EXTERIOR WALL ASSEMBLY
PREAMBLE
EXCERPT FROM ARTICLE BY JOSEPH LSTIBUREK
Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Massachusetts. He has twenty-five years of experience in design, construction, investigation, and building science research.
Joe is an ASHRAE Fellow and an internationally recognized authority on indoor air quality, moisture, and condensation in buildings.
Joseph Lstiburek,
Air Barriers vs. Vapor Barriers
The key to moisture control is the control of air transported moisture. There is a big difference between air transported moisture and
vapor diffusion. Proponents of vapor barrier systems tend to confuse these two transport mechanisms, do they control the vapor
diffusion part, the air transport part or both? It is not always clear. What is clear is that air transport is far more significant than vapor
diffusion. What is also clear is that you do not need a 6-mil polyethylene barrier to control air transported moisture.The key point that
needs to be made is that although air barriers are a good idea everywhere, vapor barriers are not. Moisture flow by vapor diffusion is
governed by the second law of thermodynamics. Moisture will flow by diffusion because of a concentration gradient as well as a
temperature gradient (from "more to less" as well as "from warm to cold"). This means that it tends to go from the inside out up north
and from the outside in down south. In the middle of the country part of the year it goes from inside out and part of the year it goes from
outside in. You certainly do not want to put a vapor barrier on both sides of the wall. It's pretty obvious that you do not want a vapor
barrier at all in most assemblies in mixed climates. "Flow-through" design makes more sense in mixed climates where vapor diffusion is
slowed down, but not stopped. Hence, the term vapor retarders. Maybe we should use an insulating sheathing that is vapor
semi-permeable on the outside with sufficient thermal resistance to elevate the temperature of the condensing surface during the
heating season, and have a vapor permeable interior finish to permit drying to the interior? One of the problems in the building industry
is that it still views the answer to all moisture problems as the installation of a polyethylene vapor barrier on the inside of buildings. This
is responsible for many more building failures than building successes.
BRITISH COLUMBIA BUILDING CODE REFERENCES
PART 9
9.25.1. Scope
9.25.1.1. Application
1) This Section applies to thermal insulation and measures to control heat transfer, air leakage and condensation.
2) Insulation and sealing of heating and ventilating ducts shall conform to Sections 9.32. and 9.33.
9.25.1.2. General (See Appendix A.)
1) Sheet and panel-type materials shall be installed in accordance with Sentence (2), if the material
a)has an air leakage characteristic less than 0.1 L/(s•m2) at 75 Pa,
b)has a water vapour permeance less than 60 ng/(Pa•s•m2) when measured in accordance with
ASTM E
96, “Water Vapor Transmission of Materials,” using the desiccant method (dry cup), and
c)is incorporated into a building assembly required by Article 9.25.2.1. to be insulated.
2) Sheet and panel-type material described in Sentence (1) shall be installed
a)on the warm face of the assembly (see also Article 9.25.4.2.),
ROL FIELDWALKER
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 1
EXTERIOR WALL ASSEMBLY
BRITISH COLUMBIA BUILDING CODE REFERENCES
b)except as provided in Sentences (3) to (5), at a location where the ratio between the total thermal resistance of all
materials outboard of its innermost impermeable surface and the total thermal resistance of all materials inboard of
that surface is not less than that required by Table 9.25.1.2., or
c)outboard of an air space that is vented to the outdoors and, for walls, drained.
3) Wood-based sheathing materials not more than 12.5 mm thick and complying with Article 9.23.16.2. need not
comply with Sentence (1).
4) Where the mild climate indicator, determined in accordance with Sentence (6), is greater than 6 300, the
position of low air- and vapour-permeance materials within the assembly relative to the position of materials
providing thermal resistance shall be determined according to Part 5, where
a)the intended use of the interior space requires the indoor relative humidity to be maintained above 35%
over the heating season and the ventilating and air-conditioning system is designed to maintain that relative
humidity, or
b)the intended use of the interior space will result in an average monthly indoor relative humidity above 35%
over the heating season and the ventilating and air-conditioning system does not have the capacity to
reduce the average monthly relative humidity to 35% for any period over that period.
