Single-skin and double-skin insulation for steel buildings describes how many insulation layers fill the roof cavity: one faced blanket in a single-skin build, or a faced layer plus an unfaced overlay in a double-skin build, and the choice changes what the facing has to do. In a single-skin system the same facing carries the vapour retarder, the abuse resistance and the finished appearance. In a double-skin build the cavity is filled in two passes, usually with the vapour retarder membrane separated from the thermal layer, which cuts the number of seams that have to be sealed on site. System selection happens before facing selection, which is why this decision is the one worth arguing about first.
Quick answer: what should you know about single skin vs double skin insulation
- Single skin means one insulation layer with the facing doing every job. Double skin adds a second, usually unfaced, layer to fill a deeper cavity.
- Three installation families sit underneath that choice: over-purlin sandwich, liner system with a continuous membrane below the purlins, and long tab banded with faced blanket between the purlins.
- Measured on the same nominal R-value, a liner system has been published around U-0.035 while a realistic long tab assembly performs around U-0.046, roughly 25 percent worse.
- Gaps between the facing and the purlin start costing performance beyond 2 mm and degrade sharply beyond 10 mm, so tab discipline is a thermal issue, not a tidiness issue.
- The facing specification changes with the system: tab width for long tab work, bay-width rolls for liner systems, tensile and puncture resistance for over-purlin work.
- Write the system into the tender, not just “insulation with facing”, or you will receive quotes for three different products at three different prices.
First, settle what the words mean
The phrase gets used two ways, and the confusion costs money.
In pre-engineered building markets, “single skin” and “double skin” often describe the building envelope: a single-skin building has one sheet of metal roof and wall cladding, while a double-skin building adds an interior liner panel. Insulation sits between them.
In insulation practice the same pair of words describes the fill: one layer of faced blanket versus a two-layer build-up, typically a faced layer plus an unfaced overlay, which is how higher R-values are reached in a deep cavity without a single impossibly thick blanket. An R-38 roof, for example, is commonly shipped as a two-layer combination rather than one piece.
This article is about the second meaning, because that is the one that determines the facing specification.
The three systems
Over-purlin sandwich. The faced blanket lays over the purlins and gets sandwiched between the purlin flange and the roof panel. Full blanket thickness survives between purlins and compresses only at the flange itself. This is the fastest method and the one most exposed to workmanship, because the facing is being pulled over steel edges under time pressure.
Liner system. A continuous vapour retarder membrane is made to fit an entire bay and installed entirely below the purlins, isolating the steel from the conditioned space. Unfaced insulation goes on top, filling the cavity between the membrane and the roof panel. The membrane is sealed around the perimeter of the bay and essentially nowhere else.
Long tab banded. Faced blanket runs parallel to the purlins with extended facing tabs that have to be adhered over the purlin tops, suspending the insulation in the cavity between purlins. The purlins remain inside the conditioned envelope, and every tab is a seam that somebody has to seal.
| Over-purlin sandwich | Liner system | Long tab banded | |
|---|---|---|---|
| Insulation layers | One or two | Membrane plus unfaced fill | One faced blanket per bay |
| Vapour retarder position | On the blanket, above purlins | Continuous, below purlins | On the blanket, tabs over purlin tops |
| Sealing work | Panel laps and blanket seams | Bay perimeter only | Every tab, plus perimeter and butt joints |
| Purlins in conditioned space | No | No | Yes |
| Compression risk | At the flange only | Low, cut to fit | At bracing and flange |
| Published performance | Good if cavity is filled | Around U-0.035 in one R-19 + R-11 comparison | Around U-0.046 in the same comparison |
| Facing demand | Tensile and puncture | Bay width, tear resistance | Tab width and adhesion |
The numbers that should drive the decision
Two figures from published industry comparisons are worth carrying into a design meeting.
The first is the U-value gap. For an R-19 plus R-11 assembly in a standing seam roof with a thermal spacer block, a liner system has been measured around U-0.035 while a realistic long tab system lands around U-0.046. Same nominal R-value on paper, roughly 25 percent worse in service. The reason is geometry, not material quality: a long tab assembly only performs if the facing sits in perfect contact with the purlin, and site conditions do not deliver perfect contact.
