Building Science Principles for Home Retrofits

The Home as a System

The building enclosure, also called the envelope, separates conditioned space from outdoors and unconditioned areas. Conditioned space means the parts of the home intentionally heated or cooled.

In a typical vented attic, the air and thermal boundaries belong at the ceiling below the attic, not at the roof. Identify the intended boundary before choosing where to seal or insulate. ^[1]

Example: Adding loose-fill fiberglass or cellulose over ceiling air leaks leaves a path for indoor moisture to reach cold roof surfaces. Air sealing, insulation, moisture control, and ventilation must be planned together. ^[2]

Four Control Functions: Water, Air, Vapor, and Heat

A wall, roof, or floor assembly is the combination of materials that makes up that part of the building. It needs four control functions, but not necessarily four separate products: one material may perform more than one job. ^[3]

• Water control: Keep liquid water out and provide drainage for water that gets behind exterior finishes. Flashing directs water away from openings; capillary breaks interrupt water wicking through porous materials such as concrete or wood.
• Air control: Limit unintended airflow through joints, gaps, and penetrations. Infiltration is air leaking in; exfiltration is air leaking out.
• Vapor control: Manage diffusion, the movement of water vapor through materials. A vapor retarder slows diffusion; an air barrier controls air leakage, which can also carry moisture.
• Thermal control: Use insulation to slow heat flow. Keep it continuous and aligned with the air barrier so moving air cannot bypass it.

An air barrier is not automatically a vapor barrier. Vapor-control materials and their location depend on the climate, existing materials, and the assembly's ability to dry; adding plastic to every wall is not a safe general rule. ^[4]

Continuity matters: Connect each control layer across roof-to-wall joints, wall-to-foundation joints, and window and door openings. A material cannot perform its intended function where joints or transitions are left unfinished. ^[3]

Heat Transfer and Insulation

Heat flows from warmer areas toward cooler areas. It can move through walls and floors as well as ceilings; heat does not only move upward.

• Conduction: Heat passes through materials, including glass, framing, and insulation.
• Convection: Moving air or liquid carries heat, such as air circulating beside a cold window.
• Radiation: Heat transfers by electromagnetic waves, such as sunlight warming a floor or a warm radiator heating nearby surfaces.

R-value measures resistance to heat flow: higher means greater resistance under comparable conditions. Gaps, compressed batts, and air moving around or through insulation can reduce installed performance. ^[5]

Insulation and air sealing are different jobs. Fiberglass and cellulose are not substitutes for an air barrier; stuffing insulation into a hole does not seal it. ^[1]

Thermal Bridging and Insulation Quality

A thermal bridge is a path through a more conductive part of an assembly, such as wood or steel framing interrupting cavity insulation. Filling the space between studs does not insulate across the studs themselves. ^[6]

• Continuous insulation extends across framing rather than only between it. Exterior rigid foam or mineral wool boards can reduce thermal bridging, with drainage, fastening, and moisture-control details designed for the assembly.
• Dense-packed cavity insulation can improve existing walls and reduce airflow through them. It does not eliminate framing bridges or replace a continuous air barrier. ^[7]
• Installation quality means filling the intended space without voids or compressed batts, meeting the product's specified thickness and density, and fitting insulation around obstructions.

Advanced framing reduces unnecessary framing and creates more room for insulation. It is mainly an option for new construction or substantial rebuilding, not a reason by itself to dismantle a sound existing wall. ^[6]

A cold patch is a reason to investigate, not proof of a thermal bridge. Record what you observe and check for air leakage, missing insulation, and moisture before selecting a repair.

Air Sealing: Practical Methods and Sequencing

Air leakage needs both an opening and a pressure difference. In winter, stack effect can push warm indoor air out through upper leaks while drawing colder air in below; wind and exhaust fans also change pressure and airflow. ^[2]

Use the ABCs: Attic, Basement or crawlspace, and Conditioned spaces as an inspection reminder. Look for gaps at the chosen enclosure boundary, including attic hatches, pipe penetrations, exterior openings, and rim joists, the framing around the outside edge of a floor.

