 Should Extend to Unand a vapor permeable compor form. It is transported by derside of Roof Deck 2 bination of sheathing and and be Sealed air moving from a higher air 1 building paper. Exterior Sheathing pressure to a lower air pressure Return Plenum Operuntil it contacts a cool surface Rule Two: Always vent ates Under Negative Pressure Relative to claddings, especially reserwhere it converts back into a Metal Stud Wall Occupied Space and Cavity Insulation voir claddings. Remember, liquid and causes mischief. Exterior that to vent the cladding, you Suspended Ceiling Rule Three: Never suck on Top Chord Bearing need an air gap behind the the exterior building encloRoof Truss material along with an air insure with the mechanical sysInterior Gypsum let, an air outlet and a clear tem. Do not connect drop Figure 2: Return plenum connected to exterior wall. path connecting the two. ceiling return plenums to exterior walls. Make sure interCondensation in the Negative Air Pressure Field Trick secting interior walls do not contain or are connected to Let’s stay with the rain on the brick veneer over a cavity leaking return ductwork or to any leaking exhaust chases or example a little longer. Assume that same building is ducts. Make sure all return ducts are sealed tightly with equipped with a dropped ceiling return air plenum. A return mastic on all joints. plenum operates under negative air pressure. It’s unlikely that the interior gypsum board extends to the underside of Groundwater in the Sand Over Polyethylene Trick the roof/floor assembly and is sealed with an airtight mateAll too frequently, we can create a water problem in the rial. The usual practice is to shove some mineral wool in the middle of a desert. There are many variations. Consider founflutes or floor decking above the top of the gypsum board dations. Normally, we do not worry about groundwater in the and hope for the best. desert. Now, let’s plant shrubbery beside a concrete foundation Given the poor air seal, negative pressure in the ceiling ple- slab. Since we have very little rain, let’s irrigate the plantings num sucks saturated air out of the wall cavity into the plenum so that the ground beside the foundation becomes saturated. where it can condense on piping, ductwork and anything else The water then wicks into the porous concrete from the satuit contacts with a surface temperature below its dew point (Fig- rated ground by capillary suction and moves upward into the ure 2). We don’t even need the brick veneer to be saturated to wall assembly (Figure 5). The symptom is a musty odor inside. cause this problem. We don’t even need brick — any cladding If the wall is opened, the bottom plate is typically saturated. 38

ASHRAE Journal

w w w. a s h r a e j o u r n a l . o r g

December 2002

Moisture

‘Alarmingly,

Dropped Ceiling (Return Plenum)

Partition Wall (Demising Wall)

Exterior Air Pressure Field (Ambient or Neutral Pressure)

Fan-Coil Unit

Exterior Wall

it is common to have the de facto fresh air intake of many buildings be the underside of the parapet flashing.’

Hotel Room (Interior Air Pressure Field)

Bathroom

Corridor

• Room is at positive air pressure relative to exterior due to hallway pressurization/

Exterior Wall

This is a simple problem to diagnose and undercut door and due to air accidentally supplied to room by fan-coil unit pulling air from exterior through the demising wall. fix. Stop the water. Kill the plants. Or move • Fan-coil unit depressurizes dropped ceiling assembly due to return plenum the plants well away from the foundation pedesign. rimeter. Install a capillary break under the bot• Demising wall cavity pulled negative due to connection to dropped ceiling return plenum. tom plate of the wall to separate the wood from the wet concrete. Finally, damp proof the edge Figure 3: Pressure field due to fan-coil unit (plan view). of the slab with an acrylic latex paint or some other damp proofing material. Bathroom Hotel Room Bathroom Symptoms of this problem become more Ducted to Exhaust Rooftop complicated with the installation of an imperGrille Exhaust Fan meable vinyl floor cover or tile flooring. The Service Shaft water can be wicked inwards laterally from Central Partition Wall (Demising Wall) Exhaust the foundation perimeter leading to discoloration of the flooring and loss of adhesion of Exterior Air Bathroom the flooring. In some cases, the lateral wicking Pressure Exhaust Grille can spread many feet. Then the problem is Field (Ambient or manifested in mold under kitchen cabinets loCorridor Hotel Room Bathroom Neutral (Interior Air cated in “islands” far away from slab foundaPressure) Pressure Field) tion perimeters. Water problems in the middle of slab foundations can be due to a water • Leakage of central exhaust duct pulls air out of service shaft depressurizing shaft and demising walls. source at the building perimeter as a result of this lateral capillary movement of water. How- Figure 4: Pressure field due to central exhaust (plan view). ever, the solution to the problem is the same — keep moisture from entering the slab by using capillary tion, the liquid water cannot drain out of the sand layer. The breaks and reducing the amount of water soaking the soil near only mechanism of drying the sand layer is upwards through the foundation’s edge. the concrete slab by vapor diffusion (Figure 8). Next, consider a bad practice that has been established in Moisture diffuses upwards through the top surface of the some U.S. building codes. West of the Mississippi River, it is concrete slab as well as through floor surface treatments. This common to place slab foundations over a sand layer installed leads to mold and other microbial contamination problems. over a polyethylene vapor barrier. This is rarely done east of The rate of wetting of the sand layer by the gravity flow the Mississippi. liquid water wetting mechanism is several orders of magniThe sand layer becomes a reservoir for water in the liquid tude greater then the rate of drying of the sand layer by the state that enters the sand layer by gravity flow from the top, vapor diffusion drying mechanism. The sand layer becomes a sides and bottom (Figure 6). Where does this liquid water come water reservoir that continually supplies water for the upward from? We often wet cure these slabs from the top with sprin- flow through the concrete slab by vapor diffusion. A couple of klers, or pond water on top of them. We often over irrigate hours of liquid water gravity wetting yields a couple of years perimeter plantings (see previous). Plumbing pipes leak. And, of diffusion drying. even in the desert it can rain during construction. The liquid Picture the sand layer as “blotter paper” that once wetted water that inevitably finds its way into the sand layer is both does not let water drain out of it. The only method of drying held in the sand layer and redistributed within the sand layer available to the “blotter paper” is evaporation. In the case of by capillarity (Figure 7). Additionally, due to capillary suc- the sand layer, the only method of “evaporation” is upwards December 2002

ASHRAE Journal

39

Stucco

Unfaced Batt Insulation

‘A couple of hours of liquid water gravity

Building Paper

Interior Gypsum Board

wetting yields a couple of years of diffusion drying.’

