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Study: Vented vs Unvented Roofs

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Things are moving along at 4600 Augusta Ave: all of the windows and doors are in, and we’re quickly approaching the time to insulate. Pinning down the best method for insulation has been a surprisingly difficult process… while building science reports have increasingly advocated tight full-home conditioned envelopes using unvented roofs and conditioned attics, we’ve found contractors to be generally skeptical of the spray-foam technique, citing the lack of ventilation along the sheathing as cause for concern about shingle temperature and degradation. Shingle manufacturers themselves have expressed confidence in their products in the assembly; however, given the degree/prevalence of contractor pushback we decided to throw this one over to the R&D department here at R. Michael Cross | Design Group, and enlisted the invaluable help of Guy DuBois of Commonwealth Building Sciences for a sort of case study to examine the effect of underside ventilation (or lack thereof) on roof surface temperature.


To break this down a little, I should explain that vented attics were mandated by code for years as a means of preventing roof damage resulting from trapped moisture. In this method, insulation gets put down between the second floor ceiling joists, vents are installed in the eaves and gable ends, and the attic is left essentially unconditioned.  There are a few main thoughts behind the original mandate: preventing the buildup of moisture during the colder months by allowing it to escape, and preventing ice damming at the eaves in the winter by keeping the entire roof cold. Dominion Power further explains:


At some point, the original purpose for attic ventilation was forgotten and/or replaced, in cooling climates, with the belief that it was to reduce roof and attic temperatures, thus lowering cooling expenses and increasing shingle life. This was further compounded by the leap of faith that increased or powered ventilation would be even better. While sounding logical, there is simply no research to validate it. In fact, scientific testing has shown that attic ventilation has almost no effect on roof surface/ shingle temperatures and very little effect on attic temperatures.


 While Dominion’s research would seem to answer our question… we decided to move forward anyway. Because we’re scientists, dammit, and we need some of that good empirical stuff.


The key to unvented roof assemblies and conditioned attics is managing the temperature of the first condensing surface either with rigid insulation on top of the sheathing, or with an impermeable barrier of spray foam insulation adhered directly to the underside of the sheathing. In the case of rigid insulation, this keeps the condensing surface at the roof sheathing sufficiently warm (45°F) to minimize or prevent condensation. In the case of applied spray-foam, this moves the condensing surface from outside of the insulation (the roof sheathing) to the inside of the spray-foam, which is essentially self-insulated by the rest of the foam behind it.


This method of keeping the condensing surface below significant insulation also allows the exterior roof surface to remain relatively cool, preventing ice dams. Furthermore, the absence of direct attic ventilation helps minimize uplift pressure in high wind events.


To study the surface temperature performance of unvented roof assemblies compared to unvented roof assemblies, we constructed two 2’-0” x 4’-0” mockup sections of roof, one with two inches of spray-foam applied directly to the sheathing, one without.
 



From there we set the roof panels up at a 12:12 slope and set up matching heat lamps (250 watts). Guy spent some time calibrating the whole set-up, making certain both panels received equal heat exposure. After heating both panels for about 15 minutes, the heating lamps were turned off and initial surface temperature readings were taken.












We found that both panels had a surface temperature within 1°F of 181°F. We chose to continue measuring at intervals over the next half hour to see if the panels rejected surface heat at the same rate. 









Ultimately, the panels performed almost identically, with the unvented panel cooling slightly slower, presumably because of the foam’s thermal mass. Measurements taken behind the panel showed an expected but marked difference in hot spots, with the insulated panel performing significantly better. 







We admit that this investigation could have been more controlled, and that the panels may not exactly reflect the performance of an actual enclosed roof assembly; however this was a helpful exercise in essentially pointing towards a confirmation that unvented roofs won’t melt your shingles off. At the end of the day, we believe the positives of an unvented spray-foam assembly present a better solution for residential roofs in most situations.


We wish we had somewhat more conclusive results, and may still pursue further tests on these panels. In fact, we’d love to hear any of your suggestions and ideas for testing methods, or even general questions that you don’t think we have addressed. You can also email us at blog@rmichaelcross.com.


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9 Responses to "Study: Vented vs Unvented Roofs"

  1. va011101 Says:
  2. Nice bit of research - Andrew
  3. ethan Says:
  4. I'm glad I came upon this article. I've been going back and forth, talking to colleagues, etc. about this very issue for a week. Granted, I am in the Pacific NW which has some different weather characteristics - namely consistently wet winters that very rarely dip below freezing. Also, I am working on a retrofit with no plan to modify existing roof assembly above the sheathing. I have come to the conclusion that XPS (Extruded Polystyrene) rigid board installed directly under the roof sheathing is an appropriate application method to achieve maximimun insulation in a relatively small cavity. This would be similar to a spray-foam application, in that they are both open celled insulations, though the spray would achieve a better coverage. We have learned to approximate spray foam by filling any gaps in the rigid with small amounts of sprayed insulation. Any thoughts about the performance of rigid and its application with no venting between the underside of sheathing and the insulation in contrast to the sprayed in that you used in your comparison study?
  5. R. Michael Cross Says:
  6. Ethan.
    We are glad that you came upon this article as well. As this article suggests, despite the absolutes given in the building science reports there are few absolutes found amongst practitioners opinions. It is my understanding that you are located in the Pacific Northwest, which may put you in a "marine" environment according to the Energy and Environmental Building Association. As you mention, this type of environment has unique concerns that are not found in the majority of the country. The use of extruded polystyrene is virtually the same as the use of spray foam with respect to its thermal performance, and likely very close to the air sealing performance when detailed correctly. Not being familiar with the specific needs of the Pacific Northwest I hesitate to make any statements about design solutions, however, I highly recommend referencing the EEBA's Builder's Guide for your region, as they discus such details at length and each of their books are tailored to specific regions.
    http://www.eeba.org/bookstore/cat-Builders_Guides-4.aspx
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