Countless historic masonry buildings dot the American landscape, a large number of them falling under some form of aesthetic scrutiny prohibiting exterior insulation. The only option to make these buildings energy efficient is to insulate them on the interior. 475 High Performance Building Supply works with architects and builders to insulate these walls safely from the interior – considering moisture drive, bulk rain water issues and insulation materials – while avoiding the dangers of freeze-thaw and mold. Using WUFI hygrothermal modeling, we show how effectively INTELLO, an airtight and "vapour-intelligent" membrane, provides the necessary drying reserve to keep newly insulated historic structures from falling apart (see blog post here). Preservation and high-performance working together for a truly sustainable future.
College campuses have all manner of these types of historic masonry structures, from academic buildings to dormitories. It's therefore not surprising that campuses have gigantic energy costs. Dartmouth College is one of New Hampshire's biggest energy users. So there's plenty of incentive to lower the heating and cooling load of campus buildings - without destroying them in the process.
Smith & Vansant Architects, based in Norwich VT, was hired to renovate an existing campus structure at 4 North Park Street in Hanover NH. Built in 1928 as the Whitaker Apartments, it was designed by Jens Frederick Larson, famous for iconic Dartmouth landmarks such as Baker Library and Memorial Stadium. While the $3.6 million pricetag could possibly have paid for a brand new building as the campus LGBTQ living and learning program house, the school decided to keep the historic exterior. They also aimed to make it greener - more sustainable, energy efficient and comfortable. (Saving the structure from demolition was in itself one of the greenest strategies of the project!)
Like many building professionals, lead architect John Vansant expressed concerns about applying closed cell foam insulation at the interior to bring it to higher performance, and was interested in a different approach. Having worked with Dartmouth College on a number of retrofit projects, he invited members of the campus buildings & grounds operations to attend 475's High Performance Historic Masonry Retrofits presentation. They decided to specify the INTELLO Plus interior airtight system with densepack cellulose for both the existing masonry walls and the roof. The existing walls were an unconventional hybrid of face brick over hollow terracotta tile (see wall section drawing). In a case such as this where the moisture risks are even less well understood, INTELLO provides the drying reserve needed to overcome any unanticipated moisture stress.
For optimal airtight installations, 475 offers a comprehensive guide: the Masonry Retrofit Smart Enclosure. Start here to get an overview of the concepts, detail drawings, 3-D diagrams and best practice recommendations. The general contractor, Estes & Gallup, did a great job tailoring 475 recommendations to the realities of a project on a tight budget and deadline. The project was a resounding success, and the Dartmouth community celebrated the "Triangle House" opening on Nov. 1, 2014 - delivered on time for the opening of the Fall semester. Follow along for a look at the project.
Masonry Walls: bulk water, windtight brick, interior INTELLO Plus and densepack cellulose
Step 1: The first step in any historic wall retrofit project is to evaluate and address bulk water issues. This includes assessing the quality of the brick, as well as restoring the traditional water shedding details to good working order. Fortunately, the brick exterior of this 86-year-old building was still in very good condition. The interior terracotta was also reasonably "windtight" - helping to ensure that wind-driven moisture would have minimal impact.
Step 2: In preparation for the interior airtight INTELLO Plus layer, 2x4 furred walls were offset a couple inches from the masonry. With no shortage of obstacles - from new and existing structural members to framed interior walls and existing floors - obtaining a perfectly continuous INTELLO layer was a challenge. (Note: our best practice recommendation is to cut back floors and interior walls to allow for optimal airtight taping and INTELLO continuity across such areas.) At below grade walls, rigid mineral wool board was first added to hedge against bulk water issues, then furred 2x4 walls were built over that.
Step 3: Without service cavities (another 475 best practice recommendation), Lessco airtight utility boxes were installed and taped to INTELLO.
Step 4: INTELLO was stapled and taped with TESCON Vana across the inside face of the studs, with staples aligned vertically to resist tearout when densepacking. This differs from traditional installation of Insulweb netting, which is often inset (lip stitch) stapled to the studs. CONTEGA HF solid acrylic caulk was used to connect INTELLO securely and flexibly to subfloors and other rough/uneven areas. INTELLO Plus was folded and taped into window openings with TESCON Profil for full airtight integration. Densepack cellulose was blown into the bays to a density of 3.5 lbs per cubic foot and the holes were taped with airtight patches. Airtight, vapour retarding and foam-free rim joists could have been achieved with a combination of INTELLO and tape, but due to time constraints closed-cell foam was used in those areas. A very small fraction of the insulation in this retrofit was foam!
Gambrel Roof: vented cathedral ceiling, interior INTELLO Plus and densepack cellulose, rigid mineral wool
Gambrel roof drawing showing existing "double roof" venting detail
Roof insulation retrofits can be challenging - insulation code requirements, airtightness, vapour control, fire resistance and other needs have to be met. For cathedral roofs that don't resort to closed cell foam, venting outboard of the roof decking is always desirable. However, in this project there were no plans to touch the existing slate roofing. Fortunately, the designers of the original gambrel roof presented the architects with a gift - in the form of a "double roof" with a vented gap in between. With venting already built in, it would be code compliant to densepack cellulose in the existing rafter bays. (Note: by keeping moisture levels low, INTELLO would have been safe to use even without venting - see here).
As with the walls, INTELLO Plus provided the airtightness, intelligent vapour control and reinforced membrane for a practical, cost-effective and safe way to insulate the dormitory roof with densepack cellulose. An additional layer of Roxul ComfortBoard was added inboard to provide a thermal break along with fire rating for the attic.
Step 1: After taping/caulking surfaces were swept clean (dust is not airtight, so don't tape to it), INTELLO Plus was stapled to rafters and caulked to the floor, then seams taped. Structural and service penetrations were diligently taped.
Step 2: A number of brick interior and end walls interrupted the roof. Brick work is pretty leaky - the mortar joints are air transfer paths. It has been reported by PHI that plastering the brick for 3'-6" (1m) will make the brick/mortar joints sufficiently airtight. There are several options to connect this plaster airtightly to the roof INTELLO. For this project, INTELLO was taped to the brick with TESCON Vana and TESCON Primer RP, then the fleece side of the Vana was plastered over.
Step 3: After densepacking behind the INTELLO Plus and taping the blow holes, the slight bulges were rolled flat. Then the 2" rigid mineral wool boards were secured through to the studs. Mineral wool boards were also added at end walls.
Conclusion
While no blower door tests were done to verify the final airtightness, Jon Haehnel from Zero by Degrees issued a comprehensive report of pressurized fog tests. He commented that the "air barrier system is performing well and that there has been careful attention to detail when making transitions in the air barrier". The architects also praised the foam-free interior, noting that it "felt different" from spray foam projects.
Historic buildings cannot take a pass on addressing climate change mitigation. We can – and should – safely make our historic masonry walls more energy efficient (using WUFI as a guide). And historic roofs can also be brought to high performance - and done reversibly, with sustainable fibrous insulations such as cellulose and mineral wool.
