Insulation by Location Reference

Why this matters

Insulation is right for some locations + wrong for others. Same R-value of fiberglass vs spray foam in the rim joist vs the attic floor delivers very different results. The HVAC or service tech who sees what's installed where + recognizes mismatches catches energy + comfort issues the customer has been chasing. This is the field card.

R-value basics

R-value measures thermal resistance for the thickness installed - higher = better insulation. The per-inch figure quoted for a material is R-per-inch (1/k), and total R is R-per-inch × thickness, plus framing/structure deductions.

Climate zone determines code-minimum R:

• Climate Zone 1 - 2 (hot): R-30 attic minimum (IECC)
• Climate Zone 3 (moderate): R-38 attic
• Climate Zone 4 - 5 (cold): R-49 attic
• Climate Zone 6 - 8 (very cold): R-49 to R-60 attic

Walls are typically R-13 to R-21 in framed cavity, depending on stud depth (2x4 vs 2x6).

But code minimum is the floor, not the optimum. Many builders go beyond for performance.

Insulation types + characteristics

Fiberglass batt: pink or yellow paper-faced batts; common; cheap; R-3 per inch. Susceptible to air leakage around it (doesn't air-seal). Common in 2x4 wall cavity (R-13) + 2x6 wall (R-19).

Mineral wool (Roxul, Rockwool): similar form to fiberglass batt; better fire + sound + somewhat better air resistance. R-3.5 per inch. More expensive than fiberglass.

Blown cellulose: recycled paper, blown loose-fill into attic OR dense-pack into wall cavity. R-3.5 per inch. Good thermal mass; treats mold + fire (borate). Settles over years on horizontal applications.

Blown fiberglass: looser than cellulose; faster to install; lower density. R-2.5 per inch.

Open-cell spray foam: lower density, R-3.5 per inch, expands a lot; less expensive; partial air seal. Vapor-open (not vapor barrier).

Closed-cell spray foam: high density, R-6 to R-7 per inch, structural strength; full air seal; vapor barrier itself. More expensive; the premium choice for high-performance.

Rigid foam board (XPS, polyiso, EPS): panels at varying R. XPS R-5 per inch (pink/blue); polyiso R-6 per inch (foil-faced); EPS R-4 per inch (white "beadboard"). Air-sealing when properly taped.

Reflective / radiant barrier: foil-faced products; works by reflecting radiant heat. Best in attics in hot climates where radiant gain is dominant.

Insulation by location

Attic floor (most common in residential)

Best for cold climates: blown cellulose OR fiberglass + air-seal underneath. R-49 to R-60 in cold zones.

Open-cell spray foam at the roof deck (creating a conditioned attic) is an alternative - moves the insulation from the floor to the roof, makes the attic semi-conditioned. Higher cost; valuable when ductwork is in the attic.

Red flag: fiberglass batt with paper facing turned UP in attic (the kraft paper faces the wrong direction - should face the conditioned space).

Walls (existing homes)

Hard to retrofit:

• Drill + dense-pack cellulose into wall cavity (common retrofit)
• Spray foam into cavity (open-cell typical for retrofit)
• Add external rigid foam during siding replacement (CI - continuous insulation)

Red flag: empty wall cavity (no insulation at all, common in pre-1970 homes); poorly-installed batt with gaps or compression.

Walls (new construction)

Closed-cell spray foam in cavity + rigid foam exterior (highest performance), OR fiberglass/mineral wool batt in cavity + house-wrap (standard performance).

R-21 cavity + R-5 exterior continuous = R-26 effective wall assembly.

Basement / crawl space wall

Closed-cell spray foam OR rigid foam board against the wall, then framed wall in front. Avoids moisture trap in the wall cavity.

Avoid: fiberglass batt against concrete (moisture transfers through, batt becomes wet + moldy).

Red flag: paper-faced fiberglass in basement / crawl wall (paper degrades when damp).

Crawl space (sealed encapsulation)

Insulation moves to the perimeter walls (rigid foam OR closed-cell spray) + the crawl becomes semi-conditioned. Old floor-joist batts may be removed OR left for air-seal protection.

Covered in crawl space encapsulation.

Rim joist (band joist)

The single highest-impact spot in many older homes. Closed-cell spray foam OR rigid foam cut to fit each bay + sealed at edges. Open-cell is OK but less ideal (air seal less complete).

Red flag: insulation falling down, gap visible, OR no insulation at all in rim joist of older home.

Cathedral ceiling

Closed-cell spray foam at the roof deck (creating an unvented assembly) OR vented assembly with insulation between rafters + air channels above. Specialty install; common in modern construction.

