Condensation & Inner Liners
Why a fabric building sweats, what the inner liner does about it, and what it does not do. Straight answers, including the ones that cost us a sale.
The short version
Any unheated building can sweat — fabric, steel or wood. It is condensation, not a leak: on a cold night the roof surface falls below the dew point of the air beneath it and that air gives up its water. Ours do it a little, depending on the region, and it is not severe. The answer is an inner liner: a 400 g fabric hung inside the trusses with about a six-inch air gap behind it, so the cold surface is no longer the surface above your equipment. It runs roughly half the price of the main cover. It is not insulation, and we do not sell insulation packages.
Why A Building Sweats
Most people meet condensation and think the roof is leaking. It is not. Air holds water — the warmer it is, the more it holds — and every parcel of air has a temperature at which it can hold no more. That temperature is the dew point. Cool any surface below it and the air touching that surface gives up its water there.
On a clear night a roof radiates heat to the sky and drops below the outside air temperature. Meanwhile the ground, the equipment, and anything living or green inside is still giving off moisture. The coldest surface in the building is the underside of the cover, so that is where the water goes. It is the same reason a cold glass of water beads on a summer afternoon.
Where the dew point sits
| Air inside at 10°C, 70% humidity | dew point 4.8°C |
| Air inside at 10°C, 80% humidity | dew point 6.7°C |
| Air inside at 15°C, 60% humidity | dew point 7.3°C |
| Air inside at 2°C, 90% humidity | dew point 0.5°C |
Read the middle line: at 10°C and 80% humidity, any surface colder than 6.7°C grows water. On a clear night a roof can sit several degrees under the air temperature, so it does not take a cold snap — it takes a still, clear night and moisture already in the building.
Standard psychrometrics, shown to explain the mechanism — not a prediction for your site. What your building actually does depends on your region, your ventilation, and what you keep in it.
Fig 1 — No liner. The coldest surface is directly over the load.
What The Inner Liner Is
A second, lighter fabric hung on the inside face of the trusses, running the length of the building. It is roughly 400 g against the 750 g of the main cover — 900 g on the 80-wide series — because it is not fighting weather. It is a ceiling, not a roof.
Hanging it inside the trusses leaves an air gap of about six inches between the liner and the cover. That gap is the working part of the assembly. The liner is simply the surface you look at from inside the building, and the surface your equipment sits under.
The assembly, outside in
750 g PVC cover on the outside of the frame → the trusses → ≈6 in air gap → 400 g liner on the inside face. Two fabrics, one cavity, one frame carrying both.
Specified with the building. A liner is ordered as part of the build so it ships and hangs with the cover. Adding one to a building already standing is a conversation about that frame — tell us the size and we will tell you plainly whether it makes sense.
Fig 2 — Liner hung inside the trusses, gap between the two fabrics.
What The Air Gap Actually Does
Two things change the moment there is a second layer.
The cold surface is no longer over your load. Whatever forms on the underside of the cover now forms above the liner, inside the cavity, and runs down the inside of the gap rather than falling on your equipment. That is the practical result owners actually feel, and it is why the honest claim is that the liner largely prevents condensation drip — not that it repeals the physics.
The liner face stays closer to inside temperature. Still air in the cavity is a poor conductor, so the surface you see from below sits nearer the air around it than the cover does. A surface nearer the room's temperature is a surface further from its dew point.
What it does not do is make the building warm. Six inches of still air is not a thermal package, and we do not publish an R-value for it, because we would be inventing one.
Fig 3 — The cavity takes the condensation the cover makes.
What It Is Not: Insulation
We do not sell insulation packages. Not as an upgrade, not as an add-on, not quietly bundled into a liner and called something else. If a supplier is selling you an insulated fabric building, ask exactly what is in the wall and what it is rated at.
The inner liner is a condensation answer. It carries no R-value from us and it will not turn an unheated building into a heated one. If you put a heater in a lined building it will feel better than the same building bare — a second layer and a still-air cavity always do — but that is a side effect, not a specification, and we will not quote you a number we have not measured.
Some owners fill the six-inch cavity with insulation themselves after the fact. That is their project. We do not supply the material, install it, or warrant what happens in that cavity afterwards — it is, as Peter puts it, a whole other beast. What we will do is tell you honestly what the gap is and let you decide.
The line we hold
The liner largely prevents condensation drip. It is not insulation, carries no R-value claim, and is not a heating solution. Anything past that line is somebody else’s sales copy.
Fig 4 — The two columns we will not blur.
What It Costs
An inner liner runs roughly half the price of the main cover for the same building. It is lighter fabric over the same footprint, so the number tracks the size of your building rather than a fixed list price — which is why we quote it per building instead of publishing a figure that would be wrong for most of them.
Ask for it on your quote and it comes back as its own line, beside the building price and the cover, so you can see exactly what you are deciding on. No bundling, no upgrade tiers.
