ᐅ What is the recommended sd-value for a vapor retarder?

Hello everyone,

I have already searched for information but haven’t found anything that fits perfectly.

I’m wondering which vapor retarder I should choose. So far, I have two offers: one is a top foil with an sd-value greater than 100, and the other is the Siga Majpell with an sd-value greater than 5. I was personally thinking about one with a variable sd-value. However, the plasterer expressed concerns, saying that such a vapor retarder might open up during the cold season and moisture could then cause problems.

My assembly is as follows:
Delta Maxx Plus
200mm (8 inches) of insulation board with a thermal conductivity of 0.035 W/(m·K)
Vapor retarder with questionable sd-value
40mm (1.5 inches) counter battens with a thermal conductivity of 0.035 W/(m·K)

Siga offers the Majpell with an sd-value of 5 and also with 25. Which one should I use, or what should this decision depend on?

Regards

I don’t understand the construction. Vapor retarder or vapor barrier? For a vapor retarder, I would choose one with an sd value of 2–3 meters (6.6–9.8 feet), maybe a Proclima DB.

MiIng schrieb:
I was thinking about a variable sd-value membrane.

Why? What advantages do you expect from that? There is no critical construction that would allow condensation or moisture accumulation inside the insulation.

The further outward the membrane is placed within the insulation layer, the more vapor-permeable it should be. For insulation placed under the rafters, you should never use a vapor barrier (sd >1000 m (3281 ft)). Everything else depends on the roof’s ventilation situation, existing insulation above the rafters, the sd-value of the insulation between the rafters, and the sd-value of the underlay membrane.

I don’t see why you would need more than 5 m (16 ft) sd-value. Everything beyond that usually has an sd-value of around 0.5 m (1.6 ft) at most and is ventilated.