Insulating a metal roof is a different problem from insulating a membrane roof, and the reason is the panel itself. A membrane sits on top of its insulation on a solid deck, warm and dry, with nothing passing through it. A metal panel is a thin cold sheet spanning an open steel frame, fixed or clipped at close centers, expanding and contracting every day, with moisture forming on its underside whenever the weather is right for it.
So the questions change. On a flat roof the argument is about thickness, cover boards and drainage. On a panel roof it is about where the insulation sits relative to the purlins, how the steel path through it is interrupted, and what happens to the water that condenses behind the sheet. Our roof insulation page covers the membrane case; this one covers panels.
The options, and where each one sits
| Option | Where it goes | Suits | Watch for |
|---|---|---|---|
| Faced blanket over purlins | Draped over the frame, panel fixed through it | New build, unheated or lightly heated | Compression at the purlin line, torn facing |
| Blanket between purlins | Held in the purlin depth on a support mesh or liner | Retrofit from inside, moderate performance | Gaps at the ends, sagging over time |
| Rigid board over a liner panel | Liner, insulation, then outer panel on spacers | Heated buildings, higher performance | Spacer design, fixing length, alignment |
| Rigid board under a standing seam roof | Over a deck or liner, clips through it | Where performance and watertightness both matter | Clip bridging, board movement |
| Spray foam to the underside | Applied to the panel from inside | Awkward geometry, air leakage problems | Access, inspection of the panel afterwards |
| Loose fill or blanket at ceiling level | On the ceiling, not the roof | Where the roof space can be left outside | Ventilating the space above properly |
The last row is worth noting because it changes the problem rather than solving it. Insulating at ceiling level leaves the roof space cold, which is fine if that space is ventilated to outside air, and a condensation trap if it is not.
Where the performance actually goes
A metal roof assembly rarely performs at the number printed on the insulation. The steel is why. Purlins run continuously from the cold panel to the warm interior, clips and fasteners pierce the insulation at close centers, and steel moves heat orders of magnitude faster than the material around it.
Three details recover most of what would otherwise be lost:
- Thermal blocks on every purlin line, breaking the steel path at the point where it is widest.
- Standoff or thermally broken clips on standing seam roofs, which raise the panel and reduce the conduction through each fixing.
- Two layers of board with staggered joints rather than one thick layer, so no joint runs all the way through.
What to ask for on a specification is the performance of the complete construction, framing and fixings included, rather than the rating of the insulation on its own. Those are different numbers and only one of them describes your building.
Condensation is the real failure mode
Most metal roof problems that are reported as leaks are not leaks. Warm moist air inside the building rises, reaches the underside of a panel that has been cooled by a clear night sky, and gives up its moisture on the steel. It runs down the pan and drips, generally in cold dry weather rather than during rain, and generally across the whole roof rather than from one point.
The evidence is easy to read once you know what to look for. Rust forming on the points of fasteners seen from below, staining in lines that follow the purlins, damp insulation facing with no hole above it, and complaints that arrive in winter and stop in spring. Our page on commercial metal roofing covers the panel system itself, and metal roof flashing covers the details where genuine leaks do start.
Four things reduce condensation, and they work together rather than in competition:
- Reduce the moisture being released into the building, which is usually the cheapest intervention by a wide margin
- Break the thermal path so the inside surfaces are not as cold
- Control the vapor with a sealed layer on the warm side of the insulation
- Ventilate the space behind the panel so whatever does get through can dry
Vapor control, and getting it continuous
In a heated building the vapor control layer goes on the warm side of the insulation, below it, and its value depends entirely on being continuous. Laps sealed, penetrations sealed, edges turned and sealed at the perimeter. A layer that is carefully laid and then opened up for every service and cleat without being closed again is performing a fraction of its job while appearing on the drawing as if it were complete.
Where the building below is humid by its nature, the requirement rises sharply and the consequences of getting it wrong rise with it. That is a design decision, not a roofing decision, and it belongs in the specification rather than in the method statement.
Adding insulation to a roof you already have
Two routes, and the roof’s condition usually decides which.
From inside, adding blanket or board between or beneath the purlins. The roof stays watertight throughout, the cost is lower, and it needs headroom, access, and care not to place new insulation on the cold side of an existing vapor barrier. On an occupied building it can usually be done in sections.
From above, using the existing panels as a base for rigid insulation and a new panel or membrane over the top. This is the right answer when the panels are also at the end of their life, because it deals with both problems in one visit. It needs the structure checked for the extra load, and every detail on the roof raised to suit the new height. Our page on roof recover against tear off covers that decision, and types of roof flashing covers the details that have to be rebuilt when the roof gets thicker.
