Matt Melton lives in central Washington state in a 3-year-old house with a roof made of structural insulated panels (SIPs) that are 12 1/4 inches thick. The pitch of the roof is very low, only 1/2 inch-in-12, and the metal roofing has been applied directly over the SIPs with no air channel beneath the roofing for ventilation.
When the house was built, Melton sealed the seams between roof panels with tape, applied on the bottom of the panels. The top of the roof was completely covered in a peel-and-stick membrane. In both cases, Melton now says, the seams of the tape and the roofing membrane didn’t seem to stick very well.
This year, there’s trouble. For the first time, Melton has noticed an ice dam on the roof. Worse, he reports water leaking through a seam between SIPs, although the leak appeared on a roof overhang and not in the interior of the house.
“Now, I’m wondering should I pull all the roofing off once it warms up and let the thing dry out and also to see if there is any mold,” he writes in a Q&A post at Green Building Advisor. “Then maybe replace with a better underlayment and a TPO roof?”
There’s one more complication: Melton has sold the house and is now renting it back from its new owners. They apparently plan to take possession in May.
How should Melton deal with this developing problem? That’s the question for this Q&A Spotlight.
A site visit is a good first step
GBA senior editor Martin Holladay makes five observations: installers used an interior tape with adhesion problems; there is evidence of air leakage at the seams between SIPs; it appears that melting snow on the roof caused by air leaks led to ice dams; the roof pitch is very shallow; and the peel-and-stick membrane was installed in a way that isn’t waterproof.
“It’s not a good sign that your roof has ice dams — especially if the house is only three years old,” Holladay writes. “It’s worrisome that you have seen water on the underside of your roof assembly. I’m not going to give advice on the moral or ethical issue of what you should tell the new owners of the building. I’ll leave that issue to your conscience or your lawyer.
“Lots of problems,” Holladay continues. “It would take a site visit to determine whether there is any way to seal the air leakage at the SIP seams from the interior. That would be Step One.”
Melton says his goal is to address the problems and pass along a well-functioning home to its new owners.
“I’m definitely not looking for legal advice on GBA,” he says. “I’m simply trying to get an idea of the severity, timeline and (possible) solution to the problem. I AM assuming liability based on the fact that I built the house (as a homeowner, not GC) and that I want it to be fixed because its the right thing to do.”
Why choose SIPs in the first place
Melton’s post prompts Malcolm Taylor to wonder why homeowners are building with SIPs at all.
“It must be apparent to regular readers here that SIP assemblies represent more than their fair share of the problems needing remediation — and that they are generally serious problems,” he writes. “In the face of that I still see posters proposing to use them on their new builds. There seems to be a real disconnect. What are the features that draw people to use SIPs over equally efficient but much more resilient methods of building? I’m genuinely stumped.”
Well, Melton replies, speed of construction and thermal performance are two reasons for using a SIP roof assembly.
“As for my decision to use SIPs, I like the idea of a R-55 roof installed very quickly,” Melton says. “We set five 63-foot beams and a 32×64 roof in two days. Which may or may not be fast, but it seemed pretty good. Sure, in hindsight, SIPs may not have been the best choice, but it seemed good at the time and I didn’t see/do enough research to convince me otherwise.”
Taylor likens the risk of using SIPs to sport.
“There are a number of extreme sports, like rock climbing, that are very safe when performed by experienced, well-trained participants,” Taylor says. “That doesn’t negate the reality that they are inherently more risky that other activities. To me, SIPs are like that. Installed by experienced, diligent, conscientious workers they perform as expected (your house is a perfect example). But the chances of success are so much lower than assemblies that are inherently more robust.
“The frequency of questions posted on GBA are no statistical indication of how often these problems occur in the wider building world, but it is clear that there are assemblies like, say, cathedral roofs that cause more concern than others,” he continues. “I can’t imagine a case being made that SIPs weren’t a risky way to build compared to most others.”
Doing SIPs the right way
Peter L makes these suggestions for a trouble-free SIP roof:
(1) Tape all exterior panel seams with a good-quality, vapor-permeable product that allows trapped moisture to escape (he recommends SIGA Wigluv tape).
(2) Install a vapor-permeable roof membrance (he suggests SIGA Majcoat, which has a vapor permeance of 34 perms) to allow drying if the roof becomes wet.
(3) Create an air space between the roof membrane and the finished roofing. As little as 1 inch of space will allow for drying.
(4) Seal and tape all interior panel joints with spray foam and a good-quality tape.
“If the above is done, a SIP roof can be as long-lasting as a truss roof design with an attic,” Peter says. “SIPS have their place, but just need the extra TLC and steps to keep them properly sealed and dry. If SIP roofs are done incorrectly, then yes, they can be a problem. Like anything in building science, there is usually a solution; just apply it properly.”
John Stephany writes that the Madison College Construction Program has been using SIPs for years.