• Caulk: Seal suitable small, stationary joints with a product compatible with the materials and exposure conditions.
• Weatherstripping: Seal moving joints at doors, operable windows, and access hatches without preventing them from opening.
• Foam and solid blocking: Use foam rated for the application; close larger openings with suitable solid material and seal its edges. Follow the manufacturer's limits rather than trying to span every gap with foam. ^[9]

Do not seal intentional ventilation, combustion-air, or drainage openings. Chimneys, hot flues, and recessed lights require approved details and clearances, not ordinary spray foam applied indiscriminately. ^[1]

A qualified assessor can use a blower door, a calibrated fan that creates a controlled pressure difference, to measure air leakage before and after work. Complete the ventilation and combustion-safety checks as well; a lower leakage reading alone does not establish safe performance. ^[9]

Moisture Moves Four Ways

Start by identifying how the moisture is moving. The repair for rainwater entering a wall is different from the repair for humid indoor air condensing on a cold surface.

• Bulk water: Rain, snowmelt, groundwater, and plumbing leaks move as liquid water. Use roof drainage, flashing, site drainage, and plumbing repairs to keep water out or direct it safely away.
• Capillary action: Liquid water can wick through porous materials such as concrete, masonry, and wood. A capillary break interrupts that path with a material or air space that water cannot readily wick across.
• Air-transported moisture: Air leakage can carry water vapor through cracks and openings. When that air reaches a cold surface, condensation may occur. Control this pathway with a continuous air barrier and pressure management.
• Vapor diffusion: Water vapor can move through materials even when there is no air leak. A vapor retarder slows diffusion; whether one is needed and where it belongs depends on climate and assembly design.

Do not treat every moisture problem as a vapor-barrier problem. First identify whether the source is liquid water, capillary movement, air leakage, or diffusion. ^[1]

Condensation, Drying & Relative Humidity

Condensation forms when moist air contacts a surface cold enough for water vapor to change to liquid. A wet window in winter may indicate high indoor humidity, a cold window surface, poor air circulation at the glass, or a combination of these conditions.

For homes, EPA recommends keeping indoor relative humidity below 60% and, where practical, in the 30–50% range. In cold weather, the safe indoor humidity level may need to be lower because windows and exterior wall surfaces are colder. ^{[2] [3]}

• If condensation appears: Reduce the moisture source, increase appropriate ventilation, and investigate unusually cold surfaces or missing insulation.
• If materials get wet: Stop the water source and dry the materials promptly. Do not cover wet or mold-damaged materials with new finishes or insulation.
• Allow assemblies to dry: Wall, roof, and foundation materials must be able to release incidental moisture. Adding low-permeance materials without understanding the existing assembly can trap moisture rather than solve it.

Cold-climate example: A home may be comfortable at 40% relative humidity during mild weather but show window condensation during a very cold spell. Lowering indoor humidity can reduce the condensation risk while the underlying window or insulation condition is evaluated. ^[3]

Indoor Air Quality: Source Control, Ventilation & Filtration

Indoor air quality problems can come from moisture, cooking, combustion, cleaning products, furnishings, building materials, soil gases, outdoor air, and occupant activities. EPA organizes IAQ improvement around three strategies. ^[4]

1. Source control: Remove or reduce the pollutant at its source. Fix water leaks, control mold-producing moisture, choose lower-emitting products when practical, and address combustion or radon problems directly.
2. Ventilation: Exhaust pollutants where they are produced and bring in outdoor air in a controlled way when needed. Kitchen and bathroom exhaust fans should discharge outdoors, not into an attic, crawlspace, or wall cavity.
3. Filtration and air cleaning: Filters and portable air cleaners can reduce particles and some other pollutants, but they supplement source control and ventilation rather than replace them.

A HEPA air cleaner is not a ventilation system. It recirculates and filters indoor air in the area it serves; it does not by itself remove a moisture source, exhaust cooking pollutants, or provide outdoor air. ^[5]

Common IAQ Concerns & What to Do

• Radon: Radon is a radioactive soil gas and the second-leading cause of lung cancer in the United States after smoking. Testing is the only way to know the level in a home; EPA recommends taking action at 4 pCi/L or higher. ^[6]
• Mold and dampness: Mold needs moisture. Correct leaks, condensation, and persistent high humidity rather than relying on air cleaners to solve the moisture source.
• Cooking and combustion pollutants: Use kitchen exhaust that vents outdoors and maintain fuel-burning equipment. Source control and local exhaust are more direct controls than trying to filter the whole house afterward.
• VOCs and other gases: Paints, cleaners, furnishings, and other products can release gaseous pollutants. Reduce or remove the source where practical and provide ventilation during activities that generate pollutants.
• Particles and allergens: Appropriate HVAC filtration or a properly sized portable air cleaner can reduce airborne particles. Filters must fit the equipment, be maintained, and be replaced as required.

