Wrapping an Older House with Rock Wool Insulation
The designer of a deep energy retrofit project in Massachusetts specified 6 inches of mineral wool insulation for the exterior side of the walls
When I first met Chris Gleba and Kris Erickson in December 2011 to discuss their plans for a deep energy retrofit, Chris told me that he had been remodeling his modest two-bedroom house in Lowell, Massachusetts, for over ten years. He had painstakingly rewired and re-plumbed the house and had made energy efficiency improvements (including the installation of a high-efficiency natural gas boiler and radiant in-floor heating). He had also devoted much sweat equity towards upgrading the interior finishes of the kitchen and baths.
Chris was aiming to achieve a high-performance, low-energy home, yet he understood that his early remodeling decisions limited his options moving forward. One of these limiting decisions was the installation of a polyethylene vapor barrier on the inside face of his exterior wall framing as he gutted and remodeled each room. This construction approach was not a mistake in and of itself, but it required that the wall assembly be designed to dry to the exterior to avoid trapping moisture in the walls.
Chris had held off replacing the exterior wall finishes of his house, as he hoped to build a second-floor addition on an existing one-story section of the house. Now that he was married with two young children, his expanded family made the addition more urgent. As he wondered when he would be able to afford the planned work, along came National Grid’s Deep Energy Retrofit (DER) Pilot Program.
Incentives for deep energy retrofits
National Grid, a Massachusetts electric utility, had established a DER pilot program that provides generous incentives for comprehensive projects that lower the energy demand of single- and multi-family residences. Although the primary objective was to improve energy performance (at least 50% better energy performance than a house that barely meets the building code), the program offered both financial incentives and integrated designBuilding design in which different components of design, such as the building envelope, window placement and glazings, and mechanical systems are considered together. High-performance buildings and renovations can be created cost-effectively using integrated design, since higher costs one place can often be paid for through savings elsewhere, for example by improving the performance of the building envelope, the heating and cooling systems can be downsized, or even eliminated. technical support to improve indoor air quality, durability, and overall occupant comfort. Armed with an educated National Grid program staff, the program was strengthened with help from employees of the Building Science Corporation (BSC) who serve as program consultants to evaluate designs (and protect applicants from themselves).
The program was a perfect match for Chris’s project goals. There were up to $42,000 of incentives available to meet the aggressive program goals, including rigorous air sealing targets, R-40 above-grade walls, R-5 windows and doors, and R-60 roofs. Enticed by the available funds, Chris moved towards an “all in” position on his exterior improvements.
Chris Gleba contacted my company, Verdeco Designs, as our company had been pre-qualified by the DER program as designers and builders with proven experience on high-performance projects.
Homeowners: Chris Gleba and Kris Erickson
National Grid staff: Oliver Klein and Marie McMahon
Building Science Corp. consultants: Cathy Gates and Ken Neuhauser
Energy consultants: Jason DerAnanian and Mathew Monaghan (Simpson, Gumpertz, & Heger)
Structural engineer: Mark Webster (Simpson, Gumpertz, & Heger)
Framing and siding: Steve Jordan & Jeremy Melansony
Insulation supplier: Frank Novogratz, IDI
Specialty hardware (Heco Topix screws): Albert Rooks, Small Planet Workshop
Electrician: Neal Boissonneault (Boissonneault Electric)
Mechanical contractor: Allan Berry (Berry Mechanical)
Waterproofing: John Lapan (Lapan's Waterproofing)
Throughout my 28 plus years of designing and building structures in different regions of the United States, I have been blessed with many excellent clients. In many cases, the goals and needs of my clients set me on a path of discovery, a path where I was forced to challenge my preconceived notions and embrace the concept that I am never too old to learn new things about my trade. Sometimes the path leads me to new materials and new methods of construction and sometimes the path leads me to using old materials in new ways. It is through these clients and these projects that I find my greatest inspirations in work and reaffirm that I chose the right profession.
I designed the first Passive House certified in Massachusetts and multiple deep energy retrofit projects, but to be honest, I had never designed a deep energy retrofit project with exterior walls that could only dry to the exterior.
