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 resistance to heat flow per inch - higher = better insulation. 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

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