CATHEDRAL CEILING VENTILATION -CONTENTS: How should I vent a cathedral ceiling?
Does air movement (wind washing) through a vented cathedral ceiling cause heat loss through the insulation? Should I install an air barrier in the cathedral ceiling above the insulation? Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices & Best Practices Guide to cathedral ceiling ventilation designs, Steven Bliss
The link to the original Q&A articles in PDF form immediately below is followed by an expanded/updated online version of each article.
Accompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Sketch above showing venting in a steep-slope cathedral ceiling roof is courtesy of Carson Dunlop Associates.
The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Wind Washing: Does airflow in a vented cathedral ceiling cause unwanted heat loss from the building?
Question about wind washing at building eaves:
Cathedral ceilings are vented at the ridge and eaves to allow airflow through the ceiling.
Doesn't this airflow remove heat from the fiberglass insulation during the heating season?
Is it better practice to separate the cathedral ceiling insulation from the air flow with a permeable fireproof material such as drywall, leaving approximately 1/2 inch between the drywall and the bottom of the roof sheathing? -- Larry Gunther, Neola, UT
Reply: The Effects on Insulating Value of Airflow Across Fiberglass Insulating Batt Surfaces
Anything that induces (causes) airflow through a fiberglass insulating batt will reduce the effectiveness of that insulation, since fiberglass works by reducing air movement. Air flowing across a fiberglass batt, however, should not seriously reduce the insulation's R-value except with very thin batts and very fast air movement.
In laboratory experiments, a 3-inch fiberglass batt on an attic floor had a 29-percent reduction in R-value caused by air flowing across it at 2 mph.
In another experiment, 6 inches of loose fill fiberglass had a 14 percent reduction at similar airspeeds.
With the thicker fiberglass insulating batts in contemporary use, the reductions in insulating value caused by air movement across the fiberglass surface will be less.
Also it is unlikely that airflows across insulation in a cathedral ceiling will reach this speed, even under extreme conditions.
Baffle Designs to Stop Airflow Effects Across Insulating Fiberglass Batts in a Cathedral Ceiling
To be safe, you could build a baffle such as you suggest, or use manufactured baffles,such as ProperVent.
If you are going to the trouble of making your own baffle, use something like perforated foil board (Dennyboard or Thermoply). You will have an effective summertime radiant barrier as well.
See RADIANT BARRIERS.
Drywall is probably not an ideal product for an inter-rafter air baffle over the cathedral ceiling insulation since drywall can get soggy if there is excess condensation or a roof leak. Also, unless the drywall were perforated it may trap moisture in the ceiling cavity.
At INSULATION LOCATION for CATHEDRAL CEILINGS we describe a high-labor method that combines providing an air barrier above fiberglass insulation in a cathedral ceiling to avoid heat loss by air movement in the roof, combined with the addition of solid foam insulation to provide a much higher R-value for the completed cathedral ceiling structure. -- DF
Bliss points out in the original Q&A on cathedral ceiling ventilation, the risk of forming a vapor barrier sandwich around fiberglass ceiling insulation (foil faced insulation above and poly vapor barrier below) is that any moisture entering the insulation will be trapped, risking future mold or rot problems.
Therefore the pink "fiberglass insulation" shown above should be installed using un-faced batts or blown-in chopped fiberglass or perhaps blown-in loose fill cellulose. Do not build a ceiling or wall with multiple layers of vapor barriers.
The risk of hidden mold and rot due to moisture entering from the building involves two concepts:
Interior sealing: Be sure that the room side of the cathedral ceiling is sealed meticulously against air leaks, such as by using a 6-mil polyethylene vapor barrier on the warm side of the insulation (in a heating climate) and careful workmanship, gaskets, sealants around any ceiling penetrations for electrical wiring or fixtures.
Exterior inspection: monitor the condition of the roof from above, with an annual inspection - unattended leaks on any roof structure can lead to costly rot, insect, or mold damage, but on a well-insulated, sealed cathedral ceiling roof, leaks from above can go for quite a while without notice from inside the building.
Another possibility would be to perforate the kraft paper on the insulating batts and face the paper up in the roof cavity. Of course you will have to install a continuous poly vapor barrier below the insulation as well.
Adequate Air Space over Cathedral Ceiling Insulation
It is unlikely that your proposed 1/2" air space between the top of the fiberglass insulation and the underside of the roof sheathing will provide an effective airflow between the building eaves and ridge. As you'll notice in Carson Dunlop's sketch at the top of this article, experienced home inspectors and other building professionals recommend a 3-inch air space.
The question-and-answer article about the effects of airflow on loss of R-value in fiberglass insulating batts, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.
Use of Radiant Barrier Foil in a Cathedral Ceiling?
