The Problem With and Solutions for Ventilated Attics

The Problems With and Solutions for
Ventilated Attics
RCI 30TH INTERNATIONAL CONVENTION - SAN ANTONIO, TX
MARCH 9, 2015
GRAHAM FINCH, MASC, P.ENG – RDH BUILDING ENGINEERING LTD.

Presentation Overview
 Current Issues with Ventilated Attics
 Case Study of Repairs
 Attic Roof Hut Research & Monitoring Study –
Key Findings
 Performance of Potential Solutions
 Ongoing Research & Field Trials

Influencers of Moisture Problems in Attics
What influences attic moisture issues &
what can we control by design?
 Roof orientation (solar radiation)
 Roof slope (solar radiation)
 Roofing material/color
 Adjacent buildings – shading
 Trees – shading & debris
 Outdoor climate
 Indoor climate
 Roof Leaks
 Insulation R-value
 Air leakage from house
 Duct leakage in attic
 Duct discharge location
 Vent area and distribution
 Sheathing durability
 Roof maintenance
 Other things

Where are we Seeing the Biggest Issues?
• Air leakage (ceiling details)
• Exhaust duct leaks & discharge location (roof, soffit, or wall)
• Inadequate venting provisions (amount, vent location, or materials)
• Outdoor moisture: night sky condensation on underside of sheathing
• Wetting through shingles/roofing (tipping the moisture balance)

Influence of Solar Radiation & Night Sky Radiation

Industry Trends – Less Heat Flow into Ventilated Attic Spaces
<1970’s attic construction with
excessive air leakage and heat loss
into the attic
1980’s to 1990’s attic construction with
moderate air leakage and heat loss into
the attic
2000’s attic construction with minimal air
leakage and heat loss into the attic

Standard Faith-Based Ventilation Approach
Air-sealing details, duct
exhaust details often not
provided & left up to the
contractor

Alternates to Ventilated Attics
Unvented Hot Roof
(Sprayfoam applied to
underside)
Exterior Insulated &
hybrid approaches

Typical Ventilated Attic Moisture & Mold Issues

Typical Issues – Impact of Orientation
North = Soaked
South (Partially
Shaded) = Damp

The Localized Nature of Air Leakage Condensation

The Localized Nature of Leaking Penetrations

The Localized Nature of Outdoor Moisture Wetting &
Night Sky Condensation

It Happens in Ventilated Low Slope & Cathedral
Ceilings Too

So When Does it Become a Problem?

Case Study: Two Steps Forward, One Step Backwards
• 2007 investigation of 5 yr old large
townhouse complex
• Was experiencing all of the problems we
were and still are currently seeing

An Assortment of Typical Issues – Exhaust Ducts

An Assortment of Typical Issues – Exhaust Duct Details

An Assortment of Issues – Inadequate Ridge ‘Vent’ Material
Almost no effective net
free area, 5 layers of
filter fabric

An Assortment of Issues – Ceiling Air Leakage

Attic Remediation – 2 years Later

Full Review of Initial Contributing Factors, Air-sealing

Retrofit Ceiling Air Sealing - Sprayfoam

Ceiling Air Sealing – Poly Bags of Fun

Mold Remediation - Dry Ice Blasting

New Exhaust Vent Hoods & Attic Ridge Vents

The Localized Nature of Wetting

Water Leaks Around Plugged Dryer Exhaust Ducts

Lack of Dryer Exhaust Duct Maintenance

Soffit Exhaust Vent Hood Configuration Issues

Questioning the Effectiveness of Dry Ice Blasting against Mold?

Water Seepage through Aged & Saturated Asphalt Shingles?

Key Findings from Field Investigations & Field
Monitoring
 Seeing widespread issues with
mold growth in newer wood-frame
attics in Pacific Northwest
 Wetting is exceeding drying
capacity provided by ventilation
 Problem is most often NOT due to
a lack of ventilation
 Usual culprits of air-leakage
condensation (leaky ceiling, leaky
ducts & discharge point)
 Also seeing supplemental exterior
moisture sources (night sky
condensation, rainwater seepage)

Research Study Premise
 Controlled field monitoring study to
isolate exterior wetting
mechanisms from interior sources
(air, vapour)
 To specifically evaluate impact of
orientation, slope (3:12, 4:12 and
6:12) & shingle underlay
 Remove influence of air leakage or
heat gain from house
 Monitor the performance of
surface treatments
Typical ventilated attic
construction with air
leakage and heat loss into
the attic
Theoretical attic with no air
leakage or heat loss into the
attic and unrestricted
ventilation – Study setup
here

