How to Install Wall Insulation Batts
Step-by-step guide to measuring, cutting, and friction-fitting thermal insulation batts between timber wall studs without compressing fibres or leaving gaps.
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🧰 Required Tools & Equipment
- ✓ Insulation cutting knife or long serrated blade
- ✓ Tape measure
- ✓ Straight edge or wide timber offcut
- ✓ Safety glasses
- ✓ P2 particulate respirator
- ✓ Heavy-duty work gloves
📦 Materials & Supplies
- ✓ Wall insulation batts (sized to framing depth, e.g. R2.2-R2.8 90mm)
- ✓ Plastic strapping or string (optional, for non-standard framing)
⚠️ Safety Checkpoints & Warnings
- • Wear a P2 respirator, protective glasses, and long sleeves to prevent glasswool fibre irritation.
- • Isolate power at the mains board before working near exposed electrical wiring or junction boxes.
- • Ensure adequate ventilation in the room while opening and handling compressed insulation packs.
📋 Step-by-Step Guide (5 Steps)
Measure the Stud Cavity Dimensions
Measure the height and width of each individual stud bay between the timber framing members. Note any variations, as stud spacing can fluctuate around doorways, windows, and internal corners.
- • Tape measure extended from inner stud edge to inner stud edge across top, middle, and bottom.
- • Measurements written down or referenced against standard batt sizing.
- • Assuming all stud cavities in the room have uniform widths.
Cut Batts to Required Dimensions
Place the batt flat onto a scrap board or timber offcut. Compress the batt firmly using your straight edge along the cut line and slice through cleanly in smooth strokes with the serrated knife.
- • Straight, square edge across the full width and depth of the batt.
- • No torn or ragged chunks missing from the cut line.
- • Trying to hack through uncompressed insulation, causing torn, uneven edges.
Slit Batts Around Electrical Cables and Pipes
Where electrical wiring runs horizontally through the studs, measure the cable height and slice the back half of the batt. Position the wiring in the centre of the batt thickness rather than crushing the insulation behind or in front of the cable.
- • Cable sits nestled in a horizontal cut halfway through the batt core.
- • Front and back faces of the batt remain flat and uncompressed around the wire.
- • Stuffing the entire batt in behind or over the top of the cable, creating massive compression zones.
Friction-Fit Batts into the Cavity
Tuck the top corners of the batt into the top plate cavity first. Work down both sides, gently tucking edges behind the framing faces so the batt sits completely flat and fills all corners without bowing outward.
- • Insulation fills the cavity from top plate to bottom plate without sagging.
- • Front face of the batt is perfectly flush with the front timber framing edge.
- • Pushing the batt too deep, creating a cold void at the front or pressing against the building wrap.
Fill Gaps and Verify Envelope Continuity
Inspect every bay for gaps around framing junctions, noggings, and corners. Cut small precision offcuts to fill narrow voids, ensuring a continuous thermal barrier without packing pieces too tightly.
- • Zero visible timber cavities or daylight behind studs.
- • Smooth, continuous wall plane ready for internal plasterboard lining.
- • Leaving narrow vertical gaps beside double studs or corner posts uninsulated.
Credit: inspired by an original tutorial by Mitre 10 New Zealand — watch the original on YouTube.
Don’t have everything?
- No Insulation cutting knife? Long serrated bread knife
- No Straight edge? Straight length of 90x45 timber offcut