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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Building cooling cost reduction through attic venting:

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:

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


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