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Rainscreen Performance Monitoring:Continuing Research
Current Masters Thesis Research Highlights
Presented by:
Graham Finch, Dipl.T, BASc
University of Waterloo, MASc Student
May 2006 - BCBEC Symposium 2
Introduction
Background Current Research Highlights
Exterior Gypsum Hygrothermal Modeling Building 3 – A Case Study
Monitoring Program Improvements Still to Come
May 2006 - BCBEC Symposium 3
Background
Building Monitoring Program RDH Building Engineering (RDH) Canadian Mortgage and Housing Corporation
(CMHC) Homeowner Protection Office British Columbia Housing Management Commission
Designed and installed on five buildings in Vancouver, BC being constructed or rehabilitated using a rainscreen wall assembly.
Data collected includes temperature, relative humidity, moisture content, wetness, pressure, wind, rain, and driving rain.
May 2006 - BCBEC Symposium 4
Background
University of Waterloo MASc Student Rainscreen Performance Monitoring Study part
of Graduate Thesis work Build on initial RDH work Further work as part of thesis
Further data analysis Trends, Normals, Abnormalities Wetting and Drying Rates
Hygrothermal Modeling Validation Material Testing
May 2006 - BCBEC Symposium 5
Presentation Outline – Research Highlights
Measuring moisture content of exterior gypsum using electrical resistance
Hygrothermal modeling of ventilated rainscreen walls Seasonal performance of Building 1 Improving performance by design
Building 3: A case study Field validation of monitored results
May 2006 - BCBEC Symposium 6
Exterior Gypsum Sheathing Properties
Purpose Measure performance of exterior fiberglass
faced gypsum exposed to humid conditions Correlate electrical resistance of gypsum with
gravimetric moisture content Well established correlation for wood More difficult with gypsum
Provide approximate sheathing moisture contents for Buildings 3 and 5 to assess performance
May 2006 - BCBEC Symposium 7
Exterior Gypsum Sheathing Properties
Physical Properties Strength loss with elevated moisture
content As a result of high relative humidity or
liquid water exposure Levelton study results (Later today)
Other Issues Mould Growth Corrosion when in contact with metals
ie. Steel studs
May 2006 - BCBEC Symposium 8
Why does it matter?
Significant strength loss with as little as 1 - 2% moisture content
Saturated = Destroyed
Exposed to 100% RH for 1 year
May 2006 - BCBEC Symposium 9
Mould Growth
Possible under humid conditions and prolonged periods of time
4 years
4 months
May 2006 - BCBEC Symposium 10
Sorption Isotherm for Fiberglass Faced Exterior Gypsum
0
1
2
3
4
5
6
7
8
9
10
0 10 20 30 40 50 60 70 80 90 100
Relative Humidity
Mo
istu
re C
on
ten
t (%
)
When is it an Issue?
May 2006 - BCBEC Symposium 11
How Long does it take?
Gypsum boards relatively permeable to water vapour 1000-2000 metric perms
Fast response to moisture Wetting - 2% moisture content
increase (from dry) in 2 days exposed to 100% RH
Even faster drying rates Likely prevent very high MC levels from
being achieved in the field
May 2006 - BCBEC Symposium 12
Moisture Content versus Time - 100% Relative Humidity
0.0%
0.5%
1.0%
1.5%
2.0%
2.5%
3.0%
3.5%
4.0%
4.5%
0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384
Hour from beginning of test
Mo
istu
re C
on
ten
t
1 week 2 weeks
May 2006 - BCBEC Symposium 13
Wetting (50 to 100% RH) and Drying (100% to 50% RH) Rate Comparison
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.028
Nov
29 N
ov
30 N
ov
01 D
ec
02 D
ec
03 D
ec
04 D
ec
05 D
ec
06 D
ec
07 D
ec
08 D
ec
09 D
ec
10 D
ec
11 D
ec
12 D
ec
13 D
ec
Time - Days
Res
ista
nce
log
(Oh
ms)
0%
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
MC
%
Wetting to 100% RH
Drying to 50% RH
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Moisture uptake rate much slower than drying rate
10 days to wet, 1 day to dry half
May 2006 - BCBEC Symposium 14
Moisture Content and Electrical Resistance
Used to correlate measured electrical resistance (ohms) with an approximate gravimetric moisture content for field monitoring studies
Determine “how wet” the gypsum is without destructive testing
Handheld moisture meters give only relative idea of moisture content Different meters, different scales
May 2006 - BCBEC Symposium 15
Moisture Content vs Log Resistance for Fiberglass Faced Exterior Gypsum
0%
2%
4%
6%
8%
10%
12%
14%
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0
Log Resistance - Log(Ohms)
Gra
vim
etri
c M
ois
ture
Co
nte
nt
Critical Moisture Content (2%):
Below 5 log Ohms (100 Kohms)
May 2006 - BCBEC Symposium 16
Hygrothermal Modeling
Purpose: To correlate field results with those predicted by hygrothermal simulation
Can we accurately model walls with ventilated claddings? ie Rainscreen Walls Can you accurately model a 2D problem with
1D software? Ventilation cannot be neglected
Current software has limitations
May 2006 - BCBEC Symposium 17
Modeling Requirements
Modeling ventilated wall assemblies with 1D software Cladding input into model with an
“effective permeance” which accounts for an assumed ventilated rate through cladding vent openings
Literature available for equivalent permeance values typically in range of 1000 + perms depending on flow rate
May 2006 - BCBEC Symposium 18
Modeling Requirements
Modeling correlation with field results Effective permeance method works on
average however: Ventilation is a dynamic variable - Wind
and temperature differences drive pressures which change on a daily basis
Better correlation achieved by using actual temperature/relative humidity values from ventilated cavity/drainage space
May 2006 - BCBEC Symposium 23
Discussion of Results
Stucco, Vinyl, and Cement board clad buildings all had similar annual trends and similar moisture levels of the sheathing
High RH (80-100%) and cool temperatures in the ventilated cavity space result in sheathing moisture contents between 20-25% during winter months
May 2006 - BCBEC Symposium 24
Discussion of Results
Correlation of hygrothermal simulation with field data is good
Material properties are important to correlation Moisture Isotherm for plywood/OSB
have direct impact on results
May 2006 - BCBEC Symposium 25
Uses for Hygrothermal Modeling
How can we improve the performance of ventilated rainscreen walls? Insulated Sheathing Is a polyethylene vapour barrier
required? Would painted drywall work instead?