Ventilation After Air Sealing

Air leakage is not a ventilation strategy. Leaks provide uncontrolled airflow that changes with wind, temperature, and equipment operation; they do not reliably remove pollutants from bathrooms and kitchens or distribute outdoor air where it is needed.

• Local exhaust removes moisture and pollutants near the source, especially in kitchens and bathrooms.
• Whole-house ventilation provides planned outdoor-air exchange using exhaust, supply, or balanced systems.
• HRVs and ERVs are balanced ventilation equipment that recover energy from outgoing air. Module 4 covers equipment selection in more detail.

When a retrofit substantially reduces air leakage, check the home's ventilation against the applicable code, program, or design standard. Do not assume the remaining random leakage provides adequate fresh air. ^[7]

Start with the Existing Window or Door

Do not start with the assumption that every old window needs replacement. Inspect the opening from indoors and outdoors, then match the repair to the condition you find.

• Water: Look for staining, peeling finishes, soft or rotted wood, failed sealant, damaged flashing, and signs that water is entering above or around the opening.
• Air leakage: Check weatherstripping, sash-to-frame joints, door sweeps, thresholds, and gaps between the frame and surrounding wall.
• Operation: Confirm that windows and doors open, close, latch, and lock properly. A unit that cannot close fully will be difficult to air seal.
• Glass and frame condition: Record cracked glass, failed insulated-glass seals, deteriorated sash or frames, and damaged sills before deciding whether to repair, add a storm, install an insert, or replace the full frame.

Pre-1978 homes require another check. Window replacement and other work that disturbs painted surfaces can be subject to EPA's Renovation, Repair and Painting requirements. Paid contractors must follow the applicable lead-safe certification and work-practice rules unless the affected components have been properly determined to be lead-free. ^[1]

Read the Window Performance Label

The National Fenestration Rating Council (NFRC) provides standardized ratings for windows, doors, and skylights. Use the rating for the whole product, not a glass-only value, when comparing products. ^[2]

• U-Factor: Measures heat transfer through the product. Lower is better for reducing conductive heat loss and gain.
• Solar Heat Gain Coefficient (SHGC): Measures how much solar heat passes through the product. Lower SHGC blocks more solar heat; higher SHGC admits more.
• Visible Transmittance (VT): Indicates how much visible light passes through. Higher VT means more daylight, but VT by itself does not tell you how energy-efficient the window is.
• Air Leakage: Where rated, lower values indicate less air passing through the manufactured product under the test conditions.
• Condensation Resistance: Where shown, a higher rating indicates greater resistance to interior condensation under the rating conditions. It does not guarantee that condensation can never occur.

A single U-Factor or SHGC target does not fit every project. ENERGY STAR criteria vary by climate zone, and SHGC selection can also be affected by window orientation, shading, and cooling loads. Use current climate-appropriate criteria rather than assuming that every cold-climate window should have the same rating. ^[3]

Repair, Add a Storm, or Replace?

Replacement is one option, not the starting point. The condition of the frame, sill, sash, flashing, and surrounding wall determines which retrofit makes sense.

• Repair and weatherstrip: Appropriate when the frame and sash are serviceable but joints, glazing, seals, or weatherstripping need attention.
• Add a storm window or panel: A useful option when an older window is in sound condition but needs better thermal performance, air control, comfort, or sound control. Low-e storm products can improve performance without removing the original window.
• Insert replacement: Replaces the sash and operating unit while retaining much of the existing frame. The retained frame and sill must be sound, and hidden weight pockets or perimeter gaps should be air sealed and insulated as part of the work.
• Full-frame replacement: Consider when frames or sills are badly deteriorated, water-management details need reconstruction, or the project requires access to the rough opening for new flashing and air sealing.

Single-pane glass alone does not prove that full replacement is the best first measure. A sound existing window may be a candidate for repair plus a storm window; a newer-looking replacement unit with poor flashing or a rotten sill can still require major corrective work. ^[4]

Installation: Connect the Control Layers

A high-performance window installed into a leaking opening can still produce drafts, water damage, and comfort complaints. Installation must connect the opening to the wall's water, air, and thermal control layers.

