Chapter 4 Key Takeaways: Insulation and the Building Envelope
The Three Mechanisms of Heat Transfer
- Conduction — heat through solid material. Resisted by insulation (higher R-value = more resistance).
- Convection — heat through moving air. Resisted by air barriers and air sealing.
- Radiation — heat through electromagnetic waves. Addressed by radiant barriers in high-solar situations.
Effective thermal performance requires addressing all three. Insulation alone is not enough.
R-Value Fundamentals
- R-value measures resistance to conductive heat flow. Higher is better. R-values of layers are additive.
- U-factor is the inverse of R-value (U = 1/R). Used for windows. Lower U-factor is better.
- Rated R-value (on the bag) is not the same as real-world R-value, which is reduced by compression, air bypass, thermal bridging, and settling.
- The appropriate R-value depends on your climate zone. Check the DOE climate zone map for your zip code.
The Critical Importance of Air Sealing
- Air infiltration accounts for 25–40% of heating/cooling load in most older homes.
- The biggest leaks are typically hidden: open top-plate penetrations, unsealed attic hatches, old recessed lights, rim joists, and HVAC duct boots.
- A blower door test (ACH50) is the only reliable way to measure whole-house air leakage.
- Seal before you insulate. Air sealing first, then insulation, is the correct sequence.
- The stack effect pulls cold air in at the bottom of the house and pushes warm air out at the top — seal both ends.
Insulation Types at a Glance
| Type | R/inch | Best Use | DIY? |
|---|---|---|---|
| Fiberglass batt | 2.9–3.8 | New construction wall/floor cavities | Yes |
| Mineral wool batt | 3.0–4.0 | Rim joists, sound walls | Yes |
| Blown cellulose (attic) | 3.2–3.8 | Attic floors, retrofit walls | Yes (attic) |
| Dense-pack cellulose | 3.2–3.8 | Retrofit wall cavities | Caution |
| Open-cell spray foam | 3.5–3.7 | Walls, attic deck | No |
| Closed-cell spray foam | 6.0–7.0 | Rim joists, crawlspaces, below-grade | No |
| EPS rigid board | 3.6–4.2 | Continuous exterior, below-grade | Yes |
| XPS rigid board | 5.0 | Exterior walls, basement walls | Yes |
| Polyiso rigid board | 5.6–6.5 | Exterior walls (above cold zone) | Yes |
Vapor Control — Get This Right
- Vapor control strategy depends entirely on your climate. What's correct in Minnesota is wrong in Florida.
- In cold climates: vapor retarder on the interior (warm) side to slow inward-to-outward vapor diffusion.
- In hot-humid climates: no interior vapor barrier; control is on the exterior side or the assembly dries inward.
- Never create a double-barrier assembly (impermeable on both sides) — it traps moisture with no drying path.
- Smart vapor retarders (variable permeance) are the safest choice in mixed climates.
Thermal Bridging
- Wood studs bridge your insulation at R-4 compared to R-15 in the cavity. A 2x4 framed wall with R-15 batts has an effective R-value of approximately R-9 to R-10 for the whole assembly.
- Steel studs are far worse — they reduce a nominal R-13 assembly to effective R-5 or R-6.
- Continuous exterior insulation is the most effective solution: it covers every stud simultaneously and dramatically improves effective R-value.
Improvement Priority Order (Typical Older Home)
- Air sealing (attic penetrations, rim joists, recessed lights, attic hatch) — best ROI
- Attic insulation to recommended R-value for your climate zone
- Rim joist and basement/crawlspace insulation
- Wall insulation — only if re-cladding creates the opportunity at low incremental cost
Financial Reality Check
- Air sealing DIY: $200–$800 in materials. Payback often 1–3 years.
- Attic insulation (professional): $1,500–$4,000. Payback typically 3–7 years.
- Rim joist insulation (DIY): $200–$400 in materials. Payback typically 2–5 years.
- Wall insulation retrofit: $4,000–$10,000. Payback often 20+ years unless combined with re-cladding.
- Check for federal tax credits (Inflation Reduction Act) and utility rebates before starting any project.
Warning Signs to Act On Now
- Visible frost on interior surfaces of rim joists or walls in winter — moisture risk, accelerating wood decay.
- Condensation on interior window surfaces every morning — excessive humidity or extreme thermal loss.
- Rooms that are consistently cold despite adequate heating — likely air bypass or missing insulation.
- Significant settled or compressed fiberglass in attic — measure depth and calculate actual R-value; it's probably much lower than you think.
- Any insulation work on a wall assembly without understanding the vapor management for your climate — stop and consult a building scientist first.