Fact or Fiction - Cool Roofs Cause Condensation? · Fact or Fiction - Cool Roofs Cause Condensation? Phil Dregger, PE, RRC, FRCI Pacific Building Consultants Technical Roof Services,

Fact or Fiction - Cool Roofs

Cause Condensation?

Phil Dregger, PE, RRC, FRCI

Pacific Building Consultants

Technical Roof Services, Concord, CA

July 17, 2012

International Roof Coatings Conference, Baltimore, Maryland

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Phil Dregger, PE, RRC

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Walking the Tight Rope…

“West Coast” Roof Deck Construction

Roof Membrane

OSB/Plywood Sheathing

F/G Batt Insulation w Facer (~retarder)

Not Vented to Exterior.

Typical Winter “Wetting”, Summer “Drying”

Often Delicate Balance Avoids Problems (e.g., inside RH, reflectance, air exchange (HVAC), air intrusion).

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“Cool” Reroof – Balance Breaker?

Old non-cool BUR got hot, helped drying.

New “cool” roof stays much cooler, slows

drying (including drying of “leaks”).

Anecdotal Evidence – Often this makes

the difference between wood decks that

barely drying out and those that develop

fungal growth and/or deteriorate.

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Condensation Control

Roofs and walls need to be designed/constructed to

avoid excessive accumulation of condensation.

Cold side vapor retarders (e.g., roof coverings)

coupled with air intrusion from below can result in

severe condensation – even in moderate climates.

It only gets worse if you add a “cool” roof,

a M.A. S/Ply, or some small roof leaks.

Today focus on potential impacts of installing

“cool roofs” on low-slope commercial roofs over

wood decks with insulation below.

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Water Vapor Condenses

On Cold Surfaces.

Air can hold only so much

water vapor.

Warm air can hold more water

vapor than cold air.

Two Glasses – one with ice

and one without. Water vapor

condenses only on the sides of

the glass with the ice, because

the air next to the that glass is

cooler (below its dew point).

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Condensation Analysis - Winter (ASTM C755 procedures)

(60°F, 54%)

(70°F, 38%)

Lack of “continuity”.

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Max Vapor Pressure

(100% RH)

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Strategy 1 – Install Vapor Retarder (or dehumidify)

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0.1 perm

V-Retarder

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Strategy 2 – Ventilate Below Deck (with outside air)

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Strategy 3 – Add Insulation Above

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~R19

Would adding a vapor retarder and/or

below deck venting have helped?

Probably. Were they required? Maybe, maybe not.

One thing IS clear…the roof system “designer” is

responsible to comply with applicable codes...

Ventilation of “enclosed” rafter spaces.

Vapor retarders in certain constructions.

Seals against air leakage in building envelope.

Projects w/o Architect, Specifier, or Consultant…

Who is the designer - Owner? Contractor? Manufacturer?

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2009 Int’l Code Council (ICC) Codes

Attics and Enclosed Rafter Spaces.

2009 Int’l Building Code (~2010 CBC)

Chapter 12, Interior Environment

1203.2 Attic Spaces - Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof framing members shall have cross ventilation for each separate space… [1503.5 refers to…]

What exactly is “an enclosed rafter space”

Does reroofing require retrofitting to comply?

Ounce of Prevention Tip

When in doubt… obtain interpretation of AHJ.

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2009 Int’l Code Council (ICC) Codes Vapor Retarders – Definitions

2009 Int’l Building Code (IBC)

202 Definitions - Vapor Retarder Classes

Class I: 0.1 perm or less (e.g., polyethylene sheet).

Class II: 0.1 < perm ≤ 1.0 perm (e.g., kraft facer).

Class III: 1.0 < perm ≤ 10 perm (e.g., latex paint).

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2009 Int’l Building Code

Chapter 15, Roof Assemblies…

Definitions – A roof assembly includes a

“vapor retarder”.

Rest of Chapter - Silent regarding vapor

retarder requirements.

Tip – Check for local amendments.

2009 Int’l Code Council (ICC) Codes Vapor Retarders – Roofs

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2009 Int’l Energy Conservation Code

Silent regarding vapor retarder requirements in roofs except:

Table 502.2(2), metal roofs with glass fiber batts

between purlins, “continuous vapor retarder… below

purlins...”

Includes consistent requirements to seal

building envelope against air infiltration

(~air retarders or vapor retarders).

2009 ICC Codes (~2010 CA Energy Code)

Vapor Retarders – Roofs

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2009 Int’l Energy Conservation Code

Chapter 5, Commercial

502.4 Air Leakage – The building thermal envelope

shall be durably sealed to limit infiltration… The

following shall be caulked, gasketed… or otherwise

sealed…

(1) All joints, seams, and penetrations.

(10) Attic access openings.

(11) Rim joints.

2009 ICC Codes (~2010 CA Energy Code)

Vapor Retarders / Air Leakage

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Air Leakage/Infiltration

Importance of Energy Code requirements to seal

“joints, seams, and penetrations” in building

envelopes under-appreciated, if even “known”.