Plywood

Mold on Gypsum Board

Bottom Plate Decay

Impermeable Floor Covering (Vinyl) Concrete Slab

Plantings

through the concrete slab due to the presence of the polyethylene vapor barrier under the sand layer. The following reasons generally are cited for using a sand layer over a polyethylene vapor barrier: 1. The sand layer controls bleed water with high water-tocement ratio concrete slabs. 2. The sand layer reduces curl with high water-to-cement ratio concrete slabs when top-side curing is not controlled. 3. The sand layer reduces plastic shrinkage cracking with high water-to-cement ratio concrete slabs. 4. The sand layer protects the polyethylene vapor barrier from punctures. The first three reasons are based on sound technical arguments. However, each of the first three are based on the condition that the sand layer be prevented from getting wet

Ground Saturated Adjacent to Moisture Flow Slab Foundation due to Irrigation Water

Mold Under Floor Covering

Figure 5: Slab edge capillarity.

during the construction process and beyond and are typically associated with floor slabs that are placed “after the

Advertisement in the print edition formerly in this space.

40

ASHRAE Journal

December 2002

Moisture

Concrete Slab Sand Layer

Footing

1 2 4 3

Polyethylene Vapor Barrier

Liquid Wetting by Gravity Flow 1. Through top of slab by rain, curing processes, cutting, finishing, cleaning, water testing, etc. 2. Through slab/footing interface by irrigation water, surface water and groundwater. 3. Through joints, penetrations and punctures in polyethylene vapor barrier by groundwater. 4. Around the edge of polyethylene vapor barrier at footing edge.

Undisturbed Soil/Compacted Earth

Figure 6: Slab wetting mechanisms with a concrete slab on a sand layer over a polyethylene vapor barrier.

Diffusion and Capillary Flow Upward Through Slab Concrete Slab Sand Layer

Concrete Slab

Polyethylene Vapor Barrier

Sand Layer

• Water migrates laterally — pulled by capillary action. • Water is unable to drain downwards through any gaps, penetrations, or punches in the polyethylene vapor barrier due to “blotter paper” effect (capillary forces) of the sand layer.

Polyethylene Vapor Barrier • Sand layer becomes water reservoir to supply water for upward flow through concrete slab by vapor diffusion and capillary transmission. • Drying downward by vapor diffusion or capillary action is not possible due to polyethylene vapor barrier.

Figure 7: Water distribution in sand layer.

Figure 8: Upward drying through slab.

building is enclosed and the roof is watertight.” Additionally, the first three are based on the condition that wet curing, such as ponding or continuous sprinkling, will not occur or that joint sawing using wet methods or power washing will not occur. The first three also are conditional on slab and foundation designs that will not be sensitive to groundwater wetting from local water tables and local irrigation. In other words, the first three reasons are based on conditions that rarely occur in the real world. The fourth reason, “puncture protection,” is based on incorrect physics. A sand layer is not necessary to protect polyethylene vapor barriers. Vapor diffusion is a direct function of surface area. Rips, holes, tears and punctures in sheet polyethylene vapor barriers constitute a very small surface area of vapor transmission compared to the total floor slab area. If 95% of the surface area of the slab is protected by a vapor barrier, then that vapor barrier is 95% effective. This holds true only if airflow or air leakage is not occurring through the vapor barrier. This is the case where concrete is in direct contact with the polyethylene vapor barrier. Airflow is not occurring. The concrete slab is an “air-bar rier” and the polyethylene is the “vapor barrier” — an effective vapor barrier even if the polyethylene has many punctures. In this example, the water enters the sand layer under the slab in the liquid form by gravity. The water is redistributed in the sand layer by capillarity and migrates upward by vapor diffusion.

Rule Four: Never place concrete on a sand layer installed over a polyethylene vapor barrier. Always place concrete in direct contact with plastic vapor barriers. Use a low waterto-cement ratio concrete, less than 0.45, and top cure the slab with damp burlap, just like the old wise concrete types once did.

December 2002

Conclusions

Water can be tricky to track when diagnosing moisture-related building problems because it constantly changes its form and behavior. Investigators have to “get smart” to outthink the water and find the root of the problem. When faced with a seemingly complex problem, go back to first principles. References 1. Kumaran, M.K., G.P. Mitalas, and M.T. Bomberg. 1994. “Fundamentals of transport and storage of moisture in building materials and components.” ASTM Manual Series: MNL 18, Moisture Control in Buildings. 2. Straube, J.F. 1998. “Moisture control and enclosure wall systems.” Doctoral Thesis in Civil Engineering, University of Waterloo, Waterloo, Canada. 3. Wilson, A.G. 1965. “Condensation in insulated masonry walls in summer.” CIB/RILEM Symposium on Moisture Problems in Buildings (NRC 9130). 4. Lstiburek, J.W. 1998. “Pressure response of buildings.” ASHRAE/ BETEC, Thermal Performance of Buildings VII. ASHRAE Journal

41