Match the material to the attic assembly
| Material | Often fits | Watch for |
|---|---|---|
| Loose-fill cellulose | Open attic floors and irregular framing | Dust, installed density, and settled depth |
| Loose-fill fiberglass | Open attics and top-up work | Wind washing near eaves and enough installed depth |
| Fiberglass or mineral-wool batts | Clear, regular joist bays | Gaps, compression, wiring, and cross-bracing |
| Spray foam | A deliberately unvented roofline assembly | Roof leaks, combustion safety, drying path, and cost |
A material with a strong laboratory R-value can still perform poorly when installers leave gaps around plumbing, bury unsafe heat sources, block soffit ventilation, or compress batts under flooring. The installed assembly matters more than a dramatic product claim.
Do the work that disappears under insulation
Before adding depth, seal penetrations through the ceiling plane and install safe blocking around chimneys, flues, and non-insulation-contact light fixtures. Repair roof leaks and bathroom fans that dump moisture into the attic. Once loose-fill is knee-deep, those defects become slower and more expensive to reach.
Measure the existing material in several locations, identify it, and set a target R-value from current local code or a climate-based program. ENERGY STAR notes that common fiberglass, cellulose, and mineral-wool products are often around R-3 to R-3.5 per inch, but the exact product documentation should control the estimate.
Situations that change the answer
- Vermiculite or any suspected asbestos-containing material should remain undisturbed pending professional guidance.
- Active knob-and-tube wiring may restrict whether insulation can be added around it under local rules.
- A roofline conversion needs a building-science design for ventilation, moisture, and any combustion appliances in the attic.
- Frequent attic access may justify dams, walkways, and a protected service route before insulation is installed.
Primary sources
Product details, health guidance, and code references were checked against these sources.
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