Cathedral Ceilings Insulation Suggestions for Older Homes InspectAPedia® -
Un-Vented Roof Solutions: how to avoid condensation, leaks, attic mold, insulation mold, & structural damage to roof framing when roof venting is not possible
Hot roof designs: suggestions for un-vented or hard-to-vent building roof cavities
How to detect roof venting deficiencies, attic insulation defects, and attic condensation problems
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Here we provide suggestions for insulating cathedral ceilings on older homes, providing under-roof ventilation for cathedral ceilings while obtaining higher insulation values or R-Values for these areas. This article describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings.
Readers should also see our "hot roof design concerns" described at Correcting Roof Ventilation, see Roof Venting: Un-Vented Roof Solutions, and see Heat Tapes: Use on Roofs for Ice. Our page top photo shows a difficult-to-vent cathedral ceiling with multiple skylights.
Cathedral roof and knee wall insulation & venting air path options in older homes
At Correcting Roof Ventilation we argued that while some experts like the "hot roof" design that omits attic or under-roof ventilation entirely, that approach risks surprise rot, mold, or insect damage on buildings where leaks and moisture are trapped in building cavities and remain un-noticed.
Some buildings, by their shape or design, simply don't make it easy to install continuous intake venting at the eaves or lower roof edge, or continuous outlet venting along a ridge. For example, a house which has no roof overhang at all makes intake venting at the eaves difficult. A house with a pyramid roof shape or complex roof shapes makes outlet venting at a ridge difficult.
Worries about the "hot roof" un-vented Cathedral Ceiling Designs
As explained in our "hot roof" discussion at Correcting Roof Ventilation, we don't have confidence in the long term durability of "hot roof designs" because any future roof leak into this cavity produces trapped moisture and rot. In particular, using fiberglass insulation in an un-vented cathedral ceiling is most likely to give way to discovery of advanced hidden damage and/or mold over the life of the building, and may give shorter roof shingle life as well as causing higher indoor temperatures than with a vented roof cavity design.
We call this a "hot roof" design because failing to vent the roof from below not only misses a chance to avoid ice dam leaks and condensation damage in cold climates. In hot climates the roof temperature will be much higher on an un-vented roof, resulting in shorter shingle life. This is less of a concern for slate and similar product roofs less affected by heat.
Foam-filled cathedral ceilings may make sense in some cases. In buildings where there is no roof venting anyway, and where ventilation is difficult or impossible to achieve, an un-vented, well insulated "hot roof" can provide a high-R-value ceiling and may be the second-best alternative to preventing ice dam related leaks in cold climates. Be sure to inspect the roof surface from outside for leaks and damage every year and to fix any damage quickly.
Suggestions for Retrofit Addition of Cathedral Ceiling Insulation & Ventilation for Older Homes
When renovating an older home whose roof rafters are only 2x6" (really 5 1/2" x 1.5") in dimension, the available space for insulation and ventilation is limited, therefore limiting the options for a combination of good under-roof ventilation and high insulation R-values in the cathedral ceiling. Here are some older home cathedral ceiling roof insulation and ventilation options that you may consider:
Less insulation: Reduce under-roof insulation level to 3 ½” fiberglass (R-11) under the cathedral ceiling (not good energy savings) - not recommended but functional, just poor insulation value.
2 Baffles: Provide double-baffles between the rafters, eaves to ridge (rafters are roughly 24” o.c.) – this solution will leave portions of the roof deck un-vented and risk future mold growth - not a very effective venting solution, especially where we have already had a mold problem
1 Baffle: Provide single baffles between the rafters, eaves to ridge – this solution will leave even larger portions of the roof deck un-vented and risk future mold growth - not recommended
High labor: Nail 1” furring along every rafter side, against the roof deck; cut and insert 1” or thicker high-R foam insulation sheets (R-8 per inch) against the furring, giving a 1” or greater air path between the insulation and the roof deck under the cathedral ceiling. (In the knee wall area the insulation is in the floor and on the back of the knee wall). Fill the remaining space with fiberglass or cellulose. With 2x6 rafters this will give about an R-19 roof, higher if you use thicker solid foam insulation. This gives the best air path, eaves to ridge, but costs more for the labor to nail furring and cut insulation into pieces to fit between rafters. – works well, costs more.
Solid Foam: Install solid foam spray-in insulation under the roof – a “hot roof” design. This eliminates the entire plan of ventilation, gives maximum R-value for the space, probably costs the most, and risks shorter shingle life (hotter roof) and future hidden damage if leaks occur into the roof cavity – works, not a solution we like as we prefer to ventilate roofs for longer shingle life and cooler house in summer. See Roof Venting: Un-Vented Roof Solutions.
