When it came time to replace the 30-year-old asphalt roof on my old Cape, I knew I wanted to address the ice dams and many icicles that formed every winter—sometimes nearly reaching the ground. When I bought the house back in 2007, I had done some basic weatherization and air-sealing (insulating the walls and rafter bays with dense-packed cellulose) but I knew the house was still leaky and we had only 5 in. of cellulose (R-17, at best) in the sloped sections of the roof, along with loose-fill cellulose in the flat of the attic.
To design a comprehensive roof retrofit solution I hired New Frameworks, a design/build firm located here in central/northern Vermont. I was interested in trying to retrofit the roof without using any foam, due to the embodied carbon and greenhouse gas emissions associated with foam, and I knew New Frameworks was experienced in working with a variety of carbon-storing materials. We landed on an approach using Steico wood-fiber board as continuous exterior insulation.
A rundown of the work
To start, they removed the existing shingles down to the sheathing (a plywood layer had been added above the original board sheathing back in the 1990s, when they last replaced the roof) and removed the existing soffit and fascia. A mason arrived to add 18 in. to the two chimneys, since once the ridge of the roof was raised, more clearance was needed to meet code. Once the demo was done, the crew focused on air-sealing—adding a layer of Solitex Mento 3000, a vapor-open water-resistive barrier (WRB).
The Mento is taped to the top plate and the top of the wall’s exterior frieze board and in between the rafter bays, and is sealed to the frieze and sheathing on the gable ends. The Mento is fastened to the top of the sheathing; a horizontal slit was cut in the sheathing across the width of the roof on both sides and the Mento runs through the attic, above the loose-fill cellulose. Before taping together the two sides of Mento in the middle of the attic, the crew blew in an additional 10 in. of loose-fill cellulose and built a catwalk for working in the attic.
With the Mento in place, they added two layers of Steico Universal Dry wood-fiber board to the lower two-thirds of the roof slope. (It wasn’t necessary to insulate the section above the attic plane because there was cellulose there.) With two layers (40 mm and 60 mm, respectively—just under 4 in.), the Steico added approximately R-12 in continuous exterior insulation. On top of the wood-fiber board the crew added 2x4s on the flat to create a vent plane. They also had to furr out the 2x4s as much as 2 in. in places to even out a bow in the roof and ensure the new roof was level all the way across.
This was an opportunity to replace two 30-year-old skylights with new operable triple-pane units from Fakro, a Polish manufacturer distributed in the US by 475 High Performance Building Supply. This involved building out a new plywood buck above the sheathing and new interior trim since the skylight well inside ended up about 6 in. deeper than before.
The new thickness of the roof also required a 6-in. thicker fascia profile, and they replaced the soffit and added a soffit vent.
Once the insulation was installed, they added a second sheathing layer of 1/2-in. plywood over the 2×4 sleepers with a layer of roofing felt. Then the new skylights went in, and the roofers could finally install the standing-seam metal roof.
Once all the trim was complete, we ran a blower-door test and learned the work had resulted in a 10% reduction in air leakage. The project finished up just as the first snowfall of the year arrived. After the first winter with a nice thick roof, I can happily report there were no ice dams.
Additionally, choosing wood-fiber board over EPS or polyiso proved effective in terms of keeping the project’s embodied carbon down. The continuous insulation covered approximately 700 sq. ft. Using the BEAM Calculator, I found that the wood-fiber board stored 1079 kg CO2e compared to polyiso, which would have emitted 394 kg CO2e, or EPS, which would have emitted 449 kg CO2e.
That one choice in materials saved close to 1500 kilos of carbon, in addition to reducing the heat load of the house, and its associated operational carbon emissions. Objectives met.
Kate Stephenson is CFO/COO at Helm Construction Solutions. Photos courtesy of the author.
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