Garage ceiling (over conditioned space below) + bonus room over garage

Closed-cell spray foam ideal (air seals + insulates). Fiberglass batt insufficient because air leakage dominates.

Ductwork in unconditioned space (attic, crawl)

Insulation thickness rated R-6 to R-8 jacket common. Older ductwork often inadequately insulated, leaking conditioned air + collecting condensation.

Around plumbing pipes

Closed-cell foam pipe insulation for hot water lines (saves energy, prevents condensation on cold lines). Particularly important for pipes in unconditioned spaces (crawl, attic) - frost protection.

Behind kitchen + bath vent fan ducts running through unheated spaces

Insulated duct (R-6 jacket minimum) prevents condensation inside the duct in winter.

Around recessed lights in attic floor

IC-AT (insulation contact, air-tight) rated cans only. Non-IC cans must have insulation cleared around them (creating a thermal gap). Retrofit with airtight covers if can't replace.

Around chimney chase + flue pipe

NEVER cover with combustible insulation. Maintain clearance per appliance specifications. Use mineral wool (non-combustible) for any insulation contact.

Around fan-rated boxes + bath fan housings

Air-tight rated fans only get standard insulation. Non-airtight boxes need insulation pulled back + box sealed before insulating.

Insulation by climate zone

Hot climates (Zone 1 - 3):

• Attic R-30 to R-38 minimum
• Radiant barrier in attic adds value
• Sealed attic with foam on roof deck - viable
• Wall insulation: standard R-13 cavity OK

Moderate climates (Zone 4):

• Attic R-38 to R-49
• Walls: R-13 or R-19 cavity, possible R-5 exterior CI
• Crawl: sealed + insulated

Cold climates (Zone 5 - 7):

• Attic R-49 to R-60
• Walls: R-19 OR R-21 cavity + R-5 to R-10 exterior CI for high performance
• Rim joist + crawl: closed-cell spray
• Basement walls: insulated full height

Very cold (Zone 8):

• Above all + extra. R-60+ attic, R-30 walls, etc.

Common mistakes you'll see

Insulating before air sealing. The single most common one. Insulation slows heat transfer, it does not stop air movement. Blowing R-49 over unsealed top plates, wire penetrations, chases, and dropped soffits buys a fraction of what the customer paid for. Seal first, then insulate.

Compressed batts. A batt squeezed into a cavity that is too shallow, or crammed behind wiring and pipes instead of split around them, loses a chunk of its rated value. So does insulation crushed under attic storage decking. If the customer wants attic storage, that area needs a raised platform, not compressed insulation.

Gaps, voids, and unsupported batts. Small gaps cost far more than their area suggests, because air moves through the gap and around the batt. Batts falling out of floor joists over a crawl are doing nothing.

Blocked soffit ventilation. Loose fill or batts pushed into the eave choke off the intake vents and set up moisture and ice-dam problems. Baffles at every rafter bay, installed before the insulation goes in.

Wind washing at the eaves. Even with baffles, low-density loose fill at the perimeter gets stirred by incoming air and loses effectiveness right where the ceiling is coldest.

Covering things that must not be covered. Non-IC-rated recessed lights, knob-and-tube wiring, chimney and flue clearances, and non-airtight fan housings. Each is a fire or code issue, not a performance issue.

Two vapor retarders. Facing on the cavity plus a poly sheet, or facing on both sides, traps moisture in the assembly. One retarder, on the correct side for the climate.

Foam left exposed. Spray and rigid foam in a living or accessible space generally requires a thermal or ignition barrier over it. Crews forget this in basements, crawls, and garages.

Insulating the attic floor while the ductwork stays in the attic. Now the ducts are outside the thermal boundary in a space that got hotter or colder. Either bring the ducts inside the boundary or seal and insulate them properly.

Forgetting the attic hatch and the kneewalls. An uninsulated hatch and unsealed kneewall backs undo a large share of a good attic job. Kneewall batts need an air barrier on the attic side, not open fiberglass facing an unconditioned space.

Adding depth over settled or wet insulation. Find out why it is wet first. Burying a roof leak or a bath fan dumping into the attic hides the real problem and grows mold under new material.

References

• IECC (International Energy Conservation Code) insulation requirements
• ENERGY STAR + Building America insulation guides
• Building Science Corporation building enclosure research
• ASHRAE Fundamentals (thermal performance)
• Manuall internal: Attic Moisture + Ventilation, Ice Dam Prevention, Crawl Space Encapsulation