What a specification should state
- Insulation type, thickness and where it sits relative to the purlins
- The performance of the complete construction, not just the material
- Thermal blocks, and on which lines
- Clip or spacer type, and whether it is thermally broken
- The vapor control layer, its position and how it is sealed
- Whether the space behind the panel is ventilated, and how
- Fastener type and length for the final build up depth
Metal roof cost explains why two quotes on the same building separate, and the insulation build up is one of the larger reasons.
Where to go next
If the roof is a new build or a full replacement, the insulation decision is made at the same time as the panel decision, and standing seam metal roofing covers the system that handles both best. If the building is already dripping in cold weather, get the assembly looked at rather than the roof repaired, because sealing a roof that is condensing makes the complaint worse.
Common questions
Why does a metal roof drip when it is not raining?
That is condensation, not a leak. The underside of a metal panel gets colder than the air inside the building, moisture in that air reaches its dew point against the steel, and it runs down the pan and drips off the laps or the fixings. It appears on clear cold nights rather than in storms.
- Appears in cold clear weather, not during rain
- Spread evenly across the roof, not at one point
- Worst above humid areas and over open water
- Rusted fastener points below are the usual evidence
- A leak tracks from one place and stains in a line
Telling the two apart matters because the fixes have nothing in common. A leak is a roofing repair. Condensation is an assembly problem fixed by reducing the humidity inside, interrupting the thermal path so the steel is not as cold, adding a vapor control layer on the warm side, or ventilating the space behind the panel. Sealing the roof harder makes condensation worse rather than better, which is why it is worth establishing which one you have before anybody is paid to stop it.
What is a thermal block, and why does a metal roof need one?
A thermal block is a strip of rigid insulation laid along the top of each purlin, under the panel or under the clip. It separates the cold steel panel from the warm steel purlin, which is otherwise a direct thermal path straight through the insulation either side of it.
- Sits on the purlin line, under the panel or the clip
- Breaks the steel to steel path through the assembly
- Recovers insulation value the purlins would waste
- Keeps the purlin warmer, so it condenses less
- Raises the panel, so the fixings must suit the depth
Leaving them out is a quiet saving that shows up two ways. The assembly performs below the figure on the drawing, because the purlin line is a continuous bridge across the roof. And the inside face of the purlin runs cold enough to condense, which is why rust and staining on an older building so often form neat lines across the ceiling rather than patches. If a bid for a new panel roof does not mention thermal blocks, ask whether the quoted performance assumed them.
Can insulation be added to a metal roof without taking it off?
Often yes, in one of two ways. Insulation can be added below, between or beneath the purlins from inside the building, or the existing panels can be used as a base for rigid board and a new panel or membrane over the top. Both avoid a full tear off.
- From inside: blanket or board between or under purlins
- From above: board over the old panels, new covering on top
- Working from inside needs headroom and access
- Working from above needs the structure checked for load
- Either way the vapor control layer has to be thought about
The decision usually comes down to what else the roof needs. If the panels are sound and the only goal is thermal performance, working from inside is cheaper and keeps the roof watertight throughout. If the panels are corroding, leaking at the fixings or near the end of their life, adding insulation above and recovering is one job instead of two. What makes either go wrong is putting insulation on the cold side of an existing vapor barrier, so that is the first thing to establish.
Does a metal roof need a vapor barrier?
In a heated building, almost always. Warm humid air from inside moves up into the roof, reaches the cold panel and condenses there. A vapor control layer on the warm side of the insulation stops most of that air getting to the cold surface in the first place.
- Goes on the warm side, below the insulation
- Laps and penetrations must be sealed to work at all
- A torn or discontinuous layer performs close to nothing
- Humid processes below raise the requirement sharply
- Ventilation behind the panel handles what gets through
The part that gets missed is continuity. A vapor layer is only as good as its weakest run, and a sheet carefully installed across the field and then cut around every pipe, light fitting and purlin cleat without being resealed is doing a fraction of its job. Where the building houses something humid, a pool, a laundry, a kitchen, food processing, the requirement goes up and so does the cost of getting it wrong. That is a design question rather than a roofing one, and worth asking early.
What is thermal bridging on a metal roof?
It is heat taking a shortcut through the metal rather than passing through the insulation. Purlins, clips, fasteners and panel ribs are all steel running from the cold outside to the warm inside, and steel carries heat far faster than any insulation sitting around it.
- Purlin lines are the largest and most continuous bridge
- Clips and fasteners bridge at close centers
- The installed performance is lower than the material rating
- Thermal blocks and standoff clips reduce it
- A cold bridge is also where condensation forms
This is why the number on an insulation product and the performance of the finished roof are different things, sometimes substantially. A specification that states only the insulation thickness has described the materials rather than the assembly. What to ask for instead is the performance of the whole construction including the framing and the fixings, and what has been allowed for to interrupt the metal path. The same detail that recovers the heat loss also keeps the inside face warm enough not to condense, so it is worth paying for twice over.