UPDATED on April 8, 2016
There are lots of ways to insulate a low-slope roof, and most of them are wrong. In older buildings, the usual method is to install fiberglass batts or cellulose on top of the leaky ceiling, with a gap of a few inches (or sometimes a few feet) between the top of the insulation and the roof sheathing. In some cases, but not all, there is an attempt to vent the air space above the insulation to the exterior.
It’s rare for anyone to inspect the roof sheathing — unless, of course, the boards gets spongy enough to be noticed when the building is re-roofed. If there were any way you could squeeze into the tiny attic under the flat roof, however, you would probably see evidence of mold or rot.
Defining our terms
What’s a low-slope roof? It’s a roof that is flat or almost flat. This type of roof is common in urban areas (for example, on triple-deckers in Boston and row houses in Philadelphia), as well as in the Southwest. Some of these roofs have parapets — perhaps on just one side of the roof, or perhaps on three or four — while others have no parapets at all.
These roofs are either framed with deep roof trusses, or are framed with roof rafters that are separate from the lower ceiling joists (creating a cramped attic between the flat roof and the ceiling). In some of these buildings, the attic is high enough to allow a person to climb into the attic through a hatch; in others, the attic is too cramped for human access.
Venting the attic under a low-slope roof is possible but difficult
So what’s wrong with insulating a flat roof the traditional way? Nothing, really — as long as the job is done correctly: that is, with an airtight ceiling and adequate attic ventilation.
The problem is that most of these roofs aren’t built correctly. The ceilings leak air, and the attic ventilation is inadequate. There’s just enough ventilation to pull warm, moist interior air through ceiling cracks; once the moist air is in the tiny attic, the moisture accumulates in the cold roof sheathing. The result is rot and mold.
Here’s some advice from Joe Lstiburek, a principal at the Building Science Corporation: “If you have an airtight ceiling, and you have an air gap of at least 6 inches between the top of the insulation and the roof deck, and if you have perimeter air coming in at vents at the soffit or fascia above the insulation, and if you also have ventilation openings near the center of the roof through some kind of cupola or doghouse — not just a whirlybird turbine vent — there is nothing wrong with your roof assembly,” Lstiburek told me recently. “You can build a 2 foot by 2 foot doghouse that sticks up a few feet, and put in some rectangular vents. If the ceiling is airtight, then the makeup air comes from the outside. That’s the least expensive way to do things.”
Lstiburek continued, “The problem with this type of roof is that it is rarely executed correctly. Usually, architects don’t want to provide any ventilation around the perimeter. Or the architect won’t provide a deep enough truss to get enough insulation. If you just have a few whirlybird vents and a leaky ceiling, the whirlybirds will suck moisture-laden air out of the building and the roof will rot.”
Bruce Harley, the technical director for residential energy services at the Conservation Services Group, shares Lstiburek’s contempt for turbine vents. “I dislike turbine vents,” Harley told me. “I’d prefer a big mushroom vent or two over a turbine vent.”
The right way to vent a low-slope roof
If you want to build a low-slope roof that is insulated with fluffy insulation, here are the details you need to include:
- Specify very deep roof trusses. The trusses should be deep enough for 12 to 16 inches of insulation (depending on your climate), plus room for an air gap of at least 6 inches between the top of the insulation and the roof sheathing. Even better: frame the roof separately from the ceiling, so that there is an attic that is deep enough for human access.
- Provide vents at the perimeter of the shallow attic. These can be soffit vents, fascia vents, or wall-mounted vents, as long as the vents allow exterior air to connect with the air gap between the top of the insulation and the underside of the roof sheathing.
- Provide one or more vented cupolas (“doghouses”) in the center of the roof. Most building codes require 1 square foot of net free ventilation area for every 300 square feet of attic floor area; half the ventilation area should be located at the perimeter of the building, and half of the area should be located at the cupolas near the center of the roof.
- Perform air sealing work at the ceiling before the insulation is installed. Pay close attention to electrical penetrations, plumbing vent penetrations, the top plates of partition walls, and access hatches. The ceiling should be airtight as you can make it.
Bruce Harley emphasizes the importance of air sealing. He said, “Besides the standard bypasses — the partition walls and plumbing penetrations — remember that these older masonry buildings often have furring strips at the perimeter walls, and the cavities created by the furring strips may reach into the attic and need to be air sealed.”
If you forget to vent the attic, lots of things can go wrong
Builders in Arizona often use open-web trusses to frame low-slope residential roofs. Some of these builders cut corners: they omit the air space above the insulation and don’t bother to install any ventilation openings. They just jam the fiberglass batts up against the underside of the roof sheathing, wire the insulation in place, and cross their fingers.
