ARCH-432
Vapor Retarders and Air Barriers
Attendance
In what modern day
country was the first
cavity wall developed
and used? For what
purpose?
A.
B.
C.
D.
E.
Spain
Rome
Greece
Turkey
Macedonia
Add Infiltration Through Open
Door
Determine Door Usage

‫ = ת‬Number of People per Minute
Determine CFM per person (D)
CFM = ‫ ת‬x D
Infiltration by Crack Method
Add CFM from Crack losses to CFM for
Open Door losses
Mitigate These Losses
How do you reduce
or mitigate these
losses?
Mitigation Strategies
Pressurization
QS = 1.1 x CFM x (T2 – T1)
Vs.
You own the
variables!
QS = 1.1 x (CFH/ft of crack x ft of
crack)/60 x ΔT
LEED Points Available
Environmental Quality (EQ) “Ventilation”
 Credit 1 – Install permanent CO2 monitoring
system with feedback to ventilation system
 Credit 2 – Ventilation system 30% above ASHRAE
Standard 62.1
Infiltration Variables
Wind velocity and direction
Stack effects
Corner rooms
Exhaust fans on or off
Pressure zoning
Frequency of use
Maintenance
Stack Effect
What You Need To Know
The cause and effects of infiltration and
exfiltration on moisture migration
What Do You Need To Be Able
To Do
Be able to calculate the dew point
within a composite wall or ceiling
Properly place an air barrier in a wall
Properly place a vapor retarder in a
wall, if one is needed
Terms
Temperature gradient
Vapor barrier
Vapor retarder
Air barrier
Design Goals
Keep water from getting in
Let water out if it gets in

Wetting prevention versus drying
Select materials and locate vapor
retarders to allow the assembly to dry

Different for each climate zone
Temperature Gradients
The temperature differential between two
separate surfaces as heat transmits across
them.

A Proportional calculation
 Uses Indoor/outdoor DB Temperatures
 R values

Purpose
 Determine surface temperatures


MRT
Dew Point
Thermal Breaks
Significantly reduce
the Resistance of
the wall assembly
Concrete Wall
Failure Analysis
Prof. Kirk’s one-of-akind, surefire
process guaranteed
to result in a mindnumbing law suit.
At inspection,
you see…
Siding pulled away
North Side
Siding warps
You see…
You note that the
snow extends into the
closet
North Side
Windows
Check for condensation within
walls
Temperature
80oF
70o
60o
50o
40o
30o
20o
10o
Heat Transmission and
Exfiltration (ventilation)
Infiltration
IA: DB=75oF WB = 56oF
OA: DB=-5oF
0o
-10o
-20o
DP: 39oF
Definitions
Vapor Retarder – The element that is
designed and installed in an assembly
to retard the movement of water by
vapor diffusion



Class I – .10 perm or less
Class II – .10 perm to 1.0 perm
Class III – 1.0 perm to 10.0 perm
Vapor Barrier – A Class I vapor retarder
Air Barrier
Air barriers are systems of materials
designed and constructed to control
airflow between conditioned and
unconditioned spaces
An air barrier is not necessarily a vapor
barrier (i.e., they can be permeable)
Air Barrier Rules
Water vapor moves from the warm side
of building assemblies to the cold side


Climate dependant
Season dependant
 In cold climates (winter) humid indoor air can
condense inside a cold wall
 In hot and humid climates (summer) humid
outdoor air can condense inside a cold wall
Air Barrier Rules
In hot and humid climates, place the air
barrier/vapor retarder on the outdoor side of
the insulation or wall sheathing
In very cold climates, place the vapor barrier
on the indoor side of the insulation
Placing Air Barriers
For a climate that is
both hot/humid and
cold, where do you
place the air barrier?
Construction and Installation
The air barrier on
the outside must be
continuous


Individual sheets
must be taped
together
Entire sheet must be
taped to all window
and door frames,
foundations and
roofing
Construction and Installation
Dry all material before interior finish is
applied

Problems often occur on accelerated or
fast-track projects
Seal all ductwork and its connections
Placing Vapor Retarders
Air barriers are more
effective than Vapor
Barriers



Vapor permeation is
minor compared to
infiltration
Vapor retarders make a
wall more fragile
Vapor retarders can
prevent seasonal drying
Vapor retarders can
be very important


Cold climates
Any building with a
high indoor moisture
level
Goal
Keep high humidity out of the walls
Placing Vapor Retarder Guidelines
Locate only on the warm side of the wall
(except basement)
Avoid vapor barriers where vapor retarders
will work
Avoid vapor retarders on both sides of
assemblies


Avoid vinyl wall coverings on the inside of airconditioned assemblies
Avoid polyethylene vapor barriers, foil faced batt
insulation and reflective radiant barrier insulation
on the interior of air-conditioned assemblies
Placing a Vapor Retarder
1A – Very hot, humid
1B – Very hot, dry
2A – Hot, humid
2B – Hot, dry
3A – Warm, humid
3B – Warm, dry
3c – Warm, marine
4A – Mixed, humid
4B – Mixed, dry
4C – Mixed, marine
5A – Cool, humid
5B – Cool, dry
5C – Cool, marine
6A – Cold, humid
6B – Cold, dry
7 – Very cold
8 - Sub arctic
Vapor Barriers
Where would you
put a vapor
retarder in the
proposed
assembly?
8" Face Brick
1" air gap
6" Batt insulation
5/8" gypboard
Cold and Very Cold Climates
Open cell
Drainage Planes
Weep holes and
slots

They allow the wall
to drain
 From leaks
 From condensation
What about Concrete Floors