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Ventilating roof panels #7849 03/02/99 05:22 PM
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Paul Freeman Offline OP
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Here's a good topic to start off this forum. I am interested in people's opinions and experiences regarding venting or not venting under roof finishes over stress-skin panels.

Re: Ventilating roof panels #7850 03/03/99 12:04 PM
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Brian Wormington Offline
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My experience has been that unventilated roofs are a bad idea in any climate. In hot climates, the unventilated roof will have an excessive variation in the temperature of the roof covering (shingles) leading to premature failure and in cold, snowy climates you will certainly have ice dams without ventilation.-- In heavy snow areas you'd need an R-90 panel to eliminate damming via insulation.

The problem for timberframes, with stress skin roof panels, is that venting the roof is expensive. You cannot use the simple soffit vent approach that is commonly used in stud construction because the integrity of the foam core must be preserved to provide strength on overhangs. The best solution I know of is to use venting drip-edge and strapping on the existing roof deck with a second layer of sheathing over top of it all. --- in effect adding a second roof to the house and destroying the low-labor appeal of the roof panels. Rumor has it that some panel manufacturers are working on ventilated roof panels, but I don't think that any are on the market yet.

Re: Ventilating roof panels #7851 03/04/99 10:36 PM
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Rudy R Christian Offline
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Actually, there is at least one ventilated nailbase panel on the market (ThermaCal by Cornell Corp.)

Unfortunately it still can't solve the problem of the "chimney effect" that a vented roof panel is prone to.

We have one customer with a premature roof failure. They are currently investigating replacing the shingles with a standing seam system. It's more out front, but I think it will payoff in the long run, and solve the premature degradation problem. Some of the new systems are pretty installer friendly too.

Re: Ventilating roof panels #7852 03/05/99 12:42 PM
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Paul Freeman Offline OP
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Thanks for the responses. You both bring up some good points. Brian you have identified the heat issue and its effect on shingles and problems with ice damming. Ice damming problems are true for both stress-skin and conventional roofs. One effective technique is to apply a layer of bituminous "paper" such as "Ice and Water Shield" on the lower 3 feet of the roof, instead of building felt. By the way, felt should not be used at all under fiberglas shingles (on panels) because it deforms excessively from the "hot roof" condition.

Rudy points out the problem of the "chimney effect", while this effect is precisely what we are trying to create, conversely it brings with it the hazard of fire promotion. If fire managed to get into an eave, the convection of air and flame through our intentional vents from eave to ridge would cause rapid spread of the fire. This problem is exacerbated in ventilated panels where toxic gasses can be given off, and structural integrity more rapidly lost from the melting foam.

Ventilated panels? I have literature from Branch River printed in 1987 advertizing their Air-Flo Roof Panel which contains air channels under the upper layer of osb. Rudy mentioned Cornell Corp.'s ThermaCal and I expect there are more. Winter Panel recommends a "cold" (ventilated) roof. However some shingle manufacturers have begun including their warranty on panel roofs, such as Elk Corporation.

According to the Journal of Light Construction, Bill Rose of the University of Illinois has run a ten year side-by-side study with no noticeable difference. He has found however that by the time the air reaches the top of the ventilation channel it is so hot that their is little to no cooling effect on the upper shingles.

If a client wants a shingle roof, but are concerned about the life of the shingles, I recommend that they use an "Architectural Grade" shingle, light in color. I suspect that if one invested the money that it would take to build a "second roof", in 20-30 years at even a modest interest rate, there will be enought money to replace the shingles with a superior roof material . . . . whatever that is in 2020 . . . . probably a silicon, self healing, solar collecting shingle, with built in satellite signal receivers, and chameleon pigment displaying your favorite screen saver on the hour!

[This message has been edited by Paul Freeman (edited 03-05-99).]


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