Metal Building Insulation Options Compared: Spray Foam, Fiberglass Blanket, Board and Panels
The short answer by use
There is no single best way to insulate a metal building. Metal building insulation comes in six common forms, and the right choice depends on what happens inside it and whether the building is already standing.
- Heated, or damp inside (a shop, a barn with animals, an office): closed-cell spray foam on the steel is usually the strongest fit, because it insulates, air-seals and keeps damp air off the metal in one layer.
- Unheated storage with no dripping: often a faced blanket, or nothing at all. Do not pay to insulate a building that has no problem to solve.
- Not built yet: decide at order time. Factory blanket or liner systems and insulated metal panels go in while the building goes up, before the panels make the frame hard to reach.
- Standing, with the panels staying on: your choices narrow to what can go on from inside: spray foam, a liner system hung from the purlins, or board on the walls.
Our crews install one thing: closed-cell spray foam. We do not sell or install blankets, liner systems, board, panels or foil. This page compares all of them anyway, so you can see where foam fits and where it does not.
Metal building insulation options matrix
Each row is one way to insulate a steel building. "Rated R" is the number on the product; "whole-assembly" values are the published default results once the roof is put together. Every row ends with when to pick it.
| Option | How it goes in | R you actually get | Air sealing | Condensation control | Can it go into a standing building? | Pick this when |
|---|---|---|---|---|---|---|
| Faced fiberglass blanket (draped over purlins and girts) | Rolled across the frame, then the steel panels are screwed down on top of it (NAIMA MB316) | Less than rated: the blanket is squashed where panel meets purlin. A rated R-10 blanket in a through-fastened roof has a default U-factor of 0.184, about R-5.4 for the whole roof (Seattle Energy Code 2021, App. A, Table A102.2.5) | Only as good as the taped and stapled seams of the facing (MB316) | The facing is the vapor retarder; it works while it stays continuous and undamaged (NAIMA MB306) | Not without taking panels off; it has to sit between panel and frame | The building is being built or re-roofed, it is lightly heated or not heated, and the facing will be sealed and kept intact |
| Liner system (a membrane below the purlins, unfaced blanket on top) | A continuous membrane is installed below the purlins and the blanket rests on it, uncompressed between the purlins (Seattle App. A, A102.2.5.4) | Much closer to rated. An R-19 + R-11 liner system in a through-fastened roof has a default U-factor of 0.044, about R-22.7 for the whole roof (same table) | Depends on the membrane being continuous and sealed at the edges | Good while the membrane stays intact as the vapor retarder | Often yes: support systems that hang from the bottom of existing purlins exist for buildings whose roof panels block access from above (US patent 4,375,742; Metal Architecture, 2019) | You want a high roof R-value with a finished membrane ceiling, the roof panels are staying on, and the walls can be handled another way |
| Rigid foam board | Sheets fastened to the frame or laid over a solid roof deck; continuous board is "uninterrupted by framing" in the code tables (Seattle App. A); one of the standard insulation types (DOE Energy Saver fact sheet) | The board's labeled R, when it is continuous and not crushed | Only at taped or sealed joints; every board edge is a seam | Works where the joints are sealed so damp air cannot get behind it to the steel | Walls from inside, yes, fastened to the girts; roofs usually during a re-roof over a new solid deck (GreenBuildingAdvisor, 2013) | You are re-roofing over a solid deck, or you want a hard, flat surface on the walls and will seal every joint |
| Insulated metal panels | Factory panels: two metal skins bonded to a foam core, installed as the wall or roof itself (Metal Construction Association, 2012) | Continuous insulation with consistent R across the wall (same source); the rated value comes from the panel maker by thickness (Metal Construction News, 2013) | Panel joints are factory-made and tested for air and water (MCN, 2013) | Strong: the foam core sits between the skins, with concealed fasteners that limit thermal bridging (MCA, 2012) | Only by replacing the cladding; it is the wall, not something added to it | You are ordering a new building or re-siding and re-roofing anyway, and want the insulation and the finished panel in one step |
| Reflective (bubble foil, radiant barrier) | Rolls of foil-faced material hung under the roof or across the frame (DOE Energy Saver fact sheet) | Depends on an air space next to the foil; the federal R-value rule bases a reflective product's R-value on the air space beside it (16 CFR 460.5) | Only at sealed seams and edges | Can warm the inside surface and act as a vapor retarder when installed properly (RIMA condensation FAQ); gaps at seams let damp air through | Yes, with retrofit clips on purlins and girts (Metal Architecture, 2019) | Summer radiant heat from the roof is the main problem and the product can be hung with the air space its R-value depends on; radiant barriers are "especially useful in hot, sunny climates" (DOE) |
| Closed-cell spray foam (what we install) | Sprayed as a liquid onto the inside of the steel, where it expands and bonds (DOE Building America guide, 2009) | The labeled R for the thickness sprayed, from the product's data sheet; at the frame there is no squashed layer, because the foam covers the purlins and girts too | Yes, as part of the same layer; it fills laps and gaps as it goes (DOE guide, 2009) | At typical thickness, closed-cell foam counts as a vapor retarder (same source); it stops the drip when it is continuous and thick enough (GreenBuildingAdvisor, 2013) | Yes, from inside, onto clean steel; old blanket or foil comes out first | The building is heated or damp, it is already standing with the panels staying on, or it drips and you want one layer that handles heat, air and moisture |
About the R numbers. The whole-roof figures are the default U-factors for metal building roofs in Appendix A of the 2021 Seattle Energy Code, a code document anyone can read free (whole-assembly R is roughly 1 divided by U). A U-factor describes how the roof is built, not the local weather, so the same roof build-up has the same U-factor in Kansas. They are published defaults, not field tests, and the code your city or county adopted is what applies to your permit. They are here to show one thing: where the blanket gets squashed, you get much less than the number on the roll.
