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Insulating Low-Slope Residential Roofs

Unless you’re careful, your low-slope roof can end up with damp sheathing

Posted on Apr 12 2013 by Martin Holladay

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 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. . 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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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 insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection.. 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 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. 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?”

      Click here to follow Martin Holladay on Twitter.

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Apr 12, 2013 8:59 AM ET

Some observations that were not publishable
by Bill Rose

Good job.

Whenever you do research you always have the recorded findings and then some side observations — never enough for publication, but they get filed away for later (or not). We had two pertinent observations in this Arizona study, without enough support to draw publishable conclusions:

1) Venting didn't work. It may have had a slight effect of warming the underside of the roof, but with low-slope it's all wind and no buoyancy, and at night the air is quite still. Plus, they were adding vents without adding an air space. Not helpful and not publishable.

2) Cellulose insulation was installed in one single case. The cellulose was at the bottom of the assembly rather than at the top, of course. That assembly had no problem at all and stayed really dry. I think we underestimate the impact of even seasonal moisture storage in cellulose.

Bill Rose

Apr 12, 2013 11:14 AM ET

I sure hope that nobody is
by Jin Kazama

I sure hope that nobody is still building "vented flat roofs" nowadays.
steel deck + vapor membrane and exterior insualtion is the only way to do it.

Apr 12, 2013 11:20 AM ET

Martin: just a quick thought,
by Jin Kazama

Martin: just a quick thought, the same "dew point" rule/calculation applies to roofing assembly then walls ?
If so, it would mean that some quantity of interior insulation could be possible even if within the final vapor membrane ?

I do like the steel roof deck used a a reflective radiance material though ...would loose this property if packed up with some insulating materials ??

Apr 12, 2013 11:41 AM ET

Dimensional Stability
by Skylar Swinford


Thanks for the great article. I noticed in the photos that the roofers/insulators are using sheets of EPS installed in staggered layers that are considerably smaller than an average 4'x8 sheet, but you didn't touch on this detail in your article. The Manual of Low-Slope Roof Systems, has a great section explaining the importance of using smaller insulation boards to improve dimensional stability. For example, the National Roofing Contractors Association (NRCA) recommends using rigid insulation boards that are 2'x4' to reduce stress on roof membranes. Lstiburek does a great job explaining why rigid foam insulation expands and contracts in his "correction" to Foam Shrinks, and Other Lessons. The Manual of Low-Slope Roof Systems also has valuable information on inverted (IRMA) or protected membrane roof (PMR) systems that readers in more extreme climates may find interesting. Plus the manual contains more great info on the pitfalls of ventilating low-slope roofing and the authors do a great job hammering home the importance of preventing ponding with adequate slope and drainage.

So it looks like the roofers/insulators in the photos got the smaller rigid foam boards right; however, there doesn't appear to be a structural air-barrier at the roof sheathing. Perhaps they are relying on SPF or SIGA tape below...any thoughts?

Readers may appreciate the article: Breaking With The Code for more observations regarding the typical ineffectiveness of ventilating low-slope roof assemblies.

Apr 12, 2013 12:09 PM ET

Response to Jin Kazama
by Martin Holladay

Q. "Does the same dew point rule/calculation apply to roofing assemblies as walls?"

A. No. All of this is explained in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing. While most of the article talks about walls, there is a section ("Is there a similar chart for unvented cathedral ceilings?") that talks about roofs.

The reason that the recommendations differ for roofs is that rafters are usually deeper than studs. It's common for rafters to have more insulation than studs; that makes the sheathing colder, which means that the rigid foam has to be thicker.

Apr 12, 2013 12:16 PM ET

Response to Skylar Swinford
by Martin Holladay

Thanks for the added information, and for the useful links.

You're right that the roof in the photo would probably have benefited from an air barrier between the roof sheathing and the insulation (or at least taped plywood seams).

For more information on dimensional instability in roofing foam, see my July 2000 article on the topic, "Shrinking Insulation Boards Plague Roofers." The article was published in the "Notebook" section of JLC; scroll down to the end of the section to find the article.

Apr 12, 2013 3:01 PM ET

Edited Apr 12, 2013 3:11 PM ET.

Expanding on Martin's response to Jin Kazama
by Dana Dorsett

Not only are the insulation depths bigger, in a roof assembly there is an extremely low-permeance exterior finish surface (the roofing material itself) that is often wet with dew/rain/snow anyway. Literally ALL drying of the assembly is toward the interior in an unvented roof.

This can be true of wall assemblies with foil-faced foam on the exterior too. The IRC prescribed minimums are just that, MINIMUMs, not optimal, not necessarily the best. Personally I'd normally shoot for more exterior R, and a more vapor-permeable exterior R. (EPS is about permeable at R15 as XPS is at R8) When there are foil facers in the mix I prefer to go 25-50% more than the IRC prescriptive min- margin counts.

But the moisture buffering of cellulose also counts. At IRC prescribed minimums there's a real difference in the seasonal moisture cycling of the sheathing between mineral/glass wools and cellulose (any density) for the fiber layer of the stackup. Bill Rose's second observation in the first comment is not a fluke.

