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Fibreglass vs Mineral Wool Insulation in Ontario: The Batt Decision, R Per Inch, Cost Per Square Foot, Fire, Sound, How Each One Gets Installed Badly, and Which Goes Where in the Houses I Build
Insulation & Air Sealing
Fibreglass vs Mineral Wool: Same R on the Label, $0.90 a Square Foot Apart, and the Difference Shows Up in a Fire, a Flood, and the Bedroom Next to the Laundry
Two batts, both R-22, both cut to fit a 2×6 bay. The pink one costs about $1.50 a square foot installed and the brown one about $2.40. For most of a house the pink one is fine and the money is better spent on the air sealing that either batt depends on. For some of the house, the wall against the garage, the partition between a bedroom and a bathroom, the ceiling under the laundry, anything near a chimney, the brown one earns its price in ways the R-value does not show. Neither of them stops air any more than a sweater does, and that is the first thing to get straight before choosing.
The numbers
$1.50fibreglass R-22, per sq ft installed
$2.40mineral wool R-22, per sq ft installed
R-17effective, either batt, 2×6 wall at 16 in. o.c.
STC 45mineral wool 2×6 wall, vs 39 to 42 fibreglass
| Fibreglass batt | Mineral wool batt | |
|---|---|---|
| R-22, 2×6 wall, installed | $1.25 to $1.75, default $1.50 | $2.00 to $2.75, default $2.40 |
| R-14, 2×4 wall, installed | $1.10 | $1.80 |
| Material share | roughly half the installed rate | about double the fibreglass material; labour is the same |
| R per inch | 3.0 to 3.7 | 3.7 to 4.3 |
| Effective R, 2×6 wall | about R-17 | about R-17 to R-18 |
| Fire | Glass does not burn; it softens and slumps around 540 C; kraft facing burns | Non-combustible; melts above about 1,000 C; used in rated assemblies |
| Sound, 2×6 wall | STC 39 to 42 | STC 45 to 48 |
| Wet | Slumps, holds water in the bay, slow to dry | Drains, keeps its shape, dries |
| Handling | Soft, compresses, sags in a loose bay, itches | Stiff, friction-fits, cuts with a bread knife, dustier |
| Air barrier | No | No |
| Vapour barrier | No; 6 mil poly, $0.50 | No; 6 mil poly, $0.50 |
Both batts need the poly on the warm side and a sealed air barrier to work as advertised; what nominal and effective R mean is on the R-value page, and the whole-wall numbers come from the wall assembly comparison calculator.
What the two actually are
Fibreglass is spun glass, mostly recycled bottles, held together with a binder. Mineral wool is spun rock, basalt and slag from steel mills, made the same way at a much higher temperature. That is why one melts at a house-fire temperature and the other does not, and why the brown batt is dense enough to stand up on its own in a bay while the pink one needs the poly to hold it off the floor.
The R per inch gap is real but small in a wall. A 5.5 in. bay of fibreglass is sold as R-20 or R-22; the same bay of mineral wool is R-22 or R-24. Run either through the framing arithmetic, with about 23% of the wall being studs, plates and headers at R-1.2 an inch, and the wall comes out R-17 or a point better. The code and the heat-loss calculation use the effective number. If you are paying $0.90 a square foot more for mineral wool, you are not paying for R; you are paying for fire, sound and water.
Installation is most of the R
A batt that is 90% of the bay is not 90% of the R. Fold a corner over, leave a 1 in. gap at the top, push a batt behind a wire so it is squashed to half its thickness for a 2 in. strip, and the cold air finds the gap and runs the whole bay. The inspector is looking for full contact on all six sides, batts split around wires and pipes rather than stuffed behind them, no compression, no gaps at the plates, and the poly sealed afterward. Fibreglass is easier to get wrong because it is soft: it slumps if the bay is wider than 16 in., it droops in a ceiling, it pulls away from a stud it was not cut tight to. Mineral wool is cut 1/4 in. oversize and friction-fits; a bay done badly is still visibly done badly, but it takes effort.
Either way, the batt is a filter, not a barrier. Air goes through it. A batt in a leaky wall is a wool sweater in the wind, and fibreglass does not stop air any more than a sweater does. The air barrier in a frame wall is the sealed poly or the taped sheathing, and it is the thing to inspect before the drywall goes on. The whole subject is on the walls and airtightness hub.
