Attic OSB boards measuring device

Regarding the question of the OP.

In most cases, using OSB as a floor covering in an attic with a cold roof is to be avoided. Here, tongue and groove boarding on cross battens with a 1 cm gap is the method of choice. The requirements/recommendations are made by the house manufacturer. As I see here, it concerns a timber frame construction trade.

Reason: Of course, heat and some moisture from below reach the attic. Vapor barrier or not. Especially in the area of the upper floor ceiling / attic floor, the dew point is often reached (warm, dry air from below, cold and mostly moist air in the attic).

OSB cannot store moisture. Unlike tongue and groove boarding.
The moisture that then condenses does so in the insulation of the upper floor ceiling towards the attic. This often leads to mold, along with the successive destruction of the insulation material due to moisture (which increases over time). Tongue and groove boarding, on the other hand, can absorb and release moisture, so nothing of that kind happens here.

You can borrow a moisture meter (architect, tool store, internet). However, it will not help you much. What do you want to measure? The moisture of the OSB board? The moisture that is already in the insulation? Well – you already have such a device. In this case, I consider it not sensibly usable.

As painful as it now sounds: I would remove the OSB panels there and replace them with tongue and groove boarding (cost in DIY work in the range of 250-400 euros + disposal of OSB).
Please talk again with your house manufacturer about what they say (I suspect: the same).
With high probability, if you leave the OSB on the floor of the cold roof over the years, you will suffer a considerable building damage. This will then be much more expensive and complex to repair than "just replacing a few OSB panels" (as painful as that is, as mentioned).

Maybe Mr. Pickartz will be so kind as to add something more professional if necessary.

Regards
Thorsten

To return to the initial question:

Basically, OSB sheathing as the upper finish of the insulated collar beam layer to the cold attic is structurally not optimal and potentially harmful. The mechanism of moisture ingress has been described by T21150. OSB boards are – compared to tongue and groove boards (spruce solid wood) – possibly relatively more vapor-retardant: μ-value OSB, depending on the manufacturer, between 50/30 and 550/300, the μ-value of wood is at 20/50, whereby the lower value relates to wet material and the higher to dry.

The so-called sd-value (diffusion-equivalent air layer thickness) is calculated from the μ-value (water vapor diffusion resistance number) x material thickness in m, for a dry 21mm OSB board, e.g., 50 x 0.021 = sd-value 1.05. Materials are considered vapor-retardant from an sd-value of 0.5 upwards.

Depending on the OSB manufacturer, the board is therefore significantly more vapor-retardant than tongue and groove. The OSB board indeed absorbs less water than solid wood, i.e., the sd-value decreases correspondingly less (water absorption coefficient (kg/m2 x h x0.5) OSB 0.05 / spruce 2.0-3.0. In addition, the board is dense, installed with tongue and groove, and has a significantly lower joint ratio than tongue and groove, through which moisture could escape.

In short: the construction is not recommended and can lead to long-term moistening of the insulation. If it is mineral wool/glass wool, it is diffusion-open but not capillary-active, so it does not support drying out again.

The correct setup would be a ventilated OSB covering, comparable to a ventilated facade, but then with a windproof yet diffusion-open wind barrier foil on the beam layer. Alternatively, a strongly vapor-retardant or humidity-variable vapor retarder on the underside of the beams, which then must be installed extremely carefully and flawlessly. I try to avoid all these foils as much as possible. They are not error-tolerant.

Good! - and a pity that the building physics proved to be right. You can of course use the OSB boards for building the shelf, just not as the bottom closure (base plate) directly on the tongue and groove panel. The OSB boards must be ventilated on all sides. Good luck.

Can I at least use the OSB boards as shelves in the cold attic/loft?

After the cause of the mold has been found so easily, the existing mold must still be removed before further steps. If necessary, also from the OSB boards. After that, you can use them (see #16).