Metal building insulation facing stops condensation when it is a Class I vapor retarder (0.1 US perm or less, and 0.02 perm for high-humidity interiors) installed on the warm side of the blanket, with every seam, tab and penetration sealed. The perm number alone will not save you: the failures that actually produce a dripping roof are an open air path at the seams and local cold spots where the blanket is compressed or bridged by steel. Insufficient insulation thickness is rarely the culprit.
Quick answer: the six decisions that determine whether the facing works
Six decisions decide the outcome, and they come in a fixed order because each one narrows the next.
- Set permeance at 0.10 perm or less for normal interiors and 0.02 perm for high-humidity interiors, measured to ASTM E96. That is the NAIMA recommendation.
- Put the retarder on the side where vapor pressure is highest, which in cold and temperate climates is the interior face of the blanket.
- Keep it continuous. A torn or unsealed facing is worse than none, because it admits moist air and then slows its escape.
- Ask for ASTM C1136 type compliance, which covers puncture resistance and tensile strength rather than permeance alone.
- Meet the interior finish class the code requires, normally Class A to ASTM E84, when the facing is the exposed interior surface.
- Confirm the insulation keeps the interior surface above the dew point of the interior air everywhere, including over purlins and girts.
Why metal buildings condense in the first place
Steel is a good conductor, so a roof panel tracks the outdoor temperature almost exactly. On a clear night it can even fall below air temperature, because the panel radiates heat to a cold sky. Warm, moist interior air rises, meets that cold surface, and if the surface is below the air’s dew point, water forms.
Dew point is the number to work with, not relative humidity. At an interior of 20 °C (68 °F), the dew point is 9.3 °C (48.7 °F) at 50 % RH, 12.0 °C (53.6 °F) at 60 %, 14.4 °C (57.9 °F) at 70 % and 16.4 °C (61.5 °F) at 80 %. A roof panel at −5 °C (23 °F) is below every one of those.
Moisture usually comes from inside the building, not from outside: curing concrete, unvented propane heaters, vehicles drying after rain, stored green timber, livestock, and washing-down operations. And air movement carries far more moisture than diffusion does. A gap at an eave or an untaped lap moves water vapor orders of magnitude faster than diffusion through an intact 0.02 perm facing, which is why air sealing outranks perm rating in practice.
You can tell condensation from a roof leak by its pattern. Condensation appears across wide areas after cold clear nights, drips from purlins and fasteners, and dries up by mid-morning. A leak tracks rainfall and follows a single path from a penetration or seam.
What the facing does, and what it does not
A laminated facing on a metal building blanket has four jobs:
- vapor control. A low-permeance film or foil limits vapor diffusion into the cold plane.
- Air barrier continuity. Sealed seams stop humid air short-circuiting the insulation. This is the job most specifications omit, and the one that fails most often.
- Light reflectivity. A white or foil surface lifts interior illuminance and lowers the lighting load. Worth having, though it should not drive the choice.
- Handling and durability. The scrim gives puncture and tear resistance so the blanket survives installation and stays intact in service.
What it does not do: the facing is not the insulation. A reflective foil surface only delivers a radiant benefit when it faces an air space; pressed flat against a panel or a liner it contributes essentially nothing to thermal resistance. Treat the R-value as coming from the blanket, and the facing as moisture and air control.
Step 1: Decide which side the retarder goes on
| Condition | Where the retarder goes | Interior-side facing | Why |
|---|---|---|---|
| Heating-dominated climate (most of North America, Europe, northern China) | Interior face of the blanket | The retarder itself | vapor pressure is highest indoors in winter; stop moisture before it reaches cold steel |
| Hot-humid climate (Southeast Asia, Gulf states, Florida) | Often the panel side of the blanket | Perforated or breathable | vapor drive reverses; the assembly needs to dry to the interior |
| Cold storage, coolers, controlled atmosphere | Both sides, or a purpose-designed double-retarder system | Retarder | vapor drive reverses seasonally; a single retarder would load the core with moisture |
| Unheated or intermittently heated sheds | Interior face, plus ventilation | The retarder itself | Night-sky cooling puts the panel below ambient; ventilation removes moisture the retarder cannot |
Never place two vapor-tight layers on opposite sides of an absorptive blanket unless the assembly was designed for it. Moisture that gets in has no way out, and the insulation becomes a sponge.
