(C) Daniel FriedmanIs Roof Venting Needed?

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Requirements & choices of methods for roof ventilation: this article discusses the options for venting versus un-vented roof.

What problems can we expect for buildings with hot roofs or un-vented roofs?

What problems occur on un-vented roofs? - are "hot roofs" really so hot?

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

The Need for Roof Ventilation or Other Options for Un-Vented & Hot Roofs

Ridge vent on modern home (C) Daniel FriedmanSketch 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.

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

Roof Vent Debate Conclusions

Readers should not fail to review the more extensive information about the need for and methods of roof ventilation found at ROOF VENTILATION SPECIFICATIONS. Also see ICE DAM PREVENTION and Ice Dams: Comparing Two Houses.

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.

Original article

Roof Ventilation Articles


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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