How to Refresh Cellulose Insulation in the Attic

In this video, This Old House general contractor Tom Silva demonstrates a cost-effective way to save energy.

Pro Tip: This Old House general contractor Tom Silva notes that cellulose insulation “works like a vacuum cleaner in reverse”—a blower machine sucks the insulation down through the bottom and pushes it out through the hose. Renting one of these machines is what makes this a manageable weekend project for homeowners.

Steps for Refreshing Cellulose Insulation

Pro Tip: Before adding any new insulation, make sure you address air sealing first. As demonstrated in a This Old House project, wherever air is leaking from the living space into the attic—especially around chimney chases and other penetrations—you want to seal those gaps nice and tight before insulating. Skipping this step means your new insulation won’t perform as well as it should.

1. Measure the depth of the existing blown-in insulation. If necessary, add more cellulose insulation.
2. Dump cellulose insulation into each joist bay and spread it evenly with a leaf rake. Be sure the insulation is slightly above the top edge of the joists.
3. Lay batts of un-faced, 9½-inch-thick fiberglass insulation perpendicularly across the joists.
4. Cut the batts of insulation to fit tightly together.
5. Continue to cover the attic with fiberglass batts, including any wood subfloor.
6. Build a storage-platform frame out of 2x10s.
7. Fill the interior of the platform frame with fiberglass insulation.
8. Cover the platform with 7/16-inch oriented-strand board (OSB) or plywood.
9. To create a walkway to the platform, cover the wood subfloor with pieces of 2-inch-thick polystyrene.

Always use unfaced batts when layering new insulation over old. Place the new layer perpendicular to the old layer to cover any gaps in the lower layer. Adjoining batts should be butted snugly together, but not tight enough to compress them. Never lay heavier batts, such as cotton, over lighter ones like fiberglass—you’ll compress the lower layer and reduce its effectiveness. Always cut batts to fit around obstructions and penetrations; stuffing or cramming them around ducts, piping, and the like will compress the air-trapping pockets in the material, reducing its insulating properties.

If you insulate the attic floor and then place stored items directly on top of the insulation, the insulation will not be effective—you cannot compress it without diminishing its R-value. That’s why building a raised storage platform is essential: it gives you a dedicated area to store things without crushing the insulation underneath. If you need more storage space than the platform provides, you’ll need to build up the frame high enough to fit the full depth of insulation beneath it before adding plywood on top.

Materials

• Dust mask
• Leaf rake

Tools