5) W here the mild climate indica tor, determined in accordance with S entence (6), is less than or equal to 6 300, the
position of low a ir- and vapour-permeance ma teria ls within the a ssembly relative to the position of ma teria ls providing
therm al resistance shall be determined a ccording to P art 5, where
a)the intended use of the interior space requires the indoor rela tive hum idity to be m aintained above 60% over
the hea ting sea son and the ventila ting and air-conditioning system is designed to ma inta in that rela tive
humidity, or
b)the intended use of the interior space will result in a n average monthly indoor rela tive humidity a bove 60% over
the hea ting sea son and the ventila ting and air-conditioning system does not ha ve the capacity to reduce t
he
avera ge monthly relative humidity to 60% over tha t period.
6) The mild clima te indicator (M C I) sha ll be ca lculated a ccording to the following formula:
M C I= a bs(2.5% JM T) x 200 + D D , where
abs (2.5% J M T)
DD
= absolute value of 2.5% Ja nuary mean tempera ture, and
= degree-da ys
F or Lions B a y: M C I= (2.5% x19) x200 + 3000
M C I = (.475 x 200) + 3000
= 3095
MILD CLIMATE INDICATOR (MCI) FOR LIONS BAY IS 3095
ROL FIELDWALKER
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 2
EXTERIOR WALL ASSEMBLY
BRITISH COLUMBIA BUILDING CODE REFERENCES
Table 9.25.1.2.
Ratio of Outboard to Inboard Thermal Resistance
Forming Part of Sentence 9.25.1.2.(2)
Heating Degree-Days
of Building
Location(1), Celsius
degree-days
up to 4 999
Minimum Ratio, Total Thermal
Resistance Outboard of Material's
Inner Surface to Total Thermal
Resistance Inboard of Material's
Inner Surface
5 000 to 5 999
0.20
0.30
6 000 to 6 999
0.35
7 000 to 7 999
0.40
8 000 to 8 999
0.50
9 000 to 9 999
0.55
10 000 to 10 999
0.60
11 000 to 11 999
0.65
12 000 or higher
0.75
( Lions Bay has 3000 Heating Degree Days, therefore the ratio outboard to inboard
thermal resistance required is 20%)
Architect's Note: From the above code references in PART 9 : None of the conditions regarding Reletive
Humidity described in sentence 9.25.1.2.5 apply to this project in Lions Bay BC. Lions Bay is a location with
3000 Heating Degree Days, and the RH of the interior is expected to be maintained at approx. 50% by the
ventilating and air conditioning system.
The amount of thermal resistance outboard of the low permeance materials in the wall assembly relative to the
thermal resistance inboard required by the Part 9 criteria is a ratio of 20% , the proposed wall has a ratio of
approx. 56%, and more than satisfies the BCBC 2006. The exterior wall assembly proposed, and the amount
and location of the thermal resistance in the wall, has been designed by the principals for exterior wall design
outlined in PART 5, and PART 5 -APPENDIX A of the BCBC 2006.
ROL FIELDWALKER
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 3
EXTERIOR WALL ASSEMBLY
BRITISH COLUMBIA BUILDING CODE REFERENCES
5.5.1. Vapour Barriers
5.5.1.1. Required Resistance to Vapour Diffusion
(See Appendix A.)
1) Where a building component or assembly is subjected to differentials in temperature and
water vapour pressure,the properties and position of the materials and components in those
components or assemblies shall be such that they control vapour diffusion or permit venting to
the exterior so as to minimize the accumulation of condensation in the building component or
assembly.
PART 5-APPENDIX A -DIVISION B
A-5.3.1.2.(1)
Use of Thermal Insulation or Mechanical Systems for Environmental Control.
Where the exterior design temperature is mild, such as in south coastal British Columbia, the interior RH during the
heating season may well be around 55%. With an exterior temperature of -7°C (19.4F), the materials in the
environmental separator would have to provide a mere RSI 0.082 ( R=.46 /1/2" plywood)
to avoid condensation on the interior surface. Depending on the specific properties of the material, this RSI might
be provided by 10-mm plywood. Therefore, materials generally recognized as thermal insulation would not be required
only to limit condensation on the warmer side of the building envelope
The word “minimize” is used in Sentence 5.5.1.1.(1) because not all moisture accumulation in an assembly need be of
concern. Incidental condensation is normal but should be sufficiently rare and in sufficiently limited quantities, and
should dry rapidly enough, to avoid material deterioration and the growth of mould or fungi.