• Water control: Flash the sill, jambs, and head so water drains toward the exterior and the opening ties into the wall's drainage plane or water-resistive barrier.
• Drainage: Preserve designed drainage paths. Do not block manufacturer weep holes or seal locations that are intended to drain to the exterior.
• Air control: Seal the interior perimeter between the window or door frame and the rough opening with an appropriate system such as backer rod and sealant or low-expansion foam rated for the application.
• Thermal control: Insulate accessible gaps around the frame without bowing the frame or interfering with operation.
• Manufacturer instructions: Follow the product's installation details for fastening, flashing, sealants, clearances, and drainage.

Exterior caulk is not the entire water-management system. Properly lapped flashing and a drainage path are intended to direct water back out if it gets behind exterior finishes. ^[5]

Drafts, Cold Glass & Condensation

A person can feel uncomfortable next to a window even when little outside air is leaking through it. Cold glass can cool nearby room air and surrounding surfaces, creating a downdraft and radiant discomfort that feels like a draft.

• Room-side condensation: Usually means the interior glass or frame surface is cold enough, relative to indoor humidity, for water to condense. Check both indoor humidity and window surface conditions before choosing a repair.
• Condensation between panes: Persistent fogging or moisture inside a sealed insulating-glass unit generally indicates a failed glass seal rather than excessive room humidity.
• Air leakage: Damaged weatherstripping, loose sash, failed perimeter seals, or gaps around the frame can produce a true draft and should be diagnosed separately.

A cold surface is evidence to investigate, not proof of one specific defect. The cause may be the glazing, frame, installation gap, missing insulation, indoor humidity, or several conditions at once. ^[6]

Doors Follow the Same Building-Science Rules

Exterior doors also interrupt the enclosure. Check the door slab, frame, glazing, threshold, weatherstripping, flashing, and surrounding wall rather than judging performance from the door material alone.

• Air sealing: Weatherstripping should make consistent contact when the door is latched, and the sweep or threshold should control leakage at the bottom without preventing operation.
• Water management: Inspect the sill and lower jambs for staining or rot, and make sure exterior water is directed away from the opening.
• Thermal performance: Compare rated U-Factor and SHGC when selecting a new exterior door, especially when the door contains substantial glazing.
• Installation: Seal and insulate the perimeter while preserving the flashing and sill-pan details needed to drain water outward.

ENERGY STAR door requirements vary with the amount of glazing, so there is no single U-Factor or SHGC number that applies to every exterior door. ^[3]

Start with the Load, Not the Equipment

A home's heating load is the rate of heat it needs during cold design conditions; the cooling load is the rate of heat and moisture that must be removed during hot design conditions. Air sealing, insulation, windows, orientation, internal gains, and climate all affect these loads.

Do not size new equipment from square footage alone or simply replace the old unit with the same capacity. Recognized residential load calculations, such as ACCA Manual J, use the actual characteristics of the home. Oversized equipment can short-cycle, reduce comfort, and operate less efficiently. ^[1]

• Before replacement: Record the existing equipment type, capacity, age when known, fuel or energy source, distribution system, and known comfort problems.
• After enclosure improvements: Recalculate the load when major air-sealing, insulation, or window work materially changes the home's heating or cooling needs.
• Equipment selection: Match rated capacity at the relevant design conditions to the calculated load and account for the distribution system and any planned backup heat.

Heating and Cooling Systems: Match the Technology to the Home

Efficiency ratings describe equipment performance, but they do not determine whether a system is properly sized, installed, or economical to operate in a specific home. Energy prices, climate, distribution losses, controls, and backup operation all matter.

• Air-source heat pumps: Move heat rather than create it by electric resistance. Ducted and ductless models can provide both heating and cooling. Cold-climate models are tested for low-temperature performance, but capacity and efficiency still change as outdoor temperature falls. Review manufacturer performance data at the home's winter design temperature and plan backup heat when the load exceeds available heat-pump capacity. ^[2]
• Furnaces: Heat air and distribute it through ducts. Replacing the furnace does not correct undersized, restricted, or leaky ductwork.
• Boilers: Heat water for radiators, baseboards, fan coils, or radiant floors. A condensing boiler captures additional heat from water vapor in the flue gases, but it must operate at sufficiently low return-water temperatures to spend meaningful time condensing. PNNL notes that return water below about 130°F promotes condensing; emitter capacity and control settings must still meet the home's load. ^[3]
• Ground-source heat pumps: Exchange heat with the ground rather than outdoor air. They can be efficient, but site conditions, loop design, drilling or excavation, and project cost determine whether they fit a particular home or community system.