FWIW: A few years ago I dense packed a portion of the underside of 2x10 rafters 12" o.c of a north side flat roof shaded by the taller portion of the building. It has tapered EPS on top of the roof deck under a membrane that has since been replaced. Other parts of that roof had ~R10 rock wool batts between the rafters. Some of the roof decking on the portion with the batts became punky and had to be replaced during the re-roofing, but so far the dense-packed portion remains solid. This is in a building with low occupancy rates and few interior moisture sources, and though the foam/fiber ratio wouldn't cut it from a code point of view in a residential building, I'm not too worried- it seems to be doing the job.

Apr 13, 2013 9:26 AM ET

Seriously, how much more
by Jin Kazama

Seriously, how much more labor is is to make a vented roof and install cellulose/batts on ceiling than
it would be to simply use more rigid insulation on top.
With the current EPDM and other membrane prices and technology ...

I am considering a balasted ( pebles ) epdm on top of a "to be determined" R value of rigid insulation
that will be fastened to the steel deck through a peel stick membrane that will serve as inside vapor barrier.

I don't see how one could do simpler than that labor wise.

wood sheathing has no place in a roof, always ends up leaking somehwere and then it needs to be torned out again whereas a leak on a steel deck can usually be fixed without destroying the roof.

Apr 13, 2013 3:27 PM ET

Bill's Sample of One
by Dan Kolbert

I realize it would be dangerous to draw much of a conclusion from the one cellulose-insulated roof, but it certainly is interesting and begs for more investigation. Bill H. is chuckling to himself somewhere in the background.

Apr 17, 2013 8:08 PM ET

A 12" SIP can solve all the moisture issues and support itself
by Kevin Dickson, MSME

An EPS structural insulated panel for the roof/ceiling system can really make it easy. Because of all the issues it solves, the extra cost is warranted. If the engineering is done right, the SIP will save a lot of money on roof joists and beams because of its inherent stiffness.

A SIP is also the most elegant way to eliminate thermal bridging at the roof/wall connection.

At R-48, however, it's a little less than optimum. Is anyone making a 12" polyurethane SIP? Or a 16" EPS SIP?

Cover it with white EPDM, and you should be good for 50-70 years, even in hail country.

Apr 18, 2013 5:36 AM ET

Edited Apr 18, 2013 5:37 AM ET.

Response to Kevin Dickson
by Martin Holladay

As I wrote in the article, "The insulation methods [for low-slope roofs] are similar to those used to build an unvented cathedral ceiling." And in my article on cathedral ceilings, I wrote, "Another possibility, of course, is to build your roof with structural insulated panels (SIPs)."

SIPs are rarely used for commercial low-slope roofs, because they are more expensive than the more common approach (installing rigid foam on top of the roof sheathing). The other problem with SIPs is the tricky matter of sealing all of those seams.

Considering the higher cost of SIPs and the seam-sealing problem, most designers will probably stick with more conventional approaches to insulating low-slope roofs.

Apr 18, 2013 11:58 AM ET

Low-Slope Venting: "All Wind, No Buoyancy"
by Rich Backus, GMB CPHC

Bill, Martin:
If we define 'low-slope' as 'flat, or nearly so', do we state a pitch < 1 in 12 ?
And to avoid a nerd-fest on that definition, the question is this: At what pitch would we expect buoyancy / air density differential to create ventilation air movement worth considering as a mechanism for drying ? (ignoring for discussion wind pressure differentials as a factor, and assuming Joe's 6" air space as entirely inviolate/ extant, except for dust and pollen particles, and the stray African Swallow carrying coconuts.)

Apr 18, 2013 12:25 PM ET

Response to Rich Backus
by Martin Holladay

Your question may be of interest to researchers, but probably not to builders. For builders, such hair-splitting falls into the category of "misplaced accuracy."

If you are worried that your roof won't get enough air flow for proper venting, the solution is simple: build an unvented roof.

For a builder, a low-slope roof is like pornography for a Supreme Court justice: you know it when you see it.

Apr 18, 2013 3:39 PM ET

Air-cooled insulation
by John Walker


Isn't free ventilation pulling perimeter air up and through "dog-houses" reducing the insulation value of the fiberglass/cellulose insulation layer? Moving air is the enemy of effective insulation...

Apr 18, 2013 3:56 PM ET

Response to John Walker
by Martin Holladay

Q. "Isn't free ventilation pulling perimeter air up and through "dog-houses" reducing the insulation value of the fiberglass/cellulose insulation layer?"

A. The degree to which wind-washing degrades the performance of air-permeable insulation depends on many factors. Ideally, the incoming ventilation air is introduced above the top of the insulation layer; if this is the case, then a ventilated low-slope roof is no different (at least in theory) from a ventilated attic.

In a classic ventilated attic, wind-washing does slightly degrade the performance of the insulation near the soffit vents -- especially if the insulation is fiberglass, and especially if there is no wind-washing dam. If a wind-washing dam has been installed, or if the builder has chosen cellulose insulation instead of fiberglass, the thermal degradation caused by wind washing is much less.

In general, there are three ways to minimize the negative effects of wind-washing:

  • Choose cellulose, not fiberglass
  • Install wind-washing dams near soffits
  • To make up for possible wind-washing effects, make the insulation a little bit deeper than you otherwise would.

Apr 23, 2013 8:59 AM ET

Open vs closed-cell foam on underside of a low-slope roof
by Mitchell Daniels

Good morning.

Planning an addition in Montreal, Canada, with a low-slope roof (Soprema modified bitumen roofing on plywood or OSB on engineered joists).

Was planning on spraying about 6 inches of closed-cell foam on the bottom of the plywood deck (as air and vapor barrier).

