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 = thermal resistance for the thickness installed; higher = more insulation. R = thickness / k, so R-value scales with thickness. The per-inch figure quoted for a material is R-per-inch (1/k); total R is R-per-inch x thickness, less framing and structure deductions.
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 scope
Price these by the square foot of area treated and by access difficulty, using your own local labor and material costs. What follows is the scope of each job and how the jobs compare to each other in cost.
Attic blow-in insulation upgrade (R-19 to R-49): blown cellulose or fiberglass over the existing attic floor, plus baffles at the eaves to keep the soffit vents clear, plus dams around recessed lights, the attic hatch, and any chimney or flue. Priced per square foot of attic floor. The least expensive of these jobs per square foot, and the one customers most often ask for by name.
Attic floor air sealing (caulk, gaskets, spray foam): sealing the top plates, penetrations, chases, can lights, and the attic hatch before any insulation goes down. Labor-intensive, low material cost, and priced by the hour or by the count of penetrations. Costs a fraction of the insulation job it precedes, and doing the insulation without it wastes most of the insulation's value.
Spray foam encapsulated attic (vented to conditioned): foam at the roof deck and gable ends, the soffit and ridge vents closed off, and the attic brought inside the thermal boundary. Several times the cost of blow-in per square foot. Requires an evaluation of every combustion appliance in the attic and of the roof assembly's drying path before it is specified.
Wall insulation (blown into wall cavities): drill-and-fill through the siding or the interior wall, dense-pack cellulose or foam, then patch. Cost varies more with the siding type and the patching than with the insulation itself, and it is the reason two houses with identical wall area quote differently.
Whole-home air sealing + insulation (deep retrofit): the attic floor sealed and insulated, the rim joist sealed and insulated, the walls filled, the ducts sealed, and a blower door test before and after to document the improvement. The largest of these packages by a wide margin, and the one that requires a ventilation check afterward, because a house tightened this far may need mechanical fresh air.
Energy audit (blower door + IR camera + report): a half-day visit that produces a measured ACH50, thermal images of the failure points, a duct leakage number if a duct blaster is run, and a written report ranking the recommended work by payback. Priced as a flat fee, the smallest line in this list, and often credited back against the retrofit if the customer proceeds. Sell it first; every job above is easier to sell once the customer has seen their own numbers.
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