A-5.5.1.2.(1) Vapour Barrier Materials and Installation. In the summer, many buildings are subject to conditions where
the interior temperature is lower than the exterior temperature. Vapour transfer during these periods is from the exterior
to the interior. In general, in Canada, the duration of these periods is sufficiently short, the driving forces are sufficiently
low, and assemblies are constructed such that any accumulated moisture will dissipate before deterioration will occur.
Architect's Note:
For comparing the dew point locations of various exterior wall assemblies an interior design temperature of
70F with a relative humudites of 60%, 50%, and 30%; and an outside design temperture of 19F have been
utilized to calculate the dewpoint locations in Diagram #1, #2, #3. With these conditions the dew point falls
inside the stud space, inside the interior face of the plywood sheathing for all the code conforming
configurations shown. These values have been assigned in accordance with ASHRAE standards.
In Diagram #4, the indoor relative humidity is assumed to be 50%, which can be maintened by the air
conditioning system proposed for the project, and the average outdoor tempertature in January in Vancouver of
39F. from the National Research Council Canadian Building Digest "Humidity in Canadian Buildings", by N.B.
Hutcheons. With these conditions the dew point in the project wall asembly falls outside the plywood sheathing.
ROL FIELDWALKER
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 4
EXTERIOR WALL ASSEMBLY
DEW POINT DIAGRAMS
65
60
RH AT 60% / DEW POINT = 55 F
55
RH AT 50% / DEW POINT = 50 F
50
45
40
TEMPERATURE
GRADIENT THRU WALL
ASSEMBLY / WINTER
HEATING SEASON
30
AIR BARRIER MEMBRANE / MOISTURE BARRIER
"DOW WEATHERMATE PLUS HOUSEWRAP"
60
55
50
RH AT 60% / DEW POINT = 55 F
TEMPERATURE
GRADIENT THRU
WALL ASSEMBLY /
WINTER HEATING
SEASON
RH AT 50% / DEW POINT = 50 F
RH AT 30% / DEW POINT = 37 F
35
30
PVC VAPOUR BARRIER
HERE
6 MIL PVC VAPOUR BARRIER
25
MOISTURE BARRIER /
2 LAYERS 30 MIN
BLDG. PAPER OVER
PLYWOOD
SHEATHING
20
15
10
0
65
40
25
5
70 F
45
RH AT 30% / DEW POINT = 37 F
35
TEMPERATURE GRADIENT THRU WALL
ASSEMBLY /SUMMER COOLING SEASON
INSIDE
VENTED 1/2" AIR SPACE
70 F
1/2" PLYWOOD SHEATHING
GYPSUM B'D.
R 3.8 OUTBOARD INSUL.= 20% R19 INBOARD INSUL
INSIDE
1/2" PLYWOOD SHEATHING
In the summer with air
conditioning, outside temp. at
70F, and inside temp. at 65F,
TEMPERATURE GRADIENT THRU WALL
RH at 60%, the Dew Point is
ASSEMBLY /SUMMER COOLING SEASON
approx. 10F below the
temperature inside the wall,
indicating no condensation will
occur during the cooling
season.
VENTED 1/2" AIR SPACE
GYPSUM B'D.
R 19 "BATT INSULATION
IN 2X6 STUD SPACE
WALL COMPONANTS TO SCALE OF THE R VALUE OF THE INSULATION
Residence at Lions Bay BC. showing dew points for standard
insulated 2x6 wall with vapour barrier on inside face of studs
Int. Temp 70F / RH 60% / RH55% RH 30%, Outside Design
Temp 19F.
DEW POINT DIAGRAM #1
WALL ASSEMBLY / DIAGRAM #1
1/2" GYPSUM BOARD
6 MIL VAPOUR BARRIER
R-19 BATT INSULATION
1/2" PLYWOOD SHEATHING
2 LAYERS 30 MINUTE BLDG. PAPER
1/2" VENTED AIR SPACE
3/4" CEDAR CLADDING
OUTSIDE DESIGN
TEMP @ 19 F
20
15
10
6 "BATT INSULATION / R 19
IN 2X6 STUD SPACE
5
0
OUTSIDE DESIGN
TEMP @ 19 F
MOISTURE BARRIER /
ALUMINUM FOIL ON
OUTSIDE FACE OF THERMAX
INSULATION
WALL COMPONANTS TO SCALE OF THE R VALUE OF THE INSULATION
Residence at Lions Bay BC. Int. Temp 70F / Outside Design Temp 19F/
20% of the insulation is outboard of the low permeance barrier (plywood)
as per BCBC requirements.