I am being told that 12 inches of open-cell foam on the bottom of the plywood deck (as air barrier) with a paint vapor barrier may be a better option.

The logic, as I understand, is that if there is a leak in the roof and closed cel foam was used, the owner will have no idea of the problem. The moisture will be trapped above the foam and the plywood runs the risk of rotting.

However if open-cell foam was used with a paint vapor barrier, if a leak occurs, there is a much better chance that the owner will see a stained ceiling and take action before rot sets in

Any thoughts/comments? Thanks in advance!

Apr 23, 2013 9:16 AM ET

Response to Marc Daniels
by Martin Holladay

I strongly urge you to choose the closed-cell spray foam.

In your climate, open-cell spray foam (which is vapor-permeable) allows interior moisture to reach the roof sheathing by diffusion. Over the course of the winter, the sheathing can get wet enough to begin rotting.

One solution that was recommended in the past (even I recommended it until I learned why it is bad advice) is to spray vapor-retarder paint on the underside of the cured spray foam. The only problem with this solution is that it doesn't work.

Engineers at the Building Science Corp. conducted tests that showed that vapor-retarder paint only works when you spray it on gypsum drywall (a smooth surface). When you spray the paint on an uneven porous surface like cured spray foam, it is worthless as a vapor retarder. The interior vapor goes right through the paint.

One possible solution (if you really want to use open-cell spray foam) is to cover the cured foam with a layer of gypsum drywall, and then to spray the drywall with vapor retarder paint.

Better yet, just choose closed-cell spray foam.

More information here: Creating a Conditioned Attic.

Apr 23, 2013 1:08 PM ET

Response to Martin Holladay
by Mitchell Daniels

Thank-you Martin.

I was not clear in my description. As for the open cell foam option, the plan was to fix drywall to the bottom of the engineered joists and then apply a vapour barrier paint to the drywall. The idea being that if there is a roof leak, the moisture would travel through the plywood roof deck and through the open cell foam and down to the drywall. The owner would then be aware of a leak and would able to fix before rot sets in.

I would prefer to use closed cell foam. The concern in this case is that if there is a roof leak, water would be trapped in the plywood deck by the closed cell foam. The owner would have no idea of this until the plywood deck would rot out.

Further thoughts?

Apr 23, 2013 1:29 PM ET

Edited Apr 23, 2013 1:47 PM ET.

Response to Marc Daniels
by Martin Holladay

The question you raise is a thorny one. Many people have proposed the same theory: that roof leaks will show up faster if you use open-cell foam rather than closed-cell foam.

Maybe. However, I have my doubts that the situation is as simple as the theory proposes.

I used to work as a roofer, and I know from experiece how hard it can be to trace a roof leak by looking at ceiling stains.

Some roof leaks show up as ceiling stains or ceiling drips immediately. Those are the easy cases.

Far more common are the roof leaks that start slow and aren't noticed for a year. This happens all the time, even with conventional building materials like wood framing and cellulose insulation. When the ceiling stain finally appears, it can be 20 feet sideways from the leak. Water moves, and water does tricky things. Many building materials are absorbent, complicating the situation.

It's even possible that closed-cell foam, being more waterproof, is more likely to protect vulnerable building materials from damage than open-cell foam.

The fact is, there are too many variables to make a clear-cut ruling on this issue. The bottom line: no matter what type of roofing, sheathing, or insulation materials you use, you want to repair any roof leaks as fast as possible. Sometimes fast repairs are possible because the owners are paying attention, and sometimes fast repairs are possible because of luck.

Other times, even when you think you're paying attention, a hidden roof leak can do considerable damage before anyone notices.

Finally, if you are really worried about this issue, there is a simple solution: design a building with a ventilated, unconditioned attic, and go up in the attic with a flashlight regularly to inspect the roof sheathing.

Apr 23, 2013 2:09 PM ET

Response to Martin Holladay
by Mitchell Daniels

Thanks again - very much appreciated!

May 21, 2014 3:01 PM ET

Insulating a cool, self-drying, vented, low-slope roof
by Confused Homeowner

I own a mid-century modern home in the Seattle area with a 1/12 pitch roof over vaulted ceilings separated by 2x6 joists with R13 batt insulation, venting at the soffit and ridge, and a condensation issue that started after the modified bitumen roof was replaced last June with a white TPO membrane. I've been reading all of Martin's articles and I'm still not satisfied that I have the best solution. It seems that the root of my condensation problem is that we've lost sight of the self-drying properties of a torch down or modified bitumen roof assembly because we're so focused on cool roofs using membranes with lifetime material warranties. If you read some of these articles below, you'll see that the heat absorbing properties of these older roof systems are part of the design. You may also notice the precautions for colder climates in the U.S. Department of Energy Guidelines fo Selecting Cool Roofs.
Unfortunately, I put complete trust in my roofer and now I have a leak free lifetime roof that condensates inside and doesn't dry out very quickly. In my search for a solution, I've found that most, if not all, of the technical documentation for TPO membrane are focused on commercial roof assemblies and they require an "adequate" insulation layer above the roof deck. It seems to make sense that some insulation above the roof deck would be appropriate when replacing a modified bitumen roof with a cool membrane roof to make up for the difference in solar reflectance and thermal emittance properties. However, residential roofers tend to dismiss the need for insulation above the roof deck as a waste of R value. What do you think? If I re-roof again this summer, should I go back to modified bitumen or should I add a layer of insulation above the roof deck and use a darker color of TPO or PVC? I've also considered replacing the batt insulation, either with a dense packed blow-in-blanket system followed by 3" of rigid insulation above the roof deck or maintaining the vented system with 4" of closed cell spray foam above the ceiling, or 4" of rigid insulation sealed in above the ceiling. These other options are a lot more expensive and are still not foolproof. I've already ruled out the option of removing the ceiling to fix the problem from underneath because it's too disruptive.