Where each goes in the houses I build
My exterior walls are ICF, above and below grade: 2.5 in. of EPS each side of a concrete core, about R-22 to R-24 effective with nothing added and no poly on the wall. So the batt decision inside one of my houses is never about the exterior wall. It is about the interior partitions, the garage, and the floors. The same rooms are the ones to think about in a frame house too, after the exterior wall is sorted.
- Bedroom and bathroom partitions: mineral wool, R-14 at $1.80. A master bedroom sharing a wall with the ensuite toilet or the laundry is quiet with wool and not with fibreglass. 600 sq ft of partition is $1,080, and it is the cheapest quiet money buys.
- The wall and ceiling between the garage and the house: mineral wool. The separation has to be gas-tight under the code and it is where a car fire starts. Wool does not feed it. The floor over the garage under a bonus room gets 2 in. of closed cell against the subfloor first, for the air seal, then R-28 wool or fibreglass below.
- Around a chimney or a B-vent: mineral wool, with the 3 in. (75 mm) clearance the code asks for still kept. Non-combustible is not the same as “allowed to touch”.
- Floors over an unheated space: R-31 at $2.00 in fibreglass, supported so it stays up against the subfloor. Wool here only if sound or fire is the reason.
- The attic: neither. Blown cellulose or blown fibreglass to R-60 over the ceiling poly. Batts in an attic leave gaps at every joist and are the worst value on this page.
- A frame exterior wall, if you have one: fibreglass R-22 with poly, $2.00 all in, and spend the $0.90 saved per square foot on air sealing and 1 in. of continuous exterior foam, which takes the wall from R-17 to about R-21.5 effective. That is a better wall than mineral wool alone at R-18.
The whole house, both ways
A 2,000 sq ft two-storey frame house has about 2,400 sq ft of net exterior wall. Fibreglass R-22 at $1.50 is $3,600 and the poly at $0.50 is $1,200, so $4,800, which is the number in the new-house package on this site. Mineral wool at $2.40 is $5,760 plus the same $1,200, $6,960. The difference is $2,160 before HST for about a point of effective R and a wall that burns last.
I would spend the $2,160 differently: $1,080 of mineral wool in the partitions where sound matters, and the other $1,080 toward the air sealing package, which does more for the heating bill than either batt. The sweater is only warm if the wind cannot get through the coat.
Where the money goes wrong: mineral wool everywhere because it sounded premium, and the poly stapled with 3 in. gaps at the plates. The wall is R-18 on paper and R-10 on a windy night. Fire, sound and water are what you buy with wool. Airtightness is what you buy with sealant, tape and the inspector’s flashlight.
Questions people ask
Is mineral wool better than fibreglass?
It is non-combustible, quieter by 5 or 6 STC points, holds its shape when wet and a little more R per inch. It costs about $0.90 a square foot more installed. In an exterior wall with a sealed air barrier, both come out about R-17 effective and the money is better spent on air sealing. In partitions, garage separations and near chimneys, wool.
How much does mineral wool cost compared to fibreglass?
R-22 in a 2×6 wall: $1.50 a square foot for fibreglass, $2.40 for mineral wool, installed. R-14 in a 2×4 wall: $1.10 against $1.80. The material is about double; the labour is the same.
Does mineral wool need a vapour barrier?
Yes, the same as fibreglass: 6 mil poly on the warm side with acoustical sealant, or a smart retarder, and a sealed air barrier. Neither batt stops air or vapour on its own.
Is fibreglass insulation fireproof?
The glass does not burn, but it softens and slumps at house-fire temperatures and any kraft facing burns. Mineral wool stays in place past 1,000 C, which is why fire-rated assemblies specify it.
Can I mix the two in one house?
Yes, and I do. Fibreglass where cost per R matters and the bay is protected; mineral wool where sound, fire or possible wetting is the reason. Both must be fitted full and tight or the R on the label never arrives.
Put the batts beside the foam
If a quote is offering spray foam where a batt would do, or a batt where only foam belongs, the spray foam cost calculator prices the foam side surface by surface so the two can be held side by side at the rates above. Three quotes with three different batts and three different R-values are what the $99 review was built for.
Sources and further reading
- Owens Corning Canada: fibreglass batts, blown fibreglass and rigid foam product data: R-value per inch, thickness and coverage.
- Rockwool: mineral wool batts and boards: R-value, fire rating, sound ratings and installation.
- Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
