FAQ
Insulation Questions Homeowners Actually Ask
Straight answers, no sales pitch.
What R-value do I need for my attic in my climate zone?
The DOE working numbers: Zones 1-2 aim for R-30 to R-49, Zones 3-4 aim for R-38 to R-60, and Zones 5-8 aim for R-49 to R-60. Most attics we walk into were built to half of that. Measure your current depth in inches and multiply by about 2.5 for old fiberglass (about 3.2 for cellulose) to see where you are starting. Six inches of matted old batts is roughly R-15 — about half of what Zone 5 wants.
Can I put new insulation over old insulation in my attic?
Yes, with rules. The new layer must be unfaced — no paper or foil backing — or you trap moisture between layers. Run new batts perpendicular to the joists so they bridge the framing. If the old stuff is wet, moldy, or full of rodent droppings, remove it first. And if it looks like gray-brown pebbly vermiculite, stop and get it tested before touching it; some vermiculite contains asbestos.
Do I need a vapor barrier for my basement walls?
Depends on your climate and your assembly, and this one trips people up. Above grade in cold climates (Zone 5 and up), codes generally want a vapor retarder on the warm-in-winter side. Below grade, against cold concrete, a stud wall with poly behind it can trap construction moisture against the concrete forever. Many builders now use rigid foam directly against the concrete and skip the batt-plus-poly combo entirely. Call your local building department before you pick an assembly — answers vary a lot by region.
How do I insulate around electrical wires and outlet boxes?
Do not stuff the whole batt behind the wire. Split the batt through its thickness — half tucked behind the wire, half laid over it, so the wire ends up surrounded. On faced batts near boxes, slit the paper flanges and fold them around. In attics, fire-stop caulk around wire bundles passing through top plates before insulating; it is code in most places and it is cheap insurance. Wires resting on top of insulation are fine. Insulation crammed inside a box is not.
What is the difference between mineral wool and fiberglass?
Mineral wool is denser, noticeably better at blocking sound, fireproof to around 2,150°F, shrugs off water, and cuts cleanly with a bread knife. Fiberglass costs about half as much, weighs less, and is stocked everywhere. Budget exterior walls favor fiberglass. Bedroom walls, bathrooms, and anywhere fire matters are where mineral wool earns its markup.
Can I insulate my walls without tearing down the drywall?
Yes — dense-pack cellulose or injected foam through drilled holes, then plug and paint. Crews either drill from inside and hide the plugs under paint, or pull the siding and drill through the sheathing from outside. It works, and it is a standard retrofit. Just know that patching and repainting are part of the deal either way, so if your walls are already open for a remodel, batts beat injected products on price every time.
Why are my roof shingles curling after I insulated the attic?
Because your attic got hotter. If insulation blocked the soffit vents or baffles were skipped, intake-to-ridge airflow stopped, and heat that once leaked through a drafty ceiling now bakes the roof deck from underneath. In winter that same failure builds ice dams. The fix is ventilation, not less insulation: one baffle per rafter bay, clear soffit vents, and a working ridge or gable vent. Plan roughly 1 sq ft of net venting per 150 sq ft of attic floor.
Do I need to wear a mask when installing blown-in cellulose?
Yes — an N95 minimum. Cellulose dust is fine and it carries borate fire retardants you do not want deep in your lungs. The blowing machine kicks out a big dust cloud the first seconds of every bag, and goggles matter as much as the mask. Rent the machine with a helper: one person feeds the hopper, one works the hose. Blowing cellulose solo takes twice as long and is twice as miserable.