What to have ready: your building size, whether it will be heated, and what you are storing. That is enough for a real number.
Do You Need One?
Plenty of buildings never get a liner and their owners are happy. The question is not whether condensation can happen — it can, in any unheated building — but whether a drip would actually cost you anything. Here is how the conversation usually goes.
| What the building does | What owners tell us | Where it usually lands |
|---|---|---|
| Equipment & implement storage | A drip on a tractor hood is a non-event | Often runs bare |
| Hay & dry feed | Depends on the region and how dry it goes in | Worth asking about |
| Livestock & calving | The animals themselves put water in the air | Liner comes up early |
| Shop or work area | People are under it all day | Most common liner build |
| Anything that rusts or spoils | Tools, feed, packaged goods, inventory | Liner worth the line item |
| Heated space | Warm moist air against a cold cover is the worst case | Liner, and talk to us first |
| Riding arena / open span | Big volume, lots of air movement | Depends — ask |
This table is a starting point for a phone call, not a rule. Two buildings the same size on the same road can behave differently depending on drainage, prevailing wind and how the doors get used. Tell us the job and we will tell you what we would do.
Before You Buy Anything
Some condensation problems are use problems, and they cost nothing to fix. If your building is sweating more than you think it should, work through these first — we would rather you solve it for free than sell you a liner you did not need.
- Move air through it. A sealed building traps the moisture it makes. Cross-ventilation — an opening low at one end and high at the other — carries damp air out instead of holding it against the roof.
- Know what you put in. Green feed, fresh-cut lumber, wet equipment, damp soil and livestock all release water into the air for days. What comes in wet has to go somewhere.
- Get the load off the ground. Bare soil and gravel wick moisture up all year. Pallets, a granular base or a pad under the stored material cuts a surprising amount of it.
- Watch the doors on the wrong days. Opening up on a warm humid morning invites moist air into a building whose surfaces are still cold from the night. That is the one day sealing it up wins.
- Check the drainage around it. Water pooling against the base is a moisture source the building has to deal with every day. That is a site fix, not a fabric fix.
If you have done all that and it still sweats where it matters, that is when a liner is the right answer — and we will say so.
Not Sure Whether You Need A Liner?
Tell us the size, the region and what goes inside. We will give you a straight answer either way.
Get Your Price Call 587-800-4629Condensation Questions, Answered
Do fabric buildings sweat inside?
They can, a little, depending on the region and what is stored inside. It is condensation, not a leak: on a cold clear night the cover drops below the dew point of the air under it, and that air gives up its water on the underside. It is not severe. When it matters, an inner liner is the fix.
What is an inner liner?
A lighter fabric — roughly 400 g against the 750 g main cover — hung on the inside face of the trusses, leaving about a six-inch air gap between the liner and the cover. The liner is the surface you see from inside the building. The gap between the two layers is the part that does the work.
Is an inner liner the same as insulation?
No, and we do not sell insulation packages. The liner is a condensation answer, not a thermal package: it carries no R-value claim from us and it will not make an unheated building warm. Some owners fill the six-inch cavity with insulation themselves afterwards — that is their own project, outside what we supply, install or warrant.
How much does an inner liner cost?
Roughly half the price of the main cover for the same building. The exact figure depends on the size, so it is quoted per building rather than published as a list price. Ask for it on your quote and it comes back as its own line.
Does a liner stop condensation completely?
It largely prevents condensation drip — the water that would otherwise land on what you are storing. The physics does not go away: warm moist air meeting a cold surface still makes water. What changes is where that surface is. It is now above the liner, inside the cavity, instead of directly over your equipment.
Can I add a liner to a building I already own?
Ask us with the building’s size and age. A liner is normally specified with the building so it ships and hangs with the cover. An existing building is a conversation about that frame and the access — not a catalogue item to add to a cart — and we will tell you plainly whether it is worth doing.
What can I do about condensation without buying anything?
Move air and let moisture out. Cross-ventilate rather than sealing the building tight, keep stored material off bare ground and dry going in, and fix drainage that pools against the base. Green feed, fresh-cut lumber, wet equipment and livestock all put water into the air the building has to shed. Many owners never need a liner because of how they use the building.
Which buildings most often get a liner?
The ones where a drip actually costs something: shops and work areas people spend the day in, buildings storing anything that rusts or spoils, and livestock buildings, where the animals themselves put moisture into the air. Open equipment and hay storage in a dry region often runs bare for years without a complaint.
The Rest Of The Building
The cover over the liner is 750 g PVC on a polyester mesh — 900 g on the 80-wide series — not the polyethylene of a hardware-store shelter. Whether you call it a fabric building, a fabric shelter, a hoop shelter or a tension fabric building, the liner works the same way in all of them. How the building is held down is covered on the anchoring page, and full specifications for each size are on its product page, held to The MAX Standard.