DEW POINT DIAGRAM #2
WALL ASSEMBLY / DIAGRAM #2
1/2" GYPSUM BOARD
6 MIL VAPOUR BARRIER
R-19 BATT INSULATION
1/2" PLYWOOD SHEATHING
R 3.8 INSULATION
2 LAYERS 30 MINUTE BLDG. PAPER
1/2" VENTED AIR SPACE
3/4" CEDAR CLADDING
Architect's Note: Both the assemblies shown in diagram #2 and #3 conform to the BCBC
2006, with a 6mil poly. vapour barrier on the inside face of studs, a min. R 20 rating for the
assembly, and a 1/2" ( 10 mm) vented air space between the 2 layers of building paper on the
plywood sheathing and the cladding.
Both of these assemblies result in the dew points for varying reletive humidities of 60%, 50%,
and 30% falling inside the stud space, for the design temperatures of 70F inside and 19F
outside.
If the interior vapour/air barrier has openings that allow air transported moisture, or water
vapour into the stud space, condensation will occur inside the extrior wall. With the interior
poly. vapor/air barrier on the inside face of the wall it is difficult for this moisture to dry to the
inside. The plywood is a reletively good air/vapour barrier on the outside face, and prevents
drying in this direction.
It is a difficult challenge to construct a fully functioning interior air/vapour barrier, and there is
likely to be some moisture in the studs and plywood from rain during construction. This
moisture in the wood should be reduced to 17% before interior finshes are intalled.
ROL FIELDWALKER
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 5
EXTERIOR WALL ASSEMBLY
DEW POINT CALCULATIONS
1/2" PLYWOOD SHEATHING
GYPSUM B'D.
INSIDE
INSIDE
70 F
70 F
65
50
45
40
25
TEMPERATURE
GRADIENT THRU WALL
ASSEMBLY / WINTER
HEATING SEASON
10
50
BY CALCULATION THE TEMP AT THE
INSIDE FACE OF PLYWOOD IS 41.43 F
RH AT 30% / DEW POINT = 37 F
MOISTURE BARRIER / ALUMINUM FOIL ON OUTSIDE FACE OF THERMAX
INSULATION
RH AT 60% / DEW POINT = 55 F
DEW POINT = 50F / RH AT 50%
TEMPERATURE GRADIENT THRU WALL
ASSEMBLY / WINTER HEATING
SEASON
BY CALCULATION THE TEMP AT THE
INSIDE FACE OF PLYWOOD IS 52.64 F
35
NO ADDITIONAL PVC VAPOUR BARRIER HERE / THIS STUD SPACE TO BE
ABLE TO DRY TO THE INTERIOR
TAPED 1/2" GYPSUM B'D., PAINTED.
OUTSIDE
DESIGN
TEMP AVG
FOR JAN
VAN.
@ 39 F
20
15
1.5 "THERMAX EXTERIOR INSULATION/ R 10
5
AIR BARRIER MEMBRANE / MOISTURE BARRIER
"DOW WEATHERMATE PLUS HOUSEWRAP"
40
25
OUTSIDE DESIGN
TEMP @ 19 F
3.5 "BATT INSULATION / R 13
IN 2X6 STUD SPACE
45
30
NO ADDITIONAL PVC VAPOUR BARRIER HERE / THIS STUD
SPACE TO BE ABLE TO DRY TO THE INTERIOR
TAPED 1/2" GYPSUM B'D., PAINTED.
20
15
55
RH AT 50% / DEW POINT = 50 F
35
30
60
MOISTURE BARRIER /
ALUMINUM FOIL ON
OUTSIDE FACE OF THERMAX
INSULATION
RH AT 60% / DEW POINT = 55 F
VENTED 1/2" AIR SPACE
55
65
AIR BARRIER MEMBRANE / MOISTURE BARRIER
"DOW WEATHERMATE PLUS HOUSEWRAP"
60
1/2" PLYWOOD SHEATHING
In the summer with air conditioning, outside temp. at 70F, and inside temp. at 65F, RH at 60%, the
Dew Point is approx. 10F below the temperature inside the wall, indicating no condensation will
occur during the cooling season.
TEMPERATURE GRADIENT THRU WALL
ASSEMBLY /SUMMER COOLING SEASON
VENTED 1/2" AIR SPACE
GYPSUM B'D.