The exterior walls need to dry outward
Chris’s existing exterior walls were constructed with standard 2x4 framing and R-13 fiberglass batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. and he understandably did not want to rip out the interior finishes he had installed over the years.
Since the exterior walls had an interior polyethylene vapor barrier, we could not simply wrap the exterior of the house with 4 inches of polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. rigid foam (an approach that seems to becoming the standard for DER projects in the U.S.). These walls needed to be able to dry to the exterior, so all of the exterior wall components need to be vapor-permeable.
Rock wool to the rescue
In recent years, I’ve heard about the use of exterior rock wool insulation, and in my initial conversation with Chris, I was happy to hear that he was interested in working with the product. Along with being vapor-permeable, rock wool has excellent fire retardant qualities. It’s also moisture- and mildew-resistant, so it is an excellent candidate for wrapping the exterior of a building shell, perhaps as an integral component of a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. design.
I have read that the use of rock wool is common in Canada and Europe, where it is sometimes installed on the exterior side of the water-resistant barrier (WRB). It is also used as batt insulation and even as a blown-in product within masonry veneer air spaces. With all of these positive qualities, we felt committed to using it for this project. Most rock wool board and batt products average around R-4.2 per inch, so the challenge then became: How would we cost-effectively wrap approximately 6 inches of rock wool insulation on the exterior of a wood structure while anticipating some way to attach the siding?
After discussing siding options and preferences for an exterior aesthetic with Chris and his wife, Kris Erickson, we agreed to develop a “modern farmhouse” design solution with vertical metal paneled siding. These vertical panels are typically fastened through horizontal furring strips installed 24 inches on center.
We envisioned a simple installation with wood furring strips holding the rock wool insulation directly over the Tyvek Commercial Wrap, our chosen WRB. The Tyvek would adequately serve as our air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. and WRB, and it had the high permeance that we desired for the “vapor-out” assembly.
Batts or panels?
We first considered using rock wool batts, as they typically have the same thermal value per inch as the denser board products, while being less expensive and more readily available. Although we found examples of similar installations in Europe, our building science consultants (Cathy Gates and Ken Neuhauser) were concerned that wind-washing through the less-dense batts would degrade the thermal performance of the insulation.
We considered incorporating rock wool batts within Larsen trusses, but the added material and labor cost of custom truss fabrication and a new layer of plywood sheathing seemed expensive.
We then focused on the rock wool board products, because their increased density would minimize the wind-washing concern and their increased stiffness might facilitate the installation of the furring strips and siding. Roxul manufactures several rock wool board products with greater density than their batts, and after analyzing cost and availability, we selected 4-inch Cavity Rock DD for our inner layer and 2-inch Cavity Rock MD for our outer layer.
We planned to install the Tyvek directly over the existing 1x6 sheathing boards. These boards had an irregular surface, and we thought that the medium density of the MD board, when slightly compressed, might help eliminate possible air gaps between the rock wool insulation and the irregular sheathing.
Searching for a fastening system
We still needed to figure out how to mechanically fasten 6 inches of rock wool to the exterior of the house. We needed a system that would support the vertical load of the insulation and the siding as well as horizontal lateral loads exerted by wind forces.
The Cascadia Window & Door Company offers an excellent fiberglass clip product prepped to receive metal hat channel for claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. attachment. We strongly considered this product, which would have met our structural requirements as well as our need for a proper fastener. However, we wanted solution that was simpler, cheaper, and would avoid the need to notch the insulation boards around the fiberglass clips.
As we continued to review various long fasteners, most all of which would need some sort of squash block to help maintain uniform depth and assist in lateral bracing, we discovered the Heco Topix-T fastener (see image below). This German fastener is a dual-threaded screw with a “broken” thread or smooth shaft between the top and bottom threads. The smooth shaft helps set the furring depth off the substrate as the screw is driven into the framing member. This fastener allowed us to continuously install our insulation with minimal notching and resolved concerns of furring attachment and lateral bracing.
Although the weight or dead load of our exterior assembly (6 inches of rock wool, 1x4 wood furring, and galvalume metal siding) was not significant, we still had to structurally account for this vertical load. Upon consultation with our creative structural engineer, Mark Webster, we designed a 4-foot-on-center wood bracket system at the first-floor rim joist that could support the metal siding by fastening a small steel angle to the face of the wood bracket (see image below). In turn, we screwed a perforated metal shelf with a nylon insect screen to support and protect the bottom edge of the rock wool insulation while providing ventilation and weep openings for the rainscreen design. We used the same simple bracket system at the window and door headers.
