(C) Daniel Friedman Cathedral Ceiling Ventilation Specifications

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Best way to vent cathedral ceilings:

This article describes the best methods for ventilation in a cathedral ceiling and the effects on heat loss of air movement through a ventilated attic or cathedral ceiling.

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Advice on Venting Cathedral Ceilings

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?

Air wash steals heat from insulation (C) Carson Dunlop Associates

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

Sketch (left) showing how wind washing steals heat from fiberglass insulation is courtesy of Carson Dunlop Associates.

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.

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.

Cathedral ceiling insulation detail (C) S Bliss D Friedman 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

You can see a schematic for this approach at left. We also discuss this insulation and ventilation approach for cape cod style homes in a comopanion article:

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:

  1. 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.
  2. 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 Associates' 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?

Cathedral ceiling insulation detail (C) S Bliss D FriedmanThe link to the original Q&A articles in PDF form immediately below is followed by an expanded/updated online version of each article.

[Click to enlarge any image]

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.

See RADIANT BARRIERS for details about radiant barriers.

Tight Ceiling - Cathedral Ceiling Advice for Venting & Moisture

As explained in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

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.

At ROOF VENTILATION IMPROVEMENTS we give suggestions for improving the performance of roofs that are not easily vented.

Penetrations & Roof Ventilation Challenges - Common Air Leak Points in Ceilings

Figure 2-53: Typical ceiling air leak points (C) J Wiley, S Bliss

Pay special attention to penetrations in the ceiling plane, particularly in cathedral ceilings.

Chimneys, recessed lights, plumbing chases, and holes drilled through top plates for plumbing or wiring should all be sealed (Figure 2-53 at left).

Plug holes with durable materials, such as expandable urethane foam, foam backer rod, EPDM, or sheet metal, and use long-lasting sealants such as high-quality urethanes, silicones, and butyls.

With a tightly sealed ceiling, attic moisture is no longer a significant problem.


Attic ventilation is still recommended for these other reasons:

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:

Original article - PDF:


Continue reading at CATHEDRAL CEILING INSULATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see ROOF FRAMING TIES & BEAMS for a discussion of proper framing of a cathedral ceiling

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