What is the impact of the indoor relative humidity and temperature?
May 2006 - BCBEC Symposium 26
Can Insulated Sheathing Improve Performance?
Base case R-19 (2x6 wall) Compare to R-12 (2x4 wall) R-19 stud insulation plus vapour
permeable R-8 insulation on exterior (no poly)
R-12 stud insulation plus vapour permeable R-8 insulation on exterior (no poly)
Vapour permeable R-12 on exterior only (no stud space insulation, no poly)
May 2006 - BCBEC Symposium 29
Role of Vapour Control Strategy
Typical R-19 insulated wall assembly (ventilated rainscreen)
Remove interior polyethylene vapour barrier
Use 50, 250 and 400 metric perm vapour retarding paints on drywall
May 2006 - BCBEC Symposium 31
Impact of Interior Conditions
250 metric perm paint layer (interior latex paint)
3 indoor cases analyzed using real vapour pressure data for Vancouver Poorly ventilated (avg. winter RH 57%) Building 1 as measured (avg. winter RH
39%) Well ventilated (avg. winter RH 34%)
May 2006 - BCBEC Symposium 34
Other Simulated Cases
OSB vs. Plywood, negligible difference in RH or MC results Using standard OSB and Plywood
properties from WUFI 3.3 database
May 2006 - BCBEC Symposium 35
Results
Insulated sheathing improves the performance of ventilated rainscreen walls
A paint vapour retarder can be used as a replacement for poly, however exterior insulation and designed ventilation are both required
May 2006 - BCBEC Symposium 39
Problems
High relative humidity within stud space 80-100% during winter months (All 8 monitored locations)
Corresponding high moisture content of fiberglass faced exterior gypsum
Interior suites – High relative humidity during winter (50-70%)
May 2006 - BCBEC Symposium 43
Field Openings
Interior openings made in January 2006 During seasonal period of elevated
moisture levels within wall assembly Confirm presence of moisture within
stud cavity Observe interstitial wall conditions
after 4 years of service
May 2006 - BCBEC Symposium 49
Suite Observations
Interior of all suites had high interior relative humidity
Condensation on window frame and glazing surfaces
Mould growth on interior drywall surfaces at corners
May 2006 - BCBEC Symposium 50
Wall Opening Observations
Openings confirmed fiberglass faced exterior gypsum is getting wet 80-100 relative moisture level (Delmhorst BD-
10) Calculated 1-2% moisture content (up to 6% in
some locations) Surface corrosion on steel studs Sensors are returning valid data Problematic details also contributing to
moisture problems (thermal bridging)
May 2006 - BCBEC Symposium 51
Corner Detail
Condensation on steel studs and gypsum sheathing observed
May 2006 - BCBEC Symposium 52
Thermal Modeling
Interior 19C, Exterior 5C
Interior Dewpoint 10C
Temperature Isotherms – THERM 5.2
Failed Air Barrier @ Corner = Condensation
May 2006 - BCBEC Symposium 53
What went wrong?
High interior relative humidity/dewpoint during the winter – Poor Ventilation
When building was retrofit in 2002, original R-8 insulation was left in stud cavity and polyethylene vapour barrier was removed
Wall Design Flawed? Morrison Hershfield (next) will talk about
potential rehabilitation strategies and improvements to mitigate the high wintertime RH in next presentation
May 2006 - BCBEC Symposium 54
Prevention by Design?
Use hygrothermal modeling (WUFI) to analyze the impact of modifications on the original design What if the poly was left in? What if the batt insulation were
removed? Vapour Permeable Air/Water Resistant
Barrier in lieu of peel and stick
May 2006 - BCBEC Symposium 55
Hygrothermal Results
Leave in Poly – dual vapour barrier, in theory would work (perfect system) however in practice would fail, small leaks
Remove R8 batt Insulation – increases the temperature of the sheathing and improves drying
Trowel or Spray applied Air/Vapour/Moisture membrane (300-600 metric perms) in lieu of peel and stick - improves drying even with high indoor RH
Must improve indoor ventilation – lower RH during the winter
May 2006 - BCBEC Symposium 56
Monitoring Program Suggestions
Data Collection interval (1 hour vs. 15 minute)
All data collected with loggers No separate Hobos for interior or
exterior data Collect Solar Radiation Data Monitor all elevations, not just wind-
driven rain exposed, ie North
May 2006 - BCBEC Symposium 57
Research Still to Come
Analyze wetting events and material response
Analyze drying rates Further hygrothermal modeling Final report of results and
recommendations