Oops. About seven years ago, this insulation method was implicated in a cluster of wet-roof failures in Arizona. The first signs of problems were drywall cracks at the intersections between ceilings and partition walls — classic signs of truss uplift. (Truss uplift occurs when the top chord of a roof truss experiences different humidity conditions from the bottom chord; the humidity difference causes the trusses to deform.)
Uncertain of the cause of the drywall cracks, one of the builders called in William Rose, the well-known building scientist from the University of Illinois, to investigate. Rose discovered that the homes had wet roof sheathing — due in part to the type of roofing installed on the affected homes (white membrane roofing). “In December, January, and February, the fiberglass was wringing wet,” Rose told me. “In this climate, radiant effects become really important. There is nothing standing in the way of the roof radiating out to space. You have a whole lot of heat loss from the roof surface, day and night. With this white roofing, 80 percent of the heat that hits the roof is reflected. The sun can’t keep up with the heat losses to the sky. What you’ve created is a sky-powered cooling coil, and the fiberglass insulation is like a dirty condensate pan. The roof sheathing gets so cold that it is sucking wetness out of dry air.”
John Tooley, a senior building science consultant at Advanced Energy Corporation in Raleigh,
North Carolina, was also called in to help investigate the case. “At one roof I investigated — it was a flat-top roof assembly with a hot tar membrane roof coated with an off-white elastomeric coating — we pulled the roof off to take a look,” said Tooley. “The roof deck was totally saturated, and there was mold growth all over the bottom of the sheathing. The moisture content was greater than 30 percent. The fiberglass insulation was totally wet. This was in a house that was less than a year old.”
Tooley told me that this type of failure was common. “If you busted open roofs all over the Southwest, you’d find that the lower the pitch of the roof, the more you would see the evidence of moisture,” he said. “All of these roofs get wetting and drying cycles. If the wetting cycle is long enough, mold will grow and the insulation will get wet. I think if you cut roofs open, you will often find out that they are wet.”
The recommended solution to the problem of these wet Arizona roofs was to add a layer of rigid foam insulation above the roof sheathing.
What if you don’t want to depend on roof venting?
Let’s face it — it’s hard to vent a flat roof. That’s why most commercial low-slope roofs, including the roof on your local WalMart, are unvented.
In many ways, it’s easier to build an unvented low-slope roof than a vented low-slope roof. If you go this route, there are several possible ways to proceed:
- You can install a thick layer of rigid foam insulation (6 inches or more) above the roof sheathing.
- You can install a more moderate layer of rigid foam insulation (2 to 4 inches) above the roof sheathing, supplemented by a layer of air-permeable insulation below (and in direct contact with) the roof sheathing.
- You can install a layer of closed-cell spray polyurethane foam roofing on top of the roof sheathing, supplemented by layer of air-permeable insulation under the roof sheathing. (For more information on spray-foam roofing, see Spraying Polyurethane Foam Over an Existing Roof and Roofing With Foam.)
- You can install a thick layer of closed-cell spray polyurethane foam on the underside of the roof sheathing.
- You can install a more moderate thickness of closed-cell spray polyurethane foam on the underside of the roof sheathing, supplemented by a layer of air-permeable insulation below that.
Of course, the total R-value of your roof insulation must at least meet minimum code requirements. Moreover, if you install a combination of foam insulation and air-permeable insulation, you need to be sure that the foam insulation is thick enough to keep the roof sheathing (or the lower surface of the foam insulation) above the dew point during the winter. The minimum R-values for the rigid foam insulation needed for this type of roof assembly are shown in the table below.
These roof assemblies dry inward
The insulation methods described above — those used for unvented low-slope roofs — are similar to the methods used to create an unvented cathedral ceiling. To read about the methods in greater detail, see How to Build an Insulated Cathedral Ceiling.
While vented roof assemblies are designed to dry to the exterior, unvented roof assemblies are designed to dry to the interior. That’s why an unvented roof assembly should never include interior polyethylene. (If a building inspector insists that you install some type of interior “vapor barrier,” you can always install a smart vapor retarder like MemBrain to satisfy your inspector.)
For more information on roof assemblies with exterior rigid foam, see these two articles:
- How to Install Rigid Foam On Top of Roof Sheathing
- Combining Exterior Rigid Foam With Fluffy Insulation
A (somewhat) controversial approach
What if you need to insulate an existing low-slope roof with attic access on only one side of the building? This type of attic might be 3 feet high on the high side, but might taper down to only 6 or 8 inches on the low side.