The link to the original Q&A articles in PDF form immediately below is followed by an expanded/updated online version of each article.
Question: Is the foil-based radiant barrier in my cathedral ceiling in a useful place?
In my proposed cathedral ceiling detail [sketch at left], I want to use a reflective foil between the insulation and the roof sheathing to double as a radiant barrier and a cathedral ceiling ventilation space liner. Is the radiant barrier in a useful place? James French, Suffern, NY.
Reply: Yes but ...
The foil is in the right place to block radiant heat across the air space. But there are some other problems.
First, radiant barriers are not really cost effective in cold climates.
Second, in winter a continuous layer of foil above the insulation in a cathedral ceiling creates a cold-side vapor barrier, vulnerable to condensation on the underside. [Foil has a perm rating of about zero.] In other words, your vapor barrier is in the wrong place and you could end up with trapped moisture, wet insulation, and even rot and mold in the ceiling cavity over time.
Most builders and designers feel that a lining material is not necessary in the airspace of a cathedral ceiling except at the eaves [to avoid wind-wash or moved insulation]. At the eaves a rigid baffle is recommended to keep the insulation in place and to keep wind out of the insulation.
For extra protection from wind, some builders line the space with a housewrap material such as Tyvek.
Although the code-mandated ventilation rate has proven
adequate under normal conditions, homes with highmoisture
levels and air leaks in ceilings may still experience
problems such as moldy sheathing. Cathedral ceilings
are at the greatest risk due to the limited ventilation path.
The best defense against problems is to create a continuous
air and vapor barrier between the living space and attic or
roof cavity by carefully sealing all air leaks. The ceiling air
barrier may consist of foam insulation with taped seams,
taped polyethylene sheeting, or finished drywall that is
sealed at corners and top plates with gaskets or sealants.
Allowing roof components to dry out in the event of a
Reader Question: my contractor proposes cutting the cathedral ceiling in half to improve ventilation
I have a peaked ceiling (insulation directly against wood underside of roof. metal roof)
During times of high humidity the ceiling boards get wet yet it appears that there are no leaks so I figure it is a condensation problem.
Our contractor wants to replace our R30 insulation with R15 so there is more room for air to flow. Does this sound like a reasonable solution? Thank you. - M.R. 7/28/2013
Well yes, you'd possibly increase air flow, and no, halving the insulation in a roof/cathedral ceiling is probably not the best approach unless you live in an area whrere the insulating value of your ceiling is of little concern.
You are adding air space, maybe, but also halving the R-value of the roof, increasing heat loss, increasing heating and maybe cooling bills, and increasing the chances that the ceiling cavity reaches the dew point (thus more condensation).
I would not take this approach. If you want to add air space to vent the cathedral ceiling, and you're going to the remarkable cost and trouble of removing and reinsulating, why not do the job right with high-R solid foam insulation, an air space, and careful workmanship to assure that air flows in at eaves, out at ridge, and that thdre are no air leaks into the cavity.
Check out these two articles on venting and insulating cathedral ceilings:
POST a QUESTION or READ FAQs about Venting Cathedral Ceilings - what is the proper way to vent cathedral ceilings? Does airflow in a vented cathedral ceiling remove heat from the ceiling insulation? - original PDF version, use your browser's back button to return to this page
Questions & answers or comments about the best methods for ventilationg cathedral ceilings and other sloped building ceilings
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Rose, William B., and Anton TenWolde. "Venting of attics and cathedral ceilings." ASHRAE Journal-American Society of Heating Refrigerating and Airconditioning Engineers 44, no. 10 (2002): 26.
TenWolde, Anton, and William B. Rose. "Issues related to venting of attics and cathedral ceilings." TRANSACTIONS-AMERICAN SOCIETY OF HEATING REFRIGERATING AND AIR CONDITIONING ENGINEERS 105 (1999): 851-857.
Rose, William B. "Measured summer values of sheathing and shingle temperatures for residential attics and cathedral ceilings." Performance of Exterior Envelopes of Whole Buildings VIII: Integration of Building Envelopes (2001).
Rudd, Armin. "Field Performance of Unvented Cathedralized (UC) attics in the USA." Journal of Building Physics 29, no. 2 (2005): 145-169.
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Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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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: email@example.com
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."
"Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
"Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
"Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
"Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
"Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
Insulation Types, table of common building insulation properties from U.S. DOE. Readers should see INSULATION R-VALUES & PROPERTIES our own table of insulation properties that includes links to articles describing each insulation material in more detail.
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
Ice Dam Leaks in building attics and roof cavities, how to inspect for evidence of leaks, identify causes, and correct bad attic ventilation, improper roof venting, and these causes of attic mold or roof structure damage
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: firstname.lastname@example.org. 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.
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