ROOF VENTILATION NEEDED? - CONTENTS: The need for roof ventilation, proposals for "un-vented" roofs, and the risks of omitting roof ventilation. Critique of the "hot roof" building design principle. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
The Need for Roof Ventilation or Other Options for Un-Vented & Hot Roofs
Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
"Is Roof Venting Necessary? The old rules may not apply to new roofs" - links to the original article in PDF form immediately below are followed by an expanded/updated online version of this article.
Our photo (left) shows a ridge vent on a modern asphalt shingle roof. Is this outlet vent necessary? Is it enough?
This article provides a review of roof ventilation theory, explaining the importance of preventing attic moisture (and mold), ice dam leaks, and at the same time, energy loss in buildings.
Mr. Bliss explains how moisture and the resulting condensation gets into roof cavities and building walls following moisture laden air that leaks through gaps in building drywall, around exposed beams, intersecting walls and ceilings, at light fixtures, electrical outlets, and other openings by riding air convection currents that move air in and out of building cavities as building interior conditions change.
Arguments for and about Un-Vented Roofs
Mr. Bliss points out that often the air/vapor barrier installation is imperfect or has been compromised, permitting leaks into building cavities.
He also points out that improperly installed ventilation (such as installing a ridge vent at the top of a roof without installing air intake openings at the soffits or eaves) can make building air movement, moisture, and condensation problems worse.
Bliss cites researchers at Lawrence Berkeley Labs who found that in a mild climate (3000 degree days), most attic moisture comes from the ventilation air itself, not from air inside the house.
In general attics are wetter in winter than summer (due to cooler temperatures causing condensation in that space), but in both daily and seasonal cycles, the water entering from outside vent air is stored (and later released) safely from the attic lumber and sheathing.
The "no-vent" or "hot roof" design is discussed, and the author points out that construction, including the vapor barrier, must be just about perfect for this approach to work.
Sketch (above left) is courtesy of Carson Dunlop Associates shows the two basic strategies for insulating cathedral ceilings and flat roofs.
While the article omits later field experience of experts like Henri DeMarne, we caution readers that the hot roof design is extra vulnerable to severe rot and mold damage from hidden, un-discovered leaks that cause more rapid, more extreme damage in enclosed un-vented building cavities than in well-ventilated ones such as a vented roof space.
Ice dams (see the sketch at page top), form when snow sits on a roof for three or four sub-freezing days. Light dry snow makes good insulation on top of the roof, permitting warmth from or inside the attic space below to warm and melt the underside of the snow. This water runs down the roof surface until it meets the cold roof edge or eaves where it freezes to form a dam of ice along the roof edge.
When sufficient water backs up above the ice dam, over the warmer sections of the lower roof edges, water leaks up under the shingles, into the attic, or into the building wall cavities. Mr. Bliss points out that super-insulated modern homes may be at less risk of ice dams than older homes with poorly-insulated attics or roof cavities. In theory, enough insulation can prevent ice dam formation on roofs, except probably on low-slope roofs that hold so much snow as to compete with the R-values provided by the roof insulation.
The article also cites a few complaints of rain or snow blowing in at ridge vents, though in nearly 40 years of building inspections we have almost never found building damage nor mold from this cause.
Conditions Under Which Un-Vented Roofs Might Work
Based on research in Sweden, an un-vented roof can work if:
the vapor barrier is perfect
the roof covering material is clay tiles or concrete tiles, or other materials that are not absolutely air tight
the climate is comparable to southern Sweden (6000 to 8000 degree days)
great care is taken that the lumber does not get soaked before or during construction
[added by DJF] over the life of the building, no roof leak or roof damage from any other source ever occurs
Roof Vent Debate Conclusions
Mr. Bliss concluded (back in 1985, and we agree today) that a non-vented roof is a bit risky, but with a good [read "perfect"] vapor barrier [and, if no roof leaks ever occur over the life of the building] you can probably get away with it.
Shallow low-slope roofs such as over shed-dormers should be vented if at all possible.
Where roof ventilation is not possible, as in some retrofits or in complex roof designs, pay close attention to sealing air leaks into the roof cavity through the ceiling.
Consider using waterproof membranes such as ice and water shield over hard to vent roof sections where ice dams are likely or where they have occurred in the past. [added-DJF]
In unvented roofs we would prefer (DJF and Mr. Bliss) if the roofing were air- and moisture-permeable. Wood shingles and shakes (and slate roofs installed on spaced nailers) meet these tests.
In un-vented "hot roof" designs, you should expect shorter roof covering life unless the materials are clay tiles, concrete tiles, metal, slate, or something else that is not very affected by the hotter roof temperatures that will be encountered during summer weather [added DJF)
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Continue reading at ROOF VENTILATION SPECIFICATIONS for more extensive information about the need for and methods of roof ventilation, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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(Oct 7, 2012) Paul said:
A contractor built a Lodge in the mountains of Bozeman Montana without roof venting. THe roof has no attic and it has ice shield and it has metal roofing. It saved the us (owners) about $3500. THere has been no problems that I can see to this point.
Sorry I forgot too that the cavity is about 11 1/2 inches and there were 4 inches of sprayed foam and the rest of the cavity was filled with batt insulation. IF there are problems that could occur with this I sure would like to know.
Thanks for the report Paul. I agree that in some installations a hot roof design can work well, and I too would prefer a very reliable roof covering, such as metal roofing, in such an installation. We recently built a roof of similar specs.
Though under a metal roof I'm not sure what the ice and water shield is adding for you.
With enough insulation to not reachnthe dew point in the roof cvity, and with no ceiling leaksmor penetrations the riskmof under-roof condensation is minimized.
I'd inspect the roof surface annually just to be sure it's undamaged, as leaks into an inaccessible cavity can lead to undiscoverd damage that festers until severe.
(Oct 8, 2012) Anonymous said:
Thanks for the information Dan. I haven't seen any problem in the 3 years since completion. If I saw a possible problem could just installing ridge vents prevent anything?
We keep the lodge heated during the winter to a minimum of 50 degrees and then during the spring, summber and fall the heat is off.
Question: how to add insulation without proper air or vapor barriers
Recently acquired a building with a warm pitched roof (50mm rigid board insulation over the rafters) and converting the building to a house. Wondering how to safely add insulation without using a Vapor Control Layer (VCL) ? Roof layers from outside; slates, batons, tyvek felt, counter batons, insulation, plyboard, all above the 6" x 2" rafters which are totally opened. There are knee walls. Too difficult to apply a VCL correctly. Block boards at eves presently prevent air flow under the roof. Shoulder straps allow for an attic height of 3ft. I presume the options are to add insulation and keep it as a warm roof or to create a vented roof. Not sure how to proceed with added insulation to achieve a U value of .2 (refurb. regulation). Hoping you can assist. Thanking you, Tim.
I'm not sure how to proceed either as we don't know enough about your building, how it's designed, its moisture sources, or what the risks are for you. Certainly there are several options but cost-benefit analysis is missing when we don't know the building. For example you could remove roof covering (shingles or whatever are installed) and add insulated panels (a sandwich of plywood and foam referred to as SIPs or structural insulated panels). Such panels are also sold with built-in eaves-to ridge venting though the space may be so small that the airflow is questionable.
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ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: email@example.com
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
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