Jan Verschuren has a nicely roofed older house, and a problem to go with it. Cedar shingles have been installed over skip sheathing, making for a roof that’s not only historically correct but one that allows air to circulate freely beneath the roof deck. Verschuren’s next objective is to insulate between the 2×4 rafters, and here’s where he has run into a snag.
Building codes require at least R-38 worth of insulation in the roof. Ordinarily, that wouldn’t be a problem, but Verschuren says that he has only the 3 1/2-inch deep rafter bay to work with. He can’t afford to lose any headroom by adding insulation below the rafters, and he’d rather not tear off the roof to add extra insulation on top of the rafters.
He’s zeroed in on a type of insulation called Spaceloft with a reported insulating value of R-10 per inch. The distributor’s web site describes Spaceloft as “a flexible aerogel composite blanket.”
“So, 3 1/2 iches is all we have, but we’d like to get to an R-50 or thereabouts in our upgrade of this 1925 house built in Climate Zone 4C,” he writes in a Q&A post at GreenBuildingAdvisor. “Spaceloft seems the only option.”
Verschuren thinks he can get nine layers of this material in the rafter bays, followed by one or two layers over the bottoms of the rafters.
There are a few downsides, however: The cost is “exorbitant,” and Verschuren still has to figure out how to detail the installation so it will be vapor-open and able to dry out.
Or, does he have other alternatives? That’s the issue for this Q&A Spotlight.
GBA senior editor Martin Holladay has never heard of Spaceloft, but has two other suggestions. “Three and a half inches of closed-cell spray foam would give you about R-22.7,” he writes. “That’s less than code minimum requirements. The solution is to thicken up your rafters or to install several layers of continuous rigid foam under your rafters.
“You’ll lose a little headroom, but at least you’ll have a roof that meets minimum code requirements.”
If Verschuren absolutely can’t afford to lose any headroom in the attic, Holladay adds, “it’s time to raise the roof. Again, we’re talking physics. Sometimes you just have to face facts.”
Nate G understands why Aerogel insulation performs as well as it does, but Spaceloft is another question.
“I am highly skeptical of the claims of this product because Aerogel is a rigid material, basically a mostly-hollow brick made out silicon,” Nate G writes. “This stuff [Spaceloft] comes in a flexible blanket. I have a hard time believing that it behaves the same as rigid Aerogel. The way Aerogel material works is by trapping a lot of air in billions of nanometer-sized pores. In a solid material, the integrity of these pores can be guaranteed. In a flexible sheet, how can it?”
And then there is the expense.
“But let’s say I’m wrong that this stuff really performs as advertised,” Nate G adds. “The best price they offer is $95/inch thick/square foot. By contrast, conventional insulation materials are literally in the ballpark range of to 100 times less expensive. At 3 1/2 inches thick, you’re paying $332 per square foot of roof. You could, like, demolish the house and build a whole new one for that price given an average-sized attic.”
To Holladay, that kind or price tag makes Spaceloft “fairly irrelevant to the everyday concerns of residential builders” even if it performs as advertised.
Do the math on how much you’d really save
The cedar roof cost $20,000 just five years ago, Verschuren replies, so removing it to put more insulation above the rafters isn’t an attractive idea — and it makes the Spaceloft option look a little less harebrained despite the expense. In addition, the engineer who’s designing a radiant-floor heating system for the house wants a healthy dose of insulation.
Charlie Sullivan has sharpened his pencil and done some math. After using new cost estimates provided by Verschuren, he calculates that Spaceloft would cost about $15 per board foot (1 foot square by 1 inch thick).
“That brings is back into the realm of a little eccentric, as opposed to completely wacko,” Sullivan says. “I’m comfortable with a little eccentric. My previous cost-effective calculation changes to 12 times as expensive as it should be, rather than 75 times.
“The extra heat loss from that, at a 60°F temperature difference (we still don’t know your climate, so that’s a guess) is only 265 Btu/h less than you’d get with an R-5/inch solution. The radiant engineer shouldn’t call himself an engineer (or radiant for that matter) if he thinks that an extra 265 Btu/h is going to undermine his design. That’s less heat loss than a 25 square foot high-quality U-0.2 triple-pane window.”
Nate G believes Verschuren would do himself a favor by being more flexible. “You seem to be getting yourself tied in knots because of the limitations you’ve set for yourself,” he adds. “Sometimes it’s time to admit that those limitations either preclude any good options or need to be removed.”
