Filtration as a Building System

Two Jobs, Not One

Most people picture air filtration as a single task: catch the dust. Indoor air carries more than dust. It carries gases and vapours — the VOCs released by furniture, paint, flooring, adhesives, cleaning products and a hundred ordinary materials — alongside fine particles like smoke, pollen and spores. Indoor air is commonly measured at several times the pollutant concentration of outdoor air, which matters because most people spend the large majority of their time indoors.

Doing this properly means two jobs.

Job one: clean the air already inside. A filter in the room processes the same air repeatedly. Every pass removes a little more, so the air improves the longer it runs.

Job two: control the air coming in. No building is airtight. Outside air enters through gaps around windows, doors, outlets, penetrations and the structure itself whether you want it or not. And you do want some of it — outside air is the only thing that removes the carbon dioxide, odours and humidity that recirculating filtration ignores entirely.

So the question is not whether outside air enters. It is how it enters: dirty, through random cracks, or clean, through a filter you chose.

Do only job one and you have a filtered room inside a leaky building. That is the gap this protocol closes, and it is why filtration belongs in the construction specification rather than on the shopping list.

HEPA and Carbon Do Different Jobs

The two media are not alternatives, and one does not partially cover for the other.

HEPA captures particles. Dust, smoke, pollen, mold spores, combustion particulate — the things occasionally visible in a sunbeam. True HEPA (H13 grade) captures 99.97% of particles at the most-penetrating size.

Activated carbon captures gases. VOCs, fumes, fragrance compounds and chemical vapours pass through a HEPA filter without slowing down. Only adsorptive media removes them.

This is where most consumer equipment fails. A unit ships with a good HEPA element and a token carbon layer — a thin impregnated mat weighing a few ounces — and cleans particles well while leaving the chemical load essentially untouched. For a building where chemical exposure is the primary concern, that is the wrong half.

Specify carbon by weight, not by presence. "Includes activated carbon filter" is not a specification. Pounds of carbon is.

Two variables drive carbon performance

Bed depth. Deeper beds give the air more contact with the media.

Airflow rate. Slower air spends longer in contact, so more is adsorbed. This is why a make-up air stream, which is slow and continuous by design, gets excellent performance from a moderate bed.

As a working figure: roughly 15 pounds of coconut-shell activated carbon is a practical minimum for meaningful VOC removal. For heavy loads — new construction, an active renovation, strong off-gassing — the specialist range runs 26 to 39 pounds.

The maintenance rule that gets missed

Carbon saturates. A saturated bed does not simply stop working — under changing temperature and humidity it can release previously adsorbed compounds back into the airstream. Replace or refill on schedule, and sooner if odours return. A refillable steel canister is the economical housing, because you refill media rather than replacing a cartridge assembly.

The Approach: Filter the Fresh Air, Push the Building Slightly Out

Bring in a small quantity of outside air, filter it before it enters through the same HEPA-plus-carbon combination used indoors, and bring in just enough that interior pressure sits marginally above exterior pressure.

That small pressure difference reverses the direction of every leak. Instead of unfiltered outside air being drawn in through gaps, filtered indoor air is pushed gently out through them. The only air entering the building is air that has already passed through your filters. Run the indoor recirculating unit at the same time and both jobs are covered simultaneously.

This is not a novel idea. It is the principle hospitals, cleanrooms and laboratories use to keep contaminants out of a space, and it scales down to a house or a single room without modification.

It also has a side benefit specific to this site's subject matter: a positively pressured building does not draw air out of its own wall cavities, crawl space or attached garage into the living space. Depressurisation is a common and under-recognised path for crawl space moisture, soil gas and garage exhaust to enter a house.

How Much Air, and How Much Pressure

The target is very slight positive pressure — roughly 2 to 5 Pascals. That is imperceptible to occupants. The field test is a tissue held near a gap or a cracked door: it should drift outward, barely.

In practice that means a small continuous flow: on the order of 20–50 CFM for a single room, and modestly more for a small whole house. The goal is a quiet trickle running continuously, not a burst. Continuous operation matters — an intermittent system loses pressure between cycles and the building reverts to inward leakage every time it stops.

The Fan

Specify an inline duct fan with an EC (electronically commutated, variable-speed) motor. Two properties make this the right category, and both are requirements rather than preferences.

Variable speed. You need to dial the flow down to a gentle trickle. A fixed-speed fan sized for the duct will substantially over-pressurise a house.

Static pressure capability. An inline fan can push air through the resistance of a packed carbon bed. A box fan or a bathroom exhaust fan cannot — it stalls and airflow collapses to nearly nothing while the fan appears to be running. This is the most common failure in owner-built systems, and it is silent.

As a specification rather than a product: an inline EC fan rated for at least 250–350 Pascals of static pressure, in a diameter matched to the filter housing, with continuous-duty speed control. Smaller diameters (4–6 inch) are easier to keep slow and quiet for a single room; a larger fan run far below its rating also works but is harder to trim to a low flow.

Avoid controllers that cycle the fan on temperature or humidity triggers. Continuous make-up air needs steady operation; a thermostatic controller set to run continuously works, but it is a layer with nothing to do.

The Filter Stack

Outside air passes through a sealed filter housing before it reaches the fan and the building. Order matters — each stage protects the next.

1. Pre-filter (MERV 8). Inexpensive, often washable. Captures the coarse material that would otherwise clog the expensive filters within weeks. This one stage roughly doubles the service life of everything behind it.

