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This is "Un-Vented Roof Solutions: how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible", part of our discussion of "Attic Condensation". Also see CATHEDRAL CEILING INSULATION and see our "hot roof design concerns" described at Correcting Roof Ventilation and see Heat Tapes: Use on Roofs for Ice.

This article describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Un-Vented Roof Solutions: how to avoid condensation, leaks, damage when roof venting is not possible

At Correcting Roof Ventilation we argued that while some experts like the "hot roof" design that omits attic or under-roof ventilation entirely, that approach risks surprise rot, mold, or insect damage on buildings where leaks and moisture are trapped in building cavities and remain un-noticed.

Some buildings, by their shape or design, simply don't make it easy to install continuous intake venting at the eaves or lower roof edge, or continuous outlet venting along a ridge. For example, a house which has no roof overhang at all makes intake venting at the eaves difficult. A house with a pyramid roof shape or complex roof shapes makes outlet venting at a ridge difficult.

Problems with Partial Roof Venting

On these roofs, partial venting can be worse than no venting. For example, adding a ridge vent, or several roof "spot vents" or roof turbine vents on a few roof slopes, typically mid-slope or in the upper third of the slope on roof surfaces not visible from the front of the building, may please the installer, but they are worse than ineffective.

Placing an outlet vent on a roof without adequate inlet venting (see Roof Vent Area Ratios) works against the interests of the building and its occupants. As convection currents and heat loss into the roof space or attic vent out through these vents, the intake air needed to satisfy the exhausted air leaving the building will be drawn from the building interior - increasing building heating costs and possibly increasing particle movement from basements or crawl spaces (if there is a mold concern in the building). If you can't provide enough intake venting it is probably better to not vent at all in these conditions.

What are some solutions for un-vented roofs if we want to avoid cold-climate ice dam leaks?

Our photo (below left) shows an ice-dam prone roof on a tall building with a slate roof.

1. 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.
2. 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.
3. 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.
4. 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.

5. 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.
6. 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. See Mold in Fiberglass Insulation and MOLD PREVENTION for details.
7. Use of roof de-icing cables or heat tapes to avoid ice dam leaks is described at Heat Tapes: Use on Roofs for Ice. 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.

Worries about the "hot roof" un-vented Cathedral Ceiling Designs

As explained in our "hot roof" discussion at Correcting Roof Ventilation, I don't have confidence in the long term durability of "hot roof designs" because any future roof leak into this cavity produces trapped moisture and rot. We call this a "hot roof" design because failing to vent the roof from below not only misses a chance to avoid ice dam leaks and condensation damage in cold climates. In hot climates the roof temperature will be much higher on an un-vented roof, resulting in much shorter shingle life. This is less of a concern for slate and similar product roofs.

In buildings where there is no roof venting anyway, an un-vented, well insulated "hot roof" is a second-best alternative to preventing ice dam related leaks in cold climates. Be sure to inspect the roof surface from outside for leaks and damage every year and to fix any damage quickly.

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• DCI Products' description of the SmartVent™ tapered under-shingle attic ventilation intake strip is provided at www.dciproducts.com/html/smartvent.htm. We have not see data citing actual air flow rates compared with the airflow through a typical continuous vent strip at an un-blocked overhanging building soffit, but this type of product is should be considered at least where a roof structure does not provide a soffit where intake venting can be easily installed. Out of a concern that some roof eave and ridge vent products do not pass nearly as much air as others, we'd like to see airflow data comparisons. [Thanks to G.K. for this update, August 2008]
• Alan Carson Carson Dunlop Associates, Toronto, Ontario. Mr. Carson is a home inspection professional, educator, researcher, writer, and a principal of Carson Dunlop Associates, a Toronto home inspection and education firm. Mr. Carson is a past president of ASHI, the American Society of Home Inspectors Some great illustrations of the proper under-roof ventilation pathways are offered by Carson Dunlop. Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. (727) 595-4211 mark@BestTampaInspector.com 11/06 Daniel Friedman - principal author Daniel Friedman, editing, expanding, adding to comments from John Annunziata, P.E. - NY Metro ASHI informal chapter discussions.
• Technical reviewers are invited to comment or ask questions - contact us

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INTERIORS of BUILDINGS
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR SEALING STRATEGIES
BATHROOM VENTILATION
BASEMENT WATERPROOFING
BLOWER DOORS & AIR INFILTRATION
BRICK LINED WALLS
BUCKLED FOUNDATIONS due to INSULATION?
CATHEDRAL CEILING INSULATION
FREEZE-PROOF A BUILDING
ATTIC CONDENSATION CAUSE & CURE
Air Bypass Leaks, Thermal Tracking
Blocked Soffit Intake Vents
CATHEDRAL CEILING INSULATION
Comparing Two Houses
Heat Tapes: Use on Roofs for Ice
HEAT LOSS: How to Calculate Heat Loss in a Building
HOUSEWRAP AIR & VAPOR BARRIERS
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
INSULATION INSPECTION & IMPROVEMENT
Inspect Attics for Moisture or Mold
Inspect Basements for Moisture or Mold
Inspect Building Exterior
Inspect the Ridge Vent System from the Attic
Inspect the Soffit Vent System from the Attic
ROOF VENTILATION SPECIFICATIONS
Roof Venting: Intake - Outlet Area Ratios
Roof Venting: Proper Locations
Roof Venting: Both Ridge & Eaves Venting Needed
Roof Venting: Eaves Intake if no Overhang
Roof Venting: Soffit Intake Vent-Continuous
Roof Venting: Un-Vented Roof Solutions
HEAT LOSS: How to Calculate Heat Loss in a Building
HOUSEWRAP AIR & VAPOR BARRIERS
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
INSULATION INSPECTION & IMPROVEMENT
THERMAL TRACKING & HEAT LOSS

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