Transcript BuildDynFINAL.ppt

BUILDING DYNAMICS:
Moisture, Airflows and Construction
Technology
James Biddle, Mansel Nelson
Northern Arizona University
Presentation Credit to: Joseph T. Ponessa, Ph.D.
Professor Emeritus Housing, Indoor Environments and Health
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Goals
Review basic dynamics of moisture
movement, control in buildings
 Review basic dynamics of airflows in
buildings
 Proper management of moisture and
airflows provides better buildings and
reduces callbacks
--------------------------------------------------- Understanding of these mechanisms is
essential for diagnostics
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
Objective: How does this fit with
weatherization?
Weatherization can improve comfort and
save money. Downside is ‘inadequate’
ventilation
When is ventilation inadequate?
 When it is less than prescribed ventilation
 When it is inadequate to take care of
building excesses
– Too much moisture
– Excessive pollution sources
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Overview of Today’s
Presentation
Building Science

Moisture dynamics; applications in
buildings

Airflow mechanisms; applications in
buildings
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Section I:
Moisture Dynamics
Outline

Basic moisture science: Vapor &
Liquid
– Air – vapor – temperature relationships
– Relative Humidity
– Putting it all together – Psychrometric
chart

Vapor movement
 Diffusion
 Bulk transfer – air flows
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Moisture Dynamics
Outline (cont.)
Basic moisture science (cont.)

Water movement
 Gravity; wind
 Capillary action


Moisture sources
Moisture Measurement
 Air
 Surfaces / solids
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Moisture Dynamics
Water can exist in three states

Vapor

Liquid

Solid
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Moisture Dynamics (cont.)

VaporTemperature
relationships (At
saturation)
Vapor
The amount of moisture
that air can hold increases
directly with temperature.
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Moisture Dynamics
Vapor
Dew point:
Saturation
Air cooled to the limit of
its moisture carrying
capacity releases vapor
as droplets
condensation (or rain)
This is the Dew point

temperature
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Moisture Dynamics
Vapor
So what does it all mean?
Moist air that is cooled down
OR…
Moist air that meets a cool surface will

condense!
Wet surfaces that don’t/can’t dry
rapidly will produce mold
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Moisture Dynamics
Vapor Transport
Vapor transport: how does vapor get from
point A to point B?
POSSIBLE MECHANISMS
 Diffusion
 Bulk transport: Airflow
Which is more important?
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Moisture Dynamics
Vapor Transport via Airflow
Source: USDOE
Moisture carried into wall via air flow / leakage
through openings. What is the driving force?
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Moisture Dynamics
Which mechanism is most important?

Diffusion?

Air transport?
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Moisture Dynamics
Diffusion vs. air transport
While air transport accounts for bulk of
moisture migration in most buildings,
there are circumstances when diffusion is
most important factor
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Permeability – Bldg Materials
Vapor Imperm P <0.1
Four classes of
*
vapor retarders
PE Film
have been
Glass
identified
Aluminum foil
Foil faced insul (non
Semi-imperm. P 0.1-1 perf)
Kraft –backed fiberglass insul
Oil based paint
Vinyl wallpaper (most)
Extr polystr >1” (unfaced)
P = Perm
* Vapor Barrier
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Permeability – Bldg. Materials
Vapor semi-perm P110
Plywood
Bitumen impreg kraft
OSB
Unfaced Exp polystyrene
Unfaced Extr poly <1”
Building paper
Latex paint (Most)
Building America Best Practices Series:
Volume 4 –…..Mixed-Humid Climate Version 1,
9/2005 • Design-p13
Vapor perm P >10
Fiberglass insul
Unpainted gyp board,
plaster
Masonry, Fiberboard,
Dimens. lumber
15# felt, Housewrap
Cellulose insulation
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PERMABILITY
An important note about building materials
and water