May 21, 2014 3:15 PM ET

Response to Confused Homeowner
by Martin Holladay

The problem you are experiencing is described in my article, under the subhead, "If you forget to vent the attic, lots of things can go wrong."

In those paragraphs, I describe how white membrane roofing stays colder than dark roofing, encouraging condensation. Building scientist William Rose describes this type of roofing as a "sky-powered cooling coil," while "the fiberglass insulation is like a dirty condensate pan. The roof sheathing gets so cold that it is sucking wetness out of dry air."

Soffit-to-ridge venting doesn't work on a low-slope roof, as my article explains. For all intents and purposes, your roof assembly is unvented. The solution, as you correctly realize, is to install rigid insulation above your roof sheathing.

Since your roofer recommended a defective roof assembly, your roofer should fix the problem at no charge to you.

May 22, 2014 2:56 PM ET

Insulating a cool, self-drying, vented, low-slope roof
by Confused Homeowner

Since you said, "for all intents and purposes, your roof assembly is unvented", would you recommend converting to an unvented roof assembly? I've observed that dripping and other signs of condensation damage are concentrated under roof bays that have inadequate venting. This leads me to believe that my venting does work, but it needs to be improved with additional vents above and below my chimney and skylight. I'm also afraid of what might happen in a completely unvented roof assembly if I fail to completely seal off air leaks from the ceiling.

If I stick with venting when I re-roof, then should I keep the R13 fiberglass batts if they seem to be in decent shape? Or should I replace the batts with closed cell spray foam or rigid polyiso that has been cut to fit? Both approaches would provide the benefit of a higher R value and the disadvantage of making any future changes, such as electrical wiring, more difficult. Spray foam would create a better moisture barrier, but it's more expensive and I think it would likely involve use of harmful hydroflourocarbon (HFC) blowing agents.

May 22, 2014 3:10 PM ET

Response to Confused Homeowner
by Martin Holladay

Q. "Would you recommend converting to an unvented roof assembly?"

A. Your R-13 batts do not provide anything close to minimum code requirements for roof insulation. In your climate zone (Climate Zone 4C), the 2009 IRC calls for a minimum of R-38 insulation. The easiest way to provide the insulation you need without disturbing your interior finishes is to install rigid foam insulation on top of your roof sheathing and to seal your vent openings.

Q. "If I stick with venting when I re-roof, then should I keep the R-13 fiberglass batts if they seem to be in decent shape? Or should I replace the batts with closed cell spray foam or rigid polyiso that has been cut to fit?"

A. You don't want to install polyiso between your rafters. (Polyiso is best installed in a continuous layer, not cut into narrow strips as you propose.) Since R-13 is insufficient, you should choose a method of insulation that allows you to achieve R-38 or more.

When it comes to venting this type of roof assembly, I stand by the advice given in the article. The only way you can make venting work on this type of roof is if you can maintain a gap of at least 6 inches between the top of your insulation and the underside of your roof sheathing, and if you can cut a 2 ft. by 2 ft. hole in the roof sheathing near the center of your roof, so that you can install a doghouse above the roof to act as a vent outlet.

May 22, 2014 5:26 PM ET

Batts, Dense Pack, and Exhaust Fans
by Confused Homeowner

Thanks to your advice, my solution is becoming much clearer. I understand that I'm not required to bring my mid-century home up to 2009 IRC standards, but achieving R-38 or higher does seem like a worthy goal. One concern about sealing the vent openings is that the 3.5 inch thick R-13 batts have probably been in place for over 45 years and are likely not achieving the stated R value. They also leave two inches of available space in the 2x6 joist bays. Should I replace the R-13 batts with 5.5 inch thick R-21 fiberglass batts or with a dense packed R-24 blow-in-blanket system (BIBS)? Also, since all this insulation will make my house tighter, I'm planning to replace my contractor grade bathroom exhaust fans with Panasonic WhisperFit 80 cfm fans to help control indoor humidity levels. Since these fans are 5.5 inches high, I'm concerned about how much insulation they would displace below the roof deck. Would it make sense to build dropped soffits above my shower and bathtub for exhaust fans so that only the ducts will need to penetrate the ceiling?

May 23, 2014 7:01 AM ET

Response to Confused Homeowner
by Martin Holladay

Q. "One concern about sealing the vent openings is that the 3.5 inch thick R-13 batts have probably been in place for over 45 years and are likely not achieving the stated R value."

A. Obviously, sealing up the vent openings would not be a solution if that were the only action you performed. Clearly, you need to add R-value to the existing assembly.

Q. "Should I replace the R-13 batts with 5.5 inch thick R-21 fiberglass batts or with a dense packed R-24 blow-in-blanket system (BIBS)?"

A. Either choice would be an improvement over what you have, but neither choice would meet minimum code requirements. Only you can decide whether you can afford to do better.

Q. "Since these fans are 5.5 inches high, I'm concerned about how much insulation they would displace below the roof deck. Would it make sense to build dropped soffits above my shower and bathtub for exhaust fans so that only the ducts will need to penetrate the ceiling?"

