Two approaches for insulating cathedral ceilings and flat roofs (C) Carson Dunlop Illustrated Home Attic Venting & Other Steps to Reduce Building Cooling Costs

  • COOLING LOAD REDUCTION by ROOF VENTS - CONTENTS: Reducing building cooling loads
  • Benefits of roof ventilation alone in reducing cooling cost. Benefit of roof ventilation plus radiant barriers
  • Effect of roof color on building cooling costs. Tips for un-vented hot roof designs to reduce building cooling costs

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This article describes the reduction in building cooling load and cooling or air conditioning costs from roof ventilation, radiant barriers, roof colors, and we include suggestions where roof venting is not possible - "hot roof" designs.

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Reducing Building Cooling Loads by Attic Ventilation

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

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

Experts recommend using attic ventilation in hot climates as part of an overall strategy to reduce cooling loads. Ventilation helps even more when used in combination with radiant barriers.

Benefits of Roof Ventilation Alone

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 about 5%.

Roor Ventilation and Radiant Barriers

Details about the benefits and effects of radiant barriers on heating costs, cooling costs, and roof shingle life are found at RADIANT BARRIERS. An excerpt is below.

For greater savings on cooling, consider adding a radiant barrier to the underside of the roof sheathing or draped between the rafters. This can reduce peak cooling loads by 14 to 15% and seasonal loads by an average of 9%.

By doubling the roof ventilation from 1/300 to 1/150, the annual savings from radiant barriers rises to 12%. These numbers assume R-19 ceiling insulation and cooling ducts located in the attic, which are typical in Florida. With R-30 ceiling insulation, the cooling benefits of radiant barriers are less dramatic.

Roofing Color Effect on Cooling Costs

Details about the effects of roof color on cooling costs and roof life are at ROOF COLOR RECOMMENDATIONS. Excerpts are below.

Table 2-18 Roof color and cooling Loads (C) J Wiley, S Bliss

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

Tests at FSEC also indicate that simply switching from dark to white asphalt shingles in a cooling climate can reduce peak cooling loads by 17% and seasonal loads by 4%.

The greatest savings resulted from using white metal roofing (see Table 2-18 shown at left.)

[Click any image or table to see an enlarged version with more detail.]

-- Adapted with permission from Best Practices Guide to Residential Construction.

Unvented “Hot” Roof Designs Where Venting is Difficult

Details about hot roof designs are found at HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS Excerpts are below.

In cathedral ceiling configurations where it is difficult to provide ventilation, some builders have eliminated the vent space, relying instead on careful sealing of the ceiling plane to prevent moisture problems. While experts concede that this should work in theory, most caution that it is difficult to build a truly airtight ceiling assembly.

Also, cathedral ceilings are slow to dry out if moisture problems do occur, whether from condensation or roofing leaks. If a hot roof is the only option for a section of roof, take the following precautions:

  • Install a continuous air and vapor retarder, such as 6-mil poly, carefully sealed at all junctures. See the vapor barrier and air barrier articles listed at VAPOR BARRIERS & CONDENSATION in buildings.
  • Do not use recessed lights or other details that penetrate the ceiling plane.
  • Carefully seal all penetrations in the ceiling assembly, including top plates of partitions, with durable materials. See these articles:
  • Use a nonfibrous insulation, such as plastic foam, and install it without voids where moisture could collect. Insulation choices are listed at INSULATION INSPECTION & IMPROVEMENT.

    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.
  • In regions prone to ice dams, use enough insulation to maintain a cold roof—preferably R-38 or greater. See ROOF ICE DAM LEAKS and Ice Dams: Comparing Two Houses
  • 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.
  • 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.
  • Eliminate all sources of excess moisture in the home (wet basements, uncovered crawlspaces, unvented bathrooms). See ATTIC CONDENSATION CAUSE & CURE.

-- Adapted with permission from Best Practices Guide to Residential Construction.

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