Vapor Control in Metal Buildings: Don’t Let Condensation Win — Insulation Systems That Work

Condensation in metal buildings isn’t just a nuisance. It wrecks finishes, grows mold, and corrodes steel. The fix is not one product; it’s a system: insulation + air sealing + vapor control + detailing. Here’s the shop‑floor guide we use to design, bid, and build PEMB roofs and walls that stay dry in Colorado’s freeze–thaw climate.

Why metal buildings sweat (fast physics)

Warm, moist air meets a cold metal skin → air cools → hits dew point → water condenses on the underside of roof/wall panels. Two culprits dominate:

• Air leakage carrying moisture into the assembly (bigger impact than diffusion)

• Thermal bridges (purlins/girts/fasteners) creating cold stripes where dew forms

Translation: control air, manage vapor, and cut thermal bridges.

Step 1 — Classify the building (drives the whole design)

• A. Unconditioned storage (≤ 40–50°F winter, no RH control): prioritize ventilation + basic condensation control.

• B. Semi‑conditioned shop (heated to 55–65°F, variable RH): needs continuous vapor retarder + higher R + good air sealing.

• C. Conditioned occupancy (65–72°F, RH targets for people/process): treat like a commercial envelope: continuous air/vapor control, high R, and commissioning.

Pick the class now. Don’t let “maybe we’ll heat it later” drive costly rework.

Roof systems — what actually works

1) Single‑layer vinyl‑faced blanket (a.k.a. bag‑and‑sag)

• Use: Unconditioned/lightly heated only.

• Risk: Discontinuous vapor retarder (at seams/tears); compressed R at purlins → cold stripes & drips.

• Pro tip: If used, add ridge vents and eave inlets for air exchange.

  
  

2) Long‑tab banded double‑layer blanket

• Use: Semi‑conditioned shops.

• How: Two layers over purlins, lower layer continuous with sealed tabs; upper layer perpendicular for R.

• Strength: Better R, fewer gaps; can hit R‑25–R‑30 nominal.

• Detailing must: Seal laps (tape + pressure), seal around penetrations, and use thermal blocks at purlins under roof sheets.

  
  

3) Liner panel system (interior metal liner + cavity insulation)

• Use: Semi‑ to fully conditioned spaces needing durability and continuous vapor retarder.

• Strength: The liner panel is the air/vapor layer; easy cleaning; protects insulation.

• Watch: Seal every joint and fastener; detail at eave/ridge and RTU curbs.

  
  

4) Insulated Metal Panels (IMPs)

• Use: Conditioned occupancies or strict humidity control.

• Strength: Factory‑laminated continuous insulation and air/vapor control in one; excellent thermal break.

• Watch: Joint sealant gaskets must be continuous; field cuts and terminations need manufactured trims and sealants.

  
  

Wall systems — align with the roof choice

• Blanket + compression for unconditioned or light duty; add interior FRP/liner where abuse is likely.

• Liner panel walls for shops/clinics: tough interior finish + sealed air/vapor layer.

• IMP walls for best performance and speed; integrate with door/overhead door frames using manufacturer jamb kits.

  
  

Vapor retarder placement (keep it on the warm‑in‑winter side)

• In Colorado, the vapor retarder belongs at or near the interior face of the assembly.

• Never sandwich a second, tighter retarder outside—risk of trapping moisture.

• Seams, penetrations, and transitions (eave/rake/ridge) must be taped, sealed, and mechanically secured.

  
  

Air sealing beats vapor control (most of the time)

• Seal RTU curbs, skylights, pipe/duct penetrations, and panel laps that cross the interior air layer.

• Use butyl tape + closure strips + approved sealants at panel seams.

• Add backer rod + sealant at liner panel joints; gasket fasteners where required.

  
  

Thermal bridges & accessories

• Install thermal blocks over purlins before roof sheets; consider thermal clips for girts.

• Use breaks at overhead door jambs/sills; insulate and seal hollow sections.

• Don’t skip snow/ice guards layout—water migration can find tiny gaps.

  
  

Dew point: a quick check you can do

You don’t need a full hygrothermal model to catch major risks:

1. Note interior setpoints (°F and %RH) and outdoor design winter.

2. Use a dew‑point chart/app; ensure the interior air/vapor layer temperature stays above dew point.

3. If it won’t: increase R, improve thermal breaks, or lower interior RH.

Rules of thumb:

• Shops at 60°F/40% RH in Denver winters demand continuous air/vapor control + R‑30 roof equivalent.

• New slabs raise interior RH for months—budget temporary dehumidification.

  
  

Details that make or break performance

• Ridge: Closures + tape under cap; tie interior air/vapor layer into ridge details.

• Eave: Seal liner/vapor layer to eave strut; terminate cleanly behind gutters.

• Rake/Endwalls: Seal returns; don’t leave exposed insulation; add trim back‑pans.

• Penetrations: Pre‑flashed curbs preferred; for small pipes, use EPDM boots + sealant consistent with panel chemistry.

• Fasteners: Correct length and drive torque; over‑driven screws crush insulation and create cold points.

  
  

Ventilation & dehumidification

• Unconditioned: Ridge vents + eave inlets (balanced), protect against wind‑driven rain.

• Semi/conditioned: Ventilation alone won’t solve moisture; control interior RH with exhaust or portable dehumidifiers during startup and wet trades.

  
  

Preconstruction submittals we require

• Assembly sections (roof & wall) calling out air/vapor layer continuity.

• Insulation layout (layering, thickness, tabs, banding spacing).

• Thermal block locations and thickness.

• Manufacturer details for ridge/eave/rake/curbs + sealants and closure schedules.

• Fastener schedule and pull‑out data; statement of compatibility for sealants/adhesives.

  
  

QC & commissioning checklist (field)

• Visual check of sealed seams and taped laps—no holidays or fishmouths.

• Confirm thermal blocks at every purlin line; photo log before panel install.

• Hose test at curbs/penetrations after sealant cure.

• IR camera on a cold morning to spot cold stripes (bridges) and unsealed joints.

• Monitor interior RH during first heating season; add portable dehumidifiers if above target.

  
  

Common pitfalls (and cures)

• “Bag‑and‑sag everywhere.” Cure: upgrade to double‑layer or liner/IMP for heated spaces.

• Unsealed RTU curbs. Cure: factory curbs + fully sealed liner tie‑in.

• Compressed insulation at eaves. Cure: maintain thickness with proper trims and blocks.

• No plan for wet trades. Cure: dehumidification during slab cure/paint.

• Mismatched sealants. Cure: use manufacturer‑approved sealants compatible with panel coatings and EPDM.

  
  

Quick selection guide (use this matrix)

Building Type	Good	Better	Best
Cold storage / unheated storage	Single‑layer + ridge/eave vent	Double‑layer banded	Liner roof + liner walls
Heated shop (55–65°F)	Double‑layer banded + thermal blocks	Liner roof + liner walls	IMP walls + banded roof
Conditioned occupancy (65–72°F)	Liner roof/walls (sealed)	IMP walls + banded roof (thermal blocks)	Full IMP roof + walls

What Novus delivers on PEMB vapor control

• Early envelope workshop (setpoints, use case, budget)

• Dew‑point sanity check + product/system options

• Manufacturer‑aligned details for ridge/eave/rake/curbs

• Field QC plan with photo logs and IR scan option

CTA: Planning a PEMB or metal‑skin retrofit? Ask us for a one‑page vapor control plan; system pick, detail pack, and a startup dehumidification plan tailored to your building class.