A. No one should ever install a bathroom exhaust fan in the insulated bays of a cathedral ceiling. You have two choices: you can build a soffit under the existing ceiling, or you can install a wall-mounted exhaust fan (assuming that your bathroom has an exterior wall). The fan should be installed as high in the wall as possible.

May 23, 2014 10:50 PM ET

2/12 roof--to vent or not to vent
by Robert Vandermolen

I am building a ranch style home in the Chicago area (zone 5) with a low slope roof (2/12). We are installing a white TPO or PVC roof. My builder recommends going with a non vented roof using closed cell insulation to seal air leaks and meet energy codes. My architect recommends going with a vented roof using blown in cellulose to meet energy codes. I was leaning toward the vented roof until the architect drew in this linear cupola that ruined the lines of the house. I was hoping to have my house featured in Fine Home Building and not Mother Earth News. But I also don't want to be featured as an example of what not to do.

How much rigid foam do I need to install on top of the roof to prevent the condensation effect that was describe when using the white membrane roof?

Can I go with a standard low profile ridge vent and get the needed draw on a 2/12 roof?

Or should I go with the closed cell and pray the installer seals it all off and lays it on thick?

May 24, 2014 8:36 AM ET

Response to Robert Vandermolen
by Martin Holladay

I stand by the recommendations given in the article. If you want a vented roof, you need the doghouse. If you don't want the doghouse, build an unvented roof.

Rigid foam above the roof sheathing is a much better approach than spray foam under the roof sheathing (since the rigid foam addresses thermal bridging through the rafters). This is a standard approach for commercial construction, so talk to a roofer who handles commercial roofs.

If you want to put all of your insulation above the roof sheathing, you can just consult the code books and do the math; I imagine that you will need at least R-38 in your climate zone. If you want to combine rigid foam above the roof sheathing with fluffy insulation below the roof sheathing, you will need to install at least R-20 of rigid foam above the roof sheathing in your climate zone.

Jun 5, 2014 10:08 PM ET

Weighing options to achieve similar R-Value
by Confused Homeowner

I'm almost comfortable with the idea of replacing the R-13 batts in my 2x6 roof bays with dense packed insulation, either cellulose or fiberglass, plus adding at least three inches of polyiso on top. This article by Joseph Lstiburek both confirmed this approach as well as scared the heck out of me.
Here are some of the options I've considered for increasing the total R-value of my roof assembly from least expensive to most expensive...
1. Keep R-13 batts and add 4" of polyiso above deck
2. Keep R-13 batts, fill cavity with dense packed fiberglass or cellulose , add 3" of polyiso above deck
3. Keep R-13 batts, add 2" fiberglass batts, add 3" of polyiso above deck
4. Remove R-13 batts, fill cavity with dense pack, add 3" of polyiso above deck
5. Remove R-13 batts, add 1" of closed cell spray foam above ceiling, fill cavity with dense pack, add 3" polyiso above deck
Would you rule out any of these options? Which option, if any, would you choose?

Jun 6, 2014 4:58 AM ET

Response to Confused Homeowner
by Martin Holladay

As long as you follow the advice in this article -- especially concerning the minimum ratio of foam insulation to fluffy insulation -- any of your proposed options will work.

The minimum R-value requirement for your layer of rigid foam insulation or spray foam insulation depends on your climate zone, so I can't evaluate your proposed foam thickness choices without more information.

Your choice will also depend on whether you are willing to replace your roofing.

In terms of performance, the best option would be option #4 -- as long as 3 inches of polyiso meets the minimum R-value requirements for this approach in your climate zone.

Jun 25, 2014 9:39 PM ET

No parapet wall on one side - how does insulation stay on roof?
by Jerry Chwang

This might seem like a silly question, but how do insulation boards above the sheathing stay on the roofs that don't have parapet walls all the way around? Will the ballast be sufficient?

I have a low slope roof where we've left the low side without a parapet wall, so it acts as one big scupper. Just thought about the XPS on top and wondering how I keep it from 'sliding' off? If I go with a green roof as ballast, this gets even trickier as my dirt will wash off for sure?

Jun 26, 2014 4:38 AM ET

Response to Jerry Chwang
by Martin Holladay

Rigid insulation is attached to the sheathing with cap nails or cap screws (available at any roofing supply house). The perimeter of the roof is usually trimmed with flashing. If these concepts are new to you, you should hire a roofing contractor.

