Interior cathedral ceiling (C) Daniel FriedmanCathedral Ceilings Insulation Suggestions for Older Homes
     


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Retrofit or add-on cathedral ceiling insulation:

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.

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Cathedral roof and knee wall insulation & venting air path options in older homes

Two approaches for insulating cathedral ceilings and flat roofs (C) Carson Dunlop Illustrated Home

Article Contents

  • Un-Vented Roof Solutions: how to avoid condensation, leaks, attic mold, insulation mold, & structural damage to roof framing when roof venting is not possible
  • Cathedral roof and knee wall insulation & venting air path options in older homes
  • Hot roof designs: suggestions for un-vented or hard-to-vent building roof cavities. Worries about the "hot roof" un-vented Cathedral Ceiling Designs
  • How to detect roof venting deficiencies, attic insulation defects, and attic condensation problems
  • Suggestions for Retrofit Addition of Cathedral Ceiling Insulation & Ventilation for Older Homes
  • Other Cathedral Ceiling Insulation Tips


Our page top photo shows a difficult-to-vent cathedral ceiling with multiple skylights.

At INSULATION LOCATION & QUANTITY for ATTICS we discuss the relative benefit of adding insulation to a cathedral ceiling versus building walls in the same area.

At Correcting Roof Ventilation and at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS we argued that while some experts like the "hot roof" design (right hand design in the sketch at left) 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.

Sketch (above left) is courtesy of Carson Dunlop Associates shows the two basic strategies for insulating cathedral ceilings and flat roofs.

Worries about the "hot roof" un-vented Cathedral Ceiling Designs

As explained in our "hot roof" discussion at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS, 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.

Watch out: Be sure to inspect the roof surface from outside for leaks and damage every year and to fix any damage quickly. Examples of trouble with un-vented hot roofs are
at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS

Suggestions for Retrofit Addition of Cathedral Ceiling Insulation & Ventilation for Older Homes

Venting cathedral roofs (C) Carson Dunlop Associates

Sketch (left) is courtesy of Carson Dunlop Associates shows the two basic strategies for ventilating cathedral ceilings or cathedral roofs.

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 CATHEDRAL CEILING VENTILATION

Insulated roof panelWhen 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.
See HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS

Other Cathedral Ceiling Insulation Tips

  1. 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 RESISTANT CONSTRUCTION for details.
  2. 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.
  3. 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.

    Details are at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS
  4. Use of roof de-icing cables or heat tapes to avoid ice dam leaks is described
    at HEAT TAPES & CABLES for ROOF ICE DAMS.

    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.

 

Continue reading at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS or select a topic from the More Reading links shown below.

Or see INSULATION LOCATION & QUANTITY for ATTICS to discuss the relative benefit of adding insulation to a cathedral ceiling versus building walls in the same area.

Suggested citation for this web page

CATHEDRAL CEILING INSULATION at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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