Rockwool & Glasswool
Insulated Sheets
Mineral fibre under a sheet roof
Rockwool and glasswool are mineral fibre blankets laid under the roof sheet. They are the most common way of insulating an existing or a plain single skin roof, because they can be installed without changing the sheet itself.
Both also cut noise, which is why they appear in auditoriums and workshops as often as in temperature-controlled buildings. Rain noise on a bare metal roof is considerable, and a fibre blanket makes a marked difference.
Rockwool against glasswool
Rockwool holds its shape at higher temperatures and is the choice where fire behaviour is being considered. It is denser, which also makes it better at cutting sound at the frequencies that matter in a workshop.
Glasswool is lighter and generally cheaper for the same thickness. Where the requirement is straightforward thermal and acoustic improvement without a fire consideration, it is the economical answer.
Density matters as much as thickness on both. A thick blanket at low density that compresses to nothing under its own weight over a season is not delivering the thickness on the drawing.
The two ways it fails
Compression is the first. Fibre insulation works because of the air trapped in it. Squash it and the air goes, and so does most of the value. Insulation pinched at every fixing and sagging between supports is performing well below specification.
Moisture is the second, and it is worse. Warm moist air from inside the building passing into a cold fibre blanket condenses there. Wet mineral fibre insulates very poorly, it stays wet, and it holds water against the underside of the sheet, which corrodes it from the side nobody inspects.
This is why a vapour barrier on the warm side and proper support underneath are not optional extras. They are what makes the insulation keep working after the first year.
Facings and finish
Blankets are available faced, and the facing does two jobs: it acts as the vapour barrier and it gives a tidier underside than bare fibre. Where the ceiling is visible, a liner sheet or a proper faced product is worth it, because bare mineral fibre sheds and collects dust.
What to send us for a quote
The covered area, the purlin spacing, and whether the priority is heat, noise or both. If fire behaviour is being specified, say so, because that decides rockwool over glasswool.
Frequently asked questions
Which is better, rockwool or glasswool?
Rockwool for fire behaviour and for sound at workshop frequencies. Glasswool for cost at the same thickness. Both insulate well when kept dry and uncompressed.
Can it be installed under an existing roof?
Yes, and that is the usual retrofit. It needs support underneath and a vapour barrier on the warm side, otherwise it sags and gets wet.
Will it stop rain noise?
It reduces it substantially, particularly rockwool. It will not make a metal roof silent, but the difference under heavy rain is obvious.
Why does insulation need a vapour barrier?
Warm moist air from inside condenses when it reaches the cold blanket. Wet fibre insulates poorly, stays wet, and holds water against the underside of the sheet where it corrodes.
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