Vapor Retarders - Definitions provided.

Limited where/when required in walls.

Very limited where/when required in roofs.

Guidelines [and reminders] Needed.

Summary - 2009 ICC Vapor Retarders / Air Leakage

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Vapor Retarder Guidelines

NRCA (one of oldest, useful, not discussed here)

Cold Regions Research and Engineering Laboratory (CRREL)

Oak Ridge National Laboratory (ORNL)

ANSI/ASHRAE Standards (e.g., resultant RH)

Guidelines Useful but NOT Requirements (pertinent to SOC question but not requirements

unless referenced in code or required by contract)

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CRREL – Retarder Guidelines:

Depending on inside relative humidity and location.

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ORNL Guideline - WUFI Program

Avoid High Rh and H20 Accumulation

Hygrothermal model.

Considers changes in material properties with changing temp/Rh.

Provides temperature and Rh data throughout assembly and at monitor locations.

Allows comparative trials...

Download Research and Education Version:

www.ornl.gov/sci/btc/apps/moisture/ibpe_sof161.htm

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http://www.ornl.gov/sci/btc/apps/moisture/ibpe_sof161.htm

Guidelines Useful But Be Careful

Above Guidelines have proven reasonable over

time and are reliable in terms of judging basic

“need” (or not) for vapor retarders.

Be Careful - WUFI and other less sophisticated

roof design guidelines assume no air intrusion.

Decisions based on them may be less than

conservative if (or where) assumptions not true.

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West Coast Roof Deck Construction

Roof Membrane





Sometimes Delicate Balance Avoids Problems (e.g., inside RH, reflectance, air exchange (HVAC), air intrusion).

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One Year Old, Unoccupied Warehouse/Office Springtime Dripping Water (+after insulation retrofit)

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One Year Old, Unoccupied Warehouse/Office Springtime Dripping Water (also after retrofit)

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One Year Old, Unoccupied Warehouse/Office Springtime Dripping Water (also after retrofit)

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Start sheathing at 28%

Start sheathing at 0% (overlay)

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Water Not From Original Sheathing

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Construction Along Truss Purlins

foil facer

f/b batt

BUR, nailed base

plywood/OSB

steel truss

wood block

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Best Explanation: Localized Winter Wetting

colder, less humid, some wind

warmer, more humid, no wind, “positive” pressure

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Saturated, ≥130°F at deck

Cooler, less humid inside

Best Explanation: Localized Spring Dripping (early warm weather)

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Vapor Condenses On And

Drips-Off Metal Surfaces

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“Air movement carrying the water vapor with it

is far more powerful in transporting water vapor”

[than diffusion]. (Chapter 21, page 4)

“The relative amounts of water deposited in a wall

or roof by vapor diffusion or by a current of air

cannot be calculated with certainty. Under one set

of conditions, six or seven times as much water can

be deposited as a result of air leakage as by vapor

diffusion… Under different circumstances, the rates

could be 100:1 or higher.” (Chapter 21, page 5)

Cautions about air intrusion are not new.

1989 ASHRAE Fundamentals…

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Barrel Roof – Openings (for air intrusion)

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West Coast Roof Deck Construction

Roof Membrane





Sometimes Delicate Balance Avoids Problems (e.g., inside RH, reflectance, air exchange (HVAC), air intrusion).

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“Cool” Reroof – Balance Breaker?

Old non-cool BUR got hot, helped drying.

New “cool” roof stays much cooler, slows

drying (including drying of “leaks”).

Anecdotal Evidence – Sometimes makes the

difference between wood decks barely

drying out and those that deteriorate /

develop fungal growth.

The physics agrees…

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WUFI – BUR vs. “Cool” BUR

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WUFI – BUR vs. “Cool” BUR

Bottom of Plywood - Rh Mold Growth ≥80% Rh, >41°F

BUR Cool-BUR

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ANSI/ASHRAE Standard 160 (2009) - Criteria

for Moisture-Control Design Analysis in Buildings

Section 6 “Moisture Performance Evaluation Criteria” (as revised by Addendum A, issued late 2011).

…keep 30-day average RH < 80%

when temperatures 41°F - 104°F

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This Speaker’s Suggestions

Ask about reports of Spring roof leaks or dripping.

Comply with codes – confirm local amendments and/or code

interpretations (e.g., venting of enclosed rafter spaces).

Advise owner of condensation sometimes associated with

cool roofs, recommend adding insulation above roof deck.

Seal openings that allow air intrusion into insulated spaces, install separate air retarders where needed.

Install vapor retarders only where required/needed.

Obtain assistance with mechanical systems – an important component.

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Questions?

The End - Thank You.

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Documents

Fact or Fiction - Cool Roofs Cause Condensation? · Fact or Fiction - Cool Roofs Cause Condensation? Phil Dregger, PE, RRC, FRCI Pacific Building Consultants Technical Roof Services,