KCB History Lesson: Hidden in the Walls We Found Old-school Insulation at The Revival
The KCB team was on site for a post demo inspection at The Revival in Needham, a home built in 1932. As fairly typical for this type of project, the floors and windowsills were covered with crumbs and dust, radiators and scrap copper were stacked by the side door and the dumpster was overflowing with insulation, plaster and dark brown hemlock studs. We retained some of the original plaster walls and ceilings and had marked them with bright orange spray paint. In reviewing the demo work for completeness I noticed what looked like an animal’s nest in the ceiling over the 2nd floor front closet. This one looked different from the one that we found under the old California valley on the back of the addition. Turns out it wasn’t a nest at all but the exposed eelgrass core of Cabot Quilt.
Project Manager Jed Orsini discussing the Cabot’s Quilt before demo.
The team on The Revival has opened up a lot of walls in the Boston area and beyond. We’ve found a lot of weird stuff behind 100-year-old plaster walls: blown in cellulose insulation, newspaper, rosin paper, tarpaper tacked to the inside face of studs, plaster or mortar covering the back side of the sheathing, bee’s nests filling the wall cavity, all manner of rubbish. I had never seen this before. We found this green/black/brown ⅛” wide, flat “fill” peeking out of every stud and rafter bay. When we pulled back more wallboard we found that the eelgrass fill was sandwiched between two layers of heavy builder’s paper stitched together with the imprint for “Cabot’s Quilt” inked all across the face of the batts.
The Massachusetts Cabots and the Eelgrass Cabot’s Quilt
We know Cabots here in Mass. The Cabots are a first family, the quintessential Boston Brahmins. You may know the famous saying from a toast given at a turn of the century Holy Cross alumni dinner by Dr. John Collins Bossidy: “And here is good old Boston, the home of the bean and the cod, where the Lowells speak only to the Cabots, and the Cabots speak only to God.” Well, this Cabot was speaking to us through a rare building product that may have become ubiquitous but for a parasitic fungus.
Eelgrass (Zostera marina) is a type of seaweed found in brackish water all along the Northeast coast of the Atlantic Ocean from Newfoundland down through Virginia. A great natural resource, its roots were eaten; the grass was used to smoke meat and fish; and when “cured” it was found to be rot and fire resistant. Early settlers to Massachusetts used dried eelgrass as insulation in their walls. Samuel Cabot Incorporated of Milk St, Boston MA, obtained a patent in 1891 for Cabot’s Quilt, an early batt insulation that sandwiched dried eelgrass between two layers of kraft paper with stitching to keep the grass in place…like a quilt.
Image from the Digital Commonwealth
Cabot’s Quilt was sold in 3’ wide rolls, 84’ long and in three thicknesses: Single ply (⅓”), Double ply (½”), and Triple Ply ⅔”. It was advertised as a heat and sound insulator, shipped all over the world, and famously used to build huts and shelters for Scott’s Antarctic expeditions in early 1900’s.
The Revival used single-ply Cabot’s Quilt on the inside face of the exterior wall studs with rock lathe applied directly over it, and to the ceiling joists on the 2nd floor with strapping nailed through it and rock lathe applied to that. The Cabot’s Quilt Catalog of 1928 has section drawings detailing typical installations for the quilt that show it applied under exterior wall and roof sheathing, under interior wall and ceiling plaster/lathe, between wythes in masonry constructions, and under floors for “sound deadening.”
The use of eelgrass as an industrial material was cut short by the Labyrinthula blight of 1931-32. A parasitic fungus quickly decimated 90% of the eelgrass population of New England. Cabot’s Quilt suffered along with the local ecosystem. With eelgrass diminished, the wetlands changed. Pollution and silt from the rivers flowed out into the bays burying shellfish and other marine life in a toxic sludge. Shellfish became inedible to people, predatory fish like cod and waterfowl like the Brant Goose.
Eelgrass has been making a comeback since then, with help from cultivation in Virginia and Cape Cod. However, in recent years we have seen populations at risk, decreasing again, largely attributed to warming ocean temperatures. Cape Cod stands in Wellfleet and Paradise Bay have diminished 50 percent since the 1950s. Some researchers estimate that globally we are losing one acre of seagrass per hour. One acre of seagrass can support 40,000 fish, 50 million invertebrates and sequester 740 pounds of carbon per year. This carbon capture is equivalent to the carbon emitted by a car driving 3,800 miles.
Eelgrass never took off as a building product. Now we insulate our wall cavities with fiberglass, mineral wool, cellulose and polyurethane foam in an effort to maximize R value and minimize air movement. Cabot shifted to making wood coatings and is world famous for it’s deck stains and sealers. This experience reminds me that the built environment is hyper local, regional, global and temporal. Some builder in the future will renovate the REVIVAL and have a look at how we built homes in the early 21st century. They will see some best practices, some best guesses and some mistakes. Perhaps they will learn a lesson from our successes and failures and make some improvements that carry this home into the next century.
A closer look at the eelgrass insulation.