Kevin is working up the design for a new lakeside cabin in Tennessee, with plans currently calling for a scissor-truss roof and a vaulted ceiling over the kitchen and living area. He wants a trouble-free roof assembly, but local advice on insulation and air sealing has been hard to find.
The roof starts with scissor trusses set on 8-foot centers. Between them, running horizontally, Kevin would run 2x6s on 24-inch centers (the 2x6s set on edge), and insulate the bays with fiberglass batt insulation. The bottom of the 2x6s would be covered with tongue-and-groove pine boards. On top, he’d start with OSB sheathing, then add 2 inches of rigid foam insulation, a layer of 1/2-inch plywood, then foil bubble wrap and, finally, metal roofing.
He’s ruled out the use of structural insulated panels (SIPs).
“There doesn’t seem to be anyone around here, in southern Kentucky, that cares much about good insulation and sealed houses,” Kevin writes in a Q&A post. “I don’t have any education other than applying different parts of roofing projects I have seen online and what I have seen in person. Please point out problems with this idea or a better way.”
That’s the topic for this Q&A Spotlight.
This plan has some problems
GBA editor Martin Holladay responded to Kevin’s question before Kevin had fully explained his plan to install rigid foam above the roof sheathing. Based on Kevin’s description of fiberglass batts installed between the 2x6s, Holladay noted a glaring weakness in plans: the lack of an uninterrupted air barrier in the ceiling.
“You absolutely need an airtight ceiling to prevent interior moisture from entering your roof assembly and causing moisture problems,” Holladay writes. “That means (a) You can’t just install your finish ceiling between your rafters — the ceiling has to be continuous and airtight. (b) You definitely don’t want to have tongue-and-groove boards as your finished ceiling unless you have an air barrier (usually taped drywall) above the tongue-and-groove boards. Again, this drywall ceiling has to be continuous — not installed in strips between your rafters or trusses.”
Fiberglass batts aren’t the best choice, he adds. But they can work, with a few caveats:
Taped drywall is one way of creating an effective air barrier. Another option is a layer of spray foam insulation — between 1 and 1/12 inches of closed-cell foam or 2 or 3 inches of open-cell foam.
Skip the bubble wrap
Holladay also takes issue with Kevin’s plans to use foil-faced bubble wrap insulation in the roof assembly.
“First of all, the bubble wrap is a waste of money,” he says. “My advice is to take the money you were planning to spend on bubble wrap and invest the money in thicker insulation.”
Kevin, however, isn’t so sure that bubble wrap deserves a blanket condemnation. Although he’s aware of the arguments against it, Kevin also offers an example of where it might be effective.
“I also have two warehouses, one with bubble wrap under the metal roof and one with nothing under the metal roof,” Kevin says. “The one with the wrap is no doubt at least 10 degrees cooler than the one with nothing. It’s like an absolute oven right now. So, however small and likely not the best choice, it does do something and appears to be how it is applied to the structure.”
But he’s still willing to forego the material in his cabin roof assembly, using asphalt felt instead over the last layer of sheathing and beefing up the purlins that span the scissor trusses from 2x6s to 2x8s.
Combining fiberglass and rigid foam
Kevin then provided more details on his plan to combine fiberglass batt insulation and XPS rigid foam insulation. In order for this kind of assembly to work, Holladay points out, the foam must have a minimum R-value based on the climate zone where the cabin will be built (as explained in a GBA article called “How to Install Rigid Foam On Top of Roof Sheathing.”)
“In Climate Zone 4, all you need is a minimum of R-10 of rigid foam for this assembly to work,” Holladay says. “Two inches of XPS will meet this minimum. However, you should know that green builders try to avoid using XPS because it is manufactured with a blowing agent that has a high global warming potential.”
He adds that Kevin will need a “perfect” air barrier at the OSB layer. This could come in the form of a peel-and-stick membrane applied on top of the OSB, or with the OSB seams sealed with a high-quality tape, such as Zip System tape.
Jon R recommends that Kevin include a vent channel under the metal roofing and above the rigid foam insulation. It would be best, he says, if the roof had some drying potential in both directions. The assembly, for example, might include expanded polystyrene (EPS) insulation and no peel-and-stick layer.
“With these options and the proper ratio of rigid foam to fiberglass, you should be fine with taped sheathing and no drywall or spray foam,” he says. “Not far from your plan — drop the bubble wrap, add a vent and use EPS.”
