Building Envelope Fundamentals
Why this matters
The building envelope - walls + roof + foundation + windows + doors - controls everything HVAC + IAQ + comfort + energy. A leaky envelope means the customer's heat pump runs constantly + bedrooms are cold in winter + humid in summer + the customer blames the equipment. The contractor who understands envelope basics diagnoses comfort + efficiency complaints correctly + sells whole-building solutions that include weatherization OR knows when to refer; the contractor who only thinks in terms of equipment hits the same callback over + over.
What the envelope does
Four core jobs:
- Thermal barrier: blocks heat transfer between conditioned space + outside
- Air barrier: blocks bulk air movement (infiltration + exfiltration)
- Moisture barrier: prevents liquid water from entering + manages vapor movement
- Vapor barrier (sometimes): controls vapor diffusion through wall
These four jobs are done by DIFFERENT materials + layers - they're not all the same product. A common contractor mistake: assuming insulation is also the air barrier (usually false).
Heat transfer mechanisms
Three ways heat moves:
Conduction - through solid material (wall studs, glass, insulation). Reduced by insulation (R-value).
Convection - through moving fluid (air OR water). Reduced by air-sealing + by trapping air in still pockets within insulation.
Radiation - through electromagnetic waves (sun on roof, body heat). Reduced by radiant barriers (foil), low-e coatings, reflective roofing.
A typical wall sees: 60 - 70% conduction (through studs + insulation), 20 - 30% convection (air leakage), 5 - 10% radiation (mostly summer roof heat).
R-value (resistance to conduction)
R-value = resistance to heat flow per inch. Higher = more insulation.
Per-inch values: cellulose blown-in R-3.5, fiberglass batt R-3.5, fiberglass blown-in R-3.7, open-cell spray foam R-3.8, closed-cell spray foam R-6 - R-7, polyiso rigid R-6 - R-7, XPS rigid R-5, EPS rigid R-4.
Whole-wall R-value is LOWER than insulation-only because studs (R-1.25/in) reduce effective performance - thermal bridging. A 2x6 wall with R-19 cavity has whole-wall ~R-12 - R-15.
Climate-appropriate values (2021 IECC residential): Zone 1 - 2 wall R-13 + ceiling R-30; Zone 3 wall R-15 + ceiling R-38; Zone 4 - 5 wall R-20 + ceiling R-49; Zone 6 wall R-23 + ceiling R-49 - R-60; Zone 7 - 8 wall R-23 with continuous exterior insulation + ceiling R-60+.
Air leakage (the killer)
Air leakage measured at 50 Pa (ACH50): old leaky home 10 - 30, average existing 5 - 10, Energy Star ≤ 3, passive house ≤ 0.6. A 1980s home at ACH50 = 12 has the equivalent of an open window 24/7.
Typical leak distribution: 25% attic hatch + recessed lights + duct boots, 20% basement/rim joist/sill plate, 15% windows + doors, 15% recessed lights + ceiling penetrations, 10% bath + kitchen exhaust fans, 15% other (electrical boxes, plumbing, fireplace).
Air sealing materials: caulk, spray foam, gaskets, foam tape, EPDM rubber, weatherstripping. Cheap + high-ROI; often customer-DIY or quick contractor add.
Vapor + moisture management
Vapor moves from high-humidity to low-humidity through any non-vapor-barrier material. In winter, indoor humid air → cold outdoor → vapor condenses in wall = moisture damage + mold.
Climate-based vapor strategy:
- Cold climates (5+): vapor retarder on warm side (interior); air barrier outside studs
- Warm climates (1 - 3): vapor retarder on warm side (exterior); air barrier inside
- Mixed climates (4): variable-perm vapor retarder that adjusts to humidity direction
Vapor retarder ratings:
- Class I (Vapor barrier): polyethylene sheeting (1 perm or less) - most restrictive
- Class II: kraft-faced batt insulation (1 - 10 perm)
- Class III: latex paint, varies (10+ perm)
Bulk water management (separate from vapor): roofing, siding, flashing, drainage planes; water shed away from envelope.
The pressure boundary
Air barrier + insulation should be at the same layer. Mismatched layers cause performance problems. Conditioned attic (modern best practice): insulation + air barrier at roof deck, HVAC + ducts inside envelope, no attic vents. Vented attic (traditional): insulation + air barrier at attic floor; ducts in attic = ducts OUTSIDE envelope (problem); attic vents at soffit + ridge. Conditioned crawl: insulation + vapor barrier on perimeter walls, sealed crawl with vapor barrier on ground, no vents. Conditioned basement: insulation on perimeter walls (interior or exterior), below-slab insulation if new construction.
Common envelope failures + diagnoses
Cold drafts at outlets near exterior walls: outlet box not air-sealed; backstrap with foam gasket or airtight box. Ice dams: warm air leaks into attic, snowmelt re-freezes at cold eave; air-seal attic floor + add insulation + ventilation. Mold on north-facing wall corner: cold + interior moisture condensing; insulate corner + reduce indoor humidity. Humid summer indoor: oversized AC or attic ducts; right-size + seal ducts. High heating bill cold house: high ACH50 + under-insulated walls; blower-door test + air sealing + insulation. Hot upstairs summer: under-insulated attic + leaky ducts; air seal + insulate + seal ducts.
Testing + diagnostics
- Blower door: fan in front door depressurizes to 50 Pa; measure CFM50; ACH50 = (CFM50 × 60) / house volume.
- Smoke pencil: visual identification of leak locations.
- Duct blaster: pressurize ductwork; measure leakage separately from envelope.
Insulation upgrades + air sealing pricing
Attic blow-in insulation upgrade (R-19 → R-49):
Attic floor air sealing (caulk, gaskets, spray foam):
Spray foam encapsulated attic (vented → conditioned):
**Wall insulation (blown into wall cavities):
Whole-home air sealing + insulation (deep retrofit):
Energy audit (blower door + IR camera + report):
Customer ROI conversation
Whole-home air sealing + insulation typically saves 15 - 35% on heating + cooling annually. Payback 5 - 12 years depending on energy prices + climate. Plus comfort improvement (no drafts, even temperatures, reduced humidity issues) - which is what customers actually pay for.
The single highest-leverage diagnostic on a "high energy bill" call is the BLOWER DOOR + THERMAL IMAGING combo. Customer sees the leakage numerically + visually. Without it, "your house is leaky" is just talk; with it, the customer sees ACH50 = 18 + the IR camera images of cold framing + air rushing out of recessed lights. The data sells the work. 60 - 75% of customers who see the audit data buy the air-sealing + insulation upgrade in the same week.
References
- ASHRAE 90.1 + 90.2 (envelope thermal design)
- IECC International Energy Conservation Code (residential + commercial)
- DOE Building America envelope guidelines
- Building Science Corporation (Joe Lstiburek) publications
- Manuall internal: Smart Home HVAC Integration, Heat Pump Sizing Cold Climate