To insulate a metal building, most builders roll faced fiberglass blanket over the roof purlins and down the outside of the wall girts, with the vapor retarder facing the inside, before the metal panels go on. The two things that decide whether it works are a continuous, sealed vapor retarder (to stop condensation) and keeping the blanket from being crushed flat where it passes over the steel framing.
This guide walks through the materials, the standard roof and wall methods, how to get more R-value than a single layer allows, what to do in an existing building, and the mistakes that lead to dripping roofs.
Why Metal Buildings Need Insulation
A bare steel roof and wall panel lets heat through quickly and gets cold in winter. When warm, moist inside air touches that cold steel, water condenses on it and drips. The North American Insulation Manufacturers Association (NAIMA) lists four jobs for metal building insulation: controlling heat flow, preventing condensation (when installed with a vapor retarder), reducing noise, and improving lighting by reflecting light off its white facing (NAIMA, Recommendations for Installing Fiber Glass Insulation in Metal Buildings, MB316, checked October 2026).
For a heated or cooled building, energy codes also apply. NAIMA points to ASHRAE 90.1 and the IECC, which set required U-values for roof and wall assemblies by climate. The U-value describes the whole assembly, including the framing and fasteners, not just the R-value printed on the roll. Check your local code before ordering.
The Materials
Faced fiberglass blanket
The standard product is flexible fiberglass made for metal buildings and laminated with a facing. In the US it is covered by ASTM C991, the specification for flexible fibrous glass insulation for metal buildings. Its scope notes that the listed thermal values are for the insulation alone and do not include compression at framing or heat flow through fasteners (ASTM C991-23 scope, checked October 2026). NAIMA also warns that residential-grade or unmarked insulation is not designed for metal buildings; look for the NAIMA 202-96 marking on the blanket.
The facing (vapor retarder)
The facing is what you see from inside the building, and it is also the vapor retarder. The lower its perm rating, the less water vapor passes through. NAIMA gives these typical ratings and minimum installation temperatures:
| Facing | Perm rating | Min. workability temperature |
|---|---|---|
| Vinyl | 1.0 | 40 F |
| Polypropylene/scrim/kraft (PSK) | 0.02-0.09 | 20 F |
| Foil/scrim/kraft (FSK) | 0.02 | 10 F |
| Polypropylene/scrim/foil (PSF) | 0.02 | 20 F |
| Vinyl/scrim/metallized polyester (VRP) | 0.02 | 20 F |
NAIMA treats 1.0 perm or less as a vapor retarder and recommends 0.02-perm facings where interior humidity is high (NAIMA, Facts About Moisture and Fiber Glass Metal Building Insulation, MB306, checked October 2026). Plain vinyl is the minimum; a reinforced scrim facing also resists tearing during installation.
Thermal spacer blocks
Where the blanket passes over a purlin, the roof panel crushes it almost flat, and the steel conducts heat straight through that thin spot. Rigid polystyrene foam blocks laid on top of the insulation along each purlin hold the panel up and reduce that loss.
How to Insulate the Roof
This is the over-the-purlin method described in NAIMA’s MB316 guide. It is done as the roof panels go on, one run at a time. What purlins are and how they work is explained in what is a purlin.
- Order rolls to length. Each run should reach from eave to eave plus about 12 inches of overhang at each end. Use a ridge pan if one roll cannot span the whole roof. Make the first run about one foot wider than a roof panel, so blanket seams and panel seams do not line up.
- Fix one end. Starting at an end wall, secure the blanket to the eave strut with spray adhesive, double-faced tape or a mechanical fastener.
- Roll it out facing down. Unroll across the purlins with the vapor retarder toward the inside of the building.
- Keep it taut, not stretched. Hold tension while the panels are fastened so the blanket does not sag into big voids, but do not overstretch it. NAIMA notes that overstretching over-compresses the blanket and reduces its R-value.
- Add thermal blocks on top of the blanket along each purlin if your system uses them.
- Fix the far eave the same way, then lay the next run tightly butted to the first.
- Seal the tabs. Facings usually have extra width at the edges. Two 3-inch tabs are stapled together about every 4 inches, folded and stapled again; a single 6-inch tab is glued over the neighboring roll. Patch any tear with matching facing tape.
On standing seam roofs, the clips hold the roof sheet above the purlin, so the blanket is crushed less and thermal blocks may be optional.
