Anik Teasdale-St-Hilaire, B. Eng., Ph.D., Morrison Hershfield

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Investigating the role of the vapour retarder in
the drying response of wood-frame walls wetted by
simulated rain infiltration

• Anik Teasdale-St-Hilaire, B. Eng., Ph.D.,
  Morrison Hershfield
• Dominique Derome, arch., Ph. D., Concordia
  University
• Paul Fazio, Ph. D., Concordia University
• May 25, 2006

British Columbia Building Envelope Council
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Outline

• Introduction
• Overview of experimental study
• Experimental facility
• Experimental protocol
• Experimental results
• WUFI simulations Montreal Vancouver
• Conclusion
• Discussion

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Introduction

• Vapour barriers were introduced to reduce flux of
  vapour diffusion, thus reducing interstitial
  condensation
• Required by building codes, e.g. NBCC
• However, there are other sources of moisture
• construction moisture
• air migration leading to condensation
• wind-driven rain infiltration
• Vapour barriers reduce ability of envelopes to
  dry by inward diffusion

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Overview of experimental study

• Study to investigate role of vapour retarder in
  drying response of wood-frame walls
• Experiment performed at Building Envelope
  Performance Laboratory, Concordia U.
• 6 large-scale wood-frame walls
• Pre-wetted bottom plate insert initial M
  source
• Spring weather in Montreal
• Duration 35 days
• Experimental variables type of sheathing
  wood-based sheathing, and type of vapor retarder

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Experimental Facility
Environmental Chamber, Building
Envelope Performance Laboratory, Concordia U.
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Experimental protocol
Monitoring in each wall 7 thermocouples 1 RH
sensor 6 MC gravimetric samples 1 DP
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Experimental protocol - (contd)
Bottom plate insert
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Experimental protocol (contd)
Outside

• From outside to inside
• Sbpo membrane
• exterior sheathing
• 38 mm x 140 mm wood studs
• glass fiber insulation
• 13 mm gypsum board
• primer (one coat) and latex paint (2 coats)

Inside
Polyethylene membrane, permeance 3.4 ng/m2sPa
Low permeance primer paint, permeance 35
ng/m2sPa
Typical wall assembly plan view
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Experimental protocol (contd)
Wall no. Sheathing Vapor retarder
1 OSB PE
2 Plywood PE
3 Asphalt-coated fiberboard PE
4 OSB Low perm. primer
5 Plywood Low perm. primer
6 Asphalt-coated fiberboard Low perm. primer
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Experimental protocol Wetting methodology

• based on results of 2 previous experiments
• simulated rainwater infiltration into back wall
  leads to MC accumulation in bottom plate and
  sheathing at bottom of wall
• partial immersion of six 38 x 140 x 360 mm3
  bottom plate inserts in 13 mm deep pool of water
• lead to MC in inserts of 53.0 56.8 MC

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Experimental protocol Climate loading
Period simulated Duration days Exterior T oC Exterior RH Exterior RH Interior T oC Interior RH Average DPv Pa
April 28 1.6 to 10.9 1.6 to 10.9 64 21 40 30 to 400
May 7 8.6 to 18.7 8.6 to 18.7 63 21 43 -90 to 200
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MC results of bottom plate insert for OSB walls
1 4
56.0
D 3.0
53.0
26.7
D - 4.0
22.7
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Mass loss results of bottom plate insert for OSB
walls 1 4
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MC results of bottom plate insert for plywood
walls 2 5
54.9
53.1
D 1.8
21.9
21.4
D 0.5
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MC results of bottom plate insert for fiberboard
walls 3 6
56.8
D3.2
53.6
20.7
D3.2
17.5
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Mass loss results of bottom plate insert for
fiberboard walls 4 6
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MC results explanation

• the results show that the role of the vapor
  retarder depends on the type of sheathing

Derived from Kumaran et al. 2002
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Experimental results of sheathing samples
D2.2
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Experimental results Pv in stud cavity above
bottom plate OSB walls
in
out
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Experimental results RH on surface of
OSB-sheathed walls
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Experimental results Pv in stud cavity above
bottom plate fiberboard walls
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Experimental results RH on surface of
fiberboard-sheathed walls
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WUFI simulations Montreal vs. Vancouver

• Typical wall assembly residential construction
• non-hygroscopic cladding
• air space
• spun-bonded polyolefin
• plywood sheathing
• 150 mm (6) fibreglass batt insulation
• polyethylene membrane, 6 mil
• gypsum board, primed and painted

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WUFI simulations Montreal vs. Vancouver
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WUFI simulations Montreal vs. Vancouver
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WUFI simulations Montreal vs. Vancouver
Exterior plywood surface
Interior plywood surface
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Vancouver vapour retarder vs. no vapour
retarder
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Vancouver vapour retarder vs. no vapour
retarder
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Conclusion

• Experiment performed to test role of vapour
  retarder in hygrothermal response of wood-frame
  wall
• Source of moisture was a pre-wetted bottom plate
  insert that was used to simulate the effects of
  wind-driven rain infiltration

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Conclusion

• Influence of vapour retarder permeance on
  moisture behaviour
• for OSB-sheathed walls significant influence gt
  compare avg. drying rate of
• 0.96 MC/day (wall with PE)
• vs.
• 0.75MC/day (wall with low perm. primer)
• for in plywood-sheathed walls insignificant
  influence, as shown by MC results in insert and
  sheathing, and in pv cavity measurements
• for fiberboard-sheathed walls influence very
  slightly significant

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Conclusion

• Surface water absorption of sheathing had an
  impact on the performance lower absorption
  increased stud cavity RH
• All walls showed steady drying patterns where if
  the test had been prolonged, the bottom plate
  inserts would have reached acceptable MC, i.e. lt
  20 MC, reducing risks of damage
• WUFI simulations for Vancouver show use of vapour
  retarder does decrease winter wetting by
  diffusion (as expected)

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Discussion