How to Insulate a Cathedral Ceiling with Mineral Wool
If the rafter bays of a sloped ceiling are packed with insulation, is venting the roof necessary?
John Roy is building a house in southeastern Massachusetts, and at least part of it will have a cathedral ceiling. He's thinking of insulating the ceiling with dense-packed rock wool.
The president of a local insulation company tells him there's no need to install air chutes in the rafter bays before the insulation is blown in because the insulation does not absorb water. The local building inspector is prepared to go along with the recommendation providing soffit vents are installed.
Is Roy getting good advice? That's what he's asking in this Q&A post, the subject of this week's Q&A Spotlight.
First, will that be enough insulation?
Whether unvented cathedral ceilings should be dense-packed at all is a question that's generated plenty of discussion in the past. Leaving that aside for the moment, a key question for Roy is whether mineral wool packed into a 2x10 rafter bay will provide enough thermal insulation for his locale, and the answer is no, it won't.
As GBAGreenBuildingAdvisor.com senior editor Martin Holladay points out, a 2x10 is really 9¼ in. deep. Using the values published by the National Association of Home Builders Research Center, mineral wool at a density of 2½ pounds per cubic foot would have an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of about 34 if it completely filled the rafter bay.
The 2006 International Residential Code requires ceilings in Roy's area (Climate Zone 5) to be insulated to at least R-38, Holladay says.
If part of each rafter bay is dedicated to an air chute, reducing the amount of insulation that can be added, the numbers get worse. This means that whether Roy adds air chutes or not, he'll need to find a way to add more insulation if he wants to meet IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. requirements.
Debating the merits of ventilation
Despite assurances from the insulation contractor, building codes won't allow the kind of roof assembly Roy is contemplating.
"According to my understanding of the International building codes, you can't install an air-permeable insulation like mineral wool in a cathedral ceiling unless you first install ventilation chutes on the underside of the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. ," Holladay says. "Styrofoam ProperVents are too flimsy to withstand the pressure of dense-packed insulation, so you'll probably want to install plastic AccuVent or site-built ventilation chutes.
"It doesn't make any sense to install soffit vents if you don't have ventilation chutes between the top of the insulation and the bottom of the roof sheathing," he adds. "If it were my house, I would install ventilation chutes (well sealed to prevent air leakage). I would also install 2 inches of foil-faced polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. under the rafters, held in place with 1x4 strapping, and I would blow the mineral wool through holes in the polyiso."
A poster named M S directs Roy to an article by building scientist Joseph Lstiburek of the Building Science Corp., who says flatly: "As fantastic as the dense-pack approach is for walls, it is a pretty dumb approach for unvented cathedral ceilings and flat roofs. And, no matter how hard some of us work to try to stop people from going ahead and doing it, they persist on doing it. First, it is a building code violation. Second, it violates the physics."
It would seem Roy should abandon ship on whatever plans he had for dense-packing an unvented cathedral ceiling. So what's the best way of adding ventilation?
Making airtight ventilation chutes
The most effective ventilation chutes will be site-built, Holladay says. "Usually, the builder starts by tacking up 1"x1" sticks in the upper corners of the rafter bays," he writes. "Then rigid material (Celotex, thin plywood, or rigid foam) is installed against the sticks, with caulk or spray foam to make the installation airtight."
While providing an excellent air seal, these chutes are not the fastest to install, Holladay says.
M S says chutes might be made from sheets of polyisocyanurate foam insulation, sealed at the seams with spray foam. The foam would simultaneously serve as baffle and insulation. By dense-packing the remainder of the rafter bays with cellulose, at R-4 per inch, the roof would be rated at R-39.
"Now, as Martin suggests, overlay the face of your studs with rigid insulation, either polyiso or XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.," M S says. "Putting up another 2 in. of polyiso gets you close to R-52."
What about a vapor-permeable chute?
If the ventilation chute carries away any excess moisture from the cavity insulation, Thomas Jefferson says, it would seem "wise" to make the baffle with a vapor-permeable material.
"Foil-faced polyiso does add R-value but it's not permeable," Jefferson says. "I'd suggest hardboard (Masonite) or another fiber panel to just hold back the dense pack without impeding water vapor. The dense-pack will be only ~R-31, so you'll need to furr out below the joists or add polyiso below, as Martin suggested."
Holladay disputed Jefferson's opinion that "this vent chute serves partly to carry out any excess moisture from the cavity insulation." According to Holladay, "No one wants interior moisture migrating through the insulation to the ventilation cavity — certainly not deliberately," he says. The idea is to prevent any moisture from getting into the ventilation cavity and to provide a mechanism that helps keep the roof sheathing dry.
"The purpose of the ventilation channel is to lower the humidity of the roof sheathing — in case some sloppy carpenter did a bad job of caulking the ventilation chutes in place, or in case some electrician mistakenly installs wiring or a can light in your ceiling," Holladay says.
Isn't the purpose of a ventilation chute to prevent the accumulation of moisture? asks Jefferson. "Moisture can come from a few different sources, and one could be below the chute," he says. "Why design a vent chute to intentionally exclude water vapor that just might find itself in the cavity insulation? Not that it's intentional for the insulation to be damp and need drying to the exterior, but the opportunity of drying reduces risk."
Not really, Holladay says: "The purpose of the ventilation channel is as a last-ditch method of keeping your sheathing dry in case your assembly has flaws. But you don't want to build in any paths that allow or encourage vapor to flow toward your cold sheathing."
Spray-in foam works, too
Given the difficulties of building in an effective ventilation chute, Roy asks, what about the possibility of using closed-cell foam directly on the underside of the roof sheathing?
Closed-cell foam provides an air barrier, but to control condensation it must be applied to a minimum R-value, depending on what climate zone you're building in, Holladay says. That ranges from R-5 in zones 1-3 to R-35 in zone 8.
Our expert's opinion
Peter Yost, GBA's technical director, had this to say:
Roof assemblies, like wall assemblies, should be designed and insulated to dry in at least one direction. We tend to focus on how building assemblies get wet from interior moisture during the cold winter months, but more importantly, we have to focus on how they dry from any of a variety of moisture sources, not just wintertime interior relative humidity.
Roof assemblies are like wall assemblies that never get stood up straight. So, while they do see more bulk water, they can be designed and insulated without back-venting the roof sheathing (“hot roofs”), just as walls are. There are two critical elements, however: selecting materials for ALL layers that collectively create directional drying AND a continuous and contiguous (to the insulation) air barrier in the unvented roof assembly.
On the other hand, back-venting the roof sheathing is a more forgiving approach, if the soffit-to-ridge vent chutes are continuous and well-sealed with respect to the roof cavity insulation.
You can “decouple” the roof claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. and roof sheathing (in terms of wetting and drying) from the rest of the cathedral roof assembly with either a top-side air space or a vent chute underneath the roof sheathing. Each presents construction detail challenges, and costs. And frankly, the choice of roof cladding material and your climate can strongly suggest just how important some sort of venting is to the performance of the roof assembly. For examples, see this article from Building Science Corp. as well as this one.
For more information on vapor profiles and drying, take a look at this blog.
- M S
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