Planning attic insulation the right way
Why air sealing comes before insulation, how to judge existing depth and coverage, and what ventilation must stay intact.
Last reviewed 2026-07-15 · About 7 min read
Adding insulation over an unsealed attic floor buries the problem rather than solving it. Air moving through insulation carries heat and moisture with it, and the added depth makes future sealing much harder.
Sequence is the whole point of this guide: seal, then ventilate, then insulate.
Assess what is already there
Measure depth in several locations, not one. Coverage is usually uneven, thin at the eaves, and missing entirely near the hatch and around penetrations.
Note the insulation type and whether it is compressed, damp, or displaced. Damp insulation is a symptom to investigate, not something to cover.
- Check for daylight or staining, which indicates roof leaks
- Look for rodent activity before spending money on material
- Photograph the attic before and after — this belongs in the home record
Protect ventilation
Soffit vents must stay clear. Baffles keep insulation from blocking airflow at the eaves, and without them added insulation can cause moisture and ice-damming problems.
Bath and kitchen exhaust must terminate outside the building, never into the attic.
Choose material for the conditions
Blown material fills irregular cavities well; batts suit accessible, evenly framed spaces. The correct choice usually follows access and framing, not brand preference.
Keep required clearances around recessed lights, chimneys and flues unless the fixtures are rated for direct contact.
When to call a professional
- Vermiculite or suspected asbestos-containing insulation is present
- There is active moisture, mould growth or a roof leak
- Knob-and-tube wiring is present in the insulated area
Source notes
- Recommended R-values