Jul 1, 2014 3:53 AM ET

Best option for a newly constructed low-sloped roof
by ron stanley

The more I read this and other sources on the subject of insulating, the more certain I am that it can be very easy to get it wrong. I have a 100 year old craftsman house in Tacoma (Marine 4C) that is in need of major updates (siding, windows, insulating, and roofing). I am about to start my remodel-redesign which includes changing the architectural style from craftsman to modern. The first phase will include complete removal of the 8/12 gable roof structure. In its place I will be constructing a shed style 1/12 roof structure with gray TPO roofing. The new attic space will have a 60” tall wall on the high side (North) and a 34” tall wall on the low side (South) leaving plenty of space to access all areas. I-joist will span between the two walls creating a 9 ½” cavity below the ¾” OSB deck. The current ceiling has R21 High Density glasswool batts between the 2 x 6 joist and it has ½” drywall below. There are numerous penetrations in the walls and ceiling, (plumbing, can lights, electrical, bathroom fan) none of which were carefully sealed by the previous owner.
Along with replacing the roof structure, I will also be removing and replacing all of the siding and many of the windows. I will be adding 3” of exterior rigid insulation and an open rain screen. The exterior insulation will run up the walls to the roof overhangs.
The existing attic is unconditioned and vented and shows no signs of moisture issues. Given my location and planned redesigned roof structure, what would be the least problematic approach to take after doing my best to seal all of the wall and ceiling penetrations in the attic:
1. Add more insulation to the ceiling and keep the attic space unconditioned and vented. If so is there any benefit or harm in adding a layer of rigid insulation between the OSB deck and TPO roofing?
2. Add 6” of rigid foam between the OSB deck and the TPO roofing making the attic space part of the conditioned volume by removing the existing batts and eliminating venting.
3. I’ve got it all wrong and should instead do this…
One additional note, I am trying to do all of this while my family continues to live in the house so it is important to have very limited disruption to the interior space.

Jul 1, 2014 6:19 AM ET

Response to Ron Stanley
by Martin Holladay

I stand by the advice given in my article; the details of your house don't change my advice.

Considering the penetrations you list, and the fact that your roof lacks a doghouse, it makes the most sense to follow the usual practices of commercial roofing and to install all of the insulation above the roof sheathing. If 6 inches of rigid foam is enough to get you to your minimum code requirements for R-value, then that's the way to go.

Don't forget to seal your attic vents and insulate your attic walls.

Jul 1, 2014 1:16 PM ET

Response To Martin Holladay
by ron stanley

Martin thanks for the response and this article. I have read and reread this article and all of the suggested linked articles. I have been finding it difficult to determine which advice applies for my application (low sloped roof over an attic space). I did not know that a 1/12 sloped roof would not vent soffit to soffit and would require “dog houses” mid span. I shared the details of my house because the multiple references to attic size “tiny attic” and “cramped attic spaces” made me wonder if my larger attic volume changed the equation in any way. I trust your advice and will go with the unvented more commercial style roof assembly.
If I understand the information in this article correctly (You can install a more moderate layer of rigid foam insulation (2 to 4 inches) above the roof sheathing, supplemented by a layer of vapor-permeable insulation below the roof sheathing) along with the information regarding minimum rigid insulation for unvented roofs (Table R806.5 specifies the minimum R-value for the foam installed on top of the sheathing -- R-10 for Climate Zone 4C) than can I go with 4” of rigid polyiso – R value of 23.6 according to the new LTTR on top of the deck and then move my existing R21 glasswool batts up to the bottom side of the deck for a total of R44.6? I already have enough 4” thick polyiso to cover the roof deck and it would be great if I did not need to add another 2-4 inches on top for both cost and aesthetic reasons.

In the future, would there be a problem with adding more vapor permeable insulation to the underside of the deck? It seems some people suggest having a 2/3 outside to 1/3 inside insulation ratio. I believe this is to keep from isolating the warm side of the deck and turning it into a cold surface that could allow condensation to occur.

Jul 1, 2014 1:47 PM ET

Response to Ron Stanley
by Martin Holladay

If you install R-23 of polyiso above the roof sheathing in your climate zone, your roof will exceed the minimum requirements for above-sheathing foam -- giving you a big margin to work with if you decide to add thicker fluffy insulation between your rafters. I think you will be fine.

If you really want to perform the dew-point calculations to show how far you can go with your plan, here is a link to an article that tells you what you need to know:
Are Dew-Point Calculations Really Necessary?

Aug 12, 2014 4:13 PM ET

moving forward with the unvented roof assembly
by ron stanley

The roof deck is built (3/4 inch t&g osb above 9.5 inch I-joist), the 4" polyiso is on site, and I'm looking for a few last words of advice. There was a mention in the comments and your article of placing an air barrier directly on top of the roof deck and below the rigid insulation. This is also described in, "Complex Three Dimensional Airflow Networks" By Joseph Lstiburek but not much detail about the airflow barrier is given. Also, many articles speak to using multiple layers of rigid insulation, offset, to eliminate "pathways". My plan, maybe not a good one, had been to put the insulation directly on top of the osb deck, put a layer of densdeck above the insulation, and fully adhere the tpo to that. I could change the assembly to - osb deck, air barrier, 4" polyiso, additional 1" polyiso that is faced prepared for fully adhered systems, and then tpo. My questions are: Is the air barrier recommended and if so, what type of product do you suggest? How important is it to have multiple layers of offset insulation if an air barrier is used?

I am also having trouble finding any information on the best way to build up the outer perimeter of the roof deck to form a nailing base for fascia, gutters, and flashing. Do I just build up using 2x4's or lumber to the height of the rigid insulation?

Aug 13, 2014 1:15 PM ET

Mounting Solar on Low Slope Insulated Roof
by Confused Homeowner

I'm hoping to install a solar array after re-roofing my low-slope roof but I'm a little worried about heat loss from the mounting brackets that would be bolted into the roof deck. I will have an R-24 blow-in-blanket-system in the 2x6 joist bays below the plywood deck and 3 inches of poly iso above the roof deck covered by a PVC membrane. How concerned should I be about thermal bridging from the mounting brackets? I suppose my alternative would be to use a ballasted system for mounting solar panels. A ballasted system would not penetrate the membrane and poly iso, but it would likely be a lot heavier since it uses bricks to hold the rack system in place.

