How to Install Vapor Barrier Insulation in an Erected Steel Building

Ideally, one should roll the fiberglass vapor barrier insulation over the purlins and girts during the erection process, then pin it down by installing the sheeting. However, if the steel building already exists, it is not a big deal to do afterward — though it will be a little more challenging because you need to work around the framing and ensure you maintain the vapor barrier integrity. Here’s a step-by-step guide tailored to retrofitting an existing steel structure:

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Items needed to get started correctly:

It is better to have all required materials present instead of acquiring them after you start the installation process.

• Faced fiberglass insulation rolls or batts (usually with a vinyl or foil vapor barrier)
• Insulation support bands and self-taping screws & washers (metal strapping or steel wire or mesh)
• A Vapor barrier tape matching the type of insulation facing utilized
• Utility knife or insulation cutter
• Scaffolding, ladder, or lift if needed
• Protective gear such as gloves, goggles, proper clothing, etc.

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 A Simple Step-by-Step Installation Process

1. Calculate the square footage of insulation needed to complete the project

• Measure spacing between steel members (usually 4, 5′ or 6′ apart).
• Measure wall height and roof pitch if applicable.
• Order insulation length accordingly or cut rolls at the time of installation.

2. Install Your Banding Across the I-beams to support the insulation from sagging or shifting

• Run metal strapping or steel wire horizontally across the I-beams, starting near the base and spacing it vertically approximately every 18–24 inches.
• For roof insulation, install support bands perpendicular to purlins to hold insulation.
• Secure with self-tapping screws and washers.

3. Ideally, order fiberglass pre-cut sizes to fit the spacings

• Use pre-cut rolls sized to bay widths (or cut to fit).
• Place insulation rolls facing the vapor barrier toward the interior.
• Install insulation into each cavity behind the support banding.

4. Seal the Vapor Barrier

• Overlap the facing of adjacent insulation pieces by 2″–3″.
• Seal all seams with vapor barrier tape to maintain an airtight, moisture-resistant seal.
• Pay extra attention to corners, edges, and around any penetrations (doors, windows, conduits, etc.).

5. Insulate the Ceiling or Roof Panels

• If accessible, apply insulation between purlins, held in place by support bands.
• Faced side should again face the interior.
• Seal all seams along the purlins and at overlaps.

6. Trim Around Obstructions

• Cut and fit insulation around I-beam flanges, braces, doors, windows, outlets, etc.
• Use tape to patch and seal any cut areas in the vapor barrier.

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For Best Results:

Add thermal breaks if condensation is a concern (like thermal blocks or foam board between metal sheeting and insulation). Before installing fiberglass insulation, apply double-sided tape to the steel frames. The first step is to thoroughly clean the steel members before applying tape. Once this step is finished, please take the time to cut the tape to the correct length needed. The tape will have a backing that will need to be peeled off on one side so that it can adhere tightly to the frames. Once all desired steel sections have tape applied, could you peel off the remaining backing? This will expose the adhesive. Now, you will meticulously position the fiberglass insulation onto the exposed tape. Before proceeding, make sure it sticks securely to the steel frame.

Start from the top down for walls to reduce sag.

Use faced insulation rolls with integrated tabs for stapling or overlapping.

Avoid compressing fiberglass too much — it lowers the R-value.

Ensure no gaps or unsealed seams, especially near roof-to-wall transitions.

Installation Process (New Construction) for Single Layer Fiberglass Steel Building Insulation

1. You simply place the insulation roll over the top of the roof purlins and / or wall girts. This is done prior to the exterior metal panels being installed.
2. The fiberglass rolls are essentially sandwiched between the interior metal and the exterior sheeting.
3. The vapor barrier (usually white finish) is faced towards the inside of the building, giving the building a clean finished look. We always Re-enforce this facing so that our customers have the sturdiest backing. With a single layer application, you can achieve R-Values up to R-19, but not higher.
4. The 3” tabs are to be rolled over and stapled. Orders come with double-sided tape for easy adherence to building, and a small supply of patch tape to assist with minor tears if needed

In addition to offering low steel building insulation prices, a representative is available to assist you 7 days a week. Factory Steel Overstock presents You will need to get a custom shipping cost for these states: CA, ID, MN, MT, NV, ND, SD, WI, and WY. For questions, call 1-888-512-1116.