Batt & Fiberglass Insulation

Fiberglass batts are the most common and generally the most affordable option, offering solid R-value per inch and wide availability in standard widths and thicknesses to match typical framing. Mineral wool (also called rock wool) batts cost more but bring real advantages: better fire resistance since the material itself doesn't support combustion the way fiberglass can, stronger sound-dampening performance for interior walls where noise control matters, and a rigidity that holds its shape well in a cavity without sagging.

For most standard exterior walls and attic rafters, fiberglass does the job at a lower price point. Mineral wool earns its premium in specific situations — interior walls around bedrooms or media rooms where sound matters, garages or mechanical rooms where fire resistance is a priority, or anywhere the extra rigidity makes for a cleaner installation.

Batt insulation's rated R-value assumes it's installed at full, uncompressed thickness filling the entire cavity with no gaps. In practice, that's where a lot of amateur and rushed installations fall short. Compressing a batt to stuff it into a cavity that's slightly too narrow, or cramming an oversized batt into a shallow one, reduces its effective R-value because insulation performance depends on trapped air, and compression squeezes that air out. Gaps left around electrical boxes, pipes, or where a batt is cut short leave a direct thermal bypass — a spot where heat moves almost freely regardless of how good the batt itself is.

A careful installation means cutting batts to fit around obstructions rather than just cramming them past, splitting batts to wrap wiring and boxes instead of compressing around them, and making sure facing (if used) is oriented and sealed correctly. It's a job where two crews using the identical material can produce noticeably different real-world results, which is exactly why installation quality is something worth asking a contractor about directly.

Batt insulation fits naturally into the new-construction and addition timeline — it goes in after framing and rough-in (electrical, plumbing, HVAC) but before drywall, when the cavity is fully open and easy to access. This is also the ideal window to combine batts with basic air sealing, caulking, and foaming smaller gaps around penetrations before the walls are closed up, since that access disappears once drywall goes up.

For additions in particular, it's worth confirming the new insulation meets or exceeds current recommendations rather than just matching whatever's in the older part of the house — an addition is a chance to insulate to a higher standard without any of the access challenges a retrofit would involve.