Recovery has become a working strategy across construction. Buildings throw away conditioned air every time a fan runs, and ventilation systems capture part of that waste before it leaves. Roofs that once got torn off and replaced can be recovered with a new membrane layer on top of the old assembly. Even failed land deals have recovery paths that return buyer deposits. The common thread is the same: recover what still has value before paying for a replacement.
The mechanical side starts with energy recovery ventilation systems, which transfer heat and moisture between the exhaust stream and the fresh supply stream without mixing the two. Builders add them to tight envelopes to cut heating and cooling loads while keeping indoor air fresh. They now appear in building codes and green building programs as a standard efficiency measure, not an exotic extra.
How Energy Recovery Ventilation Works
An energy recovery ventilator sits at the heart of a balanced ventilation system. One fan pulls stale air out of the building while another pulls fresh air in, and the two streams pass through a core inside the unit. In winter the outgoing air warms the incoming air; in summer the process reverses and the outgoing conditioned air cools the incoming outdoor air.
The two main unit types differ in what they transfer:
- Heat recovery ventilators (HRV) transfer sensible heat only, so temperature moves between the streams but moisture does not.
- Energy recovery ventilators (ERV) transfer both heat and moisture through an enthalpy core.
- ERV units are common in humid climates because they temper incoming humidity in summer and retain indoor humidity in winter.
Choosing between them comes down to climate and budget, and the home guide to energy recovery ventilators covers the full equipment lineup, from compact through-wall units to whole-house ducted systems.
The Core: Heat Exchange or Enthalpy Exchange
The core is the component that does the work. Plate cores pass the two airstreams through alternating channels of aluminum or polymer and transfer heat through the wall between them. Rotary wheels rotate between the streams and can move moisture as well as heat. Enthalpy cores use a membrane that lets water vapor pass while blocking odors and contaminants.
Manufacturers publish core performance as sensible recovery effectiveness. Good residential units recover 60 to 85 percent of the temperature difference between the streams, and the percentage drops as the outdoor temperature gets more extreme because the core works against a bigger gradient.
Ducted versus Ductless Units
Whole-house units are ducted to living spaces, bedrooms, and exhaust points. Ductless through-wall ERVs serve single rooms or apartments and trade lower installation cost for localized coverage. A ducted system typically moves 0.3 to 0.5 air changes per hour across the whole floor plan, while a ductless unit conditions one room at a time.
Heat Recovery versus Energy Recovery: Which Unit Fits Your Climate
Climate decides the right core. The Passive House community has documented the difference in detail, and their comparison of heat recovery and energy recovery ventilators shows why the same unit performs differently in a dry winter and a humid summer.
| Factor | HRV | ERV |
|---|---|---|
| Transfers | Heat only | Heat and moisture |
| Best climates | Cold, dry winters | Humid summers; dry winters with indoor dryness |
| Winter humidity | Can dry indoor air further | Retains indoor moisture |
| Summer humidity | Little moisture control | Tempers incoming humidity |
| Core type | Sensible plate or rotary | Enthalpy membrane or desiccant wheel |
A cold-climate home with dry indoor air in January can find that an HRV makes the dryness worse, because every exchange pushes humid indoor air out and brings cold dry air in. An ERV keeps some of that moisture inside. In a hot humid climate the ERV pulls moisture out of the incoming air before it reaches the cooling coil, which shrinks the dehumidification load on the air conditioner.
Selection checks:
- Confirm the winter and summer design conditions for the site.
- Compare sensible and total effectiveness at the same airflow rate.
- Check frost control, because cold-climate units need preheat or core defrost strategies.
- Verify filter access and maintenance intervals before committing to a unit.
Sizing and Placement for High-Performance Homes
Sizing starts with the ventilation rate the building needs. Standards such as ASHRAE 62.2 base the rate on floor area and occupancy, roughly 7.5 cubic feet per minute per bedroom plus 3 cfm per 100 square feet of living space. High-performance homes often ventilate continuously at low speed instead of intermittently, because continuous operation keeps the core at a steady temperature and recovers more energy.
The practice of heat recovery ventilation in high performance homes starts with the airtightness test, because an ERV only manages the air that actually moves through it.
