Every specification conversation in this category eventually reaches the same place: the buyer wants a finer mesh, and the fabric engineer explains that finer mesh means less air. Both are right. Below roughly 800 L/m²/s, a sleeping net starts to feel like a plastic bag in a warm climate — and a net the occupant opens at 2 a.m. provides exactly zero protection.
What air permeability measures
Air permeability is the volume of air that passes through a defined area of fabric in a defined time, under a defined pressure difference. It is expressed in litres per square metre per second, and it is measured to a published textile standard rather than estimated from the weave.
The number is a property of the whole fabric structure, not of the aperture alone. Three variables feed into it:
- Open area — the percentage of the fabric surface that is clear gap rather than yarn.
- Aperture size — how large each individual opening is.
- Fabric thickness and yarn roughness — which create drag at the boundary and reduce flow.
That is why two fabrics with the same aperture can differ meaningfully in permeability. A thin, smooth yarn at 1.2 mm flows better than a thick, textured yarn at the same 1.2 mm. Open area, expressed as a percentage, is the field that makes this visible on a spec sheet, and it is usually omitted.
Why a net gets hot
A sleeping net is a small enclosed volume with a person inside it. Heat and moisture leave that volume through the fabric, and the fabric is the only route. Reduce the flow and three things happen together:
Humidity rises. A sleeping adult releases a substantial amount of water vapour overnight. In a low-permeability enclosure it condenses on the inside of the mesh, on bedding, and on the occupant. The result is a damp, chilled, uncomfortable night even at a comfortable ambient temperature.
Carbon dioxide accumulates. In a small sealed volume with limited exchange, CO2 concentration climbs through the night. The physiological effect is subtle but real, and the perceived effect — stuffiness, headaches, waking — is not subtle at all.
Convective cooling stops. Moving air against skin is the main mechanism by which a sleeper loses heat in warm conditions. Block the flow and the body cannot dump heat, so the enclosure temperature tracks the occupant rather than the room.
All three failures are the same failure: insufficient exchange. The mesh count is irrelevant once the occupant opens the door.
The 800 floor
For sleeping applications, 800 L/m²/s is the practical floor. Above it, a net reads as airy and is used as intended. Below it, complaints cluster around heat, condensation and “feels like a tent inside a tent”.
Two cautions on treating 800 as an absolute:
It is a sleeping threshold, not a product threshold. A screened gazebo or a patio enclosure can sit lower because the volume is larger, the occupant is not inside it overnight, and the exposure period is shorter. A bivvy bag at 600 L/m²/s is a bad product; a shade structure at 600 L/m²/s may be fine.
Climate changes the threshold. In dry, cool conditions a lower figure is tolerable because the moisture load is smaller. In hot, humid conditions the same net fails badly. If your market is tropical, treat 800 as a hard floor and consider going higher.
| Product type | Aperture | Open area | Air permeability |
|---|---|---|---|
| Household sleeping net | 1.1–1.4 mm | 55–62% | 900–1100 L/m²/s |
| Camping net tent | 1.2–1.6 mm | 50–58% | 800–1000 L/m²/s |
| Midge-grade netting | ~0.6 mm | Lower — geometry limits it | Below the sleeping floor; solve structurally |
| Industrial screen | 0.8–1.2 mm | 35–48% | 450–700 L/m²/s |
Read the second row against the third. A camping net tent sits at the comfortable end of the range. A midge-grade fabric cannot reach it at any sensible yarn weight, which is why midge products depend on structure rather than fabric to stay usable.
The trade-off with mesh fineness
Halving the aperture does not halve the airflow — it does considerably worse. Open area falls roughly with the square of the aperture for a fixed yarn, and permeability falls at least as fast once boundary drag is included.
This creates the characteristic squeeze in product planning. The insect sets a maximum aperture. Comfort sets a minimum permeability. Between those two constraints there is a workable window, and it narrows sharply as the target insect gets smaller:
- Mosquito markets — a wide window. 1.2 mm at 40–50 gsm lands comfortably inside it.
- Sandfly markets — a narrower window. 1.0 mm is achievable without a comfort penalty if the yarn is kept light.
- Midge markets — the window closes. 0.6 mm at any yarn weight drops below the sleeping floor, so the product has to earn comfort somewhere other than the weave.
Above roughly 1200 L/m²/s, the reverse problem appears: the openings are likely coarse enough that the mesh count is too low for reliable protection. Very high permeability is a warning sign as often as a selling point.
Structural fixes that beat a looser weave
When the insect forces a fine weave, the airflow has to come from geometry rather than fabric. Five moves, in order of effectiveness:
- Two doors, opposite each other. Cross-ventilation through openings far larger than any mesh aperture. This is the single highest-impact change and the cheapest to specify.
- Internal volume. A net that holds mesh away from the face and body reduces the local humidity problem and removes contact bites at the same time.
- Vented ridge or roof panel. Hot air exits at the highest point. On a ridge-line hammock net, a raised panel above the sleeper does more for comfort than another 50 L/m²/s of fabric permeability.
- Raised foot end. Slight elevation increases convective flow along the length of the enclosure.
- Lighter yarn at the same aperture. Finer yarn raises open area without loosening the weave, which is the only fabric-side lever that does not cost protection.
Note what is absent from that list: making the mesh coarser. Loosening the weave to gain airflow trades the product's entire reason for existing against a comfort problem that geometry can solve instead.
How to specify it
Three fields, one test method, one acceptance rule:
- Air permeability: state a floor, not a target. “≥800 L/m²/s” is enforceable; “breathable” is not.
- Open area: ask for the percentage. It is the field that explains why two fabrics at the same aperture behave differently.
- Aperture and gsm together: permeability is meaningless without them, because the same figure can describe a coarse heavy fabric or a fine light one.
- Test method: name the standard your supplier will report to, and require the result on the batch record rather than on a marketing sheet.
If a supplier cannot give you a permeability figure, the honest conclusion is that the fabric has not been characterised — not that it is fine. Treat a blank field as an unknown, not as a pass.
What this means when you buy
Air permeability is the field that decides whether your product is used or abandoned, and it is the field most often missing from a quotation. Three practical consequences:
- Put a floor in the PO. “≥800 L/m²/s for sleeping products” is a line item. Without it, you are buying whatever the mill happened to run.
- Do not solve comfort by loosening the weave. If a fine mesh is required by the insect, recover airflow through doors, volume and venting. Loosening the weave trades protection for comfort, and the customer notices the bites before they notice the breeze.
- Match the threshold to the climate you are selling into. A net that passes at 850 L/m²/s in a temperate market can fail badly in a tropical one, where the moisture load is higher and the ambient temperature leaves no margin.
If you have a fabric in development and want a second opinion on the permeability-to-aperture balance, send us the numbers. We will tell you where the product sits relative to the sleeping floor and which structural changes would recover the gap.