Blanket insulation on steel: what the rolls do and don't do
“Blanket insulation” is the metal-building industry’s default: wide faced rolls of fiberglass, run over the purlins and girts as the roof and wall sheets go down, so the building arrives insulated the day it’s finished. Most guides on it are written by someone selling the rolls or the building. We sell neither — here’s what the product genuinely does well, and where its limits show up a few winters in.
What it is, mechanically
Fiberglass blanket for steel buildings comes faced on one side — a vinyl or reinforced facing that serves as the interior finish, a modest vapor retarder, and the thing holding the fluff up. The rolls compress under each roof sheet at every purlin, drape between them, and get taped or lapped at the seams. Installed with the shell, it’s fast and cheap per square foot — which is exactly why it’s the standard spec on pre-engineered buildings.
Where it earns its keep
- New construction, basic thermal goals. Taking a building from raw steel to “doesn’t radiate every temperature swing straight inside” is what blanket does at the lowest cost, with no extra trades involved.
- Unconditioned and lightly heated space. Cold storage, equipment sheds, a shop you heat occasionally — the economics are hard to argue with.
- The facing as a finish. A bright, wipeable interior surface comes free with the insulation. On a budget build, that matters.
Where it quietly falls short
None of these are defects — they’re the physics of the product, and they’re worth knowing before the building goes up:
- Compression is invisible R-loss. A blanket’s rated R-value assumes full loft. At every purlin and girt the roll is squeezed nearly flat between steel and steel — so the real installed performance is a striped pattern: rated R between the frames, a fraction of it along every line where the frame touches sheet. Steel conducts heat readily, and those lines are also exactly where interior moisture finds cold metal.
- It doesn’t air-seal. Seams, laps, punctures from later wiring and screws, facing tears from a decade of use — each one is a path for humid inside air to reach cold steel. Blanket slows heat; it does not stop air. On a conditioned building, air is the fight (why that matters: condensation and venting, explained).
- The facing is the vapor strategy. As long as it’s intact, it helps. Every tear, tape failure, and fastener hole degrades it — and on a working building, tears accumulate. A sagging, torn blanket roof isn’t just ugly; it’s the vapor plan coming apart.
- Time works on it. Fiberglass sags, facings loosen, birds and mice find the gaps. The product’s tenth year rarely resembles its first.
Double-layer and banded liner systems exist for new builds precisely to address the compression and sag problems — better performance, more cost, still a during-construction decision.
Blanket or foam? The honest comparison
This is the fork most owners actually face, and the answer isn’t a product — it’s the building’s job description. A budget build you won’t condition: blanket during construction is hard to beat, and paying triple for foam buys performance you may never use. A building you’ll heat or cool, a humid use like animals or washing equipment, or steel that’s already dripping: blanket’s air-sealing limits are the exact thing closed-cell foam was built to fix — with its own tradeoffs, cost and permanence chief among them. The full comparison, including when foam is not the answer, lives in the metal building insulation guide; if your building is post-frame with wood framing, the options shift — see the pole barn guide.
Two situations deserve their own pages: insulating a building that’s already up (blanket mostly isn’t the tool — the retrofit guide covers the order of operations), and foaming over old blanket (a substrate question your installer must answer in writing — the FAQ above says why).
The question to ask before signing anything
Whoever quotes your building — blanket, foam, board, or liner system — ask them one thing: “Where does inside air touch cold steel after your product is installed, and what happens to the moisture in it?” A good answer names the purlin lines, the seams, and the plan for them. A shrug means the condensation problem is being left for you to discover. If you’d rather have someone in our network look at the specific building, that’s what the form below is for.
Questions people actually ask
Is blanket insulation good enough for a metal building?
For an unconditioned or lightly heated building insulated during construction, usually yes — that's the job it was designed for. For a building you'll condition, a shop you'll heat daily, or a steel roof already dripping condensation, its air-sealing limits start to matter, and the comparison in our metal building insulation guide is the honest place to weigh it.
Why is my insulated metal building still sweating?
Blanket controls heat flow, not air. Wherever humid inside air can still reach steel that's below the dew point — torn facing, gaps at seams, compressed spots at the purlins — condensation continues. The mechanics are laid out in our condensation guide.
Can blanket insulation be added to a building that's already up?
It's a during-construction product — the rolls go on as the sheeting goes down. On an existing building, retrofitting it means working against the panels instead of with them, and other approaches usually fit better; see how to insulate an existing metal building.
Should old blanket come out before spray foam goes in?
Foam bonds to what it touches, and it inherits the condition of whatever it covers. Loose, damp, or torn blanket under new foam is a substrate problem waiting to show up later — the failure patterns in our problems guide include exactly this kind of story. Any installer quoting your job should address the old material in writing.