Yes, Unvented Roof Assemblies Can Be Insulated With Fiberglass - A WUFI Post
And Not Just In Climate Zones 2B & 3B: Cold Climates TooA post on August 21st, 2015 on GreenBuildingAdvisor.com, Martin Holladay asked the question, Can Unvented Roof Assemlies Be Insulated With Fiberglass? Martin's post focuses on the marketing of Owens Corning's ProPink Unvented Attic Insulation System. The Owens Corning system is only allowed by code in zones 2B & 3B with tile roofs. And even still, Joe Lstiburek is quoted in the post saying "I don’t believe it will work without a vapor diffusion vent,” ...“It’s too risky..." Martin, then quotes the response from Achilles Karagiozis, Owens Corning's director of building science: “Absolutely,”...“We are stuck with the code as written. It was written a long time ago, and it was written for one particular industry: the spray foam industry. Joe [Lstiburek] worked on that.” Later Martin Holladay notes:
Owens Corning says that this system works with asphalt shingle roofing in Climate Zones 2B and 3B — but using asphalt shingle roofing on this type of roof is a code violation. “We all have to follow code, but at the same time, we can provide code officials with the engineering behind our recommendations,” Karagiozis said. “If we feel it makes sense, we will help the builder to provide information to the code official to get approval. We take these WUFI files.”At 475 we agree with Joe Lstiburek, that we must make safe and robust enclosures. And 475 agrees with Owens Corning, and Achilles Karagiozis that the current code was written for the spray foam industry and we can make safe unvented roofs with fiberglass insulation given the right approach - and that educating code officials for approvals of such systems is essential. So building inspectors and plan examiners please take notice: we want to work with you and show we need not rely on toxic and unreliable spray foam but can instead safely and robustly use fiberglass insulation. But must we be stuck in climate zones 2B & 3B? What about the zones where most of you live and work: 4, 5 and 6? In this post we will show, that while the ProPink Unvented Insulation System may provide a safe roof in zones 2B &3B, if we instead combine dense fiberglass insulation (batts or densepacked) with INTELLO smart vapor retarder and airtight membrane we can make safe and robust unvented roofs in climate zone 6!
Code pushes the envelope - IECC vs IRC/IBC
Codes should provide the minimum parameters for performance, safety and durability. For instance IBC# does indicate that wood moisture content should be below 19% when installed, a dry roof (no condensation or leaks) is a healthy, structurally sound roof. However since the Energy Code continues to increase R-value minimums, the risk of condensation/moisture accumulation in roof assemblies in cold and mixed climates has increased too. The only solution the code provides to this moisture drive issue is to add "sufficient" air-impermeable insulation below the deck to theoretically keep protect the sheathing from condensation/moist air-currents. None-the-less, it is known that an air-impermeable insulation layer will only work as an air barrier/vapor control layer, if it is also installed as one (board seams taped, spray foam mixed/applied correctly). This work not only should be performed meticulously, it should be verified by a blower door. However, if the installation is sloppy or not durable (foam shrinks - cheap tape is used or worse - caulk), the "airtight" insulation will not be airtight for very long. If the airtightness fails, then the risk of damages/rot or mold in the assembly increases substantially. You might get lucky, but if things go wrong - the consequences are costly (repairs, healthy problems, etc).
IECC 2012 took a first step to acknowledge these issues, and is mandating a verification of the airtight layer installation at 3.0ACH50 (Unfortunately, a residential code mandate only, so far, that a number of states have adopted). And we do see good designers, even when using SIPs, acknowledge the importance of sealing the interior seams of this airtight insulation material with airsealing tape (see photo on right).
We have gotten assemblies approved in several jurisdictions while working with the authorities having jurisdiction (AHJ), such as the building inspector, to show that one can design and build a roof that has air-premeable insulation (fiberglass, mineral wool, sheepswool, cellulose, etc) that can be declared "air-impermeable insulation" with an air barrier which is also a (very) smart vapor retarder: INTELLO Plus as an integral part of the insulation installation. From experience, 475 knows that with the Pro Clima system, one can quickly make a very durable airbarrier. Every month we have dozens of customers that blow-by 3.0ACH50 and many reach 0.6 or better for Passive House certification - with more and more hitting 0.3 and lower. Since this membrane is part of a complete airtightness system from Pro Clima with TESCON tapes, gaskets, and adhesives - we know that the contractors can reliably build and roof assembly that is durably airtight (100 year lab tested tape) on the interior and can verify this with a blowerdoor test. Giving the AHJ confidence that the WUFI's as shown below, can be relied upon - because without a certain level of airtightness, no reliable performance (not with INTELLO, not with foams).If Owens Corning ProPink Boxed Netting is a Step Forward, Then INTELLO is a Leap Forward Joe Lstiburek called the ProPink Boxed Netting in the GBA article "brilliant" and "fabulous", and Martin Holladay notes the netting is well designed. Both may be the case. But it should be noted that it is this netting material that is the cause for the system's narrow reach. The netting is vapor open and not airtight. Airightness is to be provided outboard of the insulation at the sheathing with canned spray foam, as GBA notes. Consequently, if the ProPink system is installed in a cold climate, the conditioned air can easily reach condensing surfaces and cause moisture damage. (And of course, airsealing is so important because air can carry 100x as much moisture into the structure then diffusion will - leading to damages much faster.) But if the fiberglass is held in place with INTELLO membrane - the insulation has vapor control and airtightness inboard, where it should be - the conditioned air stays in the conditioned space and away from condensing surfaces.