Concurrent Companion Research in Pacific Northwest
 Homeowner Protection Office of BC
 RDH – monitoring field conditions & controlled field
study (covered here)
 MH – monitoring of attic moisture levels and air-
leakage from indoors into attic spaces
 FP Innovations – monitoring of effectiveness of various
surface treatments
 BCIT – modeling of attic ventilation rates & moisture
movement

Roof Test Hut Field Monitoring Setup
a) 3:12 Slope roof with roofing felt underlay b) 4:12 Slope roof with roofing felt underlay
c) 6:12 Slope roof with roofing felt underlay d) 3:12 Control roof with SAM underlay

Roof Test Hut Field Monitoring Setup

Monitoring Equipment & Sensors
Moisture Content, Temperature,
Relative Humidity and surface
Condensation sensors – north and
south slopes x 4 huts

0
5
10
15
20
25
30
09-2012
10-2012
11-2012
12-2012
01-2013
02-2013
03-2013
04-2013
05-2013
06-2013
07-2013
08-2013
09-2013
10-2013
11-2013
12-2013
01-2014
02-2014
03-2014
MoistureContent[%]
MC-FULL-S-CONT MC-FULL-S-312 MC-FULL-S-412
MC-FULL-S-612 EMC (1 wk)
Seasonal Roof Sheathing Moisture Contents
EMC calculated - Hailwood and Horrobin (1946)
0
5
10
15
20
25
30
09-2012
10-2012
11-2012
12-2012
01-2013
02-2013
03-2013
04-2013
05-2013
06-2013
07-2013
08-2013
09-2013
10-2013
11-2013
12-2013
01-2014
02-2014
03-2014
MoistureContent[%]
MC-FULL-N-CONT MC-FULL-N-312 MC-FULL-N-412
MC-FULL-N-612 EMC (1 wk)

Seasonal Average Moisture Contents
0
5
10
15
20
25
30
Fall 2012 Winter
2012/2013
Spring 2013 Summer
2013
Fall 2013 Winter
2013/2014
Spring 2014
MoistureContent(%)
MC-FULL-N-CONT MC-FULL-S-CONT MC-FULL-N-312 MC-FULL-S-312
MC-FULL-N-412 MC-FULL-S-412 MC-FULL-N-612 MC-FULL-S-612

Long Term Impacts of Elevated Moisture Contents
North 3:12 after 1 yr North 4:12 after 1 yr

Tracking Mold Growth
TABLE 2: VITTANEN’S MOLD GROWTH INDEX DESCRIPTIONS
INDEX GROWTH RATE DESCRIPTION
0 No growth Spores not activated
1 Small amounts of mold on surface (microscope) Initial stages of growth
2 <10% coverage of mold on surface (microscope) ___
3 10% – 30% coverage of mold on surface (visual) New spores produced
4 30% – 70% coverage of mold on surface (visual) Moderate growth
5 >70% coverage of mold on surface (visual) Plenty of growth
6 Very heavy and tight growth Coverage around 100%

Tracking Mold Growth – Year 1

Why is the Sheathing Wet? What is the Mechanism?
Night sky condensation!
Radiative heat loss from roof
surface to colder night sky

When Does it Occur?
0
100
200
300
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
Sol
Solar Radiation CONDENSE-Plywood-312
Increasin
0
5
10
15
20
25
30
MoistureContent[%]andTemperature[°C]
Condensation
0
5
10
15
20
25
30
MC-FULL-N-312 MC-IN-SURF-N-312 T-IN-N-312-Plywood
T-N-312-Embedded Outdoor - Temperature Outdoor - Dewpoint
COND-N-312 Sheathing Solar Radiation
Condensation

When Does it Occur on the Underside of the
Sheathing?
0
100
200
300
400
SolarRa
IncreasingSurf
0
5
10
15
20
25
30
Condensation
0
5
10
15
20
25
30
Condensation
0
100
200
300
400
500
600
700
800
900
1000
SolarRadiation[W/m2]
IncreasingSurfaceCondensation
25
30
perature[°C]
25
30
perature[°C]
0
100
200
Incre
0
5
10
15
20
25
30
Condensation
0
5
10
15
20
25
30
Condensation
Dry
Heavy Condensation
Light Condensation
Daily Solar
Radiation Cycles

Some Nuances of Condensation on Plywood

Nuances of Mold Growth on Plywood Heartwood vs Sapwood

Monitoring Night Sky Cooling Impacts

Shingle & Sheathing Temperature Depressions –
Night Sky Cooling
2.6 2.5 2.5
2.6
2.4 2.5
0.9 0.9 0.9 1.0 1.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
312-N 312-S 412-N* 412-S 612-N 612-S
TemperatureDepressionfromAmbient
(°C)
Avg Shingle T Avg Interior Sheathing Surface T
*412-N sheathing temperature unavailable due to sensor malfunction
4.5°F
On top of
shingles
<2°F
At interior
of roof
sheathing
Average Shingle and Sheathing Temperature Depression compared to Ambient
Temperature for Winter (December 1, 2013 to January 31, 2014).