The second is the gap threshold. The ASHRAE 90.1 committee has taken the position that long tab performance begins to drop once the gap between facing and purlin exceeds 2 mm, and drops substantially beyond 10 mm. Rafter flange bracing, purlin lap bolts, conduit and other attachments to the purlin web create gaps past that threshold routinely. This is an installation tolerance issue disguised as a product issue.
Sealing volume tells the same story from the labour side. In a 100 by 25 foot bay with purlins at 5 feet on centre, a liner system needs about 250 feet of membrane sealed, all of it at the perimeter. A long tab system in the same bay requires 1,050 feet of facing tabs handled on top of that same 250 foot perimeter, over five times the handling. Strapping follows: roughly 300 feet of strapping with 66 fasteners for the liner system versus about 900 feet with 198 fasteners for long tab work supported at 30 inches on centre.
One more constraint that surprises people: OSHA regulations prohibit walking on the purlins for the purpose of installing insulation, regardless of fall protection. System choices that assume foot traffic on the steel are describing a practice that is not permitted in the United States and is unsafe anywhere.
What each system asks of the facing
Over-purlin sandwich: tensile strength and puncture resistance. The facing is dragged over purlin flanges and pinched under panel fasteners. A scrim-reinforced laminate handles this; a light embossed foil will not, and the tear shows up years later as vapour leaks along a purlin line.
Liner system: width and tear resistance. The membrane has to span a full bay without a mid-bay splice, because every splice is a potential vapour leak and adds labour. Roll width should be matched to bay depth so one run goes eave to ridge. Where a splice is unavoidable, it belongs on a purlin line with a 100 to 150 mm overlap, stapled and taped.
Long tab banded: tab width and adhesive behaviour. The tabs have to follow the purlin profile and overlap the facing from the adjacent bay on top of the purlin. Too narrow and the vapour retarder is discontinuous exactly where the thermal penalty is worst.
Retrofit and the sub-purlin question
Older pre-engineered buildings typically run purlins at about 5 feet on centre, 8 inches deep, Z-shaped, spanning 25 to 30 feet between frames. When a metal-over-metal retrofit adds a new roof over the old one, sub-purlins come into play and two questions have to be answered: how the new panels behave when fixed to sub-purlins rather than full-depth purlins, and how much load the retrofit adds.
For insulation, the retrofit case is usually an under-purlin or between-purlins installation, which means working from below or from inside the building envelope. That changes the facing requirement again, because the crew is handling the material overhead and the vapour retarder ends up on the interior side of the steel. Retrofits are also where condensation risk gets recalculated: adding insulation to a building that previously had little or none moves the dew point location inside the assembly, and the vapour retarder has to be on the right side of that line.
Sizing the fill to the cavity
Insulation thickness has to match the space you actually have, and that space is set by the purlin depth. Nominal thicknesses at standard density run roughly as follows: R-8 at 2.5 in, R-10 at 3 in, R-11 at 3.5 in, R-13 at 4 in, R-19 at 6 in, R-25 at 8 in, R-30 at 9.25 in, with R-38 shipped as a two-layer combination.
Most metal buildings run purlins at 6, 8, 10 or 12 in deep. The rules of thumb:
- Over-purlin systems are forgiving. The blanket lays over the purlins, so extra thickness simply hangs below the purlin plane. R-30 works over 8 in purlins because the extra 1.25 in sits below the steel.
- Between-purlins systems are not. The blanket has to fit the cavity. R-19 suits 6 in purlins, R-25 suits 8 in, R-30 suits 9.25 in or 8 in with slight compression at the flanges.
- Compression is paid for twice. Pack R-30 into a strict 6 in cavity and you get roughly R-19 of performance from R-30 of material. You bought the thickness and lost it in the squeeze.
