Installing dense-packed cellulose in newly-framed walls
Cellulose is an excellent insulation choice because of its high recycled content, low embodied energy, and – unlike foam-based insulation – it does not contain petroleum products. Its low off-gassing potential made it the ideal choice for the owner of a full-house gut project in Leslieville, whose health condition makes her sensitive to a variety of environmental stimuli.
Cellulose also has the benefit of being a low-cost material, but it does require some special detailing to install.
Blown-in cellulose is often used in our region to insulate attics, but dense-packed cellulose is much less common, even though it is popular elsewhere in North America. We were able to locate only one local contractor, GreenSaver that was equipped with the machinery and knowledge to complete the installation. One important property to attain with the material is adequate density in order to achieve reliable thermal performance and prevent settling over time.
Verifying that adequate density has been achieved is difficult, but Greg Labbe from BlueGreen Group suggested two methods:
- The first is to calculate the density in each cavity by computing the volume and measuring the amount of material being installed. This is a laborious process, especially if your cavities are not uniform, as in our particular case, so a random sampling of a few cavities would be more manageable.
- The second method is less qualitative. A piece of wire (such as a coat hanger) is bent into a zed shape; the end, about 2″ long, is inserted into the cellulose and the wire is rotated (similar to a bicycle crank). An experienced tester can judge by the resistance to the rotation of the wire whether adequate density has been achieved.
On this project we had a number of different cases, each requiring its own specific detailing:
Furred-Out Below-Grade Walls
In the basement, we framed wood walls with studs at 24″ centres spaced approximately 1″ from the existing structural masonry walls. The interior of the masonry walls were covered by the corrugated plastic drainage mat that is part of the interior weeping tile system. This corrugated mat will ensure that moisture doesn’t enter the cavity from the exterior. To allow drying potential to the interior while still blocking moisture from the interior under normal cold-climate conditions, a ‘smart’ vapour retarder/air barrier was installed on the interior of the studs (Certainteed Membrain). Cavities at joist ends were insulated with recycled denim batt and the AB/VR was carefully taped and sealed around each joist.
The 1″ offset allows for additional R-value and interrupts the thermal bridge that would otherwise exist at each stud. We conferred with GreenSaver to ensure the 1″ gap was not so large that the cellulose could easily blow by it, making it difficult to build up the required density in each cavity. This was a semi-detached house, so we insulated the common wall two feet from the exterior by damming off the cavity with plywood.
Furred-Out Above-Grade Walls
These were similar to the below grade walls with the exception that there was no interior weeping tile system. In order to separate the cellulose from the structural masonry walls and to provide an air space, we installed a woven polypropylene mat (Mortair Vent – a veneer masonry rainscreen product) against the brick.
Newly Framed Wood Walls
At the third storey addition, we framed new 2X6 wood walls with studs at 16″ centres. Plywood sheathing and 1.5″ of exterior mineral fibreboard insulation was installed to achieve code-mandated insulation levels.
Newly Framed Flat Roof
9.25″ deep wood I-joists were used to frame the flat roof. Tyvek was installed on top of the I-joists to contain the cellulose on the top side. Above the Tyvek, tapered purlins were installed to slope the roof to shed water and to provide a ventilation cavity.
Where we had plumbing on exterior walls, a secondary service wall was framed in front of the insulated wall. This detail is not specific to cellulose but is good practice to avoid the risk of frozen pipes and discontinuities in the air barrier.
Electrical Rough-In Installation
Electrical rough-in was completed as usual. R-2000 airtight boxes were used on exterior walls. These did complicate installation of the mesh, which is discussed below. In the ceilings, air tight potlights rated for insulation contact were installed with polyethylene ‘bubbles’ around them so that air barrier continuity could be ensured.
Mesh or Plywood Installation for Cellulose Containment
In order to densely pack the cellulose, it must be packed into a defined volume – generally a stud cavity. The volume is defined by the studs and plates on four sides, the sheathing or structural masonry on the exterior and a special-purpose mesh on the interior. The mesh is air permeable and is stapled to the studs. Because the cellulose is packed in to achieve a desired density, it stretches the mesh causing ‘pillowing’ between studs. For this reason the mesh is stapled to the inside of the studs rather than the face to ensure drywall can be installed without making the wall wavy.
A wood backer must be installed at corners and intersecting walls to provide a stapling substrate. The airtight R-2000 boxes with their plastic gasketed flanges proved challenging to fasten the mesh to. This was successfully accomplished with diligence and a good amount of tape, but the pillowing effect near the boxes was still quite severe. For vent penetrations, we installed plywood plates that the ducts passed through. The plywood provided a solid substrate to fasten the mesh to. In the future, we may try installing similar plywood plates at receptacles as well.
At windows and doors, we installed plywood jamb extensions to contain the cellulose. The jamb extensions can then be used with casing or as a drywall return. In this project, corner bead was installed and mudded for a drywall return. The jamb extensions are taped to the AB/VR to ensure continuity.
For the ceiling, GreenSaver advised that because of the larger cavities and gravity, the mesh might not be sufficient to contain the cellulose. We therefore installed 3/8″ plywood on the ceiling, being careful to keep the edges of the poly at air barrier boxes and top plates exposed so that they could be taped to the ceiling AB/VR after cellulose installation. Although the plywood ensures no cellulose ‘blow-outs’, it is more difficult to verify full coverage as the cellulose is not visible as with the mesh.
After all the prep work was done, GreenSaver packed the cavities with cellulose. Note that it is an extremely dusty process! We had a few ‘blow-out’ situations where the mesh became detached or we missed a containment detail and clouds of cellulose spewed out into the air like a dust storm. Respiratory protection is a must. With this experience we can improve our detailing and minimize such incidents in the future.
Air Barrier/Vapour Retarder Installtion
Once the insulation has been installed, the AB/VR is installed as in conventional cold-climate construction. Tuck Tape, 3M tape, and AirDam were used to ensure continuity. Above grade, 6 mil poly was used as the AB/VR. Below grade, Certainteed Membrain was used to allow for drying to the interior should conditions necessitate it.
Even though the cellulose installation required more detailing and therefore greater labour inputs than some other insulation types (spray foam or batt), the low cost of the material still made it more economical than spray foam. Although, the resulting initial R-value of the assembly is less, this was deemed an acceptable trade-off with consideration for the owner’s health and the other environmental benefits of cellulose.
By Steven Gray, Construction Manager