10
3.5 "BATT INSULATION / R 13
IN 2X6 STUD SPACE
1.5 "THERMAX EXTERIOR INSULATION/ R 10
5
0
0
WALL COMPONANTS TO SCALE OF THE R VALUE OF THE INSULATION
Maryniak Residence at 160 mountain Drive Lions Bay BC.
Int. Temp 70F / RH 60% / Outside Design Temp 19F
DEW POINT DIAGRAM #3
WALL ASSEMBLY / DIAGRAM #3
1/2" GYPSUM BOARD
R-12 BATTINSULATION
1/2" PLYWOOD SHEATHING
R-10 "THERMAX" INSUL. WITH BONDED ALUMINUM FACE/ TAPED
JOINTS
"WEATHERMATE" HOUSEWRAP/ AIR BARRIER/ MOISTURE RETARDER
1/2" VENTED AIR SPACE
3/4" CEDAR CLADDING
WALL COMPONANTS TO SCALE OF THE R VALUE OF THE INSULATION
Maryniak Residence at 160 mountain Drive Lions Bay BC.
Int. Temp 70F / RH 50% / Outside Design Temp 39 F
DEW POINT DIAGRAM #4
WALL ASSEMBLY / DIAGRAM #3
1/2" GYPSUM BOARD
R-12 BATTINSULATION
1/2" PLYWOOD SHEATHING
R-10 "THERMAX" INSUL. WITH BONDED ALUMINUM FACE/ TAPED
JOINTS
"WEATHERMATE" HOUSEWRAP/ AIR BARRIER/ MOISTURE RETARDER
1/2" VENTED AIR SPACE
3/4" CEDAR CLADDING
Architect's Note:
For the proposed wall assembly shown in Diagrams #3,,an interior design temperature of 70F
and an outside design temperture of 19F have been utilized to calculate the dewpoints . These
parameters are the extreme conditions that may occur briefly on rare occasions. With these
conditions the dew point for interior Relative Humidities of 50% and 60% occur in the stud
space, inside the interior face of the plywood sheathing. With a relative humidity of 30%, the
dewpoint falls outside the sheathing.
In Diagram #4,if the indoor relative humidity is assumed to be 50%, which can be maintained
by the air conditioning system, and the average outdoor tempertature in January in Vancouver
of 39F is used as the outdoor design temperture The dew point falls outside the sheathing.
Temperature values have been assigned in accordance with ASHRAE standards and the
National Research Council Canadian Building Digest "Humidity in Canadian Buildings", by N.B.
Hutcheons. With these conditions the dew point in the wall asembly falls outside the plywood
sheathing.
ROL FIELDWALKER
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 6
EXTERIOR WALL ASSEMBLY
PROPOSED EXTERIOR WALL ASSEMBLY
TYPICAL OVERHANG ASSEMBLY:
'RHEINZINC' FLAT PANELS ON "ENKA-VENT"ON TYP.
MEMBRANE ON 1 1/2" FIR T&G DECK ON 3"X3 1/2" RAFTERS
AT 8" O/C
C
CENTER LINE OF
2X6 STUDS
1'-4"
4'-0"
1'- 4
"
1'-2"
1'- 4
"
1'- 4
4 '-0
"
"
8"
SPRAY ON POLYURETHANE FOAM
INSULATION
AT WALL & ROOF INTERSECTION
8"
8"
7 3/4"
7 1/4"
8"
8"
3 1/2"
11 3
/4"
4"
1 1/2" FIR TRIMS TYPICAL
WIDTH DETERMINED ON SITE
1 3/4"
1'-2"
8"
WINDOW COVERING
5'-3"
TYPICAL EXTERIOR WALL ASSEMBLY:
1/2 GYPSUM BOARD WITH PAINT FINISH ON
2X6 STUDS OR 1X2 STRAPPING @ 16" O/C WITH
3 1/2" FRICTION FIT BATT INSULATION (R-13)
1/2" FIRPLY SHEATHING
1/1/2" "THERMAX" EXTERIOR INSULATION (R-10)56%>20%
1 LAYER OF"WEATHERMATE PLUS" HOUSE WRAP AIR &
MOISTURE BARRIER
JOINTS SEALED WITH "WEATHERMATE STRAIGHT
FLASHING"
1/2" PRESERVATIVE TREATED FIRPLY
VENTED STRAPPING AT 16" O/C,
3/4" CEDAR CHANNEL SIDING WITH 3/4" CHANNEL
TYPICAL EXTERIOR WALL
AT EAST ELEVATION
4 3/4"
10 7/8"
FOAM SILL GASKET UNDER ALL SILL PLATES.