Step 2: Specify the permeance, and name the test method
Permeance is measured by ASTM E96 (or ISO 12572), and the result depends on the method: the desiccant (dry cup) and water (wet cup) methods give different numbers for the same material. State the method in the specification, or you are not comparing like with like.
| Class (IRC / ASHRAE) | Permeance, US perm | Metric equivalent | Typical materials | Verdict for metal buildings |
|---|---|---|---|---|
| Class I | ≤ 0.1 | ≤ 5.75 ng/(Pa·s·m²) | Unperforated foil, polyethylene sheet, PSK, MSK, FSK | Specify this |
| Class II | 0.1 – 1.0 | 5.75 – 57.5 | Kraft facing on batts, some vinyl facings | Marginal; use only with a defined drying path |
| Class III | 1.0 – 10 | 57.5 – 575 | Latex or enamel paint, some vinyl | Not a vapor retarder |
| Permeable | > 10 | > 575 | Perforated facings, WMP-30 / WMP-50 | Deliberate choice for drying assemblies |
NAIMA recommends metal building insulation be faced with a retarder of no more than 0.10 perm for normal conditions, tightening to 0.02 perm where interior humidity is high. Anything above 1.0 perm is not a vapor retarder at all.
ASTM C1136 is the specification to cite when you need the mechanical properties as well. It defines four types:
| ASTM C1136 type | Permeance, max | Puncture resistance, min | MD tensile, min | CD tensile, min |
|---|---|---|---|---|
| Type I | 0.02 perm (1.15) | 50 Beach (58) | 45 lb/in (7.9 N/mm) | 30 lb/in (5.3 N/mm) |
| Type II | 0.02 perm (1.15) | 25 Beach (29) | 30 lb/in (5.3 N/mm) | 20 lb/in (3.5 N/mm) |
| Type III | 0.10 perm (5.75) | 50 Beach (58) | 45 lb/in (7.9 N/mm) | 30 lb/in (5.3 N/mm) |
| Type IV | 0.10 perm (5.75) | 25 Beach (29) | 30 lb/in (5.3 N/mm) | 20 lb/in (3.5 N/mm) |
Typical published permeance for common metal building facings, measured by ASTM E96. Verify against the current product datasheet before writing it into a specification.
Step 3: Choose the facing construction
| Facing | Typical permeance | Strengths | Limits and watch-outs | Best fit |
|---|---|---|---|---|
| PP scrim kraft facing (PSK) | 0.02 – 0.09 perm | The industry default for metal buildings. Bright white, scrim-reinforced, good puncture resistance, economical | Polymer films have a service-temperature ceiling; check it before use near hot processes | Roof and wall blankets, general commercial and industrial |
| Foil-scrim-kraft laminates (FSK) | ~0.02 perm | Lowest permeance and highest reflectivity; excellent vapor control | Aluminium can corrode in persistently damp, alkaline or chloride-laden environments; pinholes reduce performance if handled roughly | High-humidity interiors from the process side, high-reflectance requirements |
| MPET scrim kraft facing (MSK) | ~0.02 perm | metallized polyester has no aluminium to corrode; strong puncture resistance; stable in coastal and humid climates | Reflectivity slightly below solid foil; verify the emissivity figure | Coastal projects, humid interiors, where foil corrosion is a concern |
| FSM duct wrap facing | ~0.02 perm | Good puncture resistance and a tough surface for mechanical handling | Specified primarily for duct and mechanical wrap rather than building envelope | Duct wrap and HVAC insulation |
| DS reflective laminate | ~0.02 perm | Reflective surface for radiant control plus vapor retardance | Only contributes radiant benefit when facing an air space | Assemblies with a designed air gap |
| Vinyl facings (VR) | ~0.9 perm | Tough, cleanable, good for exposed interiors | Not a Class I retarder; becomes brittle and cracks in cold handling (“cold crack”) | Warm interiors where a Class I retarder is not required |
| Perforated / WMP facings | 10 – 50 perm | Lets the assembly dry to the interior | Provides no vapor control; it is a drying strategy, not a barrier | Hot-humid climates, by design |
Selection comes down to two questions in practice. Does the interior generate a lot of moisture, through food processing, livestock, washing bays or pools? Then go tighter, 0.02 perm, and seal everything. Does the building sit in a hot-humid climate and run air conditioning? Then ask the designer whether the assembly is meant to dry inwards, because that answer flips the choice from a barrier to a perforated facing.
For high-temperature service, the limiting factor is the polymer film and adhesive, not the foil. Check the continuous service temperature on the datasheet before putting a film-based facing on hot pipework or ceramic fiber.