2. HEPA (H13). Fine particulate — smoke, pollen, spores, fine dust.

3. Activated carbon bed. Gases and vapours. The stage that matters most for chemical sensitivity, and the stage inexpensive systems omit.

The housing must be sealed. Any bypass around a filter is unfiltered air entering the building, and a leaking housing defeats the entire system while every component still appears to work. Gasket the access panel and seal duct joints.

Assembly order

Hooded exterior intake → pre-filter → HEPA → carbon bed → EC inline fan → conditioned space. Locate the intake away from driveways, attached garages, dryer and combustion vents, trash storage and the neighbour's air conditioner. An intake positioned badly filters the exhaust you are trying to avoid, and no filter stack fully recovers from a bad intake location.

Climate and Moisture — Where This Goes Wrong

Positive pressure is a moisture strategy as well as an air quality strategy, and it can be either the right one or the wrong one depending on climate. This is the section to get right, because the failure mode is mold inside a wall — the exact problem the rest of this site exists to prevent.

Cold climates (IECC 5–7). Pushing warm, humid indoor air into cold wall and roof assemblies risks condensation on cold sheathing. Keep the pressure at the low end of the range, be certain the interior air barrier is continuous, and do not attempt this in a building with a known air-sealing deficiency. Incoming air also needs tempering or the system will be uncomfortable enough that someone will switch it off.

Hot-humid climates (IECC 1–3A). Positive pressure is generally the correct direction here, because it prevents humid outside air being drawn into cool wall cavities where it condenses. But the incoming air must be dehumidified before entry, or the system imports moisture continuously. A ducted dehumidifier or a dedicated outdoor air system with dehumidification is not optional in this climate.

Mixed and hot-dry climates. The most forgiving cases. Standard cautions on tempering and intake location still apply.

Everywhere: avoid large exhaust appliances fighting the system. A high-capacity kitchen hood, a whole-house fan or an unbalanced dryer will pull the building back to negative pressure and hold it there while running. Where a large hood is specified, it needs its own make-up air provision — which is code in many jurisdictions and frequently ignored.

The Indoor Side

The recirculating half needs no engineering. It needs a unit that pairs true HEPA with a serious quantity of carbon, in an enclosure that does not off-gas materially itself — a metal or low-emission housing rather than a warm plastic shell holding new plastic filters.

Size by air changes per hour rather than by the manufacturer's room rating, which typically assumes a single air change and a low bar. For a sensitive occupant, four to five air changes per hour in the room that matters most is a reasonable target. That usually means a unit rated for a considerably larger room than the one it is in, run at a low, quiet speed — which also improves carbon contact time.

One category to avoid entirely

Do not specify active-oxidation air cleaners: PECO, PCO, UV-PCO, photocatalytic, bipolar ionisation, plasma, ozone-generating or hydroxyl-generating devices. Rather than trapping pollutants, these use reactive chemistry on the airstream, and independent testing has repeatedly found byproduct formation — formaldehyde, acetaldehyde, ultrafine particles — from the incomplete oxidation of ordinary indoor VOCs. For a building specified around chemical exposure, adding a device that performs chemistry on the air is working against the objective.

Sealed mechanical filtration only: true HEPA plus substantial carbon, no oxidation stage in the breathing air. This is consistent with the Standard's position on high-risk systems generally.

Where This Fits in a Build

Filtration is usually treated as something bought after move-in. Three decisions are much cheaper at design and rough-in:

• Intake location and duct routing — free at design, disruptive later.
• A filter housing location with service access — a unit that cannot be reached does not get its carbon changed, and a saturated bed is worse than none.
• Dedicated circuit and continuous power — trivial at rough-in.

None of it substitutes for source control. Filtration handles what remains after material selection has done its work; it is not a way to compensate for specifying materials that off-gas. See Materials to Avoid and the Paint & Finishes Protocol. A house that needs 39 pounds of carbon to be tolerable was specified wrong.

For the ducted-system side — filter grades in a central air handler, duct sealing, return placement — see the HVAC & Filtration Protocol. This page covers the make-up air and pressure strategy that sits alongside it.

The Short Version

Cleaning indoor air and controlling incoming air are not competing choices. Filter the air inside with HEPA and carbon, filter the small volume of fresh air you bring in the same way, and hold the building slightly pressurised so nothing unfiltered gets past. That combination addresses both the particles you can see and the chemicals you cannot.

Flow rate and moisture balance depend on the specific building and climate, and both have real failure modes. A qualified indoor air quality or mechanical contractor should commission the system to the actual house rather than to a rule of thumb — particularly in cold or humid climates, where getting the moisture side wrong creates the problem this site exists to prevent.

Evidence Basis

EPA Guide to Air Cleaners in the Home, and EPA Indoor Air Quality guidance on indoor versus outdoor pollutant concentrations and occupancy time. ASHRAE Standard 62.2 (Ventilation and Acceptable Indoor Air Quality in Residential Buildings) for outdoor air rates and building pressure considerations. ASHRAE Standard 52.2 and the EN 1822 / ISO 29463 classifications for filter efficiency including H13. California Air Resources Board guidance on ozone-generating air cleaners. Published peer-reviewed testing of photocatalytic and ionisation air cleaners reporting byproduct formation including formaldehyde and ultrafine particles. Building Science Corporation guidance on building pressure, mechanical ventilation strategy and climate-specific moisture risk. AHAM AC-1 CADR test method for clean air delivery rate.