ᐅ Workshop Insulation – Is a Vapor Retarder Necessary?

Hello everyone,

Next spring, I plan to insulate the ceiling of my workshop. The masonry consists of 175mm (7 inches) calcium silicate bricks, and the roof is a trussed rafter structure with a standard tile covering.

The insulation will be installed as friction-fit mineral wool batts between the ceiling joists and finished with OSB boards. The workshop will be heated on demand using a small diesel parking heater.

Now, the big question: Would you install a vapor retarder here? Most of the time, the space will not be heated. I'm concerned that moisture might condense between the vapor retarder and the insulation.

Thanks and best regards

As far as my understanding goes and as I plan to do in the garage, yes. A vapor retarder (definitely not a vapor barrier!) is absolutely necessary.

gmt94 schrieb:

Now here’s the big question. Would you install a vapor retarder there? Most of the time, the area won’t be heated. I’m worried that moisture might condense between the vapor retarder and the insulation.

The construction for heated or partially heated rooms is ALWAYS: interior side more diffusion-resistant than the exterior -> any moisture that enters the insulation can also exit back out.
OSB boards have a μ-value of about 200 and, depending on thickness, an sd-value of 1–3 m (3–10 ft).

My suggestion would be to use moisture-resistant laminated mineral wool for insulating the top floor ceiling, with fleece lamination facing upwards, and bond OSB boards from below to the ceiling, sealing around all penetrations.

The OSB boards on the inside then serve as the airtight layer and vapor retarder, allowing any moisture that enters to escape upwards. At the same time, the insulation above is fleece-laminated in case 1–2 drops of condensate or driving rain enter through the roof. If there is an underlayment membrane beneath the roof tiles, then the lamination on the insulation is unnecessary.

You don’t need an additional vapor retarder foil if you don’t have anything vapor-tight on the outside. The OSB boards are sufficient for this. More important is airtightness; otherwise, you have warm, moist air moving through the insulation, which condenses there as soon as the dew point is reached.

Edit: misunderstood at first, now corrected.