Blanket vs liner system, same roof
| Through-fastened roof assembly | Default U-factor | About this whole-roof R |
|---|---|---|
| Single layer, rated R-10 blanket | 0.184 | 5.4 |
| Single layer, rated R-19 blanket | 0.151 | 6.6 |
| Liner system, rated R-19 + R-11 | 0.044 | 22.7 |
Source: Seattle Energy Code 2021, Appendix A, Table A102.2.5, "Thru-Fastened Roofs without Thermal Spacer Blocks" rows. Whole-roof R is our arithmetic (1/U, rounded).
The lesson for a standing building: a thicker blanket draped over the purlins barely moves the result, while keeping the insulation from being squashed moves it a lot. That is the case for a liner system or for spray foam, which also has no squashed layer at the frame. The fiberglass side of that argument in depth is on spray foam vs fiberglass blanket.
Pick by your situation
You weld, grind or work in the building on cold days. You want heat kept in and no drip on your work. Spray foam or a liner system fits. Foam near hot work has its own rules: see insulating a heated workshop.
It is cold storage and the roof drips on equipment. The problem is moisture, not heat. Fix the moisture sources first, then insulate the roof steel if it still drips. Foam on the roof is covered on foam on the underside of a metal roof.
The building is not ordered yet. This is the only time insulated metal panels or a factory liner system are an easy add. Compare them with spraying later on ordering a new metal building.
It already has old faced blanket that is sagging or wet. Every option except a liner system hung below it means the old blanket comes out. PolyJacks does not remove insulation; metal building spray foam explains what has to happen before foam goes on.
It is a warehouse or other commercial building. Submittals, fire-code listings and working around people inside drive the choice: spray foam for warehouses and commercial buildings.
It is a quonset or arch building. The curved shell has no separate flat walls to hang a blanket or fasten board to, which narrows the options: arch steel building insulation.
It is a metal garage or carport kit. Light framing and a small space change the math: insulating a metal garage kit.
It is wood post-frame, not steel. The options are similar but the framing and code type are not: the best way to insulate a pole barn.
Any option that uses foam plastic, board, panels or spray, has fire-code rules that start with the building's construction type. That code path is on metal building spray foam.
What moves the size of a spray foam quote
We do not publish prices, because area, thickness, prep and access change every job. What drives the number is explained on what drives spray foam cost, and how much foam a steel shop takes is on what a steel shop takes in board feet.
Head-to-head pages
Each comparison above has a full page:
- spray foam vs fiberglass blanket: compression, facings and what the blanket's R-value becomes once installed.
- bubble foil vs spray foam: what reflective products do and the air space they depend on.
- foam board in a pole barn: board against spray in post-frame and metal buildings.
- open-cell vs closed-cell foam: why we spray closed-cell only.
FAQ
What is the best way to insulate a metal building?
It depends on use: a heated or humid building usually calls for closed-cell foam on the steel, while unheated storage often needs only a cheaper blanket, or nothing. If the building is not ordered yet, decide at order time, when panels and liner systems are easiest to add.
Can you insulate a metal building from the outside?
Yes, during a re-roof or re-side, with insulated metal panels or rigid board over a new deck, which is roof and siding work, not spray foam. We spray interiors only. If the outside panels are staying on, the choices are the inside options in the matrix: spray foam, a liner system or board on the walls.
Book a free on-site measurement
If spray foam looks like the right fit, the next step is a visit. We measure the building, look at what is there now and how you use it, and give you a written estimate. If foam is not the right answer for your building, we will say so. The visit is free; more on it at the free estimate visit.
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Pick "Spray foam insulation" on the booking form. We work within about 35 miles of Spring Hill, KS.
Spray foam insulation is a new PolyJacks service line run by its own crew and rig.
Who wrote this: PolyJacks spray foam team. Last updated: October 27, 2026. How it was made: drafted with AI assistance from the sources below, then checked against each source.
Sources
- NAIMA, Recommendations for Installing Fiber Glass Insulation in Metal Buildings (MB316): insulationinstitute.org
- NAIMA, Insulation Facts #26, Moisture and Fiber Glass Metal Building Insulation (MB306): insulationinstitute.org
- Seattle Energy Code 2021, Appendix A (A102.2.5, A102.2.5.4, Table A102.2.5): seattle.gov
- U.S. Department of Energy, Energy Saver fact sheet, Home Insulation (2021): energy.gov
- U.S. Department of Energy, Building America, "Which Spray Foam Is Right For You?" (Steven Winter Associates, 2009): www1.eere.energy.gov
- Metal Construction Association, "Top Reasons to Use Insulated Metal Panels," December 18, 2012: blog.metalconstruction.org
- Jonathan McGaha, "Insulated metal panels have many advantages," Metal Construction News, June 30, 2013: metalconstructionnews.com
- 16 CFR 460.5, R-value tests (FTC R-value Rule), Cornell LII: law.cornell.edu
- Reflective Insulation Manufacturers Association International, Condensation FAQ: rimainternational.org
- US Patent 4,375,742, Roof insulation support system: patents.google.com
- Metal Architecture, "Insulation uses clip-and-pin system," November 1, 2019: metalarchitecture.com
- GreenBuildingAdvisor, "Metal building and condensation" (Q&A, January 14, 2013): greenbuildingadvisor.com
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