Doing it the right way probably requires some ceiling demolition and a spray-foam contractor (if the work is performed from the interior), or else requires new roofing (if the work is performed from the exterior). Either approach is expensive, so some contractors have figured out a way to insulate this kind of roof without demolishing the ceiling and without installing new roofing.
Bill Hulstrunk is the technical manager at National Fiber, a manufacturer of cellulose insulation. When I interviewed Hulstrunk recently, he described a technique to insulate tapered attics with limited access. “With that type of attic, we crawl in and do as much air sealing as possible on the side with good access. Then there is a point where the attic gets too confined and you can’t crawl in there to do any air sealing. So we’ll dense-pack the side of the attic with limited access, and then we’ll blow in loose-fill cellulose on the side of the attic where there was enough access for air sealing work. It is always a good idea to have some vents on the side walls, above the top of the insulation, to provide some connection between the air above the loose-fill insulation and the outside. If we have done a good job with the air sealing, we have reduced the amount of moisture that will get up there. But in case there is some moisture that gets through, it’s good to have some way to allow the moisture to be able to make its way to the exterior.”
Hultrunk’s approach receives qualified endorsement from Bruce Harley. In his book, Insulate & Weatherize, Harley writes, “Even an excellent dense-pack job can allow some air movement. In an unvented cathedral ceiling or flat roof, this can deposit moisture at the roof deck, especially in a home with high humidity. There are two basic strategies to avoid increasing the risk of condensation and potential damage to the roof deck: using foam insulation to control condensing temperatures, and ensuring an opening from the unvented cavities into a larger, vented space. The first approach — using continuous foam insulation — is the only proven, code-approved method for an unvented roof. … The second approach is to provide a partial venting path for the closed dense-pack area. … If one end of the dense-packed area is open to a vented attic space (preferably the top), any wetting effects appear to be balanced by drying toward the vented space. This approach can also be used under low-slope roofs (for example, a row house or shed dormer), where access near the low side is impossible. Experience has shown that up to one-third of the total attic area can be dense-packed without venting, provided that the remaining attic space is vented normally. Note that this method does not conform to standard code requirements but has been accepted by many local building officials. And I would consider this approach much more risky in climate zones 6 to 8 [than in warmer zones].”
When I interviewed Harley about using dense-packed cellulose in low-slope roofs, he was cautious. “I am not comfortable trying to dense-pack an entire attic cavity, especially where it gets deeper in some areas, and therefore harder to dense-pack well,” Harley told me. “The risks are just too high. There are examples of dramatic and expensive failures. There are lots of questions: how effectively did you really dense-pack over areas where there may be air leaking? Are there small flaws in the cellulose? What is the moisture load in the house? We don’t really have control over that.”
Harley still endorses limited use of the dense-packing method, as long as a list of conditions is met: “For a common rowhouse in Chicago or Philadelphia, in climate zone 5, in a building with effective code-compliant venting of the attic space, we have seen pretty good results from an approach that includes dense-packing the lowest part of the the attic,” Harley said. “But never more than one-third of the total attic area.”
Repairing a problematic roof
What if you are called in to repair problems in an existing building with a low-slope roof that shows signs of moisture?
“We fix the problem roofs — the ones that get moldy — one of two ways,” Joe Lstiburek told me. “When the roof sheathing gets moldy, people freak out. The usual way we repair them is from the inside. We take out the gypsum ceiling and the insulation, and we spray 2 or 3 inches of closed-cell spray foam on the inside of the roof sheathing and the inside of the short walls. We encapsulate the mold. Then we repair the ceiling and we blow the space full of cellulose. If the space is too deep to fill with cellulose, we sometimes blow low-density spray foam over the high-density spray foam, because low-density foam is cheaper than high-density.”
The second way to fix this type of roof is from the exterior. “We’ll install two 2-inch layers of polyiso on top of the sheathing, then a layer of OSB, and we’ll screw it all down and install the roofing. From the inside, we’ll install fiberglass batts up against the roof sheathing, held in place with metal pins. That’s a foolproof method.”
A quirk in the code
Lstiburek also explained a code quirk that sometimes results in superinsulated ceilings. “In California, the code for multifamily construction requires that you need to install sprinklers if that ceiling space is not completely filled with insulation,” he said. “Sometimes we are called into a project with 2-foot-deep parallel chord trusses. If they install 14 inches of insulation, and a ventilated space above, the system works perfectly fine. But to save the cost of the sprinklers, they have been building unvented ceilings, putting R-20 of rigid foam on top of the roof sheathing and R-50 of fluffy insulation in the roof trusses. It turns out that R-70 insulation is cheaper than sprinklers.”
Martin Holladay’s previous blog: “Are Affordable Ground-Source Heat Pumps On the Horizon?”