Aug 20, 2014 8:48 PM ET

Low slope unvented assembly
by graham swett

Working on a townhome project. GC is onboard with doing unvented attic space below the low-slope trussed roof assembly. Climate zone 5B. Looking at using ccspf on underside of roof deck w/ white EPDM topside. Appears to be a viable assembly according to:
but I am concerned that there is no drying potential and any roof leak will probably go unnoticed.......thoughts?.......thanks.

Aug 21, 2014 5:27 AM ET

Response to Graham Swett
by Martin Holladay

Your chosen approach is one of the standard approaches for insulating an unvented low-slope roof assembly, and it will work.

Most commercial roofers prefer to install rigid foam above the roof sheathing, because that approach costs less, and still allows the roof sheathing to be inspected from the interior when necessary. But you can do it your way if you want.

You wrote, "I am concerned that there is no drying potential and any roof leak will probably go unnoticed." There are lots of things to worry about in life -- and if you're really worried about that issue, you should have built a steep roof over a ventilated attic.

Aug 21, 2014 11:21 AM ET

Response to MH
by graham swett

Thanks architects we are tasked with having to balance the demands of the client, the market, the planning and development dept, the building codes and a host of other factors that do not always align to allow us to take the path of least resistance. All we can do is approach the final design with intelligence so that the final built environment does not become a drain on the end user and a black-eye and potential law suit for the design/development team........thanks for contributing to the brain trust.........

Aug 25, 2014 12:48 PM ET

best way to insulate a converted attic with a flat roof
by sascha zerbin

Hello Martin,
I am in the middle of converting my attic into living space when I started researching about possible ways to insulate the ceiling/roof and I was reading your article with great interest! Thank you for bringing some more light to this challenging topic!

Despite the many good suggestions, I am still not 100% sure about the best way to tackle my particular problem. Here are a few facts:

The house is located in St Louis, climate zone 4,
built in 1880, with original sheating, the flat roof is Modified Bitumen with a white coating) the sloped mansard roof to side is black slate. The slate is new and the flat roof is good for another 5-7 years. The ceiling joists are 2 x 10 in size and I am planning to attach the ceiling drywall directly to save as much ceiling height as possible.

The 2 attached pictures might give you a better idea...

In order to get the R-30 for the ceiling, I could use fiber glass batts or rolls, but that would negate any form of ventilation between insulation and sheating. From what I understand the best option might be the insulation with rigid foam on top of the sheating....but there is still some years left on the flat roof, I am planning to put the insulation on top of the sheating while putting on a new flat roof with new sheating...

My questions are: What is the best and most cost effective, interim (5-7 years) insulation until I will put the new roof on?

Is it better to coat the flat roof with a darker coating to avoid the "cooling coil" effect?

Should I insulate the walls or between the sloped rafters or both?

And what type of vapor barrier is best to use in my particular case?

I also thought about creating a space of 3 inches between insulation and sheating by using 2x4 as spacers and putting a layer of rigid foam boards2-3 inches, sealing all the gaps with closed cell foam and fill the space underneath with fiberglass, than the vapor barrier and the drywall. But I might not reach R-30..

Thanks for any suggestions!

attic framing 6.jpg attic framing 3.jpg

Aug 25, 2014 1:11 PM ET

Reponse to Sascha Zerbin
by Martin Holladay

In Climate Zone 4, the 2009 International Residential Code calls for a minimum of R-38 ceiling insulation, not R-30.

I suggest that you insulate along the roofline rather than attempting to insulate the kneewalls.

Since you will be creating an unvented insulated roof assembly, you can't use fiberglass or a similar air-permeable insulation. Your only two choices are spray foam insulation or a combination of spray foam insulation and air-permeable insulation.

For more information on this type of insulation challenge, see How to Build an Insulated Cathedral Ceiling.

Aug 25, 2014 5:08 PM ET

thanks for getting back so quickly!
by sascha zerbin

Hello Martin,

the 09 IRC is amended in St Louis when it comes to insulation...

Would it be possible to use rigid closed cell boards combined with closed cell spray foam instead of just spray foam? This way I could reduce costs by 80%!!

And which class of vapor barrier/retarder would you recommend?


Aug 25, 2014 6:18 PM ET

Edited Aug 25, 2014 7:07 PM ET.

Response to Sascha Zerbin
by Martin Holladay

The method that you are proposing is called the "cut-and-cobble" approach. It is not recommended for unvented cathedral ceilings because of the risk of moisture problems and rot.

To read more about the method, and to read warnings about its use for cathedral ceilings, see Cut-and-Cobble Insulation.

If you decide to install spray foam insulation, you only need a vapor retarder if you choose open-cell spray foam. (Closed-cell spray foam is already a vapor retarder.) The usual vapor retarder for installations of open-cell spray foam is a layer of vapor-retarder paint installed on drywall. (The drywall is necessary for fire protection.)

Don't attempt to install the paint on the cured spray foam -- that method won't work. It has to be installed on drywall.

Sep 11, 2014 12:26 AM ET

I'm getting a little worried here...
by Gregg Berkholtz

While researching what you've called the cut-and-cobble method, I stumbled across this blog posting. After reading the article and then every comment, I'm a bit worried about our roof installation.

We recently purchased a low-sloped (1 in 12) house in Zone 4C (Portland Oregon, Multnomah County). This 1950's house had a leaking torch-down at our roofer's advice we steered towards TPO as a replacement surface. After tearing off existing roof to the decking, what they installed was...listed in order from decking up: 2" PolyIso, FR-10 underlayment, 60Mil GenFlex TPO.