Do not promise lower utility bills from the equipment category alone. Compare the proposed system with the existing system using local energy prices, expected operating conditions, and the home's actual load.

Ducted and Hydronic Distribution

Heating or cooling equipment only helps if the distribution system delivers that output where it is needed. Ducts and hydronic piping have different failure modes and need different checks.

• Duct leakage: Leaks can waste conditioned air and create pressure, comfort, and moisture problems, especially when ducts run outside the conditioned enclosure. A duct leakage test uses a calibrated fan to quantify leakage. ^[4]
• Duct sealing: Approved methods include mastic, UL 181 tape, and equivalent systems used according to their listing and manufacturer instructions. Ordinary cloth-backed household "duct tape" is not the same as listed HVAC sealing tape. ^[5]
• Duct location and insulation: Ducts outside the conditioned enclosure generally need more attention to sealing and insulation. Do not assume every basement is unconditioned; first identify whether the ducts are inside or outside the home's air and thermal boundaries.
• Airflow and pressure: Restricted filters, undersized returns, crushed flex duct, poor fittings, or closed doors without an adequate return path can reduce delivered airflow and create room-to-room pressure differences.
• Hydronic systems: Radiators, baseboards, and radiant floors release different amounts of heat at different water temperatures. Lower water temperatures can improve the performance of condensing boilers and many hydronic heat-pump systems, but the emitters must still be able to meet each room's load.

A hand near a duct joint can identify an obvious leak, but it does not measure system leakage or prove that airflow is correct. Use diagnostic testing when measured performance is required.

Mechanical Ventilation: HRVs and ERVs

As discussed in Module 2, random air leakage is not a ventilation strategy. Whole-house ventilation provides controlled outdoor air; local kitchen and bathroom exhaust removes pollutants near their sources.

• HRV — Heat Recovery Ventilator: Transfers heat between outgoing and incoming air streams while keeping the air streams separate.
• ERV — Energy Recovery Ventilator: Transfers heat and also transfers some water vapor between the air streams. An ERV can moderate the moisture added or removed by ventilation, but it is not a dehumidifier.
• System selection: Do not choose an HRV solely because the climate is cold or an ERV solely because the climate is humid. Indoor moisture generation, outdoor climate, equipment characteristics, other HVAC systems, and the ventilation design all affect the choice. ^[6]
• Commissioning: Measure ventilation airflow and verify that supply and exhaust paths are installed and operating as designed. Filters and heat-exchange cores also need accessible maintenance.

Ventilation rates and system configuration should meet the applicable building code, program requirement, or ventilation standard for the project. Equipment nameplate airflow is not a substitute for measuring installed airflow when verification is required.

Domestic Hot Water Is Part of the Energy Plan

Domestic hot water (DHW) is the water used at showers, sinks, tubs, dishwashers, and clothes washers. Evaluate the water heater separately from space heating even when one appliance serves both loads.

• Record the basics: Fuel or energy source, equipment type, tank size or rated hot-water delivery, location, age when known, efficiency rating when available, and signs of leakage or venting problems.
• Heat pump water heaters: Move heat from surrounding air into a storage tank. Check the manufacturer's requirements for room volume or ducting, ambient temperature, condensate drainage, electrical service, noise, and clearance. They cool and dehumidify the air around them while operating. ^[7]
• Hot-water demand: Efficiency is only part of selection. The system must also deliver enough hot water for the household's expected peak use.
• Distribution losses: Long hot-water pipe runs and continuously operating recirculation systems can waste energy. Pipe layout, insulation, and recirculation controls affect total performance.

Controls, Plug Loads & Occupant Operation

Thermostats and controls determine when equipment operates; appliances and electronics add to electric use and also release heat indoors. These loads affect utility bills and can affect cooling demand, but they are different from envelope or HVAC efficiency.

• Thermostat settings: Use schedules and setbacks that are compatible with the equipment and control strategy. A control that frequently activates expensive or inefficient backup heat can undermine the intended system design.
• Filters and maintenance: A higher-efficiency filter is useful only when the HVAC system can handle its pressure drop and the filter is replaced on schedule.
• Appliances and electronics: Efficient equipment and sensible operating habits can reduce plug and appliance loads. Record unusually large loads when they materially affect the home's energy use.
• Homeowner handoff: Explain normal thermostat operation, ventilation controls, filter locations, maintenance intervals, condensate drains, and backup-heat operation. A system that occupants do not understand is difficult to operate as designed.