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.
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
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
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 atRADIANT 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
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.
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
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.
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.
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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"Choosing Roofing," Jefferson Kolle, January 1995, No. 92, Fine Homebuilding, Taunton Press, 63 S. Main St., PO Box 5506, Newton CT 06470 - 800-888-8286 - see http://www.taunton.com/FineHomebuilding/ for the magazine's website and for subscription information.
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
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