If Verschuren could lose just a few inches of ceiling height, he adds, he’s have many more options. Or, if he needs more space, he could always build a ground-floor addition that preserves the architectural character of the house.
Go with the spray foam option
One of Verschuren’s objections to spray foam is that it would defeat the purpose of installing cedar shingles over skip sheathing by limiting their drying potential.
But Holladay has a suggestion: “If you are committed to your plan,” he writes, “install some cardboard or housewrap between your rafters followed by R-20 of spray foam, and live with the imperfect thermal performance of that type of assembly (assuming, of course, that your local building department doesn’t require a higher R-value),” he writes.
Once Verschuren has cut up all that expensive insulation and placed it between the rafters, Holladay says, there is still the problem of thermal bridging through the rafters. “Once thermal bridging is accounted for, the improved performance compared to spray foam is minor, and certainly not worth the investment.”
Nate G looks at it this way: Verschuren’s planned installation of Spaceloft, with a total price tag of $10,000 for materials alone, yields a total R-value of 38. Holladay’s alternative of closed-cell foam over cardboard plus a single layer of Spaceloft gets the roof to R-29 at a cost of $2,480.
“So for less than 25 percent of the price, taking Martin’s advice and adding only a little bit of Spaceloft would get you 77 percent of the R-value. That seems like a no-brainer to me. The differences between R-30 and R-38 are not worth paying another $7,500, in my humble opinon,” he writes. “Comfort-wise, if this is conditioned space, you won’t feel the difference. But it’s your money!”
The coup de grace for this idea?
The discussion so far has assumed Spaceloft has an insulating value of R-10 per inch, and that apparently does apply for certain versions of this product. But Bill Dietze takes a close look at the fine print and throws this curve: “Jan, the link you provide mentions the uses of Spaceloft as ‘Great for home insulation, winter clothing, science projects,’ but the link to datasheets for material with a conductivity of 0.14 W/mK (R-10 per inch) is an undersea pipe product,” he writes. “Hardly residential. If you proceed, be aware that you are probably in the ‘science project’ category.
“The Spaceloft product rated for ‘Ambient temperature walls, floors and roofs in commercial, residential and institutional building’ has a lower thermal resistance (R-8.3 per inch) and no datasheet describing the application.”
If the lower R-value is correct, it makes the case for Spaceloft even less compelling, Holladay points out.
“But it is still ahead in many ways over ‘the rest’ to use,” Verschuren says. We have the money to do it but do not want to waste it, either!”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
I decided I really needed to check in with Jan Verschuren on this one; just too many balls in the air to offer any helpful perspective. Having done that, I think the list of constraints (below) Jan poses or faces proves that if you narrow the boundary conditions on a problem enough, you can arrive at a singular solution.
Here are the factors to consider:
- The newness of the current roof cladding means you can’t build up insulation topside.
- Very low ceiling heights mean that there is little room to build up insulation on the bottom.
- The ceiling is already being built down 13/16 inches for Warmboard-R panels, primarily for radiant cooling.
- Radiant cooling for the attic space is being driven by lack of room for ducts and an already-in-place ground-source heat pump system, making for a fairly easily retrofit for radiant cooling.
- The homeowners can’t really rely upon natural ventilation (open windows) for low-grade space cooling because of urban noise issues.
- Spray foam insulation is not an option.
As Jan says, “…my hands are tied!”
So, if we can agree to accept these constraints, Jan is ending up with layers of Spaceloft in between the rafters and two layers (2 centimeters) of continuous Spaceloft to the underside of the rafters.
We confirmed that standardized independent R-value testing of Spaceloft yields about R-8.3 per inch. That makes Spaceloft significantly better per unit of thickness than any other readily available building insulation but also means the entire assembly is still not code-compliant and far from high performance.
At the end of the day, that phrase “readily available” may be the key to whether this solution will work for Jan and others considering this pricey approach to insulation: neither I nor Jan could get Aerogel to respond at all to our requests for technical information and information on the availability of Spaceloft for Jan’s project.
No magic bullet for this one, with neither the bullet or its delivery magical at all.
Editor’s note: After this article was published, GBA learned that the product formerly known as Spaceloft has be re-branded as Proloft. For more information on Proloft, visit Advanced Insolutions Inc.. Here is a link to a report on R-value testing of Proloft Aerogel Blanket.