Dean Manoogian has a Cape Cod style house in Portland, Maine, and is puzzling over the best way to retrofit the roof with rigid foam insulation.
Working with both an insulation company and a roofing contractor, Manoogian has come up with a plan: apply 2-in. rigid foam on the interior of the dormered roof and then fill the rafter bays with dense-packed cellulose.
“My question,” he writes in this Q&A post at GreenBuildingAdvisor, “is the air/moisture permeability against the roof sheathing/rafters.”
The roof has been built without ridge, soffit or gable vents. To protect the sheathing and rafters from moist air, mold, and possible decay, Manoogian proposes covering the underside of the roof sheathing and rafters with a peel-and-stick membrane before adding the foam and cellulose.
The rigid foam would be capped with 5/8-in. gypsum wallboard to meet fire codes.
Is Manoogian on the right track, or courting a problem? That’s the focus of this week’s Q&A Spotlight.
Not the most elegant solution
The plan has already won the city’s approval, but to GBA senior editor Martin Holladay, it has its drawbacks.
“No,” Holladay writes, “you can’t solve the problem by adhering Ice & Water Shield to the underside of your roof sheathing and your rafters, for several reasons: the materials would be expensive, the work would be awkward, it’s impossible to do a perfect job, and you would be creating a vapor-barrier sandwich with roof sheathing in the middle — a disastrous approach if you ever get a roof leak.”
There are only two reliable ways of keeping water vapor away from the roof assembly, says Holladay. One way is to spray closed-cell polyurethane foam on the underside of the roof sheathing. The second way is to install soffit vents, ridge vents, and ventilation baffles in each rafter bay, in order to vent the underside of the roof sheathing.
Because the roof has already been constructed, he adds, Manoogian may be forced to go ahead with his plan to install dense-packed cellulose in unvented rafter bays. But it’s not an approach that Holladay would recommend to other GBA readers.
No wait, it will work
Maine builder Dan Kolbert disagrees. “We have done many dense-packed unventilated roofs without incident (that we know of, at least),” he says. “It is indeed a controversial topic these days.”
Kolbert has discussed the issue with Bill Hulstrunk, the technical rep for National Fiber, a cellulose supplier based in Massachusetts, and while Hulstrunk comes from inside the industry he’s also seen more New England cellulose projects than anyone Kolbert knows and has a “vested interest” in their success.
“The short version is he hasn’t seen any problems with well-installed insulation,” Kolbert says. “The full ice & water job on the roof is a bit of a drag, but I think you should be OK. And I too have had local code officers sign off on ‘hot’ roofs.”
Let it dry to the interior
Hulstrunk is next to weigh in, and he emphasizes two things: the roof assembly must be able to dry to the interior, and the density of the installed cellulose is crucial.
He recommends one of several rigid foam products that will allow moisture in the roof assembly to dissipate, including glass-faced polyiso board, expanded polystyrene (EPS) and extruded polystyrene (XPS).
Avoid using a polyethylene vapor barrier, foil-faced polyios board or Ice & Water Shield on the interior, he says, because they will not allow the passage of moisture to the inside and “create the self-composting double vapor barrier situation.”
As important is the density of the cellulose. “The cellulose installed in enclosed building cavities including non-vented roof assemblies need to be dense packed at a density of 3.5 lbs/cu. ft. for a 2×6 increasing to about 4.0 lbs/cu. ft. for a 12 inch cavity to become self supporting and will not settle over time,” Hulstrunk says. “Cellulose loose blown in a flat attic will have some settling over time down to its settled density of about 1.4 lbs/cu. ft. These flat attics will need to be ventilated to the outside to prevent diffusional moisture from condensing on the underside of the roof.”
He adds that in buildings where the roof assembly will dry to the inside, the indoor relative humidity should be no higher than 50% during cooler months. That can be accomplished with either a heat-recovery or energy-recovery ventilator or even a continuous-duty bathroom fan on a timer.
“Although we would prefer having your non-vented roof assembly dry towards the exterior or even better yet be able to dry in both directions, there are thousands of examples of durable roof assemblies filled with cellulose that dry only towards the interior,” he adds. “We have eliminated the need for Ice and Water Shield products by insulating roof assemblies with insulation products that actually work.”
Our expert’s opinion
Peter Yost, GBA’s technical director, added this:
While I agree with Hulstrunk that using a vapor permeable rigid insulation with dense-packed cellulose in the roof framing assembly will allow “everything” to the interior of the membrane to dry to the interior, I have two cautions:
- All building assemblies — but particularly “hot” roofs — require a dedicated, comprehensive air barrier. Dense-packed cellulose, while less air permeable than other loose insulation, is NOT airtight enough to qualify as the dedicated air barrier. Be sure that the rigid insulation used on the interior is air sealed to become a dedicated continuous air barrier. “Hot roofs” that are not fastidiously air sealed, particularly at the eaves, can fail.
- I find it hard to imagine how tedious and expensive it would be to wrap all of the framing and underside of the roof sheathing with peel-and-stick membrane. Since the roofing is vapor-impermeable and not back-vented — and since most of the roof sheathing has peel-and-stick membrane on the top side — the roof sheathing and framing would be encased between two vapor-impermeable membranes, with no drying potential at all.
Much to my surprise, when I was unable to find any vapor permeability information on three-tab asphalt shingles and I asked John Straube of Building Science Corp. what he used for their vapor permeability, he stated, “vapor impermeable.” What?
While the shingles themselves may be vapor impermeable, I felt sure that their installation would have enough nooks and crannies for air movement that their “equivalent” vapor permeability as a construction would be at least vapor semi-permeable. John stated that when he has tested assemblies, the sealant strips do a good enough job of sealing to render three-tab asphalt shingle roofs as essentially vapor impermeable.
I would be very carefully checking the underside of the roof deck for any evidence of bulk water leakage, past or present, before dense-packing the roof cavities with the cellulose.