How to Insulate the Walls
- Cut each piece from the eave strut (or rake) to the base angle plus at least 12 inches.
- Fix the top to the eave strut with clamps or double-faced tape, facing toward the inside.
- Pull it tight down the outside of the girts.
- At the bottom, cut back about 6 inches of fiberglass from the facing without cutting the facing, fold the facing up and staple it to the side tabs, then tape it to the base angle. NAIMA recommends keeping the bottom of the insulation half an inch above the base flashing so it does not wick water.
- Fasten the wall panel, then repeat with the next run, sealing tabs as on the roof. Use foam or rubber closures at the base where there is no trim or where rodents are a concern.
Getting More R-Value: Two-Layer Systems
NAIMA notes that both two-layer systems give a significant increase in thermal resistance over a single layer, which matters when a code U-value is hard to reach with one compressed blanket. It describes two methods:
- Drape and fill. Install the faced layer over the purlins but let it sag slightly between them, then lay a second, unfaced layer parallel to and between the purlins on top of it.
- Liner system. Fasten a faced rigid board, or a support grid of banding, to the underside of the purlins, then fill the full depth between the purlins with unfaced blanket. Thermal blocks go on top of the purlins under the roof panel.
Some of these support systems are patented and their use may be restricted. NAIMA advises consulting the building manufacturer before choosing a method, because some manufacturers require specific installation procedures to keep the warranty. How the frame, purlins and girts of these buildings fit together is covered in pre-engineered metal buildings.
Insulating an Existing Metal Building
Once the panels are on, you cannot roll blanket over the purlins without taking the roof off. The usual options from inside are:
- Liner or banding system under the purlins, filled with unfaced or faced blanket, similar to the two-layer method above.
- Rigid foam or faced rigid board fastened to the girts and purlins, with joints taped to form the vapor retarder.
- Spray foam applied to the underside of the panels by a specialist contractor.
Whatever you choose, fix roof leaks first and make sure the steel is dry. NAIMA’s moisture guidance is blunt: wet fiberglass loses thermal performance, and insulation soaked by storm or flood water should be removed and replaced rather than left to dry. Before hanging anything new from the purlins, such as a heavy liner, ductwork or a ceiling, confirm with the building manufacturer or a licensed structural engineer that the purlins can carry the added load. Purlins and their bracing are sized for the original design loads only.
Common Mistakes That Cause Condensation
- Facing on the wrong side. The vapor retarder belongs on the warm-in-winter side, which for most buildings is the inside.
- Open seams and tears. Unsealed tabs, rips and unsealed penetrations let moist air reach the cold panel.
- Insulation too thin. NAIMA notes the blanket must be thick enough to keep the facing surface above the indoor dew point; the thickness needed to stop condensation can differ from the thickness needed to save energy.
- Installing wet or dirty insulation, or leaving rolls on the slab overnight uncovered. Store rolls off the ground, dry and covered.
- Working in the cold. Below the facing’s workability temperature (40 F for vinyl), the facing can crack.
Roof work on a metal building is work at height. OSHA’s steel erection rules require fall protection for workers on unprotected edges more than 15 feet above a lower level (OSHA 29 CFR 1926.760, checked October 2026). Site safety during steel building work is covered more fully in structural steel erection.
Metal Building Insulation FAQ
What is the best insulation for a metal building?
For new buildings, faced fiberglass blanket made for metal buildings is the standard because it goes in with the panels, controls condensation and acts as the interior finish. For existing buildings, liner systems, rigid board or spray foam are the practical options.
Which way does the vapor barrier face in a metal building?
Toward the inside of the building in almost all cases, which is the warm side in winter.
Can I use regular house insulation in a metal building?
NAIMA says residential-grade or unmarked insulation is not designed for metal buildings. Metal building blanket is made to recover its thickness after lamination and compression, and comes in custom widths and lengths for purlin and girt spacing.
Why is my metal building roof dripping?
Usually condensation: warm, moist air is reaching the cold underside of the roof through missing insulation, open seams or a torn facing. A roof leak can look the same, so check both.
Do I need thermal blocks?
They reduce heat loss where the blanket is crushed between the purlins and a through-fastened roof sheet. On standing seam roofs NAIMA says they may be optional. Follow the building manufacturer’s details.
Reference information only, not engineering advice. Disclaimer.