How the two differ in kind
Continuous insulation outside the sheathing is asked to do two jobs, and only the first one is the same for both boards. The first is geometric: an unbroken layer over the studs removes the timber or steel bridge that short-circuits everything in the cavity, and for the same installed resistance a foam board and a mineral wool board do that identically — the heat does not know what the layer is made of, only how much resistance is in the way and whether it is continuous. The second job is hygrothermal, and it is the one people skip. Warm indoor air carries moisture, some of it always reaches the back of the sheathing through the cavity, and if that sheathing is below the dew point of the air arriving at it, water appears there. Insulation on the outside fixes this by raising the sheathing's winter temperature — it is now inboard of a blanket rather than exposed to it — and that is the actual mechanism by which exterior insulation protects a wall rather than merely improving it. Which board you pick changes nothing about the mechanism. It changes what happens to the moisture that still gets there.
That is where the two diverge, and they diverge in shape, not just degree. Most rigid foam used for this is vapour-closed to some meaningful extent — extruded polystyrene is a retarder, foil-faced polyiso is effectively a barrier — so as well as warming the sheathing, it seals the sheathing's outward escape. Get the thickness right and that is fine: the sheathing never gets cold enough to collect water in the first place, and what does arrive leaves inward. Get it thin and you have built the bad case deliberately — a sheathing still cold enough to condense on, now with no way out except back into the room, behind a layer nobody will see again. That is why the residential code carries a minimum share of the total resistance that must sit outboard when the exterior insulation is vapour-closed, varying by climate zone, and why a token layer of foam is not a small benefit but a genuine hazard. Mineral wool has no such line anywhere on it. It is vapour-open enough that the wall keeps drying outward through it, so a thin layer is simply a thin benefit and a thick one is a thicker benefit, all the way up, with no threshold to clear and nothing to get wrong. Add to that what is already on the inside face: a wall with polyethylene behind the plasterboard, or a retrofit whose layers nobody can honestly account for, has no inward drying in reserve — and a vapour-closed board outside it produces an assembly sealed in both directions at once.
The practical differences follow from depth and from layers. Mineral wool board sits near the bottom of the resistance-per-inch range — comparable to the lighter expanded foams, below extruded polystyrene, well below polyiso — so hitting the same number takes visibly more thickness. That extra depth is not charged where people expect. The boards themselves scale with wall area, but the consequences of depth land on the perimeter: deeper window and door bucks, jamb extensions and sill pans, longer fasteners, more trim, extended returns at every eave, rake, corner and abutment. A wall that is mostly glass and corners pays for thickness many times over; a long plain gable end barely notices it. Running the other way, taped foam can serve as the water-resistive barrier and the air control layer in the same plane, deleting a separate wrap; mineral wool never can, so there is always a barrier behind it on the sheathing — which is a layer of cost, and also a proven and inspectable place to flash to. Both boards then finish in exactly the same position: buried under cladding, unreachable, and committed at the window bucks before either one is delivered. Five questions settle nearly every real case. What is on the inside face of this wall, and can it dry inward. What depth can you actually detail at the openings and the eaves. Does the code's minimum outboard share for vapour-closed insulation fit inside that depth. Does anything about the building — height, occupancy, wildfire exposure — require non-combustible. And does this layer continue down past grade.
The factors that actually differ
| Rigid foam board | Mineral wool board | |
|---|---|---|
| Which way the wall can dry | Outward drying is reduced or stopped, depending on the foam and its facer. The wall is then designed to dry inward, which means the interior finish has to stay permeable and stay that way through future decorating and future owners. | Vapour-open. The sheathing keeps its outward escape route, so the wall dries both ways and the assembly survives an interior vapour retarder, an unknown retrofit build-up, or a spell of high indoor humidity without a redesign. |
| The thickness below which it stops helping | There is one, and it is in the code as a minimum share of the total resistance that must sit outboard, by climate zone. Below that line the sheathing is still cold enough to condense on and no longer able to dry outward — worse than leaving the wall alone. | There is none. Any thickness warms the sheathing by that much and the wall still dries outward, so the decision is how much resistance you want rather than how much you must have to be safe. |
| Depth for the same resistance, and where the depth is charged | Usually the shallower of the two, decisively so with polyiso and clearly so with extruded polystyrene, which keeps the wall thin where overhangs, existing window positions, a boundary line or a planning envelope have already fixed how far out you may build. Expanded polystyrene is the exception and buys back little or no depth — if that is the foam you are pricing, this row is not an argument for it. | More depth for the same number, and the cost of it lands on the perimeter, not the area — bucks, jamb extensions, sill pans, longer screws, trim, and returns at every eave, rake and corner. Openings and edges pay for the thickness; plain wall barely does. |
| How the rating behaves cold, and over decades | Not constant. Polyiso is rated at a mild mean temperature and loses ground as it gets cold, which on this layer is at its worst in the coldest week; extruded polystyrene drifts down over years as its blowing agent leaves; expanded polystyrene is stable and slightly better cold. | Flat and dull, in the good sense. Stone fibre has no blowing agent to lose and no cold-weather derate worth planning around, so the number on the pack is the number in February and the number in twenty years. |