Placement rules:
- Locate the unit in a conditioned space, typically a utility room or attic with easy filter access.
- Keep the intake and exhaust terminals at least 3 meters apart outside so exhaust cannot short-circuit back into the intake.
- Insulate and seal ducts that run through unconditioned space.
- Balance the two fans so the building stays near neutral pressure.
Commissioning steps:
- Measure supply and exhaust airflow with a flow hood or anemometer.
- Adjust dampers until the two flows are within 10 percent of each other.
- Verify that the core drains condensate during winter operation.
- Record the filter type, change date, and fan speed settings on the unit label.
Airtightness changes the numbers. A leaky house exchanges air through the envelope whether the fans run or not, so the ventilator handles a smaller share of the total airflow and recovers a smaller share of the total energy. Sealing the envelope first makes the recovery share bigger, which is why high-performance builders test the envelope before commissioning the mechanical system.
Roof Recovery: Restoring the Assembly Instead of Tearing It Off
The same logic applies above the ceiling line. Roof recovery places a new membrane or coating over an existing roof assembly instead of removing the old layers down to the deck. Codes permit recovery when the existing roof is structurally sound, dry, and free of serious defects, and most limit how many recovered layers can accumulate. Contractors weigh roof recovery systems against full tear-off on every reroofing job.
The cost argument is direct:
- A recover typically costs 30 to 50 percent less than a tear-off and replacement, because the labor to remove and haul away the old roof disappears.
- Recovery keeps thousands of square feet of old membrane and insulation out of the landfill.
- The job finishes in days instead of weeks, which matters for occupied buildings.
- Recovery keeps the thermal barrier intact when the existing insulation stays in place.
When a Roof Is a Candidate for Recovery
Not every roof can be recovered. The existing membrane must be sound, the deck must be dry, and there must be no hidden moisture in the insulation. Contractors probe the assembly, take core samples, and check moisture readings before committing.
Candidate checklist:
- No more than one or two existing roof systems, depending on local code.
- No ponding water, blisters, or widespread membrane deterioration.
- Positive drainage to scuppers or drains once the new layer is added.
- Fastener pull-out strength adequate for the new attachment method.
Recovery Layers and Fastening
Recovered roofs are usually mechanically fastened or adhered rather than torch-applied, because an open flame near an old membrane is a fire risk and the old substrate may not accept hot asphalt. Fastener patterns, plate spacing, and lap sealant details follow the membrane manufacturer’s instructions, and the new membrane has to bridge joints and flashings without tearing.
Roof Recovery Materials and Methods
The material menu for roof recovery mirrors new construction. Single-ply membranes such as TPO and PVC, modified bitumen, and fluid-applied coatings each behave differently over an old substrate, and the roof recovery materials and methods review explains how each system performs.
| Material | Attachment | Best recovery scenario |
|---|---|---|
| TPO single-ply | Mechanical or adhered | Large low-slope roofs needing light weight |
| PVC single-ply | Mechanical or adhered | Roofs with chemical exposure |
| Modified bitumen | Adhered, cold or hot | Sloped and low-slope retrofits |
| Fluid-applied coating | Spray or roller | Metal roofs and small repair areas |
Method differences:
- Mechanically attached membranes use plates and screws into the deck, which requires the deck to hold fasteners.
- Fully adhered membranes handle wind uplift well but need a clean, dry substrate.
- Fluid-applied systems coat the existing surface and are popular on metal roofs where sheet membranes are hard to detail.
Inspection before recovery catches the problems that would otherwise appear through the new membrane: wet insulation, deteriorated decking, corroded fasteners, and failed flashings. Repair those first, then recover. Skipping the inspection saves a day and costs a roof.
Recovery thinking extends past the building itself. When a land deal falls through, buyers can recover earnest money deposits under the terms of the purchase contract, and the guide to getting your earnest money back explains the notice periods and conditions that protect the deposit. The pattern repeats across construction: energy recovery ventilation cuts operating cost, roof recovery defers replacement cost, and contract protections return capital. Each is a way to keep value that would otherwise be lost, and each starts with knowing what the existing asset is worth before deciding to replace it.