When re-roofing from the building exterior: if the roof decking and possibly roof framing have been extensively damaged by leaks it is usually necessary to repair the roof from outside the building. Before replacing roof decking, consider re- decking the roof with insulated structural panels that include eaves to ridge ventilation in their design.
What about adding solid foam high-R insulation on top of the roof when re-roofing? Adding rooftop insulating foam is a common practice on flat roof commercial buildings. The insulation used in those applications is tapered to provide positive roof drainage to roof drain points as well as to improve the building insulation system and cut energy costs.
We do not consider this a good design for a pitched roof on a residential building as it converts the roof to a "hot roof" design (shorter shingle life, hotter under-roof area in warm weather). Adding ventilation under a solid-foam insulated pitched roof in order to dry and cool the building interior would, in turn, lose the benefit of the roof top insulation.
Other Cathedral Ceiling Insulation Tips
Tips for insulating a cathedral ceiling, take care to seal ceiling penetrations such as around light fixtures or ceiling-mounted hard-wired smoke detector. More moisture enters building cavities through these cuts in the ceiling (or wall) drywall than permeates through the drywall itself.
While fiberglass insulation is an excellent and effective product for insulating most building cavities, in areas where there is extra risk of trapping moisture (and thus rot or mold infections) such as crawl spaces and cathedral ceilings where roof venting may be absent or minimal, we prefer to use closed-cell foam insulation products or spray-in icynene foam insulation: these products can seal the cavity against drafts and they do not as readily pick up moisture nor do they readily form hidden mold reservoirs. See Mold in Fiberglass Insulation and MOLD PREVENTION for details.
Ice and Water Shield: On roofs that are too difficult to vent, a second-best solution is to remove the shingles (or slates) from the lower 3 feet
of those slopes where leaks and ice dams have been recurrent, install a waterproof but nail-able membrane such as WR Grace's Ice and Water Shield (other product names from other manufacturers) which will prevent any ice dam
backup leaks from entering the building. This is basically a sticky membrane that is applied to the roof decking and through which shingle or slate nails can be nailed back onto the roof; the membrane seals around the nails so
that those penetrations do not form leaks during a water or ice backup.
Un-Vented, Hot Roof Designs Indoors, finished attic: Where the attic space is finished with drywall or other ceiling materials installed against the underside of the roof rafters, while we prefer in-floor insulation,
here we'll have to insulate the roof cavity between the rafters.
In cases where there is no under-roof venting system (no soffit intake vents, no ridge vents), a "hot roof" design is followed: the roof cavity between rafters can be filled with insulation, followed by installation of a perfect vapor barrier, followed by finish surface of drywall or whatever else.
Use of roof de-icing cables or heat tapes to avoid ice dam leaks is described at Heat Tapes: Use on Roofs for Ice. While we prefer to avoid ice dam leaks by good building design and good under-roof ventilation, where conditions require stopping ice dam leaks on an existing structure, proper installation of heating cables may be the fastest and cheapest solution.
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DCI Products' description of the SmartVent™ tapered under-shingle attic ventilation intake strip is provided at www.dciproducts.com/html/smartvent.htm. We have not see data citing actual air flow rates compared with the airflow through a typical continuous vent strip at an un-blocked overhanging building soffit, but this type of product is should be considered at least where a roof structure does not provide a soffit where intake venting can be easily installed. Out of a concern that some roof eave and ridge vent products do not pass nearly as much air as others, we'd like to see airflow data comparisons. [Thanks to G.K. for this update, August 2008]
Alan Carson Carson Dunlop Associates, Toronto, Ontario. Mr. Carson is a home inspection professional, educator, researcher, writer, and a principal of Carson Dunlop Associates, a Toronto home inspection and education firm. Mr. Carson is a past president of ASHI, the American Society of Home Inspectors
Some great illustrations of the proper under-roof ventilation pathways are offered by Carson Dunlop. 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. (727) 595-4211 mark@BestTampaInspector.com 11/06 Daniel Friedman - principal author Daniel Friedman, editing, expanding, adding to comments from John Annunziata, P.E. - NY Metro ASHI informal chapter discussions.
Technical reviewers are invited to comment or ask questions - contact us
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
Be sure to review HOW TO FIND MOLD: How to Inspect Homes and Other Buildings for Mold - the Basics of How to Find Problem Mold Indoors
in our Mold Action Guide. Here are other articles that will be helpful in evaluating attic mold presence, causes, and cures
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."
Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 AShrae Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Wiley.com and also at Amazon.com. See our book review of this publication.
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
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
More Information on Building Diagnostic Inspections and Repairs
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The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
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