| Which other layers it can replace | Taped and detailed as the manufacturer requires, it can be the water-resistive barrier and the air control layer as well as the insulation — one plane instead of three. The catch is that the building's water control is now a tape joint that will never be inspected again. | None. There must be a barrier on the sheathing behind it, and that is the plane windows flash to and air sealing seals to. An extra layer, but a conventional, inspectable, well-understood one that the insulation cannot compromise. |
| Getting wet, and getting dry again | Water reaching it runs down an impermeable face to whatever flashing is there. The board itself is unbothered, but anything damp behind it stays damp, and a rainscreen cavity plus meticulous flashing is doing all the work. | Hydrophobic and draining. Water passes through and out, the board sheds it and dries, and the assembly forgives the wet fortnight, the imperfect flashing lap and the leak found late — which is why it is the softer landing on a retrofit over unknown sheathing. |
| Fire | Combustible. It needs a thermal barrier on the room side, and on taller or multi-occupancy buildings a tested exterior wall assembly — several jurisdictions now restrict combustible insulation in external walls above certain heights outright. | Non-combustible stone fibre that does not burn, does not contribute smoke and holds its shape at temperatures that destroy every foam. On a building where that is required, or in a wildfire-exposed location, this factor alone can end the comparison. |
| Holding the cladding plane | Stiff and dimensionally stable, so furring pulled up against it seats on something solid and the cladding plane comes out flat with ordinary care. The flip side is that it telegraphs: a bowed stud or a proud old board shows through a rigid sheet. | Semi-rigid and compressible, so it conforms over an irregular substrate — an advantage on old work — but the furring is now bedding into it and consistent screw torque is what keeps the cladding plane straight. Over-driving one screw is a visible dish in the siding. |
| Pests, ground contact and time left open | Tunnellable. Ants and termites do not eat it but move through it readily, and a foam layer continuing past grade is a concealed route into the frame — restricted or inspection-gapped in some jurisdictions. It also degrades in sunlight, so a wall left open ages in the wrong direction. | Not a tunnel and not a meal, and indifferent to sun and to a wet exposed spell. But it is not the default below grade, so the run down to the foundation usually changes material anyway and the transition detail becomes a real one to draw. |
Which one, and when
Choose rigid foam board when…
- Depth is constrained and cannot move: existing overhangs, a boundary or planning envelope, or window positions you are keeping. Foam buys the most resistance per inch of wall you are allowed to grow.
- The wall genuinely dries inward and will keep doing so — new construction, no interior polyethylene, a permeable interior finish, and a specification that says so rather than an assumption that hopes so.
- You want the insulation to be the water and air control layer in the same plane, and you are prepared to detail the tape joints as if they were the building's weather barrier, because they are.
- The layer continues below grade or into ground contact, where mineral wool board is not the default and a suitable foam is.
- It has to come off ordinary stock and be hung by a crew that has done it before. Foam is stocked everywhere and universally understood; exterior mineral wool board is an order item in many markets and a first time for many siding crews.
Choose mineral wool board when…
- The wall must be allowed to dry outward — an existing house with an interior vapour barrier, a retrofit whose layers nobody can account for, an absorptive cladding, or a building that runs humid inside.
- The depth you can actually detail will not clear the code's minimum outboard share for vapour-closed insulation in your zone. Below that line mineral wool is the only one of the two that is still doing what it was bought to do.
- Non-combustible is required or wanted: a taller or multi-occupancy building, an assembly that has to pass a fire test, or a wildfire-exposed site where the exterior of the wall is the thing at risk.
- The substrate is irregular — board sheathing, out-of-plane studs, a century of repairs — where a semi-rigid board conforms and a stiff sheet telegraphs every high spot into the finished cladding.
- Traffic, rail or aircraft noise is part of the brief. A thick fibrous layer outboard of the sheathing does something for airborne sound that a stiff closed board does not.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which one is cheaper?
- The two sides put their money in different places, so a single figure would be a fiction — and this site does not publish one. Foam usually needs less depth for the same resistance and can delete a separate weather barrier, and both of those savings scale with wall area. Mineral wool's extra depth is charged almost entirely on the perimeter: deeper bucks, jamb extensions, sill pans, longer fasteners, extra trim and extended returns at every eave, rake and corner, which means the same thickness decision costs very little on a plain gable end and a great deal on an elevation that is mostly glass. Then there is the labour shape. Foam is light, fast to hang and slow to seal, because its performance as a barrier is in the taping. Mineral wool is heavy, has nothing to tape, and asks for care at the furring instead. And weigh the cost of being wrong, which is not symmetrical: an under-thick mineral wool wall is merely a wall with less insulation, while an under-thick vapour-closed foam wall can be a wall quietly wetting its sheathing behind cladding you will not open again. Run the board count and the package count against your own elevations and your own quotes, and price the window detailing at both thicknesses before you choose.