One point of judgement, since specification sheets do not say this: PP scrim kraft facing is the boring answer, and it is usually the right one. Move to foil or metallized polyester when something specific about the project justifies it, not because a heavier-looking laminate feels safer.
Step 4: Confirm the fire classification on the laminate, not just the film
Where the facing is the exposed interior surface, it counts as an interior finish and must meet the class the code requires, which in most occupancies is Class A.
| Standard | Region | What it measures | Common requirement |
|---|---|---|---|
| ASTM E84 / UL 723 | North America | Flame spread index and smoke-developed index | Class A: FSI ≤ 25, SDI ≤ 450 |
| NFPA 701 | North America | Flame propagation of flexible materials | Pass, for loose-hung flexible facings |
| EN 13501-1 | Europe | Reaction to fire, Euroclass | Commonly B-s1,d0 or better |
| BS 476 Parts 6/7 | UK | Fire propagation and surface spread of flame | Class 0 |
| AS 1530.1 / AS/NZS 1530.3 | Australia / New Zealand | Combustibility and spread of flame | Non-combustible or low spread, per occupancy |
Ask for the test report on the laminated assembly as installed, not on the film alone. Adhesives and scrims change the result, and a fire-retardant additive that performs when new can lose effectiveness over time in a hot, humid building.
Step 5: Check the dew point at the coldest local surface, not the average
A common assumption is that a thicker blanket fixes condensation. It usually does not. Because the interior air film dominates the temperature drop once any insulation is present, the average interior surface temperature rises quickly and then flattens.
For an interior of 20 °C (68 °F) and an exterior of −5 °C (23 °F), with an interior surface film resistance of 0.12 m²·K/W:
| Blanket RSI (m²·K/W) | Approx. R-value | Interior surface temperature | Above 14.4 °C dew point (70 % RH)? |
|---|---|---|---|
| None (bare steel) | none | 0 °C (32 °F) | No, this is the dripping roof |
| 0.5 | R-2.8 | 15.4 °C (59.7 °F) | Yes, marginally |
| 1.0 | R-5.7 | 17.4 °C (63.3 °F) | Yes |
| 2.0 | R-11.4 | 18.6 °C (65.5 °F) | Yes |
| 3.35 | R-19 | 19.1 °C (66.4 °F) | Yes |
Calculated as T_surface = T_interior − ΔT × R_film / R_total. Illustrative only; use your actual design temperatures.
Read that table carefully, because it inverts the usual assumption. Going from RSI 1.0 to RSI 3.35 buys 1.7 K of surface temperature. Going from bare steel to RSI 0.5 buys 15.4 K. Once you have a modest blanket, thickness stops being the lever. Continuity and air sealing take over.
The places that still condense are local, and they are the ones that produce the classic symptoms:
- Purlins and girts. The steel bridge bypasses the blanket, so the local RSI is close to zero and the surface sits near outdoor temperature. This is why drip lines follow structural members.
- Compressed blanket. Insulation squeezed against framing loses thickness and R-value, leaving a cold strip.
- Sagged or gapped blanket. An air pocket between the warm interior and the cold panel becomes the dew-point zone, and the condensation is hidden.
- Unsealed seams. Moist air bypasses the facing entirely and condenses on the steel.
If your building drips after insulation was installed, look for these four before you specify anything thicker.
Step 6: Detail the seams, tabs and penetrations
This is where vapor control is won or lost. Facings are normally supplied about 150 mm (6 in) wider than the blanket, either as two 75 mm (3 in) tabs or one 150 mm (6 in) tab.
Two 75 mm tabs. Pull the tabs between adjacent runs, staple roughly every 200 mm (8 in), positioned 6 – 10 mm from the base of the tab, then fold the tab over and staple again between the first staples to halve the spacing. Do not staple right at the base. The staples pull out and the seal fails.
One 150 mm tab. Overlap it onto the adjacent run. For a sealed joint, apply double-faced tape under the tab before pressing it down; for a mechanical joint only, the overlap alone is acceptable where the design allows it.
Everywhere else. Seal roof-to-wall transitions, eaves and ridge details, door and window perimeters, service penetrations and untaped panel laps. Concentrate effort there rather than in the middle of a run. Match the tape to the facing: foil tape with an acrylic adhesive for foil facings. Cloth “duct” tape is not a vapor seal and does not hold in this application. Patch any tear the day it happens.
Failure modes to design out
Almost every dripping metal building traces back to one of the eight below. They are listed roughly in order of how often they turn up on site.