Hours of Potential Condensation - Sheathing
0
100
200
300
400
500
600
700
800
0.00
0.25
0.50
0.75
1.00
1.25
1.50
North South
HoursofPotentialCondensation
TemperatueDepressionfromAmbient(°C)
3:12 Temp 4:12 Temp 6:12 Temp 3:12 Hours 4:12 Hours 6:12 Hours
<2°F temperature drop
150 to 300 hours per year

On the Flip Side: Solar Heat Gain & Drying during the
Day

On the Flip Side: Solar Heat Gain & Drying Potential
0
5
10
15
20
25
30
35
40
45
50
55
60
Aug 21 00:00 Aug 21 06:00 Aug 21 12:00 Aug 21 18:00 Aug 22 00:00
Temperature[°C]
Temperatures - 3:12 and 6:12 Slope Roofs - Early Spring Conditions
T-OUT-N-612-Shingle
T-OUT-S-612-Shingle
T-IN-N-612-Plywood
T-IN-S-612-Plywood
T-OUT-N-312-Shingle
T-OUT-S-312-Shingle
T-IN-N-312-Plywood
T-IN-S-312-Plywood
Outdoor - Dewpoint
Outdoor - Temperature
South Shingles
130 -140°F
North Shingles
100-110°F
Ambient Air
up to 79°F
South Sheathing = 98°F
North Sheathing = 90°F

Shingle and Sheathing Temperature Rise – During the
Winter
1.9
5.8
1.7
6.5
1.7
6.6
0.9
1.7
0.9
2.0
1.7 1.8
0
1
2
3
4
5
6
7
312-N 312-S 412-N 412-S 612-N 612-S
TemperatureRiseFromAmbient(°C)
Avg Shingle T Avg Interior Sheathing Surface T
Shingle Surface
Sheathing
~11°F
~4°F

Impact of Impermeable SAM vs Permeable Roofing
Felt Underlay

0
5
10
15
20
25
30
Oct 17 Nov 14 Dec 12 Jan 09 Feb 06 Mar 06 Apr 03 May 01 May 29 Jun 26
MoitsureContent(%)
MC-OUT-S-CONT MC-OUT-S-312
South Slope - Fall to Spring
Impermeable SAM vs Permeable Roofing Felt Underlay
0
5
10
15
20
25
30
Oct 17 Nov 14 Dec 12 Jan 09 Feb 06 Mar 06 Apr 03 May 01 May 29 Jun 26
MoitsureContent(%)
MC-OUT-N-CONT MC-OUT-N-312
Permeable Felt
Impermeable SAM
North Slope - Fall to Spring

Okay But.. What Happens when Shingles Start to
Leak/Seep - Long Term?

Key Findings – The Cause of the Problem in the
Pacific Northwest
 Roof sheathing in well ventilated attics (also soffits, canopies)
experiences elevated moisture levels in winter
 Occurs despite elimination of typical wetting mechanisms within
attics (air leakage, duct leakage, rain water leaks etc.)
 Moisture level is above equilibrium level indicating additional wetting
sources
 Night sky cooling causes wetting when sheathing drops below ambient
dewpoint (few hundred hours per year) in attic
 Difficult to stop it from occurring
 Fungal growth occurs due elevated moisture and condensation on
underside of sheathing
 More fungal growth on north than south – drying matters, the heat
from a ceiling above a house will also help too

Monitoring of Potential Mitigation Strategies
 Vented underlayment – de-
couple night sky radiation
cooling effects
 Surface treatments to kill &
prevent fungal growth

Night Sky Radiation De-Coupler?
 Vented shingle underlay
installed in one roof in 2nd year
of study
 Purpose: try to de-couple
night sky cooling effects
from sheathing

Vented Underlay – The Double Edged Sword
South Orientation – Vented Underlay vs Direct Applied Shingles
0
50
100
150
200
250
300
350
400
450
-5
0
5
10
15
20
25
Jan 16 Jan 17 Jan 18
SolarRadiation(W/m2)
Temperature(°C)
T-OUT-S-VENT (Shingles) T-IN-S-VENT (Sheathing)
T-OUT-S-312 (Shingles) T-IN-S-312 (Sheathing)
Outdoor T T-Drainmat-S-VENT
Solar Radiation
Shingles – direct applied
Shingles – vent mat
12°F drop in
shingle
3°F drop in
sheathing