Permability of many materials changes
when they are wet. Ppermeability of wet
plywood, for example, changes from 0.75
(dry) to 3.0 when wet
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Questions about vapor
transmission/dynamics?
While vapor migration and
condensation can play an important
role in moisture problems, most
problems are caused by rainwater
Moisture Dynamics
Liquid
Liquid water can flow via
 Gravity (or wind pressure)
 Capillary action (wicking)
= against gravity
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Moisture Dynamics
Liquid
Gravity
“It flows downhill”
Source: Builder Magazine
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Moisture Dynamics
Capillary action
Water can flow against
gravity when moving
in a tight space…
…and, by the same
process, can wick
through porous
materials
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Moisture Dynamics
Practical applications
 Water can travel up and behind flashing
that is not properly dimensioned
 Water can diffuse/wick through masonry,
adding humidity to spaces and wetting
components (e.g. sill plates)
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Moisture Dynamics
Capillary action
Source: USDOE
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Moisture dynamics
Abbey
XXX
barn grange,
Great Coxwell, England
Photo: Barn. Houghton Mifflin, 1992.
Some builders, at least, have known about
capillary action for a long time…
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Moisture dynamics
Barn interior, showing
posts set on stone
piers
This barn, built in mid13th century, in use
until 1966, when
deeded to National
trust
Photo: Barn. Houghton Mifflin,
1992
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Moisture dynamics
Note detail on top of
pier
A sacrificial wood slab
has been placed
here. Moisture
migrating through
pier will enter slab
instead of end grain
of post; slab is easily
replaced
Photo: Barn. Houghton Mifflin,
1992
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Moisture Sources
Outline for this section
 Plumbing leaks
 Rainwater
 Groundwater
 Humid air (Including embodied water)
 Mechanical equipment (Including
Combustion equipment)
 Occupant practices
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Moisture Sources:
Plumbing leaks

Plumbing leaks should be obvious but can
be in concealed spaces, and may involve
supply or drain lines

Sweating may sometimes be significant
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Moisture Sources
Rainwater
Gutters & downspouts
 Water discharged next
to foundation and /or
against building is
almost certain to enter

Most basement
moisture problems
are due to rainwater
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Moisture Sources
Rainwater
Discharge against
building may also
penetrate- masonry
is not waterproof
(Consider masonry as a
“Hard sponge”)
-----------------Low spots, backslope
next to building also
cause problems
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Moisture Sources:
Rainwater and grading
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Moisture Sources:
Humid air (Summer)
Basement ventilation may add moisture / RH
Also consider air
conditioned
interior
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Moisture sources:
Mechanical equipment
Combustion
produces a LOT
of moisture
2O2 + CH4 = CO2 +
2H2O
1 lb of nat gas
2.25 lb (1.125 Q)
water!
Further discussion
under Airflows
Gas furnace,
blocked flu,
condensation soaks
brick in & out
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Moisture Measurement
Air
measurement
 Sling
psychrometer
 Hygrometer
(electronic)
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Moisture Measurement (cont.)
Surface / material
measurement
 Electronic device
(eg Protimeter)
Measures moisture
content- wood,
drywall, masonry
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Moisture Content in
Building Materials
Mold growth can begin…
 In lumber @16% moisture content (this
represents equilibrium @80%RH)
 In gypsum sheathing @1% moisture
content
Source: Lstiburek, ASHRAE Journal, 2/02
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Keep Water Out: Drain the
Building
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Case study: Things Gone Wrong
Photo: Joe Lstiburek
Photo: Nathan Yost, BSC
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Anatomy of a Disaster
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Flashing is Key: e.g., Windows
Photo: Mark LaLiberte,
Building40Knowledge.
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Photo: Mark LaLiberte, Building
Knowledge.
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Reverse Flashing: A Common Mistake
Photo: Mark LaLiberte, Building
Knowledge
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Building Moisture: Take-away
Messages
Building components that get wet must
be able to dry out quickly. Assemblies
must be able to dry!
 If they don’t, mold and other organisms
will grow, creating health hazard for
occupants and ultimately destroying the
building
A moisture problem is like a fire: it will
not get better with time. It cannot be
ignored. Respond promptly!

