Short answer: for most existing attics in the East Bay, blown-in insulation is the better fit. It fills in around ducts, wiring and truss webs without leaving gaps, and it can go right on top of the insulation you already have. Batts work where the joists are evenly spaced and nothing is in the way, and for small spots like the back of the attic hatch. With either one, the result depends on the installation: no gaps, no squashed spots, and enough depth to reach the R-value recommended for our climate.
What each one is
Batts (and rolls) are pre-cut blankets of fiberglass, mineral wool or other fibers, sized to lay between joists. Blown-in, also called loose-fill, is fiberglass, cellulose or mineral wool in loose form, blown through a hose by a machine. ENERGY STAR lists batts for unfinished walls, floors and ceilings, and loose-fill for unfinished attic floors and other hard-to-reach places.
The fibers are similar. The difference shows up in how each product handles a crowded attic, and how easy it is to get full, even coverage.
Why blown-in usually wins in an existing attic
In many Tri-Valley homes, from Twin Creeks to Gale Ranch in San Ramon, the attic floor is busy. Heating ducts, wiring, recessed light housings and truss webs all sit where the insulation needs to go.
- It flows around obstacles. Loose-fill is blown into place, so it settles around pipes, wires and duct boots. A batt has to be split or cut at each one. The Department of Energy’s Building America guide says batts should be split to fit snugly around any wiring or pipes, and that a good installation has no gaps, voids or compression. That is a lot of careful cutting in a crowded attic.
- It goes on top of what you have. ENERGY STAR says you can add loose fill on top of batts, and you don’t have to match the type already in the attic. If the old insulation is clean and dry, it becomes the bottom layer. Our guide on whether to remove old insulation or add on top covers the cases where it has to come out first.
- It covers the joists. A deep, even layer buries the joists, so heat can’t bridge through the wood. ENERGY STAR’s quick check: if your insulation is level with or below the joists and you can easily see them, you should add more.
- It is easier to keep even. ENERGY STAR warns that some attics have plenty of insulation in the middle and very little along the eaves. Blown-in can be built up evenly out to the edges, as long as the vents there are protected (more on that below).
When batts make sense
Batts are the easier product for a homeowner to handle. ENERGY STAR rates batts as a basic-skill job and loose-fill as intermediate, and notes that blowing in loose fill takes a machine (some home improvement stores rent them). Batts are a reasonable choice when:
- The joists are evenly spaced and the attic floor is open, with few ducts or wires in the way.
- You’re insulating a small area, such as the attic hatch. ENERGY STAR suggests attaching a piece of fiberglass or rigid foam board to the back of the hatch and weatherstripping the edges.
- You’re adding a second layer. ENERGY STAR says to lay it perpendicular to the joists, and to use unfaced batts (no paper or foil backing) whenever they go over existing insulation.
The weak point is fitting. A batt stuffed into a short bay, bunched around a duct or cut short at the eaves leaves a spot where heat gets through, and across a whole attic those spots add up.
How much you need, and how to check it
The Tri-Valley sits in climate zone 3 on the DOE map. For zone 3, ENERGY STAR recommends R49 for an attic with no insulation, or R38 when you’re adding to 3 to 4 inches of existing insulation. ENERGY STAR puts R-38 at roughly 10 to 14 inches, depending on the type of insulation, so a depth on its own doesn’t tell you much. What matters is the product and its coverage chart.
A federal rule helps you check. Under the FTC’s R-value rule, an installer has to give you a contract or receipt for the insulation. For loose-fill, it must show the coverage area, the installed thickness, the minimum settled thickness, the R-value and the number of bags used. For batts, it shows the coverage area, thickness and R-value. Loose-fill settles a little, which is why the receipt lists two thicknesses. If a quote promises R49 but no one can tell you the bag count or the settled depth, ask before you sign.
The prep matters more than the product
Blown-in or batts, the job goes wrong in the same places. Before any insulation goes in:
- Seal the air leaks first. ENERGY STAR’s guidance, from the EPA, is to air seal the attic before adding insulation. Once a foot of insulation is down, the holes around wiring, plumbing pipes and the tops of interior walls are buried.
- Keep the eave vents open. ENERGY STAR calls blocked airflow at the eaves the most common mistake homeowners make. Rafter vents, also called baffles, keep a channel open so insulation can go all the way to the edge without covering the soffit vents.
- Keep clear of hot spots. Insulation should stay at least 3 inches away from recessed “can” lights unless they are rated IC (insulated ceiling). Around furnace and water heater flues, ENERGY STAR notes that building codes usually require 1 inch of clearance from metal flues (2 inches from masonry chimneys), held back with a metal dam.
- Check what’s already there. If the old fill is vermiculite, a pebble-like material that is gray-brown or silver-gold, the EPA says to assume it contains asbestos and leave it alone. Call an asbestos professional before any attic work.
Quick guide for East Bay attics
If your attic has ducts, wiring and recessed lights on the floor, which describes many homes in San Ramon, Danville, Dublin and Pleasanton, blown-in is usually the practical choice. If you’re topping up clean, dry insulation, blown-in on top or unfaced batts laid across the joists both work. If the attic is open and evenly framed and you want to do it yourself, batts can do the job, as long as every piece fits snugly and nothing blocks the eave vents.
During a free in-home estimate we check the type and depth of what’s there, how crowded the attic floor is, the ducts, the eave vents and any signs of rodents or roof leaks. You get photos of what we find and a written quote.
Not sure which type your attic needs? Call (510) 725-4073 or request a free estimate. We serve San Ramon, Danville, Pleasanton, Dublin and the rest of the East Bay. More about our attic insulation service.
Sources: ENERGY STAR, recommended home insulation R-values · ENERGY STAR, choosing the insulation type · ENERGY STAR, adding attic insulation · ENERGY STAR, attic ventilation and rafter vents · ENERGY STAR, attic air sealing · DOE Building America Solution Center, batt insulation for existing attics · FTC R-value rule, 16 CFR 460.17 · EPA, vermiculite insulation