Improving the thermal performance of an existing attic is often challenging: workers are faced with narrow cavities, low clearances, and cladding systems that make it hard to achieve desired R-values while still maintaining the necessary drying potential of the assembly.
The house at 81 Chapin Street in Brattleboro, Vermont, is no exception. It’s a 100-year-old wood-framed two-story home that Alex Beck and Candace Pearson are determined to comprehensively retrofit to high performance.
That’s where Eli Gould, owner and founder of , comes in. Eli is an architect and builder responsible for projects like the Lemon’s “Almost” Passive House and Alex and Jerelyn Wilson’s Leonard Farm retrofit (see multiple GBA Energy Solutions blogs on the Wilson project). The Brattleboro project has no shortage of challenges; for example, in subsequent blogs we will discuss the basement. Here, we’ll focus on the attic, which needed careful hygrothermal consideration.
Eli Gould lays out the roofing configuration used at 81 Chapin Street: “We decided to cavity fill with dense-packed cellulose and then — on the underside of the rafters — to run two staggered-seam, taped layers of 30 mm foil-faced rigid foam insulation.” Gould specified . (Kingspan is an international corporation headquartered in Ireland. Kingspan’s U.S. office is located in Atlanta, Georgia. Kooltherm is a type of rigid foam called phenolic foam.)
[Editor’s note: Phenolic foam rigid insulation was introduced in the U.S. in the 1980s and sold by Beazer East and Johns Manville. Serious corrosion problems of fasteners and steel decks that were in contact with the foam resulted in legal action, and both companies ceased production of phenolic foam in 1992. For more information on the history of phenolic foam marketing in the U.S., see Phenolic Foam Insulation Revisited.]
He continued, “This approach got us to the right total R-value, highest level of airtightness, and excellent drying potential to the exterior.” The exterior layers of the roof assembly include sheathing boards, building paper, and slates. Gould added, “And the clincher for the attic was that the foil-faced Kootherm up there does not need any ignition barrier.”
Candace Pearson, a former editor and research analyst for BuildingGreen, explained, “Initially, we were hoping to find a wood fiberboard insulation that could be installed to the interior [of the rafters], but when we did the modeling we found out we needed a higher R-value.”
Candace said, “We became interested in Kooltherm because, unlike XPS, Kooltherm did not use a high global warming potential (GWP) blowing agent and, unlike conventional polyiso, it did not contain added chemical flame retardants. That’s what initially prompted me to suggest the product. Plus, it is a recommended product in the .”
Kooltherm characteristics and properties
Alex Wilson of BuildingGreen has already published a . The details are really a bit hard to believe:
Let’s go after the first two of these (the most difficult to accept, frankly).
You get R-6.2 or R-6.3 per inch for 1-inch samples, and R-8.2 per inch for 3-inch samples, according to the manufacturer, because of two properties of Kooltherm:
- The insulation includes really tiny gas pockets (the smaller the pockets, the less heat flow); and
- The insulation has a high percentage of closed cells (98% are truly trapped gas pockets).
Here is a link to a .
I have the most trouble with the flammability performance claimed for Kooltherm. After all, it’s a petroleum-based polymer matrix with a hydrocarbon blowing agent. It seems as though we are getting burned, not the insulation!
When I asked Kingspan about this, they stated that phenolic foam is “inherently flame-resistant.” That seemed like restatement, not an explanation. I was then referred to one of Kingspan’s chemists, who explained it this way: “It [comes] down to the highly cross-linked thermoset PF [phenol formaldehyde] material. [With] a phenolic matrix, unlike U-F chemistry, once cured the polymeric thermoset material is irreversible. Phenolic resins … have a highly cross-linked, high carbon (char), and aromaticity level which lends itself to excellent FST (fire, smoke, and toxicity) properties. The material is stable to well over 200°C.”
That seems to address the fire properties of the phenolic foam matrix, but what about the pentane blowing agent? Craig Lynch, commercial technical director at Kingspan, responded, “The Kooltherm blowing agent is not pentane, but pentane-based. The actual blowing agent is a proprietary mix, with constituents added to — among other things — reduce the flammability of the blowing agent.”
That makes some sense to me as well, but is not wholly convincing. Being an empirical sort of guy, I decided to torch some Kooltherm.
Using a grill starter flame source, the test yields no flame, very little smoke, and only charring.
Using a propane torch, the test yields no flame, very little smoke, and only charring.
That’s pretty impressive fire resistance with both the low-temperature grill starter and the higher temperature propane torch.
To be completely fair, here is a list of fire performance standardized tests from the laboratory certification for NFPA 286:
The NFPA 286 test is the “corner” test for interior applications (for example, see ). The NFPA 285 test is the exterior assembly test (for example, see ).
- Performance in accordance with ASTM E84/UL 723 for smoke and flame development: Flame Spread Index <25; Smoke Developed Index <450
- Performance for use without a thermal barrier in accordance with IBC Section 2603.4 and 2603.5.2: Approved.
- Performance with regard to vertical and lateral fire propagation in accordance with IBC Section 2603.5.5: Approved.
- Performance with regard to ignition in accordance with IBC Section 2603.5.7: Approved.
Clearly Kooltherm — notwithstanding that it is a directly derived petroleum-based rigid insulation — has solid high-performance credentials.
Gould concludes, “Kooltherm is now my interior rigid insulation material of choice for this type of deep energy retrofit.”
In addition to acting as GBA’s technical director, Peter Yost is the Vice President for Technical Services at in Brattleboro, Vermont. He has been building, researching, teaching, writing, and consulting on high-performance homes for more than twenty years. An experienced trainer and consultant, he’s been recognized as NAHB Educator of the Year. Do you have a building science puzzle? . You can also to get a free report on insulation, as well as regular posts from Peter.