A chainsaw retrofit
With proper planning, the installation of the assembly was systematic. Once all of the exterior finishes had been stripped down to the original sheathing, we transferred the wall stud locations to marks on the foundation, removed the roof eaves with a chainsaw, and then wrapped the building shell in Tyvek like a giant birthday present, carefully taping seams and button caps, with careful attention to penetrations and water management details at door and window openings.
We then installed the bracket and metal shelf at the first-floor rim joist. This allowed us to start installing the rock wool boards by stacking them on the shelf, overlapping seams, and fastening the batten strips as we went up. The 2 ft. by 5 ft. rock wool panels were fairly easy to handle, though my carpenters found the fibers irritating to the skin if there was much direct contact. At first, we cut the panels with a hand saw specifically made for cutting rock wool. After a while the carpenters switched to a PVC hand saw, which cut the panels more easily.
The batten strips slightly compressed the rock wool panels and held the insulation in place as we continued to stack it. As the insulation went up and fully wrapped the two-story home, I marveled at how snug the continuous rock wool wrap made the structure appear, like the thick wool comforter that might have kept us warm in simpler times.
Birds love rock wool
One slightly amusing problem that we did not anticipate: birds like to nest in rock wool! As soon as we installed insulation above head height, the word got out to the bird community that there was some prime real estate to squat in! The birds would quickly burrow clean circular holes to claim their own little condo.
In the few weeks that it took to install the insulation and await delivery of the metal siding, there were a dozen or so uninvited guests in our wall assembly. Systematic eviction took place as we installed the siding. We simply filled the bird holes (and other small gaps around openings and brackets) by pressure fitting some loose rock wool material.
TJIs for the roof
To achieve our R-60 goal on the roof, we over-framed the existing roof framing with 16-inch-deep TJIs and installed steel truss rods with horizontal hold-downs to prevent the new rafters from spreading. Along with a positive connection to the existing wall plates we were able to eliminate the structural loads on the existing 2x6 roof rafters, which were both structurally undersized and unable to handle much insulation within their limited depth.
Although we did not need to design the roof system to dry to the exterior, we elected to use rock wool batts. We installed three layers of 5-inch batts, overlapping the seams within the TJI cavities. We used the standard 24-inch wide commercial acoustic rock wool batts for this vented roof application, and the batts fit snugly in our TJI framing set at 24 inches on center.
A tight building that won’t trap moisture
The project was a smashing success. With the assistance of National Grid’s dedicated staff and BSC’s careful guidance, we comfortably achieved our airtightness goal and obtained close to the maximum incentive available. The blower door test result was 320 cfm @ 50 pascals (about 0.9 ach50).
As with most deep energy retrofits, this comprehensive renovation included integrated design solutions for water management, energy systems, and controlled ventilation. Low-energy buildings always require rigorous air sealing, but this project was noteworthy for the unusual building enclosure components and the recognition that exterior rock wool is an excellent solution for “vapor-out” assemblies.
As the debate on the environmental impact of spray and rigid foam insulation continues, many designers, builders, and homeowners may prefer to let buildings “breathe” rather than wrap them in foam.
Since rock wool resists fire, mold, and mildew, designers should recognize that this material has more uses than fire-stopping and acoustical control — the uses for which it is usually specified in the U.S.
I feel fortunate to have met Chris Gleba, the client who set the stage of this investigation and helped me identify attractive options for future deep energy retrofit projects. Empowered with new knowledge, I can only look forward to my next rock wool project.
Mark Yanowitz is the managing partner of Verdeco Designs, LLC, a design-buildCompany that handles house design and construction. Since both services are provided by the same firm, integrated design can often be more easily achieved. and consulting firm in Andover, Massachusetts. The company specializes in energy efficiency and low-impact green building solutions for residential, commercial, and mixed-use projects.
- All photos: Mark Yanowitz
- Architectural drawings: Verdeco Designs
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