He refers Kevin to an article on above-sheathing ventilation available through Texas A&M University.
It sure looks like a pole barn to me
Walter Ahlgrim has been following the discussion and observes the building Kevin is planning looks a lot like a pole barn, a type of building that can be put up “fast and cheap” but has drawbacks for heating and cooling.
“Is it just me,” Ahlgrim asks, “or do I see this same question on this and other forums? ‘How can I make my pole barn into a high-performance building for a song.’ OK, that’s not how they write it, but it’s how I read the question. The owners of these building always manage to find some other way describe their building other than ‘pole barn.’ Let’s admit it’s a pole barn! Pole barns are designed and built with one goal in mind, and all other considerations are abandoned for this goal. How to build at the lowest cost per dry square foot possible.
“Is ‘insulated pole barn’ an oxymoron, like ‘jumbo shrimp’?”
Kevin points out that the photo shows truss-and-purlin construction, not the exact system he plans to use. He adds: “I currently live in a post framed house and my electric bill is usually a little over $100 a month and it was poorly built for heating and cooling purposes. Just as many traditionally built homes in my area as well. That said, if I take my time and educate myself on techniques as described on this website, a post framed building can be as efficient as any other building. I can get the structure built and enclosed in a week relatively cheaply, then take my time to finish everything. Just my idea. I may start a blog on the process as efficient affordable housing seems to be very limited.”
Another plan emerges
Yet the discussion has led Kevin to a revised plan: trusses with 2×8 purlins, then a layer of OSB sheathing covered with a peel-and-stick membrane, two layers of rigid polyiso insulation, horizontal lath screwed into the OSB through the foam, then metal roofing.
In Climate Zone 4, Holladay replies, at least 31% of the total R-value of the roof assembly must come from the rigid foam layer. If Kevin were to use two layers of polyiso insulation, with a combined R-value of 13, and batt insulation rated at R-26.8, the foam would meet the minimum requirement. It would, however, be worth checking local codes to make sure an R-40 roof is legal; many jurisdictions in Climate Zone 4 require an R-49 roof.
A better plan would be to double the amount of polyiso, creating an R-26 layer.
“I realize that you plan to make the Ice & Water Shield your primary air barrier,” Holladay writes. “But it wouldn’t hurt to include a secondary air barrier between the fiberglass and the tongue-and-groove boards, in case sloppy workmanship results in a few unplanned air leaks through your roof assembly.”
Our expert’s opinion
Here’s how GBA Technical Director Peter Yost sees it:
For starters, I think the best resources for code information are these: state maps showing details of the International Energy Conservation Code by county; and building energy codes by state, published by the U.S. Department of Energy.
In the example Kevin provided, the scissor trusses are flush with the exterior wall. That means that on the exterior his roof and eave walls are flush and in line with each other, which is a great way to get an exterior air barrier continuous from wall to roof. And with exterior rigid insulation on both the eave wall and roof, the drop-in insulation depth at the scissor truss heel won’t matter nearly as much.
I can’t tell you the number of projects I have seen with wood ceiling tongue-and-groove as the interior finish, air-permeable cavity insulation, and substantial air leakage and condensation. Even with a continuous exterior air barrier, convection currents within the interstitial roof cavities move enough moisture to cause problems. It’s logical to think that enough topside continuous insulation, as Martin prescribes, should keep everything above the dew point, but if it were my building I would definitely go with the belt-and-suspenders of both a continuous interior air barrier (taped and sealed gypsum wallboard) as well as an exterior air barrier (taped and sealed sheathing).
But what about venting this roof assembly? I just wrote a two-part article for The Journal of Light Construction on this topic. My take (along with Steve Baczek’s) is this: Vent until you can’t. If your roof is really simple, soffit-to-ridge venting is the right choice for most climates, including Tennessee, but if your roof is full of dormers, valleys, hips, and such, getting any real drying potential from air flow is just not going to happen.
Finally, there are at least two wild cards in all of this:
- Quality of construction: Having just come back from a road trip looking at a ton of custom and production home construction, I think that all of our building science logic goes to pieces when the execution completely undermines performance. Most of my building investigations/assessments are more about poor execution than getting the science wrong.
- Interior conditions: Most buildings have a ton of forgiveness built into them. But if the nature of the building’s purpose (indoor swimming pool, for example) or occupant use (750 gorgeous lush plants) is special, we move into a different region of the psychrometric chart and the margins of forgiveness get razor thin.