Home Energy Efficiency Basics

Home Energy Efficiency Basics: Terms, Insulation Values, and Window Frames

Contributed by ASES Board Member, John Avenson, CPHC (Phius and PHI), Passive House resident since 1981

Planning energy improvements for your home means learning some new vocabulary and comparing a lot of materials. This guide covers three things: the terms you’ll hear in conversations with contractors and energy auditors, how common insulation materials compare, and why the frame material matters for windows as a home ages.

1. Key Terms and Facts

How heat behaves

• Heat always flows from warmer to cooler, and the bigger the temperature difference, the faster it flows.
• “Cold” is just a lack of heat. Anything warmer than absolute zero (−460°F / −273°C) contains heat energy, which is why a heat pump can still harvest heat from outdoor air at 0°F.
• Warm air rises and cool air sinks, because air expands as it warms and becomes lighter.
• Heat moves in three ways: conduction (through solid material, end to end), convection (through moving air or fluid), and radiation (like sunlight reaching Earth).

Sizing and efficiency

• Ton: A “ton” of heating or cooling is 12,000 Btu per hour, the heat needed to melt a 2,000 lb block of ice in 24 hours (288,000 Btu per day).
• CFM: A common rule of thumb is about 400 cubic feet per minute (CFM) of airflow per ton, so a 2-ton system needs roughly 800 CFM.
• SEER2: The Seasonal Energy Efficiency Ratio is the cooling efficiency rating for a whole season. Higher is better. The “2” version, in use since January 2023, reflects real-world airflow resistance, so ratings under the newer scale run lower than the same equipment would have scored before.
• HSPF2: The Heating Seasonal Performance Factor is the heating equivalent, also updated in 2023. Higher is better.
• Refrigerants: Older refrigerants such as R-22 have been phased out because they damage the ozone layer. Common R-410A is now being phased down, and newer options such as R-32 and R-454B have a lower impact on the climate.
• Heat pump (mini-split): A heat pump moves heat rather than creating it, and it can run in reverse to heat in winter and cool in summer. A ductless “mini-split” typically comes in sizes from 9,000 Btu (¾ ton) to 36,000 Btu (3 tons).

Insulation terms

• R-value measures resistance to heat flow. Higher means more insulating power. U-value is the opposite: it measures how much heat passes through a whole assembly, such as a wall or a window including its frame. Lower is better.
• Attic insulation of R-60 or more is a common recommendation for cold climates, and Passive House targets go higher. Wall targets of R-30 or more are common in high-performance homes. Recommendations vary by climate and local code.
• A standard 2×4 wall with fiberglass batts is rated about R-13 on paper, but in practice often performs closer to R-9 to R-11 because of the framing.
• Moisture tip: Continuous rigid foam on the outside of a wall moves the dew point out of the wood structure, which helps prevent hidden mildew from cooking and shower moisture over the years.

Air sealing (tightness)
After a blower door test shows where air is leaking, common fixes include caulk, expanding foam (attic-to-room joints, baseboards, rim joists, window frames, bath fans and range hoods), house wrap, and specialized air-sealing tapes. Be cautious about window replacements that cover gaps with trim without sealing them, since this hides leaks instead of fixing them.

Windows

• Look for the NFRC label. Its U-factor covers the whole unit (frame plus glass): around 0.12 (R-8.3) is excellent, and above about 0.21 (R-4.7) is poor.
• SHGC (Solar Heat Gain Coefficient) is the fraction of the sun’s heat that passes through. At 0.41, 41% gets in.
• VT (Visible Transmittance) is the fraction of visible light that passes through. Higher means a brighter room.
• Shading options include exterior solar screens, awnings, and deciduous trees. Exterior shades keep heat out better than interior ones. Interior shades can trap heat against the glass and stress the seals of the insulated glass unit.
• Other options: reflective film, storm windows, and full replacement.

Tools used by auditors and homeowners
Indoor/outdoor thermometers, an infrared camera, a home weather station, a solar radiation meter, and a blower door. Energy modeling software such as NREL’s BEopt is used for audits. Manual J (heating and cooling load), Manual D (duct design), and Manual S (equipment selection) are the standard industry protocols.

Resilience and renewables

• For battery backup, list your priority circuits (refrigerator, sump pump, internet modem, heating system) so you can size a battery for what matters most. Home batteries commonly range from about 6 to 16 kWh.
• Solar PV panels are commonly in the 430 to 470 watt range today. South-facing windows are also a free energy source.
• For most homes, money is better spent on solar PV than on a small residential wind turbine, which needs to be 200 to 300 feet up to perform well.

2. Insulation R-Values by Material

R-value per inch of thickness, listed from highest to lowest. Values are approximate and vary by product, density, and age, so check the manufacturer’s data.

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3. Window Frames: Why Expansion Rate Matters

Materials expand when heated and shrink when cooled. A window is a glass pane held in a frame, and if the frame expands and contracts at a very different rate than the glass, the seals and joints are stressed again and again over the years until they leak. The closer a frame material’s expansion rate is to glass, the better it holds up.

Coefficient of linear thermal expansion (millionths per °C; lower means less movement):

Example: Over a 90°F (50°C) temperature swing, a 4-foot window frame changes length by about 0.3 mm for fiberglass, 0.5 mm for glass, 1.4 mm for aluminum, and 3.1 mm (about ⅛ inch) for vinyl. Fiberglass moves almost in step with glass, while vinyl moves six times as much. Over many seasons, that mismatch is a common cause of failed seals and air leaks in older windows.

Wood moves little along the grain but swells and shrinks across it with temperature and humidity, so wood frames depend on good finishing and maintenance.