Vented Underlay – The Double Edged Sword
0
2
4
6
8
10
12
14
16
6
8
10
12
14
16
18
20
22
Day Night Avg
Temperature(°C)
MoistureContent(%)
Spring
VENT-N-MC VENT-S-MC 312-N-MC 312-S-MC
VENT-N-T VENT-S-T 312-N-T 312-S-T
Vent Normal
North
South
North
South
Spring (March 1, 2013 to April 30, 2013) Diurnal Moisture Content (SURF)
and Temperature Averages for North and South Oriented Vented and
Control (Direct Applied Shingles) Roof Assemblies

Round 1 - Surface Treatment Application
 4 huts x 2 orientations = 8
applications of each
 Fungicides, Cleaners, Sealers
› Boracol® 20-2
› Boracol® 20-2 BD
› Bleach
› Thompson’s WaterSeal®
› Kilz® Paint
 Wood Preservatives
› Copper Naphthenate
› Zinc Naphthenate

Wood Preservative & Fungicide Surface Treatments
When Applied 1 Year Later
Fungal Growth observed is Cladosporium

Visual Assessment of Surface Treatment Efficacy
VISUAL ASSESSMENT OF SURFACE TREATMENT EFFICACY
Test
Roof
Surface Treatments (north left, south right)
Sansin
Boracol®
20-2
Copper
Naphthenate
Bleach
Thompsons
WaterSeal®
Kilz®Paint
Zinc
Naphthenate
Sansin
Boracol®
20-2BD
Control
3:12
4:12
6:12
VISUAL ASSESSMENT SCALE
Pristine or very light fungal growth
Moderate fungal growth
Significant fungal growth
Hence need for duplicate samples – slope not a big factor (heartwood vs sapwood is)
In our experience Kilz® & Boracol® 20-
2BD while okay here after 2 years may
not be best long term for fungal growth

Wood Preservative & Fungicide Surface Treatments
The best decay fungicide (Boracol 20-2BD) & surface
paint (Kilz) looked okay in years 1 and 2 – but not in year
3

A Caution with Surface Treatments

Summary – Mitigation Strategy Performance
 Thermally de-coupling the exposed shingles from the sheathing did
not work well
 Did not significantly “warm” sheathing temperature at night
 On flip-side – the sheathing did not get as hot during the day, so
less drying and resulting prolonged wet periods
 Surface treatments appear to be a potential viable solution if right
product is developed - current products not quite effective (nor
developed specifically for this application)
 Concurrent work by FP to perform accelerated testing of some new
biocides/fungicides
 Ongoing monitoring at our huts to monitor long-term field
performance of next generation treatments

Concurrent Fungicide/Biocide Research –
FP Innovations
Images courtesy FP Innovations
 Developed an
accelerated 12 week test
method to evaluate new
fungicides applied to
wood products
 Have already tested a
handful of newly
innovated & proprietary
fungicides & coatings
 A few promising
formulations completely
prevented mold growth
 Follow-up with field
testing

Round 2 - Field Trials & Monitoring of New Surface
Treatments

Nine New Treatments Applied to Both Cleaned &
Moldy Sheathing

Adapting Surface Treatments for the Field (Underside
Application)

Ongoing Monitoring of New Surface Treatments
Most Promising
fungicide/biocide is
water repelling &
contains several “active”
ingredients to prevent
long term mold growth.
Note it is not a wood
preservative it is a
fungicide
Currently undergoing
environmental testing &
available soon?

What A Real Roof Leak Ends up Doing to Sheathing
(6 Months)

Final Thoughts
 Ventilated attics & roof assemblies ‘built to code’ are experiencing
mold growth on underside of sheathing (plywood or OSB)
 Wetting from night sky condensation and may be exacerbated
by air leaks & water leaks
 Hard to reliably stop night sky condensation
 More ventilation makes it worse, less may also
 Mold growth may be minor but perceived as risk
 Could build other attic assemblies but unlikely to replace
ventilated attics any time soon
 Need to address durability & sensitivity of wood based sheathings
to mold growth
 Just make sure the fungicide is no more harmful to humans
that the mold is

 rdh.com
Discussion + Questions
Graham Finch – gfinch@rdh.com – 604-873-1181

The Problem With and Solutions for Ventilated Attics

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