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Section II:
AIRFLOWS
OUTLINE
 Air moves according to pressure
differences; These can be created by
– Temperature differences
– Wind
– Mechanical equipment
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Airflow
•
•
•
For air to move (leak) into
or out of building, you
need a hole and a
pressure difference
Air in must equal air out–
same for moisture, but on
a different time frame
Pathways can be direct or
indirect, natural or
mechanical
EEBA BFG
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Airflows: Driving forces:
Temperature Differences
Heating Season:
Aka THERMOSIPHONING
Source: USDOE
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Airflows: Driving Forces:
Wind
Source: USDOE
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Airflows: Driving forces:
Mechanical Equipment
Airflows can also be influenced by
 Ventilation fans
 Furnaces / boilers
 Ductwork (leaking)
 Major Appliances (dryer, water heater)
All of the above can remove air from the
conditioned space. What about makeup
air?
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Providing Fresh Air
Best practice to control moisture,
pollutants and to save energy
Build a tight building
 Tightly seal ducts (anything but duct tape)
 Ventilate by design!

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Building Ventilation by Accident
Typical
ducts can
lose 25%
of airflow
through
joint
leakage
Daylight!
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…and some ducts lose more than 25% of
their airflow!
“Needs repair”
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Typical Ventilation Rates
100-year-old house: Two ACH
 Energy-conserving house (1970’s)”: 0.1
ACH)
 Estimated optimum, 1980’s: 0.5 ACH
 Present day: ASHRAE’s engineering standard

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Building Ventilation by Design
Various choices
 Passive vent open to building
 Outside air ducted to air return; dampers,
controllers
 Sealed combustion equipment reduces
need for makeup air
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ASHRAE Residential Ventilation
Standard 62.2
Goal: Reduce indoor pollutants
Approach
 Whole-house ventilation
 50 CFM (typical house)
 Vent system rated @ 7.5 CF PP + 1CFM /
100SF (Some exceptions)

Local exhaust
 Mech exhaust, Kitchens & baths (Not
toilets, utility rooms)
Source: M. Sherman, Lawrence Berkeley Labs
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ASHRAE Residential Ventilation
Standard 62.2 (cont.)

Source control
 Some sources addressed
---------------------------------------------------Backdraft testing required in some cases
Some secondary requirements
Some flexibility
Lots of controversy
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ASHRAE Residential Ventilation
Standard 62.2 (cont’d.)
About whole house ventilation
Calculation: House, 3 BR, 1500 sq ft
7.5 CFM/ BR+1 + 1CFM/100 sq ft
7.5X4 + 100X15 = 30 + 15 = 45 CFM
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Airflows: Summary
Why be concerned about airflows into & out
of buildings?
1) Energy transfers (losses)
2) Moisture transport (into bldg or into walls)
3) Pollutant transport (eg radon, ozone, fireplace
smoke, particulates, etc.)
And on the plus side, airflows provide
1) Fresh air
2) Replacement/ makeup air

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Airflow Dynamics
Summary
“Nature abhors a vacuum”
The law that gases (and other materials)
move from a region of high pressure to
low pressure is analogous to, and as
immutable as the law of gravity
Pressures will equilibrate whenever there is
a pathway, no matter how small or
indirect
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Airflow Dynamics
Summary (cont.)
Amount of air entering (or leaving) through
various openings (such as envelope leaks)
vs. flue openings depends on relative sizes
of openings
OR
If total envelope leaks are small relative to
flue opening(s), some flues may become
main sources of makeup air–a problem if
flue is active!
Direct vent or sealed combustion better
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Airflows
Summary
Take Home Message
If building airflow is not balanced
– inflow and exhaust not equal - the building
will become pressurized or depressurized
and bad things can happen.
Likewise, pressure differentials can happen
within the building.
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References & Resources
for this Section
Building Science Corporation
wwww.buildingscience.com
 BFG: Builders Guide: Mixed Humid Climates.
Energy and Environmental Building
Association (EEBA): www.eeba.org
 Lstiburek, J. Water Management Guide.
Energy and Environmental Building
Association (EEBA): 2004. www.eeba.org
 Building America Best Practices Series:
Volume 4. USDOE Building America program:
www.buildingamerica.gov

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References (cont.)

Lstiburek, J. Moisture Control for
Buildings. ASHRAE Journal, Feb 02, pp3641.
HUD Moisture Resistant Homes. March 2006.
125 pp. Available at
http://www.huduser.org/publications/dest
ech/moisturehomes.html
Or call 1-800 245 2691, option 1 for hard copy
($5.00)

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