- Retarder on the wrong side. The most expensive common error. In a cold climate, an interior-side retarder is required; reversed, it traps moisture in the blanket.
- A second faced layer over an old faced layer. The original retarder ends up buried inside the insulation mass, where moisture collects with no drying path. Check the existing assembly before a retrofit.
- Two vapor-tight layers on opposite faces. Same mechanism as above, usually created by adding polythene sheeting over an already-faced blanket.
- Vinyl facing in cold handling. Impact and flexing at low temperature splits it; the damage is often invisible from the floor.
- Foil facing in a persistently damp, alkaline or chloride environment. Aluminium corrodes; use a metallized-polyester facing instead.
- Perforated facing where a barrier was required. A 10 – 50 perm facing is a drying strategy, not a vapor barrier.
- Expecting a reflective facing to add R-value. Without an air space there is no radiant benefit.
- Leaving seams open. The system performs to its weakest metre, not its best one.
Specification clause you can copy
Metal building insulation: mineral fiber blanket to ASTM C665 Type III, faced one side. Facing: [PSK / FSK / MSK], permeance ≤ 0.02 perm by ASTM E96 (state method), complying with ASTM C1136 Type I or II for puncture resistance and tensile strength. Surface burning characteristics Class A per ASTM E84 (flame spread ≤ 25, smoke developed ≤ 450) on the laminated assembly. Facing 150 mm wider than the blanket, supplied as [two 75 mm tabs / one 150 mm tab]; seams, penetrations, eaves, ridge and roof-to-wall transitions sealed with tape compatible with the facing. Submit: permeance test report, E84 report, roll width and length, tab configuration, and continuous service temperature of the facing.
For climate-specific projects add one line: “Retarder placed on the [interior / exterior] face of the blanket as indicated on the drawings.” Then make sure the drawings actually say which. That sentence prevents more callbacks than any other line in the spec.
FAQ
What perm rating should I specify for a metal building insulation facing?
Specify 0.10 perm or less as a baseline, and 0.02 perm where the interior runs humid, meaning food processing, livestock, washing bays or a pool. Above 1.0 perm the material is not a vapor retarder and gives no meaningful moisture protection.
Should the facing face inward or outward?
In heating-dominated climates, inward: the retarder must sit where vapor pressure is highest, which is the warm interior side. In hot-humid climates the vapor drive reverses and the retarder often goes on the panel side, with a perforated facing on the interior to allow drying. Cold stores need a retarder on both sides.
Is FSK better than PSK for a metal building?
Not universally. Foil-scrim-kraft gives the lowest permeance and the highest reflectivity, but aluminium corrodes in persistently damp, alkaline or chloride-laden environments. PP scrim kraft facing is the industry default because it is tough, bright and economical. Choose foil where reflectivity or the tightest vapor control matters, and a metallized-polyester facing where corrosion is the risk. The full comparison is in our article on FSK or ASJ facing.
Can I install new faced insulation over the existing faced blanket?
Usually not without a plan. A second faced layer buries the original retarder inside the insulation, where moisture collects with no route out. Either remove the old facing, slit it, or use a retrofit system designed for it. Inspect the existing assembly first.
Do I need a separate polyethylene sheet as well as the facing?
Rarely. A faced blanket already carries the vapor control layer, and adding a second vapor-tight layer on the opposite face traps moisture between them. A separate sheet belongs in a designed assembly, not as insurance.
Does a reflective foil facing replace insulation?
No. Reflective surfaces only perform when facing an air space; pressed against steel or a liner panel they add almost nothing thermally. The R-value comes from the blanket. See our guide to radiant barrier foil for where it does apply.
Why does my building still drip after the insulation was installed?
Four causes cover almost every case: unsealed seams letting moist air bypass the facing, a compressed or gapped blanket leaving a cold strip, thermal bridging at purlins and girts, and an old retarder buried in the insulation when the retrofit added a second faced layer. Adding thickness rarely fixes any of them.
Which standards should a metal building insulation facing be tested to?
ASTM E96 for permeance, ASTM C1136 for permeance plus puncture and tensile strength, ASTM E84 (or EN 13501-1, BS 476 or AS 1530 in other markets) for fire, and ASTM C665 for the faced blanket itself. Ask for reports on the laminated assembly, not the film alone.
Next step
Pick the permeance and the placement first, since those two drive everything else. Then match the construction to your climate and fire requirement, and spend whatever effort is left on the seam detailing. Done in that order, the roof stays dry through its first cold snap. Done backwards, you get to re-insulate it.
