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What is the thermal effectiveness of an exposed batt in a flash-and-batt hot roof?

First, this is for residences in Climate zone 5. We have seen several insulation companies recommend a flash and batt system to insulate a roof system in an unfinished attic space. The recommendation is for 3" of CC foam (R-20 to R-23) covered by an R-21 unfaced fiberglass batt for a total nominal R value of ~ R-40. The fiberglass batt is visible from the unfinished attic and according to the insulation company meets the dual role of an ignition barrier and an R-21 thermal break . EPA (energystar) standards state that fiberous insulation be enclosed on all six sides to be effective and if not there is an airbarrier mis-alginment. There are several questions and please identify the authoritative source of you answers.
1) Should the batt be considered part of the thermal layer or be ignored completely.
2) If the exposed batt is part of the thermal layer, how much should the thermal value be degraded (if at all) for being exposed (i.e. no aligned airbarrier on the bottom?
3) Does the pitch of the roof system matter to your answer in #2?
4) If the batt needs to be covered, what would you use and what are the critical properties (air barrier, vapor barrier, etc.)?
5) Would you recommend this strategy and if not what is a better strategy at comparable cost.

Asked by Jeffrey Rhodin
Posted Dec 27, 2012 4:23 PM ET
Edited Dec 27, 2012 6:59 PM ET


3 Answers

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There are no simple answers to your questions, but you're on the right track.

Fiberglass batts don't do much, because they are air-permeable and difficult to install well. If they have no interior air barrier, they do even worse. And since your proposed assembly doesn't address thermal bridging through the rafters, it has another strike against it.

The assembly would be vastly improved if the fiberglass batts were thick enough to completely fill the rafter bays, and if they were protected on the interior by a 2-inch-thick layer of Thermax polyisocyanurate. That will, of course, cost you extra.

Answered by Martin Holladay
Posted Dec 27, 2012 6:55 PM ET


thanks for the input. Would you know of any studies that could quanitfy the degradation in thermal resistance for the fiberglass?

Answered by Jeffrey Rhodin
Posted Jan 2, 2013 5:08 PM ET


When there is no air barrier on one side the thermal effectiveness varies with both the pitch of the roof, the density of the fiber, and whether it's summer vs. winter, and the difference in temperature across the batt. In winter the warmer air from the attic is more buoyant, and infiltrates the batt where the denser-colder air a the foam surface is falling. With high density ~R4/inch "cathedral ceiling" batts that effect is pretty small even at the temperature extremes, and can prettty much be ignored from a thermal point of view, but not from a moisture transport point of view.

With standard density or low density batts there is a real degradation below the rated performance in winter if the bottom side air-barrier is absent. When the roof is hotter than the attic air, the convection effects are lower, since the lighter more buoyant air is above the denser-cooler air, but there is still some.

I'm a bit surprised that unfaced batts of any density meets code for an ignition barrier if friction-fitted. (The thing has to not fall out even when the exposed rafter edge might be burning.) A full cavity fill w/gypsum (or fire-rated iso board) on the interior would be a much more reliable ignition barrier, as well as providing higher overall thermal performance.

Even though it's higher-performance, I'm reluctant to recommend the fire-rated Theramax approach, since it creates a moisture trap between the 3" of closed cell foam and interior- any moisture that finds it's way into the fiberglass filled cavity will stay there, whereas with gypsum it would always be able to dry into the attic. Any assembly that requires perfection over the long term is bound to create a problem eventually, even though that may be decades hence if the roof is well maintained.

Answered by Dana Dorsett
Posted Jan 3, 2013 12:19 PM ET

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