Where do you use the Gold. Silver or Multistrat membranes, and where those with variable SD?
Published on 22.07.2026 · Isoflect Insulation technical team
Isoflect technical guide
There is no good membrane without an address.
It is not the membrane that decides — your roof or wall decides.
The wrong question: “Which membrane is better?” The right question: “Which membrane does my build-up require?” The same product that performs impeccably in one build-up may create problems in another, and the difference lies not in the quality of the membrane, but in the address at which you install it.
Since 1993 in the insulation industry and since 2005 working with membranes, we have seen a single recurring pattern in moisture problems within structures: it was not the product that was wrong, but the match between the product and the build-up. This guide is the matching rule, explained once, so that you can apply it yourself.
What “sd” actually measures
The sd value (the equivalent air layer thickness for diffusion) indicates one thing only: how difficult it is for water vapour to pass through the material. A high sd means a strong vapour retarder; a low sd means a material that is open to diffusion. From this derive the two families of membranes:
Reflective membranes
Isoflect Gold · Silver · Multistrat
high and constant sd — vapour barriers
Practically complete vapour barrierStrongly limits vapour diffusion from the interior towards the insulation.
Genuine thermal gainThe aluminium surface, installed next to an enclosed air layer, reduces heat transfer by radiation.
Maximum performanceWhere the drying path towards the exterior exists and has been verified.
Variable sd membrane
Hygrovariable
sd that adapts to humidity
Hygrically adaptiveIn winter, in dry air, it closes and retards vapour; in summer it opens and allows the structure to dry towards the interior.
Retarder in winter, open in summerIt is not a directional valve — it reacts to the hygrothermal conditions on both faces.
Tolerance and safetyProtects the structure in assemblies without a drying path towards the exterior or with unknown layers.
The golden rule: the drying path towards the exterior
Above the insulation there is a diffusion-open membrane and a verified ventilated channel. The structure dries continuously towards the exterior. The reflective barrier on the interior is the highest-performing choice.
Above the insulation there is an impermeable layer (bituminous felt, bitumen without double ventilated roof boarding, OSB, flat roof). The only remaining drying path is towards the interior. The variable sd membrane becomes mandatory.
Quick decision guide
| Your build-up | The appropriate membrane |
|---|---|
| Roof with diffusion-open membrane + verified ventilated channel | Gold / Silver / Multistrat |
| Roof with bituminous felt or bituminous membrane, without double ventilated roof boarding | Variable SD |
| Wall or roof with OSB on the exterior, without a ventilation space | Variable SD |
| Wall with OSB + ETICS | Variable SD |
| Flat roof / impermeable roof covering on a substrate | Variable SD |
| Renovation with unknown layers beneath the roof covering | Variable SD |
| Cellulose / wood fibre insulation | Variable SD |
| New construction with a designed and verified drying path towards the exterior | Gold / Silver / Multistrat |
Note the logic of the table: the reflective membrane appears where the drying path towards the exterior exists and has been verified. Variable SD appears where the exterior is blocked or unknown. This is not a quality hierarchy — they are two tools for two different situations.
The defect that wets the structure is not the membrane. It is the untaped joint.
However correctly the membrane has been chosen, the choice is nullified by an installation that is not airtight. Warm air passing by convection through an untaped joint, an unsealed sleeve at a cable penetration or an untaped edge at the wall carries far more moisture into the structure than passes by diffusion through the surface of any membrane — irrespective of its sd value. Airtightness studies consistently demonstrate this order-of-magnitude difference between convective and diffusive transport.
The practical conclusion: joints taped along their entire length, airtight sleeves at every penetration, airtightness around the entire perimeter. In hygric terms, correct sealing is worth more than the difference between any two membranes.
Frequently asked questions
Can I install a reflective membrane if I do not know what lies above the insulation?
We do not recommend it. In hygric terms, “I do not know” means “I assume it is blocked”. In renovations with unknown layers beneath the roof covering, the variable-sd membrane is the tolerant choice: it performs correctly whether the exterior proves to be vapour-permeable or vapour-tight.
Does Reflectiva not “suffocate” the structure?
No, provided that the structure can dry towards the exterior. The vapour barrier on the interior does not hold moisture within the structure — it prevents it from entering. The problem arises only when the interior barrier is combined with a vapour-tight exterior layer: the structure is then closed on both sides, and it is precisely for that case that the variable-sd membrane exists.
Is the variable-sd membrane “more modern”, and therefore better?
It is more tolerant, not universally better. In a build-up with a verified drying direction towards the exterior, the reflective barrier provides stronger vapour protection in winter, together with the radiant component that the hygrovariable membrane lacks. Each family prevails in its own domain.
What should I do with a wall that has OSB on both the interior and the exterior?
The all-OSB configuration is common in timber structures in Romania and is the most hygrically sensitive case: the exterior OSB slows drying towards the outside. Here, the variable-sd membrane on the interior, installed with complete sealing, is the safe solution. For particular build-ups, analyse the specific case before installation.
See the complete map — which Isoflect product is installed in each area of the house →
Do not guess. Analyse the build-up.
Technical consultancy is included with every project: we analyse the actual build-up, with its layers and drying directions, before making any recommendation. Send us a section through the roof or wall — even a hand sketch.
or call directly: +40 790 644 051