What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingquilt
also insulation roll, loft roll, slab, batt, mineral wool
A British SLAB is a rigid or semi-rigid mineral wool board, not a concrete slab and not the same as a quilt. Loft roll is sold by roll length and cut on site rather than pre-cut to a bay.
Australia
Close, not identicalbatts
also insulation batts, blanket, ceiling batts
American terminology and format, with batts pre-cut for the local framing module rather than sold as roll.
United Statesyours
Same word, different thingbatt
also blanket, faced batt, unfaced batt, roll
A FACED batt carries a kraft or foil facing acting as a vapour retarder and a stapling flange — a specification decision about which side it goes, and there is no direct British equivalent product.
Canada
Close, not identicalbatt
also blanket, friction fit, unfaced batt
As the United States, with unfaced batts more common because a separate continuous polyethylene air/vapour barrier is normally specified instead of a facing.
The governing standard, by market
United Kingdom
BS EN 13162 / Approved Document L
Factory-made mineral wool products for thermal insulation, and conservation of fuel and power.
United States
ASTM C665 / IECC
Mineral-fiber blanket thermal insulation for light frame construction, and the energy code's required R-values by assembly and climate zone.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Does compressing it into the bay reduce its performance?
- Yes for compression into a shallower space, no for slight lateral squeeze, and the distinction is the one people get backwards. Fibrous insulation works by trapping still air, and the thermal resistance comes mostly from the THICKNESS of that trapped air layer — so squashing a 100 mm quilt into a 75 mm space gives you roughly the resistance of 75 mm, not of 100. But a batt slightly wider than the bay, compressed laterally so it grips the studs, keeps its full thickness and simply has a slightly higher density, which is a negligible change and is exactly how a friction fit is meant to work. The failure that actually costs performance is neither: it is a batt cut short, left with gaps at the ends, or with the ends not touching, because a small gap short-circuits a large area.
- Faced or unfaced, and which way does the facing go?
- Toward the warm side in winter, which in most of North America means toward the interior — and that is the rule, not a preference. The facing on a faced batt is a vapour retarder, there to slow water vapour moving from warm humid air into the cold part of the assembly where it would condense. Put it on the cold side and it does the opposite: it traps moisture in the insulation. That is also why British practice generally uses unfaced quilt with a separate vapour control layer, and why Canadian practice specifies a continuous polyethylene air/vapour barrier rather than relying on the facings. The facing's stapling flange is a convenience; its position is a building-physics decision, and in a mixed or hot-humid climate the correct answer may be no vapour retarder at all.
- Why does a wall never achieve the batt's R-value?
- Because the studs are not insulation and they run right through. A timber stud has perhaps a quarter of the thermal resistance of the insulation beside it, and in a wall at 400 mm centres with headers, sills, corners and junctions the framing is a substantial fraction of the wall area — commonly a fifth to a quarter in real construction rather than the tenth people assume. Heat takes the easy path, so the wall's whole-assembly U-value is far worse than the batt's nominal R-value suggests. That is why energy codes have moved to assembly performance and continuous exterior insulation rather than cavity R-value alone: a layer outside the framing interrupts every one of those bridges at once, which no amount of better batt can do.