INSULATE TO TOP OF CONC. UPSTAND
1 1/2"
TYP. MAIN FLOOR ASSEMBLY:
STONE TILE ON SCHLUTER SYSTEM ON 2" CONCRETE TOPPING WITH HYDRONIC HEATING ON
SILVERED PAPER ON 3/4" T&G PLYWOOD ON JOISTS .(SEE STRUCTURAL DWGS.)
6" BATT INSULATION TO CRAWL SPACE.
2 3/4"
*N.B.KEEP STRUCTURAL WOOD
0"
MEMBERS MIN 18" ABOVE GRADE (TYPICAL)
1'-0"
45D
1'-5"
3 1/2" FIR OR STONE TILE
BASE TYPICAL
5 1/4"
8"
1 1/2"
3"
7'-2"
8'-4"
3"X8" FIR BLOCKING PIECE BETWEEN 2-3"X8" ROOF
PURLINS, CAULKED & BOLTED CONNECTION.
1 1/2" RIGID OR FOAM INSULATION ON 8" CONC. FOUNDATION PINNED TO ROCK . PARGE
BOTTOM OF EXCAVATION TO GIVE CONTINUOUS POSITIVE SLOPE TO DRAIN TILE. 6"DIA.
PERFORATED DRAIN.
USE "PRTECTOWRAP" 6" DETAIL TAPE TO SEAL BOTTOM OF CONC WALL TO PARGING
CRAWL SPACE
ROL FIELDWALKER
WATERPROOF CONCRETE AS SHOWN ON SECTION
A-A
EXTERIOR WALL SECTION/ MAIN FLOOR
NEAR GRID LINE 'C'
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 7
EXTERIOR WALL ASSEMBLY
PROPOSED EXTERIOR WALL ASSEMBLY / SUMMARY
The proposed exterior wall assembly locates the air barrier, and 56% of the insulation outside the sheathing.
The "Thermax" exterior insulation proposed has an R value of 10, and a water vapour permeance ASTM E96,
less than .03 perms.( BCBC calls for less than 1.05 perms (60 ng (Pa.s.m2).
The air barrier, provided by "Weathermate" a housewrap, has air leakage characteristics 0 .0036 L/sqm. (
BCBC calls for less than 0 .1 l/sqm), CCMC requirement is less than .02@75 Pa). It does a allow vapour
permeance of 312 ng (Pa.s.m2).
The three most important aspects this wall assembly was designed to address are:
1.A very good air barrier to reduce air transported moisture into the wall, as air movement has the
potential to transport much more moisture that vapour diffusion.
It is easier to install and maintain the integrity of an air barrier and moisture barrier on the outside face of
the exterior insulation than on the inside face of studs, where electrical fittings create potential for
breaching this membrane, and the poly must be interwoven through framing members at floor
intersections. This assembly limits air movement, and air transported moisture through the wall .
Moisture moved by vapour diffussion into the wall is reduced by the painted gypsum board( 1140
ng(pa.s.m2) and by the fact that the stud space will be warmer, and the temperature differential driving
the diffusion from the inside is reduced.
2. Arrange the insulation in such a way as to cause the dew point to fall outside the plywood sheathing.
The wall is proposed for a residence where the air conditioning system is expected tol limit the interior
RH to 50%.The dew point tempurature then falls outside the sheathing for a outside design temperture
of 39F.
3.Reduce the thermal bridging through the studs that occur if all the insulation is in the stud space.
The fact that there is R10 insulation outside the studs reduces heat loss by thermal bridging through
the studs. Thermal transfer through the studs can reduce the effective R value of a wall assembly by as
much as 20%.
Conclusion: All the walls considered conform to the BCBC 2006. From the BCBC 2006 requirements for
exterior walls and the resultant calculated dew points,the proposed wall assembly is considered to be superior
to wall assemblies that have all the insulation on the interior of the sheathing and have vapour barriers on the
inside face of the studs.
ROL FIELDWALKER
architect
# 408, 611 Alexander St.
Vancouver, B.C. V6A 1E1
Tel: 604-813-6044
Fax: 604-266-8842
Email: rol@rolf ieldwalker.com
PAGE 7