Hot Roof Design Solutions
Design suggestions for better building cooling in hot climates, lower roof ice or condensate in cool climates
DESIGN SOLUTIONS for HOT or UN-VENTED ROOFS - CONTENTS: Un-Vented Roof Solutions: how to avoid condensation, leaks, attic mold, insulation mold, & structural damage to roof framing when roof venting is not possible. Hot roof designs: suggestions for un-vented or hard-to-vent building roof cavi
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Hot roof design problem solutions:
This article describes various solutions for un-vented cathedral ceilings and similar under-roof spaces, offering advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible.
This article series about roof and ceiling ventilation describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings.
Solutions for un-vented roofs: Avoid cold-climate ice dam leaks & reduce cooling costs in hot climates
Ice Dams on Un-Vented Roofs in Cold Climates
Grange and Hendricks (1976) recommended a combination of eaves and ridge venting to avoid ice dams on buildings.
Other authors found that ice dams seldom occur at temperatures above 22 degF. or when attic temperatures are below freezing.
Our photo shows an ice-dam prone roof on a tall building with a slate roof.
Roof Edge Sheathing Intake Vent: There is a product called Hicks Starter Vent™ and similar products such as the SmartVent™ distributed by DCI products that replace the first few inches of roof sheathing under
the shingles or slates by a louvered vent so that air can sneak into the roof cavity by that path.
It's cost-appropriate to install this when re-roofing but probably too costly to do so otherwise.
Half-Ridge Vent: a half-ridge vent, basically a conventional ridge vent but cut in half lengthwise, can be installed at the up-roof edge where a lower roof abuts a higher building wall, such as where a
roof slopes up to butt against the wall of a raised dormer.
Combined with soffit intake venting this roof vent design works well to cool and dry roof sections with this shape.
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.
Adding Attic Insulation to Avoid Ice Dam Leaks: Indoors, unfinished attic: if we add as much insulation as we can fit into the attic floor of an unfinished attic space, paying close attention to insulating under the eaves
at the lower roof edges, and making sure that the insulation blanket is absolutely complete with no missing areas or holes or leaks, we can reduce the heat loss into the attic space and thus reduce the warming of the roof underside and thus reduce future ice dam formation and its related leaks.
It's better to place insulation in the attic floor than under the roof, since in the latter location ventilation and drying of the roof sheathing are prevented and there is a greater chance of future mold growth or rot caused by trapped moisture there.
Un-Vented, Hot Roof Designs Indoors, finished attic: Where the attic space is finished with drywall or other ceiling materials installed against the underside of the roof rafters, while I prefer in-floor insulation,
here we'll have to insulate the roof cavity between the rafters.
In cases where there is no under-roof venting system (no soffit intake vents, no ridge vents), a "hot roof" design is followed: the roof cavity between rafters can be filled with insulation, followed by installation of a perfect vapor barrier, followed by finish surface of drywall or whatever else.
The vapor barrier and air sealing in particular, need to be perfectly installed to prevent warm moisture-laden air from entering the un-vented roof cavity. But even so, see Worries about the "hot roof" un-vented Cathedral Ceiling Designs, discussed below.
Tips for insulating a cathedral ceiling, take care to seal ceiling penetrations such as around light fixtures or ceiling-mounted hard-wired smoke detector. More moisture enters building cavities through these cuts in the ceiling (or wall) drywall than permeates through the drywall itself.
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.
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.
Our expert Steven Bliss commented to offer a final word on Hot Roof Designs:
I agree with you that, in the real world, this is not such a good idea. If there's a flashing leak or other roof leak, you could have a pretty soggy mess that stays wet for a long time and could cause structural decay.
Plenty of people are building hot roofs, but I wouldn't -- except maybe one with spray urethane which won't absorb much water like cellulose would. 
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