Roll width should be matched the same way a carpet is: one roll should span eave to ridge on a roof run, or floor to eave on a wall run, without a mid-bay splice. If the building is longer than a single roll, splice at a purlin line rather than mid-bay, overlap the facing tabs 100 to 150 mm over the purlin, and close it with staples plus patch tape so the vapour retarder stays continuous. Purlin spacing matters here too: if purlins sit at 5 ft on centre and the roll is 60 in wide, seams land on steel naturally. Mismatch the two and the seams float mid-bay with nothing to seal against.
Failure modes worth designing against
- Sag. Wide bays sag over years without adequate strapping or hangers. Sag opens gaps at the purlin face and closes air spaces the R-value depends on.
- Dust and discolouration. A facing that sags into view collects dust and looks failed long before it performs as failed.
- Condensation at the purlin. Systems that leave the purlins inside the conditioned envelope put cold steel in warm humid air. The facing does not fix a thermal bridge.
- Compression by bracing. Current energy codes prohibit insulation that is substantially compressed by bracing. Unfaced fill can be cut around bracing; a faced blanket cannot be cut without cutting the vapour retarder, which is the practical argument behind the liner system.
Two decisions sit upstream of the facing choice and are worth settling first. The insulation itself: our metal building fiberglass guide covers what the blanket contributes before any facing is applied. And the facing family: the trade-offs between foil and polypropylene skins are set out in our FSK and PSK comparison, which matters most on exposed roofs where appearance and glare drive the spec. For projects in Southeast Asia, the regional product guide for Indonesia adds local availability context.
How to write it in a tender
Avoid the phrase that causes most of the trouble, “insulation with facing, price per square metre”. Three suppliers will quote three systems and the comparison will be meaningless. Write this instead:
- System type: [over-purlin sandwich / liner system / long tab banded], with cavity depth and purlin spacing stated.
- Blanket: [R-value] at [thickness], unfaced where a separate membrane is specified.
- Facing or membrane: [construction], tensile MD/CD, and facing tab width where tabs are used.
- Sealing: seams on purlin lines, overlap [mm], stapled at [spacing] and taped with a matching tape.
- Strapping: spacing and fastener type.
- Bay dimensions, so roll width can be matched to a single eave-to-ridge run.
FAQ
What is the difference between single skin and double skin insulation?
Single skin uses one insulation layer to fill the cavity, with the facing carrying vapour control and finish. Double skin fills the cavity in two passes, usually a faced layer plus an unfaced overlay, which is how higher R-values are reached in deep cavities. The choice also changes where the vapour retarder sits.
Which system performs better, a liner system or long tab banded?
On the same nominal R-value, published comparisons put the liner system around U-0.035 and the long tab assembly around U-0.046, about 25 percent better for the liner. The difference comes from air gaps between the facing and the purlin, which degrade performance beyond 2 mm and degrade it sharply beyond 10 mm.
Can installers walk on the purlins during installation?
No. OSHA prohibits walking on purlins for the purpose of installing insulation regardless of fall protection, and the practice is unsafe in any jurisdiction. Systems should be planned around working from the panel side or from below.
Does the facing need to be wider for a liner system?
It needs to span the bay. A liner membrane is made to fit an entire bay and is sealed at the perimeter, so roll width should match bay depth to avoid a mid-bay splice. Every splice is a vapour leak risk and extra labour.
What facing property matters most in a long tab system?
Tab width and how the tab holds adhesion over a purlin profile. Tabs that are too short or that lift leave the vapour retarder discontinuous at exactly the location where the thermal penalty is largest.
Is more insulation always better in a metal roof?
Not if it is compressed. Packing a thick blanket into a cavity shallower than its nominal thickness costs R-value proportionally, and current codes prohibit insulation that is substantially compressed by bracing. Match the fill to the cavity or change the system.
How does retrofit differ from new build?
Retrofit usually installs between or under purlins from inside, adding sub-purlins for the new roof. It also changes the condensation calculation, because adding insulation to a previously uninsulated building moves the dew point within the assembly.
Next step
Every system above asks for a different facing, and the differences are in tensile, tab width and roll width rather than in price per square metre. Donfoil supplies reinforced facings and liner membranes in the widths, tab configurations and roll lengths these systems require. Send us your bay dimensions, purlin spacing and target R-value and we will specify the facing to match the system you are building.