- How much foam is enough, and is a little better than none?
- Not necessarily, and this is the one place on this page where the honest answer is a warning. The rule that matters is not an absolute thickness but a ratio: enough of the wall's total resistance has to sit outboard of the sheathing to hold that sheathing above the dew point of the air reaching it from inside, and the required share rises with how cold the winters are, which is why it appears in the code as a table by climate zone rather than as a number. Clear it and vapour-closed foam is a safe, excellent wall. Fall short of it and you have combined the two bad properties — a sheathing still cold enough to collect water, and no outward path for that water to leave — in an assembly that gives no sign of it until something is opened up. Look up the share for your zone before you settle on a thickness, and if the depth you can genuinely detail at the windows will not reach it, that is not a reason to fit thin foam. It is the argument for the vapour-open board.
- My wall already has a vapour barrier on the inside. Can I still put foam outside it?
- This is the condition that most often settles a retrofit, and it settles it against the foam. Polyethylene behind the plasterboard means the wall has no inward drying to fall back on; adding a vapour-closed board outside gives it none outward either, and anything that gets into that cavity — a plumbing leak, wind-driven rain past a failed flashing, air leakage condensing on a cold night — has nowhere to go. Mineral wool leaves the outward direction open and is the straightforward answer here. If foam is unavoidable for depth reasons, the honest options are to remove or deliberately perforate the interior barrier, or to get the assembly modelled for your climate rather than assumed — and older houses in particular deserve that caution, because what is actually behind the finishes is frequently not what the drawings say.
- Does mineral wool still need a housewrap behind it?
- Yes, and it is not optional. Mineral wool is insulation and nothing else — it is not a water-resistive barrier, it is not an air barrier, and it drains rather than resists, which is exactly why the plane behind it has to do both jobs. That barrier goes on the sheathing, and it is what every window flange, every penetration and every flashing lap seals to. The upside of that arrangement is that it is the conventional detail every crew already knows, done in the position where it can be inspected before it is covered. With foam the same functions can be collapsed into the insulation itself, which saves a layer and moves the entire water control of the building onto tape joints behind cladding — a legitimate approach, but one that has to be detailed and inspected as the serious thing it is.
- Does it matter which foam — XPS, EPS or polyiso?
- Enough that treating rigid foam as one product is the most common way to get this wrong. Polyiso carries the highest rated resistance per inch and is therefore the usual answer where depth is constrained, but its rating is measured at a mild mean temperature and it loses ground as it gets cold — on this layer, which is the coldest part of the assembly, that shortfall arrives in the coldest week of the year, and foil facers make it effectively a vapour barrier. Extruded polystyrene is the middle option on depth, is a vapour retarder rather than a barrier, and drifts downward over years as its blowing agent diffuses out; its blowing agents have historically been the high global-warming-potential ones, though products have been moving away from them. Expanded polystyrene is the least resistance per inch, the most vapour-open of the three, stable over time and slightly better in cold — which makes it the foam that behaves most like the mineral wool option, and the one to look at first if the reason you wanted foam was price and depth rather than a vapour-closed layer.
- How does the cladding attach through either of these?
- The same way in principle and with different care in practice. Furring strips run over the insulation and are screwed through it into the studs, so the cladding's weight hangs on fasteners working in shear at a lever arm set by the insulation depth — which is why screw length, spacing and the depth of penetration into the framing all stop being incidental once the layer gets thick, and why beyond a certain thickness the attachment becomes an engineered question with proprietary clips or thermally broken rails rather than a rule of thumb. The difference between the two boards is what the furring bears on. Over foam it seats on a stiff surface and comes out flat with ordinary care. Over mineral wool the furring compresses into the board, so consistent torque is what keeps the cladding plane straight and one over-driven screw shows as a dish in the finished siding. Neither is difficult; both punish being improvised on the wall instead of decided on paper, along with the window position, which has to be fixed before the bucks are built.
- The two calculators count different things — how do I compare the quantities?
- They do, because the two products are not bought the same way, and the two answers are not interchangeable. The foam side comes out in whole sheets: rigid board is sold in one standard face size per market, and that size genuinely differs between metric and imperial markets, so the sheet count for the same wall is worked out against the board actually sold to you rather than converted from the other market's count. The mineral wool side comes out in packages, divided against the coverage area printed on the pack — and this is the part to read carefully, because the mineral wool calculator is a batt tool and opens on a cavity batt's package coverage. Exterior mineral wool board is a different product with a different pack coverage, so replace that default with the figure from the board's own data sheet before you trust the count. Start both from the same area, too: gross wall area less openings, measured on the plane the insulation actually runs on. Neither number is a price and neither should be turned into one here — take both quantities to your own supplier, and remember that on the mineral wool side the depth decision also has a perimeter bill attached that no board count will show you.