“There are some key things: being hyper-vigilant when installing to make sure the gaskets are in the right place and in full contact,” Stephany says. “We threw out the hardware store gasket material the supplier sent out and went with a much more robust seam tape that expanded two times its original size over time.”
On the interior, Stephany applies 3M tape with a J-roller.
The roof pitch is a concern
Melton recognizes the roof pitch is low, a detail that several posts suggests is contributing to the problem.
Holladay, for example, says that a roof with a slope of 1/2 inch in 12 inches “is always going to be at great risk of leaks than a roof with a steeper roof.”
Installing metal roofing on slopes of less than 2-in-12 is inherently much riskier than using it on steeper roofs, Taylor says.
“Apart from small penetrations for vents and roof stacks, all the other trim and detailing for conventional metal roofs rely on lapping material and more importantly gravity,” he says. “Chimneys, ridge caps, valleys, gable, sidewall and eve flashing all need slopes to work. Further, unlike other common materials like asphalt, metal wicks water through capillary action, so on metal roofs water often travels uphill.”
Although there are ways to mitigate the risk, he continues, “these things are to mitigate risk that doesn’t exist in other, more robust assemblies.”
Stephany adds that most standing-seam metal roofs should not be installed on roofs with a pitch of less than 2-in-12.
“There are a few that are allowed on as low as 1/2-inch pitch, and they have robust gasket material inside the standing seam to stop leakage and are clear in their literature about the usage,” he writes. “At this point, you do need to remove the roof, and I would recommend a second layer of sheathing spaced off the first with 1x4s to create a vent space, on top of Grace Ice and Water shield, lapped 6 inches or more. And air-seal everything on the interior as best you can.”
An air channel is important in a cold climate
Holladay admits that his advice to incorporate a ventilation channel in this type of insulated roof assembly is a conservative approach. He writes that he would sleep better at night knowing that the ventilation channel was there.
He advises including an air space beneath any type of roofing over a SIPs structure in a cold climate (Climate Zones 5 through 8).
Peter L says that he’s discussed the well-publicized failure of SIP roofs in Juneau, Alaska, with one manufacturer, who said the reports were overblown. Juneau, he said, represented a “worst case scenario type of climate,” and that there are thousands of SIP roofs without ventilation channels that have not had a problem.
Further, the website for the Structural Insulated Panel Association suggests there’s nothing wrong with applying standing-seam or asphalt roofing directly on a SIP with no air channel.
“The SIP manufacturers are correct about the climate in Juneau,” Holladay replies. “It’s damp, and there are few sunny days that allow for much drying. They are also correct that there are thousands of SIP roofs without any vent channels or rot. (Those are the SIP roofs that are perfectly airtight.)”
And while SIPA may see nothing wrong with skipping an air channel beneath roofing, that doesn’t necessarily make it the right thing to do.
“SIPA is an association with the mission of promoting SIPs,” Holladay says. “Historically, SIPA has downplayed the disadvantages of SIPs and highlighted the advantages of SIPs. SIPA is not the best source of information on potential SIP failures.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
It’s unfortunate if this problem is characterized as a SIP failure; the roof represents a failure in design and detailing. Sure, SIP roofs demand rigor, but so do many high-performance systems.
Detailing for air leakage: Any high-performance assembly and building needs performance testing, particularly the first time around. The only way to know that you have superior airtightness — in this case, something that SIPs demand — is to test.
All SIP panel joints should be backed up, topside and bottom-side, by high performance PSA tapes; in my opinion that would be an acrylic tape such as SIGA, Pro Clima, 3M, or Zip. (For more on tapes, see Two Wingnuts Describe Their Backyard Tape Tests, or my BuildingGreen blog series: Sticky Business.) This means that the airtightness of the panel joints is assured by (a) the original (and often custom) panel joint detail as well as (b) flexible membranes that can withstand panel contraction and expansion at all the panel joints.
Roof pitch: I must admit that when I first read that a standing-seam metal roof system was used on a roof of this slight a pitch, I thought it was flat-out wrong. But I checked with local roofing expert, Brian Knowles of Jancewicz & Son, and here is what he had to say:
“Most roofers will say that this can’t be done (standing-seam metal roof on 1/2-inch-in-12 roof), but we are one of a few companies that have the technique and craftsmanship to tackle this. The keys are:
“1. Don’t attempt this on any roof with a ridge.
“2. Be sure that panel layout places any roof penetrations in the middle of a panel and not at a standing seam.
“3. Use a double-fold standing seam [as shown in Image #2, below] with a continuous bead of Geocel 2300 sealant in the top of the first fold.”
So what do I recommend? Definitely take the roof apart and identify the leaks, investigating both air leaks and bulk water leaks, given the patterns of wetting and the apparent lack of adequate sealing for both air and bulk water.
Here’s my take on the possible benefits of venting a roof with this low of a pitch: to get real air flow and the drying it brings, you need two holes and a driving force. It’s tough to get much driving force — in terms of force and frequency — at that low a pitch.