Grade D building paper is
an asphalt-impregnated kraft-type paper, similar to the
backing on fiberglass insulation. Unlike asphalt felt, it is
made from new wood pulp, rather than recycled material. Its most common use is under stucco in the western United
States. The vapor permeance of Grade D paper is similar
to asphalt felt. Its liquid water resistance ratings range
from 20 to 60 minutes, as measured by using the boat test
(see Water Resistance in "WATER RESISTIVE BARRIERS ON BUILDING EXTERIOR WALLS").
Because Grade D paper tends to deteriorate under prolonged
wetting, the trend in three-coat stucco is to use two
layers of 30-minute paper. Because the paper tends to
wrinkle, the two layers tend to form a small air space, creating
a rain-screen effect.
A separate Building Sciences Corporation report also elaborates the usefulness of placing a vapor barrier on the roof deck below shingles in hot humid climates. BSC points out that: [some paraphrasing -DF]
Unvented roofs with asphalt shingles in hot humid climates require a vapor
barrier between the asphalt shingles and the roof deck. This is because asphalt roofing materials
store water from dew or rain. Thus asphalt shingles
form a water reservoir not unlike wood shingle or shake roofs. The report argues that this stored moisture is driven inwards [presumably as water vapor, not liquid water] when sun strikes the damp or wet roof surface, and it continues to argue that moisture is driven through vapor-permeable roofing paper, felt, and plywood or OSB roof decking, thus ultimately into the attic space
But unlike an asphalt shingle roof nailed [over felt] directly to a roof deck, a wood shingle or shake roof that has been installed using best practices includes a disposal path for water absorbed in the roof surface: an air space between the wood roofing and the roof deck, or the installation of wood roofing over spaced nailers or "skip sheathing".
In cool or temperate climates this does not present a problem because the combination of heavy wetting from due or rain i snot combined with solar heating at high outdoor temperatures, say the authors who go on to argue that that buckled roof shingles observed in the morning (caused by moisture migrating back up from the roof deck) relax during the day. But on an un-vented roof moisture driven inwards [through the shingles, roofing felt, and OSB or plywood roof decking] in hot humid climates, needs to be addressed.
This phenomenon can
typically be ignored in climates other than hot humid climates because the combination
of extensive dew formation and solar heating at high outside ambient temperatures is not
common. In vented roofs, this is often manifested in the buckling of shingles early in the
morning as the moisture migrates in to the roof deck sheathing and the joints close. This
is followed by relaxation and opening up of the roof sheathing later in the day—the
But in un-vented roofs in hot humid climates, the authors argue that water from the roof surface is drawn upwards in liquid form, by capillary action, between plies of overlapped shingle courses where it passes ultimately through the vapor barrier and through the roof decking to the roof cavity interior.
The driving force of moisture through the roof and into the building is by solar heating according to the authors.
[OPINION-DF: from exterior roof inspections at all times of day and seasons, we have not observed this time-related morning roof shingle buckling in the Northeastern U.S. nor in Florida, nor the Southwest, though the authors report the phenomenon. It is possible that the authors are not quite correct that daily buckling and relaxing of roof shingles can be ignored on a vented roof as harmless, since certainly the product is expected to remain flat, and flexing daily might reduce its anticipated wear life.]
With unvented roof assemblies, this inwardly driven moisture must
be addressed. The preferred method is to prevent the moisture from entering the roof
deck material via the installation of a vapor barrier.
Asphalt shingles are quite impermeable to the passage of liquid water directly through
them. However the geometry of their installation allows wicking at overlaps. This inwardly driven capillary water is the source for the wetting of the
roofing underlayment and roof sheathing. The material properties of shingles change
under elevated temperatures and moist conditions due to their hygroscopic nature. The
large vapor pressures resulting from incident solar radiation and the changed material
properties are sufficient to drive moisture inward through the shingles.
Roofing felts or underlayments vary greatly in their permeability to water vapor; the
typical underlayment used under asphalt shingles in residential construction is quite
[QUESTION-DF: we note that the test chamber constructed by BSC was itself in an enclosed, air-conditioned space, and that the underside of the test chamber roof was at least in part exposed to the air conditioning. It seems possible that the reduced humidity and lower temperatures on the "interior-side" of the test roof may have contributed to moisture behaviors that vary from what occurs in the field. Attics and under-roof spaces such as in an un-vented "hot roof" cathedral ceiling are certainly not exposed to cool dry conditioned air. BSC may have addressed this concern but we did not find it in the referenced article.
The conclusion of the BSC report is an argument for installation of an impermeable moisture barrier underneath roof shingles, perhaps in place of the traditional and permeable roofing felt.
What are the differences between the test roof and a roof's behavior in the field?
How does the impermeable moisture barrier under roof remain impermeable when perforated by shingle nails or staples? What will be the market effects of suggesting entire roofs be underlaid first with a moisture barrier unaffected by roof shingle fasteners such as ice and water shield?
Why not recommend that all roofs and roof cavities include ventilation in their design, not only solving the possible driven-moisture problem discussed by BSC but also providing for longer shingle life (cooler roof surface) and in cooling climates, possibly reduced building cooling costs?]
Technical note on roof ventilation and cooling cost:
Researchers at the Florida Solar
Energy Center (FSEC) have found that adequate attic ventilation
can modestly lower sheathing and shingle temperatures,
and reduce an average home’s cooling load by
Details about combining roof color, roof ventilation, and radiant barriers to reduce cooling cost are found at: ROOF COLOR RECOMMENDATIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
This discussion of vapor barriers and condensation in buildings in this article series begins at part I, VAPOR BARRIERS & CONDENSATION in buildings, (when and why condensation occurs inside buildings, explains the problems caused by excessive indoor condensation, explains how moisture enters building wall and ceiling cavities, and summarizes the best approaches to prevention of indoor moisture and condensation problems).
The article series continues with part II at VAPOR CONDENSATION & BUILDING SHEATHING (detailed questions and answers about various building wall sheathing and insulating materials and their impact on building condensation problems).
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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: firstname.lastname@example.org
ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries. The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
2005 ASHRAE Handbook : Fundamentals: Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
"2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
"The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
The National Institute of Standards and Technology, NIST (nee National Bureau of Standards NBS) is a US government agency - see www.nist.gov
"A Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the "Moist" Transient Heat and Moisture Transfer Model [copy on file as/interiors/MOIST_Model_NIST_b95074.pdf ] - ", George Tsongas, Doug Burch, Carolyn Roos, Malcom Cunningham; this paper describes software and the prediction of wall moisture contents. - PDF Document from NIST
Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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