Since we had opened the house's interior (total gut; for new electrical & plumbing, and replacing wall-insulation), our roofer then advised us to install new high-density batts in-between 2x6 joists; leaving 1" of airspace between batts and roof deck underside. We then needed to open the soffits a minimum 1", and use a 1"-2" circular saw to open up as much blocking as possible, as close to the top plates as we can get.

From what I'm reading here, it appears our installation is likely to encounter moisture/condensation issues... Am I understandings things correctly?

Sep 11, 2014 6:33 AM ET

Edited Sep 11, 2014 6:34 AM ET.

Response to Gregg Berkholtz
by Martin Holladay

If you have rigid foam above your roof sheathing, you definitely don't want a ventilated air gap underneath your roof sheathing. So the first order of business is to close off the soffit vents in an airtight manner.

The second issue concerns the decision to combine a layer of rigid foam above the roof sheathing with air-permeable insulation under the roof sheathing. This approach can work, but according to code requirements, the air-permeable insulation has to be in direct contact with the roof sheathing. Here's how the code reads: "In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control."

Your rigid foam sheathing is fortunately thick enough for your climate zone, but the gap between the fiberglass and the roof sheathing is a code violation (as well as bad practice). You should bring this fact to the attention of your contractor, who is responsible for following the building code.

Sep 11, 2014 6:05 PM ET

Edited Sep 11, 2014 6:07 PM ET.

Response to Martin Holladay
by Gregg Berkholtz

Thank you greatly for the information. The idea of an unvented roof is completely foreign to me - so your feedback greatly helps us better understand things.

Since we have enough rigid foam sheathing above the roof deck (2"), it sounds like our best course of action is to focus on the void boxed between our rafters and roof sheathing/drywall.

In 1/3rd of the house, we've already drywalled, so it's damage control: it appears our most reasonable path is to drill & blow-in a dense cellulose to fill the gap between fiberglass bats and roof sheathing, and then seal openings once complete.

For the rest of the house, we had originally planned on a 1-2" airspace between batts and roof sheathing. Since that part of our project is just beginning (two roofs to deal with - one is on 2x6 & the other on 2x10 rafters), it appears we have additional options.

Modeling after your article's recommendations:

  1. 1) Install 5 1/2" high-density batts in the 2x6 space; filling the cavity. Then covering the rafters with plastic just before putting drywall up (this was also suggested by an insulation installer that visited today).
  2. 2) Cut-and-cobble with spray foam to to fill the gaps between boards and rafters.
  3. 3) Fill cavity with dense pack (e.g. a bib system)...this was the preferred recommendation of today's insulation installer.

I get the impression that cut-and-cobble would actually leave an airspace with 2x6 rafters (we have access to 3" PolyIso boards at $10/sheet). We understand its extremely labor intensive, but at this point its cost and damage-control for us.

There's another space where we have 2x10 rafters, still under a 1/12 flat roof. Today's insulation installer also suggested using 8 1/4" high-density batts (R30), plastic, drywall; and leaving venting/soffits open. With what I'm learning these past few days, I'm not sure how I feel about that recommendation.

Our goal is to not tear off the roof surface, again...if at all possible.

Sep 12, 2014 7:45 AM ET

Response to Gregg Berkholtz
by Martin Holladay

You're getting closer to understanding a good roof assembly -- you are almost there. There is one remaining problem: you definitely don't want interior polyethylene with this type of roof assembly. An interior vapor barrier is a no-no.

While vented roof assemblies are designed to dry to the exterior, unvented roof assemblies are designed to dry to the interior. Polyethylene would interfere with the assembly's ability to dry to the interior.

The theory behind wall and roof assemblies with exterior rigid foam is explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

On the underside of your roof assembly, it's always a good idea to pay attention to airtightness. You want your drywall layer to be airtight, but not vapor-tight. To make sure that the assembly is airtight, don't install any recessed can lights, and make sure that you seal carefully at all wiring penetrations and plumbing vent penetrations.

There are lots of articles on the GBA site that delve deeper into these issues, including:

Questions and Answers About Air Barriers

Vapor Retarders and Vapor Barriers

Forget Vapor Diffusion — Stop the Air Leaks!

Do I Need a Vapor Retarder?

Sep 12, 2014 10:53 AM ET

Response to Martin Holladay
by Gregg Berkholtz

I thought an interior vapor barrier might be a bad option, especially with a TPO roof membrane; where would any moisture escape? Although the plastic sheeting was recommended by yesterday's installation installer, so thank you very much for reinforcing my thoughts on this.

In addition, my understanding of the bib system they recommended also sounds like it'd be a problem; their design would use tyvek to hold blow-in cellulose. What's strange is that our installer said he "cleared it with his technical/legal department" while we chatted (he was exchanging photos and TXT messages). They sounded so convincing...

For our situation, to get something better than the ~R12 from existing 2" PolyIso above the roof deck, it sounds like packing the space with un-faced high-density insulation might just be our least-cost option, as long as we're careful to seal all top-plate/blocking edges (and any interior or top/bottom-plate holes) with expanding spray foam.

I'll dig into your links this afternoon - the information is much appreciated!

[Editor's note: To read the answer to this comment, and to read subsequent comments, advance to page 2 by clicking on the number 2 below.]

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